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Cardiac Emergencies: Sudden Death (FL INITIAL Autonomous Practice - Differential Diagnosis)

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Author:    Pamela Downey (MSN, ARNP)


Sudden cardiac arrest (SCA) and sudden cardiac death (SCD) refer to the sudden cessation of cardiac activity with hemodynamic collapse, typically due to sustained ventricular tachycardia (VT)/ventricular fibrillation (VF). These events mostly occur in patients with structural heart disease (that may not have been previously diagnosed), particularly coronary heart disease (CHD).

The event is referred to as SCA (or aborted SCD) if an intervention (e.g., defibrillation) or spontaneous reversion restores circulation, and the event is called SCD if the patient dies. However, the use of SCD to describe both fatal and nonfatal cardiac arrest persists by convention. The specific causes of SCA vary with the population studied and patient age. SCA most commonly results from hemodynamic collapse due to VF in the setting of structural heart disease.

The outcome following SCA depends upon numerous factors, including the underlying cause and the rapidity of resuscitation.

Most individuals suffering from SCA become unconscious within seconds to minutes due to insufficient cerebral blood flow. There are usually no premonitory symptoms. If symptoms are present, they are nonspecific and include chest discomfort, palpitations, shortness of breath, and weakness.

Helpful Acronyms

SCA = Sudden cardiac arrest

SCD = Sudden cardiac death

Abnormal Cardiac Rhythms
AFAtrial fibrillationPVCPremature ventricular contraction
AVAtrioventricular blockSQTSShort QT syndrome
CPVTCatechaminergic polymorphic ventricular tachycardiaSVTSupraventricular tachycardia
LQTSLong QT syndromeVFVentricular fibrillation
NSVTNonsustained ventricular tachycardiaVTVentricular tachycardia
PEAPulseless electrical activityWPWWolff-Parkinson-White
Cardiac/Noncardiac Diagnoses
ACSAcute coronary syndromeMIMyocardial infartion
ARVCArrhythmogenic right ventricularSTEMIST elevation MI
CHDCoronary heart diseaseNSTEMINon-ST elevation MI
CHFCongestive hear failurePTSDPosttraumatic stress disorder
CVDCardiovascular diseaseSIDSSudden infant death syndrome
HCMHypertrophic cardiomyopathySUDEPSudden unexplained death in epilepsy
HFHeart failureSUNDSSudden unexplained nocturnal death syndrome
Cardiac Interventions/Management
ACEAngiotensin-converting enzymeEMIElectromagnetic interference
AEDAutomated external difibrillatorEPElectrophysiologic
CABGCoronary artery bypass graftESUElectrosurgery unit
CCBCalcium channel blockerETTExercise tolerance testing
CMRCardiac magnetic resonance imagingICDImplantable cardioverter-defibrillator
CRPC-reactive proteinLVEFLeft ventricular carioverter-defibrillator
CRTCardiac resynchronization therapyMRIMagnetic resonance imaging
CRT-DCardiac resynchronization therapy defibrillatorPCIPercutaneous coronary intervention
DFTDefibrillation threshold testingS-ICDSubcutaneous coronary intervention
ECTElectrocardiogramWCDWearable cardioverter-defibrillator
EMSEmergency medical servicesAHAAmerican Heart Association
BLSBasic life supportHRSHeart Rhythm Society
ALSAdvanced life supportLVLeft ventricular
CARESCardiac Arrest Registry to Enhance SurvivalPAPosteroanterior
ACCAmerican College of CardiologyIVIntravenous


Various criteria have been used to define SCA and SCD in the medical literature. Difficulties in deriving a specific definition include the following:

  • Events are witnessed in only two-thirds of cases, making the diagnosis difficult to establish in many instances.
  • It is not possible to restrict the definition of SCA to documented cases of VF since the cardiac rhythm at clinical presentation is unknown in many cases.
  • The duration of symptoms before SCA generally defines the suddenness of death. However, the duration of symptoms is unknown in approximately one-third of cases.

For these reasons, operational criteria for SCA and SCD have been proposed that do not rely upon the cardiac rhythm at the event. These operational criteria focus on the out-of-hospital occurrence of a presumed sudden pulseless condition and the absence of evidence of a noncardiac condition (e.g., central airway obstruction, intracranial hemorrhage, pulmonary embolism, etc.) as the cause of cardiac arrest.

SCA is the sudden cessation of cardiac activity so that the victim becomes unresponsive, with no normal breathing and no signs of circulation. If corrective measures are not taken rapidly, this condition progresses to sudden death. Cardiac arrest should signify a reversed event, usually by CPR, defibrillation, cardioversion, or cardiac pacing. Sudden cardiac death should not describe events that are not fatal.


The following observations illustrate SCA:

  • The risk of SCA is increased 6- to 10-fold in the presence of clinically recognized heart disease and two- to four-fold in the presence of CHD risk factors.
  • SCD is the mechanism of death in over 60% of patients with known CHD.
  • SCA is the initial clinical manifestation of CHD in approximately 15% of patients.


SCA usually occurs in individuals with some form of underlying structural heart disease, most notably CHD.

Coronary Heart Disease (CHD)

Although not specifically mentioned in most of these studies, heart failure (HF) is a relatively common cause of SCD. Although the risk of arrhythmic and non-arrhythmic death can be reduced with appropriate chronic HF therapy, the SCD risk remains elevated. Thus, virtually all SCD survivors with HF receive an implantable cardioverter-defibrillator (ICD).

Examples of CHD (ischemic heart disease) include the following:

  • Coronary artery disease with myocardial infarction or angina
  • Coronary artery embolism
  • Nonartherogenic coronary artery disease (arteritis, dissection, congenital coronary artery anomalies)
  • Coronary artery spasm

Other Structural Heart Disease (Nonischemic Heart Disease)

Other forms of structural heart disease, both acquired and hereditary:

  • Acute pericardial tamponade
  • Acute myocardial rupture
  • Aortic dissection
  • Congenital coronary artery anomalies
  • HF and cardiomyopathy, SCD is responsible for approximately one-third of deaths
  • Dilated cardiomyopathy
  • Hypertrophic cardiomyopathy
  • Arrhythmogenic right ventricular dysplasia/arrhythmogenic right ventricular cardiomyopathy (ARVC)
  • Left ventricular hypertrophy due to hypertension or other causes
  • Myocarditis
  • Mitral valve prolapse

No Structural Heart Disease

Several major diseases must be considered as possible causes of SCD in patients without evidence of structural heart disease. Many of these disorders are familial and therefore are associated with an increased risk of SCD in first-degree relatives.

Brugada Syndrome

  • The Brugada syndrome is characterized by the electrocardiographic (ECG) findings of right bundle branch block and ST-segment elevation in leads V1 to V3 and an increased risk of SCD.
  • Brugada syndrome is due to a functional abnormality in repolarization. There may be some overlap with an early subclinical manifestation of ARVC.
  • The Brugada syndrome occurs in families, with genetic transmission consistent with autosomal dominant inheritance with variable penetrance. Mutations in the cardiac sodium channel gene, SCN5A, have been found in several families.
  • A sudden unexpected nocturnal death syndrome (SUNDS) has also been described in young, apparently healthy males from Southeast Asia. This disorder is closely related and may be the same as Brugada syndrome since most affected patients have the ECG manifestations of the Brugada syndrome and the same mutations in the sodium channel gene.
  • Different mutations of the same SCN5A gene have been found in a number of cardiac disorders, including the long QT syndrome, and a unique allele found in African Americans, Y1102.

Commotio Cordis (Chest Wall Trauma)

  • Commotio cordis is SCD secondary to VF's relatively innocent chest wall impact. Affected patients have no underlying heart disease, and there is no structural damage to the chest wall, thoracic cavity, or heart.
  • Early defibrillation of victims is lifesaving, despite historical evidence that resuscitation may be more difficult in commotio cordis than in other forms of SCD.

Early Repolarization Syndrome

  • Early repolarization pattern on an ECG is common.
  • Studies have shown a higher frequency of early repolarization in SCA survivors without apparent heart disease. These survivors tended to have increased incidences of recurrent VF compared with those SCA survivors with normal hearts and no early repolarization.
  • Early repolarization in the inferior and lateral leads is associated with an increased risk of SCA.
  • Early repolarization ECG pattern is especially common in athletes, and it is generally benign in these individuals. An expert consensus panel does not recommend any specific treatment for early repolarization without SCA.

Familial SCD of Uncertain Cause

  • A family history of SCD in the absence of apparent structural heart disease is associated with an increased risk for primary SCD.
  • The absolute increase in risk is quite small since primary SCD is rare in the general population. Genome-wide association studies have demonstrated an increased risk with several loci (Wei et al., 2016).

Idiopathic VF (Primary Electrical Disease)

  • If all of the above disorders are excluded, and the heart is structurally normal, primary electrical disease is diagnosed. More commonly referred to as idiopathic VF, this entity is estimated to account for 5% of cases of SCD.
  • In a review of 54 published cases with presumed idiopathic VF, the mean age was 36 years with a male-to-female ratio of 2.5-to-1.30. A history of syncope preceded the episode of VF in 25%. In a meta-analysis of 639 patients with idiopathic VF from 23 studies, 80% had an ICD for secondary prevention, 31% of patients had recurrent ventricular arrhythmias over a mean follow-up of five years (Weeu & Gyabgm 2016).

Long QT Syndrome (LQTS)

  • Long QT can be primary (genetic/inherited), i.e., Long QT Syndrome (LQTS) or secondary (acquired). It may be associated with a specific form of polymorphic VT called torsades de pointes. Among patients with inherited LQTS, the precipitating factors and prognosis vary with the genetic abnormality.
  • The majority of secondary causes of a prolonged QT interval result from an interaction with a drug/electrolyte which interferes with an ion channel involved in repolarization, the same ion channels involved in the LQTS. Most of the pharmaceutical agents which increase the QT interval are prescription drugs such as antipsychotics, anti-emetics, quinolones, antiarrhythmics, and methadone.

Short QT Syndrome (SQTS)

  • SQTS is an extremely rare inherited channelopathy associated with markedly shortened QT intervals and SCD in individuals with a structurally normal heart.
  • In contrast to LQTS, ion channel defects associated with SQTS lead to an abnormal abbreviation of repolarization, predisposing affected individuals to a risk of atrial and ventricular arrhythmias.

Polymorphic Ventricular Tachycardia with Normal QT Interval

  • Acute cardiac ischemia or
    • Catecholaminergic polymorphic VT (CPVT)
  • Ischemia is the cause in most of these patients, necessitating prompt evaluation for cardiac ischemia
  • CPVT, an inherited channelopathy, may cause those without cardiac ischemia
    • Affected patients typically present with life-threatening VT or VF occurring during emotional or physical stress, with syncope often being the first manifestation of the disease
    • Although sporadic cases occur, this is a largely familial disease. The majority of known cases are due to mutations in the cardiac ryanodine receptor, the cardiac sarcoplasmic calcium release channel. One report suggested that this disorder may account for at least one in seven cases of sudden unexplained death

Third Degree (Complete) AV Block

  • Third Degree AV Block is defined as a blockage in the AV junction where no atrial impulses reach the ventricles.
  • The potential etiologies of third-degree AV block are similar to lesser degrees of AV block and include reversible causes:
    • Pathologic causes include:
      • Cardiomyopathy (e.g., amyloidosis, sarcoidosis)
      • Endocarditis with abscess formation
      • Hyperkalemia
      • Hypervagotonia
      • Infiltrative malignancies
      • Myocardial ischemia (acute or chronic) involves the conduction system
      • Myocarditis (e.g., Lyme disease)
      • Neuromuscular diseases
    • Iatrogenic causes include:
      • Medication-related (AV nodal blocking medications)
      • Post-cardiac surgery
      • Post-catheter ablation
      • Post-transcatheter aortic valve implantation
    • Idiopathic causes include:
      • Progressive cardiac conduction disease with myocardial fibrosis or sclerosis that affects the conduction system

VF Secondary to Premature Ventricular Contractions (PVCs)

  • Short coupled PVCs have been described as a trigger of VF. Generally, these arise from the Purkinje fibers, but PVCs from papillary muscles have also been described to trigger VF. Ablation of these triggering PVCs may eliminate recurrent episodes of VF.

Wolff-Parkinson-White (WPW) Syndrome and other Forms of Supraventricular Tachycardia (SVT)

  • Both WPW syndrome and, very rarely, other forms of SVT can cause SCD.
  • The frequency with which this occurs was assessed in a report of 290 patients with aborted SCD. The mechanism was an arrhythmia associated with the WPW syndrome in 2.1% and atrial fibrillation (AF) with a rapid ventricular response which was the most common.
  • Most patients resuscitated from VF secondary to preexcitation have a history of syncope, atrioventricular reciprocating tachycardia, or AF.38 However, preexcitation and arrhythmias have been undiagnosed in up to 25% of such patients.
  • Among patients with WPW syndrome who survive an episode of SCD, ablation of the accessory pathway is the treatment of choice.

Noncardiac Disease

Fifteen to 25% of cardiac arrests are noncardiac in origin.

Noncardiac disease triggers for SCA/SCD include:

  • Autonomic nervous system activation
  • Bleeding
  • Central airway obstruction
  • Drug intoxication
  • Electrolyte disturbances (particularly hypokalemia and hypomagnesemia)
  • Intracranial hemorrhage
  • Ischemia
  • Near-drowning
  • Pickwickian syndrome
  • Pulmonary embolism
  • Proarrhythmic effect of some antiarrhythmic drugs
  • Psychosocial factors
  • Sudden infant death syndrome (SIDS)
  • Sudden unexplained death in epilepsy (SUDEP)
  • Trauma

Warning Symptoms

"Warning" symptoms may precede the SCA event in many individuals. Still, symptoms may be unrecognized or minimized by them, thus limiting the discovery of the symptoms, especially in those who do not survive the event. In addition, individuals who have SCA and are often resuscitated have retrograde amnesia and thus do not remember events or symptoms that may have been present.

As symptoms are nonspecific and may reflect benign conditions, and as these symptoms may not necessarily occur before all episodes of SCA (insensitive), their presence may not be of value in helping offset or prevent episodes. A causal or temporal relationship between symptoms and SCD has not been established.

Risk Factors

Several clinical characteristics and other factors are associated with an increased risk of SCA among individuals without prior clinically recognized heart disease. Most risk factors for CHD are also risk factors for SCA. These include:

  • A family history of premature CHD and SCA
  • Cigarette smoking
  • Diabetes mellitus
  • Dyslipidemia
  • History of myocardial infarction
  • Hypertension
  • Obesity
  • Physical inactivity

Cigarette Smoking

Current cigarette smoking and the number of cigarettes smoked per day among current smokers are strongly related to the risk of SCA in patients with CHD.

Based upon the observations that the risk of SCA is particularly high among current smokers and declines rapidly after stopping smoking, smoking cessation should be viewed as a critical component of efforts to reduce the risk of SCA and a multitude of other complications.


SCA risk is transiently increased during and up to 30 minutes after strenuous exercise compared to other times. However, the actual risk during one episode of vigorous exercise is very low (1 per 1.51 million episodes). Furthermore, the magnitude of the transient increase in risk during acute exercise is lower among men who are regular exercisers than men for whom exercise is unusual.

The small transient increase in risk during exercise is outweighed by a reduction in SCA risk at other times. Regular exercise is associated with a lower resting heart rate and increased heart rate variability, associated with a reduced risk of SCD.

One exception to the lower overall risk associated with intensive exercise occurs in patients with certain, often unrecognized underlying heart diseases. Examples include hypertrophic cardiomyopathy, anomalous coronary artery of wrong sinus origin, myocarditis, and ARVC.

Family History of SCA

A family history of SCA, either alone or with myocardial infarction, is associated with a 1.5 to 1.8-fold increased risk of SCA. The increase in risk is not explained by traditional risk factors that tend to aggregate in families, such as hypercholesterolemia, hypertension, diabetes mellitus, and obesity.

The magnitude of the increase in risk associated with the presence of family history is modest compared to the two- to five-fold increase in risk associated with other modifiable risk factors such as physical inactivity and current cigarette smoking. Few studies have examined potential gene-environment interactions related to the risk of SCD. Nevertheless, interactions of mutations or polymorphisms in specific genes and environmental factors likely influence this risk.

Serum C-Reactive Protein (CRP)

As manifested in part by higher serum concentrations of CRP, chronic inflammation has been implicated as a risk factor for various cardiovascular diseases (including ACS and stroke). Elevated serum CRP is also associated with an increased risk of SCA.

Excess Alcohol Intake

Moderate alcohol intake (e.g., one to two drinks per day and avoidance of binge drinking) may decrease the risk of SCD. In comparison, heavy alcohol consumption (six or more drinks per day) or binge drinking increases the risk for SCD.

Psychosocial Factors

Clinical observations have suggested a possible relationship between acutely stressful situations and SCA risk. Major disasters, such as earthquakes and war, resulting in a rapid transient increase in SCA rate in populations. The level of educational attainment and social support from others may alter the risk associated with stressful life events.


Excessive caffeine intake has been investigated as a potential risk factor for SCA. In the limited data available, no significant association between caffeine intake and SCA has been found.

Fatty Acids

After myocardial infarction, elevated plasma nonesterified fatty acid (free fatty acid) concentrations were associated with ventricular arrhythmias and SCD. However, nonesterified fatty acids were not associated with SCD in the Cardiovascular Health Study, a population-based cohort of older adults. In a population-based case-control study among individuals without prior clinically recognized heart disease, SCA cases had higher concentrations of trans isomers of linoleic acid in red blood cell membranes. In contrast, higher dietary intake and higher levels of long-chain n-3 polyunsaturated fatty acids (eicosapentaenoic acid and docosahexaenoic acid) in plasma and the red blood cell membrane are associated with a lower risk of SCD.


Management issues for survivors of SCA include the following:

  • Identification and treatment of acute reversible causes
  • Evaluation for structural heart disease
  • An evaluation for primary electrical diseases in patients without obvious arrhythmic triggers or cardiac structural abnormalities
  • Neurologic and psychologic assessment
  • Evaluation of family members in selected patients with a suspected or confirmed hereditary syndrome

Initial Evaluation

The evaluation begins immediately after resuscitation. The highest priority is to exclude any obvious reversible factors that may have led to the event (Table 1).

Table 1: Treatable Conditions Associated with Cardiac Arrest
ConditionCommon Associated Clinical Setting
AcidosisDiabetes, diarrhea, drug overdose, renal dysfunction, sepsis, shock
AnemiaGastrointestinal bleeding, nutritional deficiencies, recent trauma
Cardiac TamponadePost-cardiac surgery, malignancy, post-myocardial infarction, pericarditis, trauma
HyperkalemiaDrug overdose, renal dysfunction, hemolysis, excessive potassium intake, rhabdomyolysis, major soft tissue injury, tumor lysis syndrome
Hypokalemia*Alcohol abuse, diabetes mellitus, diuretics, drug overdose, profound gastrointestinal losses
HypothermiaAlcohol intoxication, significant burns, drowning, drug overdose, elderly patient, endocrine disease, environmental exposure, spinal cord disease, trauma
HypovolemiaSignificant burns, diabetes, gastrointestinal losses, hemorrhage, malignancy, sepsis, trauma
HypoxiaUpper airway obstruction, hypoventilation (CNS** dysfunction, neuromuscular disease), pulmonary disease
Myocardial InfarctionCardiac arrest
PoisoningHistory of alcohol or drug abuse, altered mental status, classic toxidrome (e.g., sympathomimetic), occupational exposure, psychiatric disease
Pulmonary EmbolismImmobilized patient, recent surgical procedure (e.g., orthopedic), peripartum, risk factors for thromboembolic disease, recent trauma, presentation consistent with acute pulmonary embolism
Tension PneumothoraxCentral venous catheter, mechanical ventilation, pulmonary disease (e.g., asthma, chronic obstructive pulmonary disease), thoracentesis, thoracic trauma

* Hypomagnesemia should be assumed in the setting of hypokalemia, and both should be treated.

**CNS: central nervous system.

History and Physical Examination

  • The patient (if conscious) and family should be questioned, with particular attention to the following:
    • Prior diagnoses of heart disease
    • Use of any medications, especially antiarrhythmic drugs, diuretics, and drugs that might produce LQTS
    • Ingestion of toxins or illicit drugs
    • Antecedent symptoms, especially evidence of ischemia
    • Antecedent stressful events or activities
  • A coherent history may not be obtainable since the cardiac arrest is frequently unwitnessed. In addition, the patient resuscitated from VF often has retrograde amnesia and cannot remember what occurred before the cardiac arrest.

Laboratory Testing

  • Immediate evaluation should include standard laboratory testing to exclude electrolyte abnormalities and, in many cases, an arterial blood gas to exclude acidosis. Any reversible metabolic abnormalities should be identified and corrected, particularly hypokalemia and hypomagnesemia, which can predispose to ventricular tachyarrhythmias.
  • Two important limitations should be considered when interpreting the test results:
    • Electrolyte abnormalities during and shortly after resuscitation may be secondary to cardiac arrest and hypoperfusion rather than a cause of SCD.
    • Electrolyte abnormalities by themselves are usually insufficient to cause SCD. Clinical settings that increase the proarrhythmic effect of hypokalemia and hypomagnesemia include acute myocardial infarction, overt HF, and LQTS.
  • Unless compelling evidence of an association, it is potentially hazardous to ascribe a cardiac arrest to an electrolyte or metabolic derangement alone. Mistaken attribution of a significant arrhythmia to an innocent or merely potentiating laboratory abnormality can place the patient at high risk if appropriate therapy to prevent recurrent SCD is delayed or not given. Importantly, electrolyte and pH abnormalities may be secondary to the arrhythmia itself.
  • This risk was illustrated in a review of 169 patients treated with an ICD for sustained ventricular arrhythmia. The plasma potassium concentration was measured on the day of the arrhythmia. The likelihood of a recurrent sustained ventricular arrhythmia was 82% at five years. The long-term risk was similar in patients with low, normal, and high plasma potassium concentrations at presentation.

Electrocardiogram (ECG)

  • An ECG can reveal evidence of both acute abnormalities and chronic conditions. It should be part of the immediate evaluation and repeated as necessary once the patient's cardiac, hemodynamic, and metabolic condition stabilizes. The ECG should be evaluated for evidence of the following:
    • Ongoing ischemia or prior myocardial infarction
    • Conduction system disease, including bundle branch block, second-degree heart block, and third-degree heart block
  • Less common abnormalities that also may be evident include:
    • ARVC is suggested by VT or ventricular ectopy with a left bundle branch block configuration and an inferior axis. In addition, abnormalities of the baseline QRS may be present, including an epsilon wave in the right precordial leads
    • Brugada syndrome, evidenced by a pseudo-RBBB and persistent ST-segment elevation in V1 to V3
    • Hypertrophic cardiomyopathy
    • LQTS, possibly with torsades de pointes
    • WPW syndrome, evidenced by a short PR interval and a slurred QRS complex upstroke known as a delta wave

Evaluation for Structural Heart Disease

Excluding patients with an obvious noncardiac etiology (e.g., trauma, hemorrhage, pulmonary embolus, etc.), structural heart disease is present in up to 90% of patients with SCD.

All survivors of SCD should undergo a complete cardiac examination to determine the nature and extent of underlying heart disease. The initial history, physical examination, and laboratory tests may provide evidence of one of these disorders, but further testing is usually necessary to confirm a diagnosis.

The standard evaluation typically includes:

  • ECG
  • Cardiac catheterization with coronary angiography
  • Echocardiography

Coronary angiography and echocardiography may be part of an urgent initial evaluation in the appropriate clinical setting.

In selected patients, cardiac magnetic resonance imaging (MRI) and, rarely, myocardial biopsy are performed.

Coronary Angiography

  • Coronary angiography is performed in most survivors of SCD for one of two indications:
    • Management of an ACS
      • SCD may be the presenting manifestation of an ACS. Among patients with an ACS, malignant arrhythmias are significantly more common in an acute ST elevation MI (STEMI) setting. Still, they are also seen in approximately 2% of patients with a non-ST elevation MI (NSTEMI). In patients with an ACS and ischemia, the arrhythmia is usually polymorphic VT, rapid ventricular flutter, or VF.
      • Any patient with evidence of a confirmed or suspected STEMI or NSTEMI following resuscitation from SCD should undergo urgent cardiac catheterization and primary percutaneous coronary intervention (PCI) or, in some cases, surgical revascularization as appropriate.
      • Patients who experience SCD during the first 48 hours after a STEMI have higher in-hospital mortality compared with STEMI patients who do not experience sustained VT or VF. However, there is little or no difference in mortality among patients who survive hospital discharge at one to two years.
    • Diagnosis of chronic CHD
      • Diagnostic angiography
        • In SCD survivors without an ACS, angiography is still considered to exclude stable, chronic CHD, which is the leading cause of SCD. Malignant arrhythmias and SCD occur in such patients, usually those who have had a prior infarction with residual myocardial scarring. In contrast to patients with an ACS, the culprit arrhythmia is usually scar-related monomorphic VT, which is not the result of ischemia. However, monomorphic VT can ultimately degenerate to VF, particularly if the arrhythmia induces ischemia. Because SCD may be the first clinical evidence of chronic CHD, most SCD survivors undergo diagnostic angiography before discharge.
      • Even for patients arriving at the hospital in refractory VT/VF who are receiving ongoing CPR, immediate coronary angiography appears beneficial in a carefully selected group of patients (18 to 75 years of age with VT/VF as initial rhythm who received three shocks and amiodarone loading dose and who can be in the catheterization lab in <30 minutes post-arrest) (Yannopoulos et al., 2017).
      • Diagnostic coronary angiography may not be necessary for selected patients without signs or symptoms of CHD if another clear cause for SCD is identified (e.g., LQTS, WPW, Brugada, HCM, left ventricular noncompaction, or ARVC).
      • Angiography is suggested in younger patients without an apparent cause for SCD, in whom angiography may also detect an anomalous origin of a coronary artery. Among competitive athletes under age 35, anomalous origin of a coronary artery was present in 13% of SCD survivors in one series.
      • Patients with stable CHD who experience an episode of primary SCD without evidence of simultaneous ischemia are at high risk for recurrent malignant arrhythmias, even after percutaneous or surgical revascularization. As a result, such patients are treated with an ICD.


  • Echocardiography can detect abnormalities that suggest or confirm the diagnosis of many of the important causes of SCD. Since global left ventricular dysfunction due to myocardial stunning can be induced by cardiac arrest, and cardiopulmonary resuscitation, evaluation of left ventricular function should be performed at least 48 hours after resuscitation.
  • Potential causes of SCD that can be detected with echocardiography include the following:
    • Aortic stenosis
    • ARVC
    • CHD – Left ventricular dysfunction with wall motion abnormalities suggest prior myocardial infarction. Dyskinetic wall motion is consistent with an aneurysm
    • Dilated cardiomyopathy
    • HCM

Cardiac Magnetic Resonance Imaging (CMR)

  • CMR is indicated for selected patients whose diagnosis is uncertain after the evaluation.
  • CMR is useful in the evaluation of the following disorders:
    • ARVC
    • Cardiac amyloidosis
    • Cardiac sarcoidosis
    • Congenital heart disease, including the anomalous origin of coronary arteries
    • Dilated cardiomyopathy
    • HCM
    • Myocarditis
  • The utility of CMR angiography as an alternative to invasive coronary angiography is not well defined. Alternatively, cardiac computed tomography angiography may be used to assess both congenital and acquired coronary abnormalities.

Evaluation for Primary Electrical Diseases

General Issues

  • Approximately 5 to 10% of SCD survivors have no evidence of a noncardiac etiology or structural heart disease after the above evaluation. Such patients are considered to have a primary electrical disorder.
  • The majority of these patients do not have "normal" hearts, but historically diagnostic tools have been unable to identify the structural or functional derangement. In the past, the etiology of many of these deaths was unknown and termed "idiopathic." Subsequent discoveries have identified the cause of death in many of these patients (Wei et al., 2016). As understanding the mechanisms of primary electrical disorders has improved, so have diagnostic capabilities, with important benefits for both the victims of SCD and their families.
  • Characteristic changes on an ECG often detect these disorders.
  • Several of the disorders that cause SCD in the absence of structural heart disease are due to abnormalities of cardiac ion channels, including the following:
    • Brugada syndrome
    • CPVT
    • LQTS
    • SQTS
  • Disorders associated with SCD that are not due to ion channel abnormalities include:
    • WPW syndrome
    • Commotio cordis
  • Patients without evidence of any of the above structural or electrical abnormalities have idiopathic VF or primary electrical disease.
  • Identification of a primary electrical disorder in an SCD survivor has two important benefits:
    • Directing medical treatment to prevent arrhythmia recurrence (e.g., beta-blockers for CPVT). Although medical therapy alone is now uncommon in SCD survivors, adjunctive medical therapy can reduce the frequency of ICD shocks.
    • Guiding the evaluation and management of family members.

Electrophysiologic (EP) Study

  • EP testing is not usually performed in patients with an established etiology of SCD. However, an EP study can be valuable in those whose initial evaluation reveals no etiology and in selected patients with a previously identified disorder.
  • In SCD survivors with a normal heart, EP testing may reveal the following:
    • Abnormalities of atrioventricular conduction.
      • The presence of severe conducting system disease suggests that a serious bradyarrhythmia may have contributed to the SCD event.
      • These patients, though, typically present with syncope rather than SCD.
      • Furthermore, VT/VF may be the culprit even when conduction disease is identified, and ventricular stimulation to induce ventricular arrhythmias may be warranted.
    • An accessory pathway in patients with WPW syndrome.
      • An accessory pathway can rapidly conduction supraventricular arrhythmia, primarily AF, producing a rapid ventricular rate that can degenerate to VF.
      • Such patients usually have evidence of preexcitation on their ECG. If preexcitation is evident on an ECG in a survivor of SCD, EP study and ablation of the accessory pathway are usually indicated.
    • Inducible ventricular arrhythmias.
      • VT and VF may be induced in patients with several underlying cardiac abnormalities. The prognostic value of inducible arrhythmias is best established in patients with prior myocardial infarction and reduced left ventricular (LV) systolic function.
      • There is evidence that inducible VF, particularly when repeatedly induced with nonaggressive protocols, suggests the diagnosis of idiopathic VF and may predict recurrent arrhythmic events.
      • In a literature review, 69% of patients with idiopathic VF had sustained ventricular tachyarrhythmia induced with a nonaggressive protocol. The induced arrhythmia was generally polymorphic in configuration and poorly tolerated.
      • In some other conditions, it is unclear whether inducible VT or VF has prognostic significance (e.g., Brugada syndrome, infiltrative diseases, HCM. Furthermore, aggressive stimulation protocols can induce polymorphic VT or VF in some individuals without cardiac disease. Thus, inducible arrhythmias can be a nonspecific finding. For this reason, the significance of inducible ventricular arrhythmias in patients with apparently normal hearts is unclear. On the other hand, the absence of inducible VT/VF may not preclude ICD implantation since lack of inducibility does not predict low risk.
    • Myocardial scar
      • Substrate mapping may identify areas of scar indicating abnormal substrate and a predisposition to ventricular arrhythmias. These abnormalities are more commonly seen in patients with CHD, HCM, ARVC, or infiltrative diseases and occur in some idiopathic VF cases.
      • In addition, some patients with idiopathic VF have other electrophysiologic abnormalities, including areas of slow conduction, regionally delayed repolarization, or dispersion in repolarization.
    • Supraventricular arrhythmias
      • Patients in whom VT or VF was not well documented at SCD may have another culprit arrhythmia, usually an SVT. In such patients, SVT may be inducible during the EP study.

Exercise Testing

  • Exercise testing is not usually part of the CHD evaluation in SCD survivors since most undergo coronary angiography. However, the provocation of ischemia with exercise, independently of coronary anatomy, is important in evaluating the SCD survivor. Although angiography alone does not prove a causal relationship to SCD or the presence of ischemia, the observation that revascularization appears to improve outcomes means that a negative exercise test in a patient with a significant coronary disease on angiography is not likely to affect the decision on revascularization. Also of great importance is the provocation of VT or VF in these patients, which would predict a higher recurrence rate. It is also a target for adjunctive antiarrhythmic therapy in addition to an ICD. While VT (often scar mediated and provoked by catecholamines) may be provoked during exercise, VF most commonly occurs after exercise in the recovery period (when ischemia is more common).
  • In patients with apparently normal hearts, exercise testing can assist in diagnosing LQTS and CPVT. It is also useful in patients with WPW patterns as the resolution of the delta wave with exercise generally correlates with a low likelihood of a rapid ventricular rate with AF, which is the etiology for VF and SCD in these patients.

Ambulatory Monitoring

  • In patients without a clear etiology for SCD, ambulatory monitoring may reveal recurrent sustained or nonsustained arrhythmias. However, most patients without an established diagnosis will have an ICD placed before discharge, and the memory features in these devices may preclude the need for ambulatory monitoring.

Pharmacologic Challenge

  • Some primary electrical disorders may still be present despite no abnormalities on any of the proceeding tests. ECG abnormalities may be intermittent or latent, and genetic testing is not comprehensive enough to exclude all possible disorders.
  • Investigators have evaluated pharmacologic challenge's role in eliciting diagnostic ECG changes or arrhythmias in selected SCD survivors. One report included 18 SCD survivors with no evidence of structural heart disease. All patients had a normal ECG, echocardiogram, coronary angiography, and CMR. Patients were infused with epinephrine (0.05 to 0.5 micron/kg per minute) and then procainamide (1 g over 30 minutes). Epinephrine was intended to induce CPVT and procainamide to induce the characteristic ECG abnormalities of Brugada syndrome.
  • The following findings were noted:
    • Ten patients were diagnosed with CPVT based upon an abnormal response to an epinephrine infusion (frequent or polymorphic ventricular ectopy, nonsustained VT, or sustained VT). Four of these patients had ventricular ectopy during exercise testing, but none had sustained or nonsustained VT.
    • Two patients were diagnosed with Brugada syndrome.
    • Six patients were left with a diagnosis of idiopathic VF.
    • Among 55 family members who were tested, eight affected members from one family were diagnosed with CPVT, and one relative was diagnosed with Brugada syndrome.
  • In summary, two-thirds of patients whose standard evaluation provided no etiology for SCD had a diagnosis established with provocative pharmacologic testing. This allowed for the addition of appropriate adjunctive therapy (beta-blockers for CPVT) and the identification of nine additional affected family members.

Minor Cardiac Abnormalities Not Associated with SCD

  • During the cardiac evaluation, minor cardiac abnormalities are often detected that do not have a clear causal relationship to SCD. These findings do not preclude the diagnosis of idiopathic VF. However, their severity must be considered, and monitoring is warranted since these disorders may be the initial manifestations of underlying structural heart disease that will become clinically apparent later.
    • Examples of such disorders which do not exclude idiopathic VF are:
      • First degree atrioventricular (AV) block
      • Transient second-degree Mobitz type II AV block without bradycardia
      • Isolated bundle branch block

Evaluation of Family Members

  • Some causes of SCD are familial, including a genetic predisposition to premature CHD. The risk of cardiovascular disease appears significantly higher in first- and second-degree relatives of the SCD victims, particularly young victims.
    • In a nationwide Danish study from 2000 to 2006, 470 victims of SCD were identified who were 35 years of age or younger. Among a cohort of 3,073 first- and second-degree relatives of the SCD victims who were followed for up to 11 years, CVD was significantly more likely to be present than in the general population. In contrast, there was no difference in CVD rates compared with the general population among relatives of elderly (> 60 years of age) victims of SCD.
  • A general cardiologic evaluation of first- and second-degree relatives of victims of unexplained SCD can yield the diagnosis of heritable disease in up to 40% of families, as illustrated by the following observations.
    • In a study of 32 families of victims of unexplained SCD, a general cardiologic evaluation (ECG, echocardiogram, Holter monitor and, less commonly, stress testing) was completed in 107 first-degree relatives. Seven families (22%) were diagnosed with a heritable disease: four with LQTS, one with nonstructural cardiac disease, one with myotonic dystrophy, and one with HCM.
    • These findings were extended in a second report that evaluated 43 families with 183 surviving first- and second-degree relatives of victims of unexplained SCD at age ≤40. Careful history identified 26 additional cases of unexplained SCD at age ≤40. Cardiology evaluation included ECG, echocardiogram, exercise tolerance testing (ETT), and measurement of serum lipids. Additional testing, as indicated, included a flecainide challenge for suspected Brugada syndrome or cardiac MR for suspected ARVC.
  • Genetic testing for the suspected disease was performed when a clinical diagnosis was established. A genetic abnormality was confirmed, additional screening was done in another 150 family members. The following findings were noted:
    • A heritable disease was identified in 17 families (40%): five with CPVT, four with LQTS, two with Brugada syndrome, one with Brugada/LQTS, three with ARVC, one with HCM, and one with familial hypercholesterolemia. Genetic analysis confirmed the diagnosis in 10 families.
    • An average of 8.9 asymptomatic carriers per family was identified through secondary genetic analysis.
    • Identifying a specific disease was more likely if ≥2 unexplained SCD events occurred in the family and more family members underwent evaluation.
    • The increased yield in the second study may reflect the more extensive evaluation, including ETT, and the inclusion of more family members.
  • Consistent with these findings, it has been recommended that first-degree family members of patients with SCD in the absence of structural heart disease be informed of the potentially increased risk. An assessment should be offered at a center with experience diagnosing and managing inherited cardiac diseases. Routine genetic screening for inherited disorders is not feasible, although, in an identifiable condition, the genetic evaluation of family members may be undertaken at some centers.

Neurologic and Psychologic Assessment

SCD survivors who have been resuscitated should be given a complete neurologic examination to establish the nature and extent of impairment resulting from the arrest. The physical examination, rather than imaging studies or other testing, is the most useful way of elucidating the patient's degree of neurologic function, mental impairment, and determining prognosis.

Primary Prevention

The optimal approach to the primary prevention of SCA varies among the following categories:

  • General population
  • Patients surviving an acute MI
  • Patients with HF and cardiomyopathy
  • Patients with one of the congenital disorders are associated with an increased risk of SCA (e.g., Brugada syndrome, congenital LQTS, WPW)

General Population

There are two approaches to reduce the risk of SCA in the general population:

  • Screening and risk stratification identify individuals who may benefit from specific interventions (e.g., stress testing, screening ECGs).
    • Among populations already known to be at an elevated risk of SCA (e.g., individuals with a prior MI), further risk stratification with various tests can identify subgroups that benefit from specific therapies, such as an ICD.
    • However, there is no evidence that routine screening with any test (e.g., 12-lead ECG, exercise stress testing, or Holter monitoring) effectively identifies populations at an increased risk of SCA in the general population without known CVD.
    • With regard to risk stratification of the general population, the following has been suggested:
      • Screening for risk factors for CVD according to standard guidelines.
      • Screening for CHD as appropriate in selected patients, according to standard guidelines.
      • Routine additional testing for SCA risk stratification is not recommended.
  • Interventions may be expected to reduce SCA risk in any individual (e.g., smoking cessation or other lifestyle modifications). Such interventions generally target the underlying disorders that predispose to SCA.
    • Many traditional risk factors associated with CHD development are also associated with SCA.
    • Management of these risk factors may reduce SCA incidence in the general public. Such interventions include:
      • Effective treatment of hypercholesterolemia
      • Effective treatment of hypertension
      • Adoption of a heart-healthy diet
      • Fish Intake and Fish Oil
        • In observational studies of populations at low cardiovascular risk, greater dietary fatty fish intake was associated with lower cardiac mortality. This benefit is due in part to a reduced risk of SCD. Based upon these results, subsequent randomized trials evaluated the benefit of fish oil supplements in various high-risk populations.
        • For most individuals, there is little evidence that the pharmacologic doses of n-3 polyunsaturated fatty acids found in fish oil supplements (approximately 10 to 20 times the nutritional dose from fish) provide more protection than the intake of one to two servings of fatty fish (e.g., salmon) per week.
          • The pharmacologic use of fish oils supplements should be restricted to individuals with refractory hypertriglyceridemia, along with the periodic monitoring of apolipoprotein B levels.
      • Regular exercise
        • There are no data from long-term exercise intervention trials among apparently healthy individuals that focus upon major disease endpoints. Nevertheless, regular exercise should be encouraged to prevent CHD and SCA.
        • Although there is a small transient increase in risk during and shortly after strenuous exercise, there is an overall reduction in SCD among exercisers compared with sedentary men.
        • It is unclear if more exercise (higher intensity or longer duration) is better than less (non-strenuous physical activity, such as walking for exercise 30 minutes most days).
        • Patients should be advised to pay attention to potential symptoms of CHD, even if they have engaged in regular exercise without limitations for an extended time.
        • Patients with known heart disease should be encouraged to exercise in a supervised setting such as a cardiac rehabilitation program regularly.
      • Smoking cessation
      • Moderation of alcohol consumption
        • Excess alcohol intake increases SCA risk, while light-to-moderate alcohol consumption (i.e., ≤2 drinks per day) is associated with a lower risk of CAD and cardiovascular mortality.
        • It is reasonable to expect that moderate alcohol intake will also reduce SCA.
      • Effective treatment of diabetes
    • There is no definitive evidence that risk factor reduction in the general population lowers the SCA rate. However, several studies have demonstrated that interventions to treat risk factors can lower total cardiovascular and coronary mortality. Since most CHD mortality is due to SCD, these results suggest that reducing risk factors will also reduce SCD rates.
      • A multifactorial, controlled, randomized trial from the Belgian component of the World Health Organization evaluated the effect of efforts aimed at reducing serum cholesterol (via dietary changes), increasing physical activity, and controlling smoking, hypertension, and weight (in those who were overweight) on risk factors and mortality. Compared to the control group, the intervention group had significant CHD and coronary mortality reductions.

Post Myocardial Infarction

Patients who have had an MI are at an increased risk of SCA. However, this risk varies significantly among post-MI patients according to several factors.

The approach to the prevention of SCA in post-MI patients includes the following:

  • Standard Medical Therapies
    • Both beta-blockers and ACE inhibitors (or angiotensin II receptor blockers):
      • Reduce overall mortality after an MI and are routinely administered.
      • Lower the incidence of SCD.
      • The benefit, however, may be limited to three years post MI.
    • Beta-blockers post-MI is useful for a longer time in patients with post-MI HF.
  • Risk stratification to identify those patients at the highest risk of SCA.
  • ICD implantation in selected patients.

Heart Failure and Cardiomyopathy

Regardless of the etiology, patients with HF and LV systolic dysfunction are at an increased risk of SCA.

  • Primary prevention with an ICD is recommended in selected patients with either ischemic or nonischemic cardiomyopathy.
  • In addition, as with patients with CHD, standard medical therapies for HF (beta-blockers, ACE inhibitors or angiotensin II receptor blockers, and aldosterone inhibitors such as spironolactone or eplerenone) may lower the risk of SCA.

Counseling Patients and Families

Given the mounting evidence related to the primary prevention of SCA, it is now clear that primary care providers can influence the occurrence of these events. There are clinical recommendations for those at risk of SCA that are likely to reduce risk.

Secondary Prevention

Implantable Cardioverter-Defibrillator (ICD) Therapy

An ICD is the preferred therapeutic modality in most survivors of SCA. The ICD does not prevent the recurrence of malignant ventricular arrhythmias, but it effectively terminates these arrhythmias when they do recur.

ICD patients with frequent arrhythmia recurrences and device discharges may benefit from adjunctive therapies, such as antiarrhythmic drugs or catheter ablation.


The main indications for the use of an ICD can be divided into two groups (Russo et al., 2013):

  • Secondary Prevention
    • Implantation of an ICD is recommended for the secondary prevention of SCD due to life-threatening VT/VF in the following settings:
      • Patients with a prior episode of resuscitated VT/VF or sustained hemodynamically unstable VT in whom a completely reversible cause cannot be identified. This includes patients with various underlying heart diseases and idiopathic VT/VF and congenital LQTS, but not patients who have VT/VF limited to the first 48 hours after an acute MI.
      • Patients with episodes of spontaneous sustained VT in the presence of heart disease (valvular, ischemic, hypertrophic, dilated, or infiltrative cardiomyopathies) and other settings (e.g., channelopathies).
      • A key issue is the prevention of total mortality (not arrhythmic or sudden death). Simply correcting VT/VF may not improve overall mortality. Therefore, patient selection for ICD implantation should consider the known risk of SCD due to VT/VF for a specific condition and the risk of total mortality from underlying medical conditions.
  • Primary Prevention
    • Implantation of an ICD is recommended for the primary prevention of SCD due to life-threatening VT/VF in patients who have received optimal medical management (including the use of beta-blockers and angiotensin-converting enzyme [ACE] inhibitors) yet remain at high risk of SCD, including:
      • Patients with a prior MI (at least 40 days ago) and left ventricular ejection fraction (LVEF) ≤30%.
      • Patients with cardiomyopathy, New York Heart Association (NYHA) functional class II to III (Table 2), and LVEF ≤35%.
Table 2: NYHA Classifications of Cardiovascular Disability
ClassNYHA Functional ClassificationSpecific Activity Scale
IPatients with cardiac disease but without resulting limitations of physical activity. Ordinary physical activity does not cause undue fatigue, palpitation, dyspnea, or anginal pain.Patients can perform to completion any activity requiring ≥7 metabolic equivalents, e.g., can carry 24 lb. up eight steps; do outdoor work (shovel snow, spade soil); do recreational activities (skiing, basketball, squash, handball, jog/walk 5 mph).
IIPatients with cardiac disease resulting in slight limitation of physical activity. They are comfortable at rest. Ordinary physical activity results in fatigue, palpitation, dyspnea, or anginal pain.Patients can perform to completion any activity requiring ≥5 metabolic equivalents, e.g., have sexual intercourse without stopping, garden, rake, weed, roller skate, dance fox trot, walk at 4 mph on level ground, but cannot and do not perform to completion activities requiring ≥7 metabolic equivalents.
IIIPatients with cardiac disease resulting in marked limitation of physical activity. They are comfortable at rest. Less than ordinary physical activity causes fatigue, palpitation, dyspnea, or anginal pain.Patients can perform to completion any activity requiring ≥2 metabolic equivalents, e.g., shower without stopping, strip and make bed, clean windows, walk 2.5 mph, bowl, play golf, dress without stopping, but cannot and do not perform to completion any activities requiring >5 metabolic equivalents.
IVPatients with cardiac disease resulting in inability to carry on any physical activity without discomfort. Symptoms of cardiac insufficiency or of the anginal syndrome may be present even at rest. If any physical activity is undertaken, discomfort is increased.Patients cannot or do not perform to completion activities requiring >2 metabolic equivalents. Cannot carry out activities listed above (specific activity scale III).

*New York Heart Association

  • Patients with nonischemic cardiomyopathy generally require optimal medical therapy for three months with documentation of persistent LVEF ≤35% at that time.
    • Some patients with HF who are candidates for an ICD also have an intraventricular conduction delay (≥120 milliseconds) and are candidates for cardiac resynchronization therapy (CRT) with a biventricular pacemaker. Such patients could be treated with a device with combined ICD and biventricular pacing functions (cardiac resynchronization therapy-defibrillator [CRT-D]).
  • Patients with syncope who have structural heart disease and inducible sustained VT or VT are going to VF on electrophysiology study.
  • Select patients with certain underlying disorders who are deemed to be at high risk for life-threatening VT/VF. These patients include:
    • Patients with congenital LQTS have recurrent symptoms or torsades de pointes despite therapy with beta-blockers or other high-risk patients.
    • High-risk patients with HCM or ARVC.
    • High-risk patients with Brugada syndrome, CPVT, and other channelopathies.

Contraindications to ICD Therapy

  • ICD therapy is not recommended in the following settings:
    • Patients with ventricular tachyarrhythmias due to a completely reversible disorder in the absence of structural heart disease (e.g., electrolyte imbalance, drugs, or trauma).
    • Patients do not have a reasonable expectation of survival with an acceptable functional status for at least one year, even if they otherwise meet ICD implantation criteria.
    • Patients with incessant VT or VF in whom other therapies (e.g., catheter ablation) should be considered first.
    • Patients with severe psychiatric illnesses that may be aggravated by device implantation. This situation is rarely encountered in clinical practice and may apply more to primary prevention than secondary prevention settings.
    • Patients with NYHA Class IV HF are refractory to optimal medical treatment and are not candidates for cardiac transplantation or CRT.
    • Patients with syncope without inducible ventricular tachyarrhythmias and structural heart disease.
    • Patients with VF or VT amenable to surgical or catheter ablation in whom the risk of SCD is normalized after successful ablation (e.g., atrial arrhythmias associated with the WPW syndrome, RV or LV outflow tract VT, idiopathic VT, or fascicular VT in the absence of structural heart disease).
    • ICD implantation should be delayed in patients with active infections or other acute medical issues. If necessary, the patient can be bridged with a wearable cardioverter-defibrillator (WCD) until ICD implantation can be carried out.

Elements of the ICD

  • The ICD system is comprised of the following elements (Picture 1):
    • Pacing/sensing electrodes
      • Pacing and sensing functions require a pair of electrodes. Contemporary pacemakers and defibrillators usually use leads with two electrodes on the ventricular lead: the distal electrode at the tip of the lead and a second electrode in the shape of a ring, several millimeters back from the tip (i.e., true bipolar leads). These bipolar leads provide accurate sensing, with high amplitude, narrow electrograms. Some ICD leads utilize integrated bipolar sensing in which the bipole consists of a single tip electrode and the distal shocking coil electrode. In addition to improved sensing capabilities, bipolar leads reduce the risk of extraneous interference, leading to inappropriate device function (e.g., inappropriate shocks delivered due to sensing of muscular activity).
    • Defibrillation electrode
      • The defibrillation function of the electrodes requires a relatively large surface area and positioning of the lead to maximize the density of current flow through the ventricular myocardium. Contemporary ICD systems use a "coil" of wire that extends along with the ventricular lead as the primary defibrillation electrode. Thus, a single transvenous lead can accomplish all pacing, sensing, and defibrillation functions.
      • Additional defibrillation electrodes improve defibrillation efficacy and reduce the defibrillation threshold. Most contemporary ICD systems have two or three defibrillation electrodes. Along with the distal coil in the RV on the transvenous lead, some ICD leads have a second defibrillation coil proximal to the RV coil. In addition, with "active can" technology, the metal housing of the ICD serves as one of the shocking electrodes. This configuration requires that the pulse generator be implanted in the pectoral region. The active can and transvenous lead systems can be combined to achieve adequate defibrillation thresholds (minimum energy required for successful defibrillation, which should generally be 10 joules less than the device's maximum output).
      • There are three types of pacing offered by current transvenous systems.
        • Single-chamber systems have only an RV lead.
        • Dual-chamber systems have right atrial (RA), and RV leads.
        • CRT, also called biventricular systems, has RA, RV, LV leads, or RV and LV leads in some patients with permanent atrial fibrillation.
    • Pulse generator
      • The pulse generator contains the sensing circuitry and the high voltage capacitors and battery.
      • Placement:
        • The development of small pulse generators (e.g., thickness ≤15 mm) has permitted placement in the infraclavicular region of the anterior chest wall in nearly all patients. The majority are placed in a:
          • Prepectoral (i.e., subcutaneous) position
          • Subpectoral position is advantageous. Most patients with the pulse generator in this location transmit the impulses generated to the myocardium via transvenous leads. Epicardial systems are still available and may be necessary due to anatomical limitations to placing a transvenous lead(s). Additionally, a subcutaneous ICD (S-ICD) system is now available in which the pulse generator is placed overlying the left lower lateral ribs.
          • A pulse generator will last for five or more years in most patients.

Implantable Cardioverter Defibrillator

Implantable Cardioverter Defibrillator (ICD)
When the ICD senses an abnormal heart rhythm, it delivers an electrical shock to reset the heart to a normal rhythm.


  • Before implanting an ICD, the healthcare provider must determine the optimal position to place the leads and the pulse generator. Most current ICD systems utilize one or two transvenous leads placed via the axillary, subclavian, or cephalic vein, attaching to a pulse generator in the subcutaneous tissue in the infraclavicular anterior chest wall. In more recent years, there has been a trend toward single coils rather than dual coils, which were favored earlier in the era of transvenous ICD systems. The rationale for this is that a proximal coil is rarely needed for defibrillation and that single-coil leads are less difficult to extract in the future if lead removal is necessary. If necessary, an additional defibrillation lead can be placed in the azygos vein, coronary sinus, or subcutaneous tissue.
  • Choosing the optimal pulse generator location
    • Modern devices are small enough to be implanted in the pectoral region of the anterior chest wall, either subcutaneously or submuscularly, similar to pacemaker implantation.
    • Transvenous devices placed in the pectoral region are associated with fewer perioperative complications, shorter procedure time, shorter hospital stays, lower hospitalization costs, and lower total costs.
    • Although implantation on the left side is preferred, a right-sided implant can be performed. The left pectoral position is usually chosen for two reasons: defibrillation energy requirement is usually lower on the left because of the location of the heart in the left chest, and there is a small risk of arm swelling due to venous occlusion, which would impact fewer people on the left as the majority of patients are right-handed.
    • Additionally, an S-ICD system is available that allows for defibrillation (though no backup pacing or anti-tachycardia pacing) without inserting a transvenous lead. The pulse generator for the S-ICD system is implanted in a subcutaneous pocket in the left lateral, mid-axillary thoracic position.
  • Choosing the optimal lead placement
    • In most new ICD implantations, the lead with the pace/sense electrodes is placed transvenously, with the distal electrode positioned on the RV apical endocardium. RV septal lead placement is also an option. In rare cases, usually due to limitations of the venous anatomy or a high risk of bacteremia and endovascular infection, the pace/sense electrodes are placed on the epicardium during surgery. The electrodes should record a ventricular electrogram of at least 5 mV. These signals should be sufficiently large for the detection of VT and VF.
    • Dual-chamber ICDs have an additional lead with another pair of pace/sense electrodes in the right atrium for atrial sensing and pacing.
    • An S-ICD has been developed with no leads placed in the heart. The subcutaneous lead, which toward its terminal end contains an 8-cm shocking coil electrode, is tunneled from the pulse generator in the left axilla to a position along the left parasternal margin. The S-ICD can sense VT/VF and deliver therapeutic shocks but cannot deliver anti-tachycardia pacing or pacing for bradycardias.

Defibrillation Threshold (DFT) testing

  • DFT testing has historically been performed at device implantation, although the necessity for this evaluation with the current generation of devices has been questioned (Wikoff et al., 2015).
  • DFT testing is generally performed in patients receiving an S-ICD.
  • DFT testing is not required and can be omitted in patients undergoing a left pectoral transvenous ICD implantation with an RV apical lead functioning appropriately.
  • DFT testing is reasonable in patients undergoing a right pectoral ICD implantation or ICD pulse generator changes (on either right or left side). The rationale for testing right-sided implants is that defibrillation may be more difficult with a right pectoral pulse generator, given that the heart lies in the left chest. There may be concerns about the integrity of the chronic leads for generator changes.
  • DFT testing should not be performed in patients with a documented nonchronic cardiac thrombus, atrial fibrillation/flutter without adequate anticoagulation, severe aortic stenosis, unstable angina, recent stroke or transient ischemic attack, or hemodynamic instability. Many centers avoid DFT testing in patients with very low LVEFs (<15%) or severe pulmonary hypertension.
  • Some electrophysiologists feel that universally omitting DFT testing might compromise the safety of certain patients, especially those with high DFTs who would benefit from a higher energy device or additional leads.
  • However, a distinction should be made between DFT testing at initial implantation and at the time of generator replacement. DFT testing at the time of generator replacement is useful in subsets of patients with leads that have a hazard alert or in patients at higher risk of DFT changes (e.g., obese patients, patients with HF symptoms, patients on amiodarone, etc.) (Phan et al., 2015).
  • Early ICD systems frequently required lead system adjustment at the time of implantation to achieve an adequate safety margin (arbitrarily set at 10 joules or greater). As technology improved, thresholds were substantially reduced. As a result, it is now unusual for the standard triad defibrillation system (two shock coils and an active can) to require modification at the time of implantation. Data regarding DFT testing on the more modern single-coil systems are limited since the available cohort and registry data predate the development of single-coil systems. On average, omitting the proximal coil in a single-coil system likely increases the DFT a few joules, which usually does not impact the safety margin but could be significant in some patients.
  • Several studies have illustrated the impact of DFT testing at the time of ICD implantation with the current generation of devices, generally showing no significant difference in outcomes (arnson et al., 2014). In a systematic review and meta-analysis of 13 studies involving 9,740 patients undergoing initial ICD implantation, there was no significant difference in mortality or adverse outcomes between patients with and without DFT testing (Phan et al., 2016).

Periprocedural Monitoring

  • Nearly all patients who undergo ICD implantation will have the device placed using local anesthesia at the site of the pulse generator insertion, with intravenous sedation provided for patients who experience significant anxiety or pain by nurse anesthetists or anesthesiologists.
  • Following ICD implantation:
    • A posteroanterior (PA) and lateral chest x-ray should be obtained to establish the position of the pulse generator and the associated lead(s) and exclude any apparent complications, including pneumothorax and lead dislodgment.
    • Patients should also have a 12-lead ECG recorded during pacing to document the ECG appearance of the QRS complex.


  • There are a variety of potential complications associated with ICDs, both at and around the time of implantation and long-term over the life of the patient and their device.
  • Periprocedural complications include:
    • Bleeding
    • Cardiac perforation
    • Infection
    • Perioperative mortality (rare)
    • Shoulder related problems include:
      • Decreased shoulder mobility
      • Pain
      • Reduced function
      • Insertion tendonitis
    • Long-term complications include:
      • Lead-related problems
        • Increased defibrillation threshold
        • Infection
        • Lead failure (resulting in failure to pace, failure to shock, or inappropriate shocks)
        • Tricuspid valve damage
        • Venous thrombosis
      • Pulse generator complications
        • Electronic circuit damage
        • Electromagnetic interference
        • Skin erosion due to the size and weight of the generator and infection of the pulse generator pocket
        • Twiddler’s Syndrome is a condition in which twisting or rotating the pulse generator within its pocket results in lead dislodgement and device malfunction.
      • Arrhythmia-related problems
        • Appropriate shocks can have an adverse effect on the quality of life, including emotional problems and driving restrictions.
        • Inappropriate shocks
        • "Phantom" shocks
      • Miscellaneous complications
        • HF
        • Quality of life -The ICD is often associated with deleterious psychosocial effects, with as many as 50% of recipients reporting elevated levels of anxiety and depression resulting from the fear of receiving a shock, device failure, decrease in physical activity, and negative lifestyle changes (such as the inability to drive or to return to work). Some patients develop severe psychiatric problems after receiving appropriate shocks.

Reuse of Explanted ICD

  • Many ICD pulse generators have useful battery life remaining at the time of a patient's death or when the ICD is explanted due to infection or device upgrade.
  • Because of concerns regarding the transmission of infectious disease from patient to patient and the lack of data regarding device reliability when used in such a fashion, reuse of explanted ICDs has not been approved by any governing or regulatory body.
  • However, due to the large numbers of patients in resource-limited settings who have indications for an ICD but cannot afford the device, there is a potential for compassionate reuse of ICDs if sterility and reliability can be assured.

ICD Functions

  • ECG monitoring and storage
    • Contemporary ICDs have more extensive storage and monitoring capacities, allowing more expedient patient management, often without requiring a face-to-face visit. Some examples include:
      • Recording and displaying stored electrograms from tachyarrhythmia events, thus allowing for the detection of "silent" or asymptomatic arrhythmias where patient management is likely to change (e.g., AF).
      • Telemetry capabilities permit easier analysis when patients receive shocks.
      • Remote monitoring capabilities via telephone or internet allow healthcare providers to review ICD parameters and events without requiring the patient to come to the office or hospital.
    • Antitachycardia pacing
      • VT, particularly reentrant VT associated with the scar from a prior MI, can sometimes be terminated by pacing the ventricle at a rate slightly faster than tachycardia. When a paced impulse enters the reentrant circuit during a tachycardia, it can depolarize a segment of the circuit, leaving that segment refractory when the reentrant wave returns, thus terminating the tachycardia.
      • Antitachycardia pacing, or overdrive pacing, refers to the delivery of short bursts (e.g., eight beats) of rapid ventricular pacing to terminate VT. Although a variety of algorithms exist, antitachycardia pacing is usually programmed to be delivered at a slightly faster rate (e.g., at a cycle length 10 to 12% shorter) than the rate of the detected tachycardia.
      • S-ICDs cannot pace for bradycardia or antitachycardic pacing.
      • Though employed substantially less frequently, antitachycardia pacing can also terminate some atrial tachyarrhythmias, and these features have been incorporated in some contemporary ICD systems.
    • Cardioversion/defibrillation
      • A synchronized shock to be delivered at the peak of the R wave is referred to as cardioversion.
      • Because VT is an organized electrical rhythm, delivering an electrical shock during the vulnerable period of repolarization can cause VT to degenerate into VF.
      • Synchronized cardioversion prevents shock delivery during the vulnerable period. Although ICDs can be programmed to deliver synchronized shocks at a range of energies up to the device's maximum output (usually 30 to 35 joules), synchronized cardioversion can often terminate VT with relatively low energy (e.g., 10 joules or less).
      • An unsynchronized shock (i.e., a shock delivered randomly during the cardiac cycle) is called defibrillation. Healthcare providers can program ICDs to deliver unsynchronized shocks for very rapid ventricular arrhythmias (e.g., heart rate greater than 200 beats/min). Because VF is not an organized rhythm, synchronized cardioversion is neither possible nor necessary. Similarly, it can be difficult to synchronize with very rapid VTs, and such rapid rhythms are unlikely to be hemodynamically tolerated.
      • ICDs are typically programmed to deliver synchronized shocks at energies approaching the device's maximum output (usually 30 to 35 joules).
    • Bradycardia pacing
      • All contemporary transvenous ICDs can pace, although S-ICDs cannot deliver pacing therapies.
      • Many patients with an ICD have a conventional indication for cardiac pacing.
      • Separate ICDs and pacemakers can produce device-to-device interactions, particularly with older models, potentially resulting in inappropriate shocks and underdetection of VT/VF.
      • Patients should have only one transvenous or epicardial device with rare exceptions, although the combined use of a leadless pacemaker with an S-ICD is under investigation.
      • Generally, however, when a patient with a pacemaker develops an indication for ICD implantation, the pacemaker is removed and replaced with an ICD.
      • The device will be programmed accordingly for patients with known AV block or sinus node dysfunction or those receiving LV pacing as part of CRT.
      • For those without pre-existing AV block or sinus node dysfunction and who presumably do not require regular ventricular pacing, the ICD will typically be programmed to minimize the amount of pacing provided (e.g., pace only for intrinsic rates less than 40 beats/min).
      • In addition to the usual indications for pacing, the ability to provide pacing also protects against bradyarrhythmias that can follow a tachycardia or shock and against ventricular arrhythmias that are bradycardia-dependent. Because of the unique physiology following a ventricular tachyarrhythmia and device shock, ICDs allow for distinct post-shock pacing programming (usually at higher outputs).
    • Cardiac resynchronization therapy (CRT)
      • CRT, which utilizes biventricular pacing, is an effective treatment for symptomatic HF in some patients with LV dyssynchrony.
      • CRT is currently recommended in patients with advanced HF (usually NYHA class III or IV), severe systolic dysfunction (LV ejection fraction ≤35%), and intraventricular conduction delay (QRS >120 milliseconds).
      • The evidence of benefit is greatest in patients with left bundle branch block and a QRS duration >150 milliseconds.
      • Pacing of the LV is most frequently achieved by transvenous insertion of an electrode into a cardiac vein via the coronary sinus.
      • Surgical placement of an epicardial lead is also an option in patients following failed efforts at transvenous lead placement or in patients undergoing cardiac surgery for another reason.
      • Improvement in HF can reduce the frequency of ventricular arrhythmias, raising the possibility that biventricular pacing may have an adjunctive role with an ICD by reducing the need for ICD therapy.
    • Perioperative ICD functioning
      • During surgical procedures, the function of ICDs may be affected by electromagnetic interference (EMI), most commonly due to the use of an electrosurgery unit (ESU).
      • ICDs with integrated bipolar sensing configuration may be more susceptible to EMI than true bipolar sensing.
      • Very rarely, direct damage from cautery to the ICD may alter its delivery of pacing or shocks or reset the ICD to an alternate or backup mode. The more common concern is that the device might misinterpret the cautery as tachyarrhythmia, which leads to withholding bradycardia pacing and perhaps inappropriate ICD shocks.

Wearable Cardioverter-Defibrillator (WCD)

  • Some patients at risk for SCD do not meet established criteria for implantation of an ICD or may require only short-term protection (such as patients awaiting subsequent ICD insertion or cardiac transplantation).
  • A wearable WCD may be preferable to either ICD insertion or bystander resuscitation in such settings.

Subcutaneous ICD (S-ICD)

  • Some patients who are at risk for SCD and require an ICD will have compelling reasons for avoiding the indwelling transvenous leads associated with a standard ICD (e.g., other indwelling leads or catheters, high risk for systemic infection, relatively young age at implant with numerous device implants anticipated over a lifetime, etc.).
  • An entirely S-ICD has been developed to provide an effective alternative means of defibrillation, albeit without some of the standard capabilities of a traditional transvenous ICD (i.e., no antitachycardia pacing or backup bradycardia pacing).

Antiarrhythmic Drugs

Antiarrhythmic drugs are less effective than an ICD for secondary prevention of SCD. Consequently, their use in the setting of SCD is limited to an adjunctive role as described above, or in patients who do not want or are not candidates for an ICD (e.g., due to marked comorbidities or end-stage HF that make death likely).

Indications for Pharmacologic Therapy

  • Nearly all survivors of SCA without a reversible cause should be evaluated for placement of an ICD. Because an ICD treats but does not prevent arrhythmias, patients with symptoms or device discharges may require adjunctive antiarrhythmic therapy.
  • In addition to ICD therapy for survivors of SCA, there are three main indications for concomitant antiarrhythmic drug therapy:
    • To reduce the frequency of ventricular arrhythmias in patients with frequent ICD shocks. In one analysis, the occurrence of frequent ICD shocks was the primary reason for adding an antiarrhythmic drug (64%).
    • To suppress supraventricular arrhythmias that may cause symptoms or interfere with ICD function, potentially resulting in "inappropriate" shocks.
      • "Inappropriate" shocks result from non-life-threatening arrhythmias which meet the programmed parameters for ICD therapy, primarily based upon rate (e.g., AF with a rapid ventricular response exceeding the programmed threshold for delivering a shock).
      • "Inappropriate" shocks have been reported in up to 29% of ICD patients and can have a substantial impact on the patient’s quality of life.
        • These shocks are caused by various arrhythmias, including sinus tachycardia, AF, and NSVT.
        • More sophisticated programming features of current-generation ICDs may allow the device to ignore clinically unimportant and non-life-threatening arrhythmias rather than delivering an unnecessary shock.
    • To reduce the ventricular rate of VT so that it is better tolerated hemodynamically or more amenable to termination by anti-tachycardia pacing or low energy cardioversion.

Choice of Pharmacologic Therapy

  • Medications used to manage cardiac arrhythmias (Table 3):
Table 3: Medications Used to Mange Cardiac Arrhythmias
Medications used to slow fast cardiac rhythms or stabilize irregular rhythms are known as antiarrhythmics. Antiarrhythmic medications are divided into classes by their primary mode of action.


  • Class I agents: Sodium channel blockers that slow electrical conduction in the heart tissue. They are primarily used to treat rapid heart rhythms originating in the ventricles.

Class I agents are subdivided into:

Ia – Quinidine, Procainamide, Disopyramide

Ib – Lidocaine, Tocainide, Mexiletine, Phenytoin

Ic – Flecainide, Propafenone, Ecainide, Moricizine

  • Class II agents: Beta blockers that reduce heart workload by blocking certain hormones that bind with beta receptors in the heart. This makes it harder for a rapid heartbeat to be triggered.

Class II agents include:

Metoprolol, Propranolol, Esmolol, Atenolol

  • Class III agents: Potassium channel blockers which slow the use of potassium in heart cells.

Class III agents include:

Amiodarone, Sotalol, Azimilide, Bretylium, Cloflium, Dofetilide, Tedisamil, Ibutilide, Sematilide

  • Class IV agents: Calcium channel blockers (CCBs) decrease the contraction of the heart and dilate (widen) the arteries thus slowing overall heart rate.

Class IV agents include:

Verapamil, Diltiazem

  • Class V agents: A category for those medications which fail to fit into other class descriptions.

Class V agents include:

Magnesium, Digitalis (digoxin), Adenosine

  • For patients with an ICD who require adjunctive antiarrhythmic therapy due to ongoing arrhythmias, treatment with the combination of amiodarone plus a beta-blocker is recommended rather than treatment with amiodarone alone or other antiarrhythmic agents. On occasion, therapy with mexiletine or sotalol may be useful.
  • In general, the class I antiarrhythmic drugs are not used as most patients with SCA have structural heart disease, and these drugs are not recommended.
  • In the form of beta-blockers and antiarrhythmic medications, pharmacologic therapy can help control arrhythmias in survivors of SCA.
    • Virtually all patients who have survived SCA should be considered for beta-blocker therapy. However, due to the efficacy of the ICD in treating sustained ventricular tachyarrhythmias and improving mortality, antiarrhythmic drugs are generally reserved for use in select patients as adjunctive therapy or as primary therapy when an ICD is not indicated or refused by the patient.

Empiric Versus Guided Pharmacologic Therapy

  • Empiric pharmacologic therapy for SCA survivors, primarily with beta-blockers or an antiarrhythmic drug, is an effective approach for survivors of SCA. They have refused ICD placement or are not candidates for an ICD.
  • Beta-blockers have some efficacy with relatively few side effects, while for most patients, amiodarone is the most efficacious antiarrhythmic drug for preventing recurrent ventricular arrhythmias.
  • However, in current practice, when pharmacologic therapy is administered to a patient with or without (because of refusal or noncandidacy for) an ICD, empiric treatment with beta-blockers or amiodarone is the preferred approach.
    • Other antiarrhythmic drugs (for example, mexiletine or sotalol) are considered if there is recurrent arrhythmia despite therapy with amiodarone or a beta-blocker.

Beta Blockers

  • Nearly all patients who have survived SCA should receive a beta-blocker as part of their therapy.
  • Beta-blockers are not generally considered adequate monotherapy and should be used with an antiarrhythmic drug for most patients resuscitated from SCA due to VT or VF. However, the associated anti-adrenergic effects of beta-blockers may effectively reduce arrhythmias and SCA when no specific antiarrhythmic treatment is given.
    • In an analysis from the AVID trial, patients discharged from the hospital on a beta-blocker had a mortality reduction compared with those not receiving a beta-blocker.
  • Most SCA survivors will have multiple indications for a beta-blocker (e.g., post-myocardial infarction, HF, etc.) from which they derive clinical benefit.
    • Beta-blockers reduce the incidence of sudden death and total mortality in patients with a recent myocardial infarction and those with symptomatic HF or congenital LQTS.
    • Beta-blockers should be used as part of the medical regimen following SCA due to VT/VF, even in the absence of any additional indications.
  • Beta-blockers can potentiate the effects of class I antiarrhythmic drugs by preventing the effect of sympathetic stimulation on reversing the depressant effect on slowing conduction. They can also potentiate the action of class III antiarrhythmic drugs by preventing the sympathetic effect of shortening repolarization.

Antiarrhythmic Drugs

  • Amiodarone is the most effective for preventing recurrent ventricular tachyarrhythmias, although mexiletine, sotalol, and dofetilide are also useful in reducing recurrent ventricular arrhythmias.
    • Empiric therapy with amiodarone is preferred for immediately following SCA in patients with recurrent ventricular tachyarrhythmias and those who have refused (or are not candidates for) ICD placement.
    • Following patient stabilization, if there are concerns about potential toxicity related to amiodarone, particularly for anticipated long-term use, mexiletine, sotalol, or dofetilide may be considered.


  • Several clinical trials and systematic reviews have evaluated the efficacy of antiarrhythmic drugs as adjuvant therapy in ICD patients. There were significant differences in trial methodologies, which limit direct comparisons. Amiodarone has generally been the most effective antiarrhythmic drug for preventing ventricular arrhythmias (and associated ICD shocks).
    • There was significant heterogeneity among the trials in one systematic review that included eight randomized trials involving 1,889 patients. Key findings included:
      • Amiodarone in combination with a beta-blocker significantly reduced the incidence of shocks compared with the beta-blocker alone.
      • Sotalol reduced the incidence of ICD shocks when compared with a placebo. There was also a trend toward fewer shocks in patients treated with sotalol versus another beta-blocker.
      • Treatment with either azimilide or dofetilide resulted in nonsignificant trends towards reduction in total ICD shocks compared with placebo. However, the incidence of appropriate ICD therapies (shocks plus antitachycardia pacing) was significantly reduced by azimilide.
      • Another major advantage of amiodarone is its very low frequency of proarrhythmia. Although amiodarone can prolong the QT interval, torsades de pointes are rare. Caution is necessary when amiodarone is given with other drugs that can prolong the QT interval or therapy is complicated by hypokalemia or hypomagnesemia.


  • When patients are started on an antiarrhythmic drug, a baseline ECG should be obtained before drug initiation and then serial ECGs obtained for the first two to three days, particularly to monitor heart rate and assess for any significant QT interval prolongation.
  • Amiodarone
    • The initial dosing of amiodarone will vary depending on the route (IV or oral), as well as the clinical situation:
      • For patients with electrical storm or incessant VT, amiodarone IV (150 mg IV push, followed by 1 mg/minute IV infusion for six hours, followed by 0.5 mg/min IV infusion for 18 additional hours) is the initial antiarrhythmic agent.
      • For patients who have been on IV therapy for more than two weeks, oral maintenance amiodarone at a dose of 200 to 400 mg/day is begun.
      • For patients who have been on IV therapy for one to two weeks, an intermediate maintenance oral amiodarone dose of 400 to 800 mg/day is begun. This should be continued until a total loading dose of 10 to 15 grams has been received, then the dose should be reduced to the usual maintenance dose of 200 to 400 mg/day. As oral amiodarone is only approximately 50% bioavailable, a total of 20 to 30 grams needs to be administered.
      • For patients who have been on IV therapy for one week or less, a full oral amiodarone loading dose of 400 to 1200 mg/day (typically in two divided doses) is started. This should be continued until a total loading dose of 10 to 15 grams has been received (and a total of 20 to 30 grams administered due to the reduced oral bioavailability), then the dose should be reduced to the usual maintenance dose of 200 to 400 mg/day.
  • Sotalol
    • Sotalol is not universally available in intravenous form. Bradycardic and proarrhythmic events can occur after the initiation of sotalol therapy and with each upward dosing adjustment. As a result, sotalol should be initiated, and doses increased in a hospital with cardiac rhythm monitoring and assessment facilities.
    • Sotalol is started at a dose of 80 mg twice daily, with dose adjustments at three-day intervals once steady-state plasma concentrations have been achieved and the QT interval reviewed on an ECG. Patients with renal insufficiency require a modification of the dosing interval.
  • Dofetilide
    • Dofetilide is only available in oral form and is not universally available in all countries.
  • Mexiletine
    • Mexiletine, a lidocaine-like antiarrhythmic drug, is only available for oral use.
    • It is often used with or without amiodarone for treating patients with an ICD who have ventricular arrhythmias that are of concern.
    • The usual dose is 200 to 400 mg PO three times daily.

Treatment of Breakthrough Arrhythmias

  • Patients with recurrent, or breakthrough, arrhythmias resulting in repeat ICD shocks or SCA despite therapy with a beta-blocker or antiarrhythmic drug represent a significant clinical challenge. As with the occurrence of any ventricular arrhythmia, any identifiable reversible causes (e.g., myocardial ischemia, electrolyte disturbances, etc.) should be corrected.
  • In the absence of any reversible causes, treatment should be approached as follows:
    • For patients who are taking only a beta-blocker, an antiarrhythmic drug, ideally amiodarone, should be added.
    • A beta-blocker should be added for patients taking only an antiarrhythmic drug.
    • For patients who are taking both a beta-blocker and an antiarrhythmic drug, treatment options include:
      • Upward titration of either or both existing drugs or,
      • Discontinuation of the current antiarrhythmic drug in favor of an alternative antiarrhythmic drug.
      • First, increasing the beta blocker's dose and the current antiarrhythmic drug to the maximum recommended dose (or maximum tolerated dose if side effects arise) is preferred. If this approach is ineffective and the patient continues to have recurrent ventricular arrhythmias and shocks, the current antiarrhythmic drug should be stopped and treatment initiated with another agent.
      • Another important option for patients with recurrent arrhythmia despite amiodarone and a beta-blocker is adding a class I antiarrhythmic agent that does not alter the QT interval (i.e., mexiletine or propafenone).

Impact on ICD Therapies

  • The primary goal of using beta-blockers or antiarrhythmic drugs in patients with an ICD is to minimize the frequency of recurrent ventricular arrhythmias, thereby decreasing the likelihood of receiving additional ICD shocks.
  • Beyond reducing the likelihood of ICD shocks, however, antiarrhythmic drug therapy may impact the efficacy of ICD therapies by potentially altering defibrillation thresholds and slowing the ventricular rates of any recurrent sustained tachyarrhythmias.

Alterations in Defibrillation Thresholds (DFTs)

  • Any antiarrhythmic drug can potentially alter the DFT. However, the effect has been most pronounced with amiodarone and its major metabolite, desethylamiodarone, increasing the DFT dose-dependent.
  • DFT testing has historically been performed at the time of ICD implantation, although the routine necessity for this evaluation with the current generation of ICDs has been questioned. However, repeat DFT testing may be warranted after the initiation of amiodarone if there is concern about rising DFT thresholds (as may occur in certain clinical situations, including AF, hypertension, and left ventricular hypertrophy).
  • The report on the efficacy of routine ICD testing is discussed above (Yannopoulos et al., 2017). patients had an ICD test due to the initiation or dose-adjustment of an antiarrhythmic drug (primarily amiodarone or sotalol), and the ICD failed to defibrillate only two patients.

VT Rate Slowing

  • In patients receiving chronic antiarrhythmic drug therapy, the rate of recurrent VT is often slower than the rate seen during the index arrhythmia. This slower ventricular rate can have both positive and negative impacts on the delivery of ICD therapy:
    • Antitachycardia pacing is often more effective for the treatment of VT with a slower ventricular rate. When antitachycardia pacing effectively terminates the VT, the ICD does not deliver a shock, and many patients will not know about the event.
    • Slower ventricular rates during VT may result in the VT falling below the previously defined rate thresholds for the ICD to detect and treat the arrhythmia. As a result, if no ICD programming changes are made, and the VT is hemodynamically significant (i.e., resulting in syncope, palpitations, chest pain, dyspnea, or even SCA), the patient may not receive an appropriately-indicated therapy.
    • Consequently, lowering the VT detection cutoff is common when initiating antiarrhythmic drug therapy. The specific detection threshold is individualized based on the specific data from prior events in the individual patient.

Prognosis And Outcomes Following Sudden Cardiac Arrest

Despite advances in the treatment of heart disease, the outcome of patients experiencing SCA remains poor (Wong et al., 2014).

  • In a Canadian study of 34,291 patients who arrived at the hospital alive following out-of-hospital cardiac arrest between 2002 and 2011, survival at both 30-day and one-year increased significantly between 2002 and 2011 (from 7.7% to 11.8% for one-year survival) (Wong et al., 2014).
  • Similarly, among a cohort of 6,999 Australian patients with out-of-hospital SCA resuscitated by EMS between 2010 and 2012, 851 patients (12.2%) survived for at least one year, with more than half of patients reporting good neurologic recovery and functional status at one year (Smith et al., 2015).

The reasons for the continued poor survival of patients with SCA are uncertain. Although some aspects of acute resuscitation have improved over time (increased bystander CPR and shortened time to defibrillation), these positive trends have been offset by adverse clinical features of patients presenting with SCA (such as increasing age and decreasing proportion presenting with VF). In addition, the response times of both BLS and ALS services have increased, possibly due to population growth and urbanization.

A pilot study comparing the feasibility of EMS transport to a regional cardiac arrest center (with increased transit time) versus transport to the closest hospital suggested no difference in 30-day mortality or major adverse cardiac events. These results should be considered hypothesis-generating for larger-scale studies (Patterson et al., 2017).

Neurologic Prognosis Following SCA

Survivors of SCA have variable susceptibility to hypoxic-ischemic brain injury, depending on the duration of circulatory arrest, the extent of resuscitation efforts, and underlying comorbidities.

Outcome According To Etiology

There is an association between the mechanism of SCA and the outcome of initial resuscitation.


When the initially observed rhythm is asystole (even if preceded by VT or VF), the likelihood of successful resuscitation is low. Only 10% of patients with out-of-hospital arrests and initial asystole survive until hospital admission, and only 0 to 2% until hospital discharge. The poor outcome in patients with asystole or bradycardia due to a very slow idioventricular rhythm probably reflects the prolonged duration of the cardiac arrest (usually more than four minutes) and the presence of severe, irreversible myocardial damage.

Factors associated with successful resuscitation of patients presenting with asystole include:

  • No further need for treatment with atropine for a bradyarrhythmia after initial resuscitation
  • Shorter arrival time of EMS personnel
  • Witnessed arrest
  • Younger patient age

Pulseless Electrical Activity (PEA)

Patients with SCA due to PEA (also called electrical-mechanical dissociation) have a poor outcome.

Ventricular Tachyarrhythmia

The outcome is much better when the initial rhythm is sustained ventricular tachyarrhythmia. Acute MI or myocardial ischemia is the underlying cause of VF for many of the patients who survive hospital discharge.

Survival is approximately 65% to 70% in patients with hemodynamically unstable VT. The prognosis may be better in patients found in monomorphic VT because of the potential for some systemic perfusion during this more organized arrhythmia. In addition, patients with VT tend to have a lower incidence of a previous infarction and a higher ejection fraction when compared with those with VF.

SCA due to Noncardiac Causes

As many as one-third of cases of SCA are due to noncardiac causes. Trauma, nontraumatic bleeding, intoxication, near drowning, and pulmonary embolism are the most common noncardiac etiologies.

Factors Affecting Out-Of-Hospital SCA Outcome

Despite the efforts of emergency personnel, resuscitation from out-of-hospital SCA is successful in only one-third of patients, and only about 10% of all patients are ultimately discharged from the hospital, many of whom are neurologically impaired (Chan et al., 2014).

The cause of death in-hospital is most often noncardiac, usually anoxic encephalopathy, or respiratory complications from long-term ventilator dependence.

In addition to later initiation of CPR and the presence of asystole or PEA (electromechanical dissociation), several other factors are associated with a decreased likelihood of survival with neurologic function intact following out-of-hospital SCA:

  • Absence of any vital signs
  • Alzheimer disease
  • Cancer
  • Cerebrovascular accident with severe neurologic deficit
  • History of cardiac disease
  • History of more than two chronic diseases
  • Prolonged CPR more than five minutes
  • Sepsis

There are also several poor prognostic features in patients with SCA who survive until admission:

  • History of class III or IV HF
  • Hypotension
  • Need for intubation
  • Need for vasopressors
  • Older age
  • Persistent coma after CPR
  • Pneumonia
  • Renal failure after CPR

VF Duration

VF in the human heart rarely terminates spontaneously, and survival is therefore dependent upon the prompt delivery of effective CPR. Electrical defibrillation is the only way to reestablish organized electrical activity and myocardial contraction.

Increasing duration of VF has two major adverse effects:

  1. VF reduces the ability to terminate the arrhythmia and
  2. If VF continues for more than four minutes, irreversible damage to the central nervous system and other organs begins. Consequently, the longer the duration of the cardiac arrest, the lower the likelihood of resuscitation or survival with or without neurologic impairment, even if CPR is successful.

It has been suggested that without CPR, survival from a cardiac arrest caused by VF declines by approximately 10% for each minute without defibrillation. After more than 12 minutes without CPR, the survival rate is only 2% to 5%.

Time to Resuscitation

These observations constitute the rationale for providing more rapid resuscitation in patients with out-of-hospital SCA. One approach is optimizing the EMS system within a community to reduce the response interval to less than eight minutes.

However, the response times of both BLS and ALS services have increased, possibly due to population growth and urbanization. Thus, bystander CPR and even defibrillation have been recommended and implemented in some settings. Such interventions permit more rapid responses than those provided by BLS or ALS personnel, with better survival as a result.

Bystander CPR

The administration of CPR by a layperson bystander (bystander CPR or bystander-initiated CPR) is important in determining patient outcome after out-of-hospital SCA. Survival after SCA is greater among those who have bystander CPR when compared with those who initially receive more delayed CPR from EMS personnel. In addition to improved survival, early restoration or improvement in circulation is associated with better neurologic function among survivors.

For adults with sudden out-of-hospital SCA, compression-only bystander CPR (without rescue breathing) appears to have equal or possibly greater efficacy compared with standard bystander CPR (compressions plus rescue breathing).

The importance of bystander CPR and support for compression-only bystander CPR comes from a combination of retrospective and prospective studies.

These observations were subsequently confirmed in larger studies (Nakahara et al., 2015).

  • In a nationwide study of out-of-hospital cardiac arrest in Japan between 2005 and 2012, during which time the number of out-of-hospital cardiac arrests grew by 33% (n = 17,882 in 2005 compared with n = 23,797 in 2012), rates of bystander CPR increased (from 39% to 51%). Recipients of bystander CPR had a significantly greater chance of neurologically intact survival (8.4% versus 4.1% without bystander CPR) (Nakahara et al., 2015). Bystanders' early defibrillation was also associated with significantly greater survival odds neurologically intact.
  • In a cohort of 19,468 individuals with out-of-hospital cardiac arrest in Denmark between 2001 and 2010, which was not witnessed by EMS personnel (from the nationwide Danish Cardiac Arrest Registry), the frequency of bystander CPR increased from 21% in 2001 to 45% in 2010, with a corresponding significant increase in survival at 30 days (3.5% to 10.8%) and one year (2.9% to 10.2%).

Despite the benefits of bystander CPR, it is not always performed. Reasons for this include:

  • The bystander's lack of CPR training.
  • The bystander's concerned about the possible transmission of disease while performing rescue breathing.
  • Neighborhood demographics (racial composition and income level) also appear to be a factor in the performance rates of bystander CPR.

Interventions that appear to improve the rate of bystander CPR include verbal encouragement and instruction in CPR by EMS dispatchers and public campaigns to promote the delivery of bystander CPR.

Chest Compression-Only CPR

Bystander CPR with only chest compressions improves survival to hospital discharge, compared with chest compressions with interruptions for rescue breathing, with an absolute improvement in mortality of 2.4% (Zhan et al., 2017).

Initial observational studies that evaluated the delivery of compression-only CPR versus standard CPR, including rescue breathing, found no significant differences in survival or long-term neurologic function between the two groups, suggesting that compression-only CPR could be safely delivered.

In a nationwide study of Japanese out-of-hospital cardiac arrest victims between 2005 and 2012, chest compression-only CPR improved the number of SCA victims receiving bystander CPR and the number of patients surviving with favorable neurological outcomes (Iwami et al., 2015). These findings hold promise for improving the delivery of bystander CPR. Further data are required to determine if bystander-delivered compression-only CPR (rather than standard CPR) will translate into better neurologic outcomes for patients with out-of-hospital cardiac arrest.

Automated Mechanical CPR Devices

Several automated devices that deliver chest compressions have been developed in an attempt to improve upon chest compressions delivered by humans, as well as to allow rescuers to perform other interventions simultaneously.

Timing of Defibrillation

VF's standard of care for resuscitation has been defibrillation as soon as possible. In the Seattle series of over 12,000 EMS-treated patients, 4,546 had witnessed VF. The defibrillation response interval was significantly correlated with survival to hospital discharge (odds ratio 0.88 for every one-minute increase in response time). Subsequent studies have shown similar benefits, with earlier defibrillation associated with improved survival (Nakahara et al., 2015).

Despite these findings, it has been suggested that outcomes may be improved by performing CPR before defibrillation, at least in patients in whom defibrillation is delayed for more than four to five minutes. An initial report from Seattle compared outcomes in two time periods. When an initial shock was given as soon as possible and subsequently when the initial shock was delayed until 90 seconds of CPR had been performed. Survival to hospital discharge was significantly increased with routine CPR before defibrillation, primarily in patients whose initial response interval was four minutes or longer.

However, in the largest study to date comparing shorter versus longer periods of initial CPR before defibrillation in 9,933 patients with SCA, patients were randomly assigned to receive 30 to 60 seconds versus 180 seconds of CPR before cardiac rhythm analysis and defibrillation (if indicated). There was no significant difference in the primary endpoint of survival to hospital discharge with satisfactory functional status.

Early defibrillation and CPR should be performed as recommended in the 2010 ACLS guidelines for SCA and ventricular tachyarrhythmia patients.

Automated External Defibrillators (AEDs)

The use of AEDs by early responders is another approach to more rapid resuscitation. In most but not all studies, AEDs have improved survival after out-of-hospital cardiac arrest.

Predictive Value of BLS and ALS Rules

The OPALS study group has proposed two terminations of resuscitation rules for use by EMS personnel. The rule for BLS providers equipped with AEDs includes the following three criteria:

  1. Event not witnessed by EMS personnel
  2. No AED was used, or manual shock was applied in the out-of-hospital setting
  3. No return of spontaneous circulation in the out-of-hospital setting

The ALS rule includes the BLS criteria, as well as two additional criteria219:

  1. Arrest not witnessed by a bystander
  2. No bystander-administered CPR

Validation of the predictive value of the BLS and ALS termination rules was performed with data from a retrospective cohort study that included 5,505 adults with out-of-hospital SCA.

However, the validity of these termination rules may be reduced with improvements in EMS and post-resuscitation care. One potential target for understanding and ameliorating current limitations to post-arrest care is the observed marked regional variation in prognosis following SCA.

Adequacy of CPR

The adequacy of CPR delivered to a victim of cardiac arrest and outcomes related to resuscitation efforts may depend on various factors (e.g., rate and depth of chest compressions, amount of time without performing chest compressions while performing other tasks such as defibrillation, etc.). The AHA 2010 Guidelines for Cardiopulmonary Resuscitation (CPR) and Emergency Cardiovascular Care emphasized early defibrillation (when available) and high-quality chest compressions (rate at least 100 per minute, depth of 2 inches or more) with minimal interruptions.

The effect of CPR quality has been evaluated in several studies (Brouwer et al., 2015):

  • In a 2013 systematic review and meta-analysis which included 10 studies (4,722 patients total, 4,516 of whom experienced out-of-hospital cardiac arrest), individuals surviving cardiac arrest were significantly more likely than non-survivors to have received deeper chest compressions and have had compression rates between 85 and 100 compressions per minute (compared with shallower and slower compression rates).
  • In a study of 3,098 patients with out-of-hospital cardiac arrest, the return of spontaneous circulation was highest at a rate of 125 compressions per minute. However, higher chest compression rates were not significantly associated with survival to hospital discharge.

End-tidal carbon dioxide levels have an excellent correlation with very low cardiac outputs when measured after at least 10 minutes of CPR. They may provide prognostic information, suggesting that the cardiac output maintained during CPR determines the outcome.

Body Temperature

An increase in body temperature is associated with unfavorable functional neurologic recovery after successful CPR. The increase in temperature may be neurally-mediated and can exacerbate the degree of neural injury associated with brain ischemia. For the highest temperature within 48 hours, each degree Celsius higher than 37ºC increases an unfavorable neurologic recovery risk.

On the other hand, the induction of mild to moderate hypothermia (target temperature 32 to 34ºC for 24 hours) may benefit patients successfully resuscitated after a cardiac arrest, although studies have shown variable outcomes.

Prehospital ACLS

The incremental benefit of deploying EMS personnel trained in ACLS interventions (intubation, insertion of intravenous lines, and intravenous medication administration) on survival after cardiac arrest probably depends upon the quality of other prehospital services.

  • In the OPALS study, ACLS interventions were added to an optimized EMS program of rapid defibrillation. No improvement in the survival rate for out-of-hospital cardiac arrest was observed with the addition of an ACLS program.
  • In a retrospective report from Queensland with an EMS program not optimized for early defibrillation, the presence of ACLS-skilled EMS personnel was associated with improved survival for out-of-hospital cardiac arrest.

Effect of Older Age

The risk of SCA increases with age, with older age being associated with poorer survival in some, but not all, studies of out-of-hospital cardiac arrests.

Effect of Gender

The incidence of SCA is greater in men than in women. The effect of gender on outcome has been examined in multiple cohorts, with the following findings:

  • Men are more likely than women to have VF or VT as an initial rhythm.
  • Men are more likely than women to have a witnessed arrest.
  • Men have higher one-month survival than women following SCA due to the higher likelihood of VF/VT as their presenting rhythm.
  • Women have a greater survival with favorable neurologic outcomes when considering only patients with VF/VT as the initial rhythm.

Effect of Comorbidities

The impact of preexisting chronic conditions on the outcome of out-of-hospital SCA was evaluated in a series of 1,043 SCA victims in King County, Washington, in the United States. There was a statistically significant reduction in the probability of survival to hospital discharge with increasing numbers of chronic conditions, such as:

  • CHF
  • Diabetes
  • Hypertension
  • Prior myocardial infarction

The impact of comorbidities was more prominent with longer EMS response intervals.

Factors Affecting In-Hospital SCA Outcome

The outcome of patients who experience SCA in the hospital is poor, with reported survival to hospital discharge rates of 6% to 15%. Several clinical factors have been identified that predict a greater likelihood of survival to hospital discharge:

  • Pulse regained during the first 10 minutes of CPR
  • VT or VF as initial rhythm
  • Witnessed arrest

Other factors have been identified that predict a lower likelihood of survival to hospital discharge:

  • Longer duration of overall resuscitation efforts
  • Multiple resuscitation efforts

Delays in providing initial defibrillation have been associated with worse outcomes. Delayed defibrillation (more than two minutes after SCA) occurred in 30% of patients and was associated with a significantly lower probability of hospital discharge survival.

Delayed defibrillation was more common with the black race, noncardiac admitting diagnosis, cardiac arrest at a hospital with fewer than 250 beds, an unmonitored hospital unit, and arrest during after-hours periods.

Multiple resuscitations involving CPR have also been associated with worse outcomes. Survival following in-hospital SCA treated with an AED has also been evaluated using data derived from the National Registry of Cardiopulmonary Resuscitation. Compared with usual resuscitative care, using an AED did not improve survival among patients with a shockable rhythm. It was associated with a lower survival to hospital discharge among patients with a non-shockable rhythm.

The AHA issued consensus recommendations regarding strategies for improving outcomes following in-hospital SCA. While the consensus recommendations focused on many of the same factors as out-of-hospital cardiac arrest (i.e., early identification of SCA, provision of high-quality CPR, early defibrillation when indicated), the authors commented on a lack of evidence specifically focused on in-hospital SCA, with many of the current guideline recommendations based on extrapolations of data from out-of-hospital SCA. Further data specifically focusing on in-hospital SCA are required before making additional recommendations.

Impact Of Arterial Oxygen Level

Arterial hyperoxia early after SCA may have deleterious effects, perhaps due to oxidative injury. In a multivariable model, hyperoxia was an independent risk factor for death. Hypoxia was also an independent risk factor.

Long-Term Outcome

The reported long-term survival of resuscitated SCD survivors is variable and may depend upon multiple factors:

  • In patients with out-of-hospital VF, was early defibrillation achieved?
  • Do the data come from randomized trials, in which many, often sicker, patients are excluded, or from community-based observations?
  • Was the patient treated with early revascularization, antiarrhythmic drugs, or an ICD?
  • Does the patient have other risk factors, particularly a reduced LVEF?
  • Do patients with seemingly transient or reversible causes of SCA have a better prognosis?
  • Did the episode of SCA begin as VF or VT?

Case Study

Scenario/Situation/Patient Description

Mr. John Jomes is a 42-year-old married father of four, a stockbroker who was in a committee meeting at work at 0845 when he suddenly collapsed, falling forward and hitting his forehead on the table in front of him. EMS was quickly called while one of his work compatriots performed compression-only CPR. He regained consciousness, with some slight confusion noted.

EMS arrived 20 minutes later. EMS personnel related to ED personnel denied chest pain or SHOB. C/o slight neck pain with a frontal HA. BP remained 120-140/66-80, HR 35-66 sinus bradycardia to NSR with infrequent PVCs. Afebrile. Slight confusion persists. A soft neck brace remains on, with the patient remaining on a backboard.

In the emergency department, the patient denies any SHOB, CP. He continues to c/o slight posterior neck pain with a frontal HA. Past medical history negative for DM, HTN, or any cardiac/pulmonary issues. Negative past surgical history. The patient denies alcohol or drug usage or exposure to toxins. He takes Tylenol prn for aches and pains. States his mother died suddenly when she was 43 years of age and his uncle (his mother’s brother) in his 30s.


12-lead ECG shows sinus bradycardia to NSR with rare PVCs.

IV of 0.9 NS infusing at 100 ml/hr.

Continuous cardiac monitoring with pulse oximetry.

Following lab tests ordered and drawn: Comprehensive metabolic panel, CBC, Lipid profile, CK-MB, CK and MB, Troponins I and T, CRP, Coagulation studies, Toxicology screen.

Soft neck brace remains in place. Patient remains on a backboard.

PA and lateral chest x-ray ordered.

PA and lateral neck x-rays ordered.

Discussion of Outcomes

12-lead ECG shows no evidence of cardiac ischemia, but the rhythm remains sinus bradycardia to NSR with rare unifocal PVCs.

All laboratory tests ordered are WNL or negative.

Chest and neck x-rays remain normal.

Cardiologist on-call to evaluate a patient in the emergency department.

Strengths and Weaknesses

Cardiologist to evaluate a patient for structural heart disease/arrhythmias/primary electrical diseases with appropriate diagnostic procedures ordered

Wife and other family members are not available at present to obtain further information on SCD in family history.


SCA and SCD refer to the sudden cessation of organized cardiac electrical activity with hemodynamic collapse.

  • The event is referred to as SCA (or aborted SCD) if an intervention (e.g., defibrillation, cardioversion, antiarrhythmic drug) or spontaneous reversion restores circulation.
  • The event is called SCD if the patient dies. However, the use of SCD to describe both fatal and nonfatal cardiac arrest persists by convention.

SCD is the most common cause of cardiovascular death in the developed world.

  • Although the risk of SCD is higher in patients with structural heart disease, as many as 10 to 15% of SCDs occur in individuals with apparently normal hearts.
  • Causes of SCD with no structural heart disease include:
    • Brugada syndrome
    • Commotio cordis
    • Early repolarization syndrome
    • Familial SCD of Uncertain Cause
    • Idiopathic VF (Primary Electrical Disease)
    • LQTS congenital or acquired
    • SQTS
    • Polymorphic VT with Normal QT Interval (CPVT)
    • Third Degree (Complete) AV Block
    • VF Secondary to PVCs
    • WPW Syndrome and Other Forms of SVT
  • Survivors of SCA should undergo extensive testing to exclude drug or toxin exposure or underlying structural heart disease that may have contributed to SCA. Therapy with an ICD should generally be recommended in survivors of SCA.
  • In families of victims of unexplained SCD, a general cardiology evaluation of first- and second-degree relatives can diagnose heritable disease in up to 40% of families.

The exact mechanism of collapse in an individual is often impossible to establish since, for the vast majority of patients who die suddenly, cardiac electrical activity is not being monitored at the time of their collapse. In studies, however, of patients with cardiac electrical activity monitored at the time of their event, VT or VF accounted for most episodes, with bradycardia or asystole accounting for nearly all of the remainder.

  • In most patients with VT/VF, sustained ventricular arrhythmia is preceded by an increase in ventricular ectopy and the development of repetitive ventricular arrhythmia, particularly runs of nonsustained VT. In about one-third of cases, the tachyarrhythmia is initiated by an early R on T PVC. In the remaining two-thirds, the arrhythmia is initiated by a late-cycle PVC.
  • There are many cardiac and noncardiac causes for sustained ventricular tachyarrhythmia resulting in SCD. Among all SCD in all age groups, the majority (65% to 70%) are related to CHD, with other structural cardiac diseases (approximately 10%), arrhythmias in the absence of structural heart disease (5% to 10%), and noncardiac causes (15% to 25%) responsible for the remaining deaths.

The risk factors for SCA are similar to those for CHD.

The approach to primary prevention of SCD varies according to a patient's clinical profile.

For the general population without known cardiac disease:

  • Apart from standard screening and management of risk factors for CHD (e.g., measurement of lipids, BP, and glucose, etc.), in patients without known cardiac disease, no additional screening tests are recommended or treatment for primary prevention of SCD.

A heart-healthy lifestyle, including habitual physical activity, a heart-healthy diet, and abstinence or cessation of cigarette smoking, is recommended for the primary prevention of SCD.

Patients with known cardiac disease (e.g., prior MI, cardiomyopathy, or HF) are at an increased risk of SCA. The approach to the primary prevention of SCA in such patients includes the following:

  • Standard medical therapies that lower the incidence of SCA.
  • Testing for SCA risk stratification in selected subgroups.
  • ICD implantation in selected patients.

The management of SCA includes acute treatment of the arrest, and for SCA survivors, a comprehensive evaluation and secondary prevention.

  • The acute management of SCA involves standard cardiopulmonary resuscitation protocols.
  • Initial evaluation of the survivor of SCD includes the following:
    • History and physical examination
    • Laboratory testing (electrolytes, blood gas, toxin screen, etc.)
    • ECG
  • Identification and treatment of acute reversible causes, including:
    • Acute cardiac ischemia and myocardial infarction
    • Antiarrhythmic drugs or other medication (e.g., QT-prolonging drugs), toxins, or illicit drug ingestion
    • Electrolyte abnormalities, most notably hypokalemia, hyperkalemia, and hypomagnesemia
    • HF
    • Autonomic nervous system factors, especially sympathetic activation (e.g., physical or psychological stress)
  • Evaluation for structural heart disease which may also include:
    • Coronary angiography
    • Echocardiography
    • Cardiac magnetic resonance imaging (CMR)
  • Evaluation for a primary electrical disease may also include:
    • Electrophysiology studies
    • Exercise testing
    • Ambulatory ECG monitoring
    • Pharmacologic challenges
  • Neurologic and psychologic assessment
  • In selected patients with a suspected or confirmed heritable syndrome, evaluation of family members

Secondary prevention of SCD, usually with an ICD, is appropriate for most SCA survivors.

  • Because of its high success rate in terminating VT and VF rapidly, along with the results of multiple clinical trials showing improvement in survival, ICD implantation is generally considered the first-line treatment option for the secondary prevention of SCD and primary prevention in certain populations at high risk of SCD due to VT/VF. However, there are some situations in which ICD therapy is not recommended, including, but not limited to, patients with VT/VF from a completely reversible disorder and patients without a reasonable expectation of survival with an acceptable functional status for at least one year.
    • The ICD system is comprised of pacing/sensing electrodes, defibrillation electrodes, and a pulse generator.
    • Most current ICD systems utilize one, two, or three transvenous leads placed via the axillary, subclavian, or cephalic vein, with attachment to a pulse generator in the subcutaneous tissue in the infraclavicular anterior chest wall.
    • DFT testing is generally performed during device implantation in patients receiving a subcutaneous ICD. It is reasonable in patients undergoing a right pectoral ICD implantation or ICD pulse generator changes (either right or left side). However, DFT testing is not required and can be omitted in patients undergoing a left pectoral transvenous ICD implantation with a right ventricular apical lead functioning appropriately.
    • Contemporary ICDs have extensive storage and monitoring capacities, the ability to deliver antitachycardia pacing (i.e., overdrive pacing) to terminate VT, the ability to deliver synchronized and unsynchronized shocks for VT/VF, and the option of bradycardia pacing.
    • There are a variety of complications associated with ICDs, including:
      • At and around the time of implantation:
        • Bleeding
        • Cardiac perforation
        • Infection
        • Perioperative mortality
        • Shoulder related problems (i.e., decreased shoulder mobility, pain, reduced function, insertion tendonitis)
      • Long-term complications include:
        • Lead-related problems (i.e., increased defibrillation thresholds, infection, lead failure resulting in failure to pace, failure to shock, or inappropriate shocks, tricuspid valve damage, and venous thrombosis.
        • Pulse generator complications may include electronic circuit damage, electromagnetic interference, skin erosion due to the size and weight of the generator, infection of the pulse generator pocket, and Twiddlers Syndrome.
        • Arrhythmia-related problems include appropriate shocks that can harm the quality of life, inappropriate shocks, usually due to the treatment of supraventricular tachycardias, and "phantom" shocks.
  • Antiarrhythmic drugs can be considered the primary therapy when an ICD is not indicated or refused by the patient.
    • Nearly all patients who have survived SCA should receive a beta-blocker as part of their therapy, which may also provide additional antiarrhythmic benefits.
    • Because an ICD does not prevent arrhythmias, patients with symptoms or device discharges may require adjunctive antiarrhythmic therapy or consideration of catheter ablation.
    • The three main indications for concomitant antiarrhythmic drug therapy are:
      • To reduce the frequency of ventricular arrhythmias in patients with frequent ICD shocks.
      • To suppress other arrhythmias that cause symptoms or interfere with ICD function (e.g., causing "inappropriate" shocks).
      • To reduce the ventricular rate of VT so that it is better tolerated hemodynamically and more amenable to termination by anti-tachycardia pacing or low-energy cardioversion.
    • For patients with an ICD who require adjunctive antiarrhythmic therapy due to ongoing arrhythmias, treatment with the combination of amiodarone plus a beta-blocker rather than treatment with amiodarone alone or other antiarrhythmic agents is recommended. This approach is especially preferred in patients with significant left ventricular dysfunction who require adjunctive antiarrhythmic therapy since amiodarone does not exacerbate HF and is less proarrhythmic than other agents.
    • Adverse effects of antiarrhythmic medications include increased DFTs and slowing of the tachycardia rate, which may preclude its recognition by the ICD.

Despite advances in the treatment of heart disease, the outcome of patients experiencing SCA remains poor. The reasons for the continued poor outcomes are likely multifactorial (e.g., delayed bystander CPR, delayed defibrillation, advanced age, decreased proportion presenting with VF.)

  • When SCA is due to a ventricular tachyarrhythmia, the outcome of resuscitation is better compared with those with asystole or pulseless electrical activity.
  • Among the many factors that appear to influence the outcome of SCA, the elapsed time before effective resuscitation (i.e., the establishment of an effective pulse) appears to be the most critical element. There are several ways to decrease the time to the onset of resuscitative efforts:
    • Rapid EMS response
      • Optimizing the EMS system within a community to reduce the response interval to eight minutes or less where possible.
    • Bystander CPR
      • The administration of bystander CPR is an important factor in determining patient outcome after out-of-hospital SCA, as early restoration or improvement in circulation has resulted in greater survival and better neurologic function among survivors.
      • Bystander CPR, however, is not always performed, primarily due to the bystander’s lack of CPR training or concerns about possible transmission of disease while performing rescue breathing.
    • Early defibrillation
      • The standard of care for resuscitation of SCA has been defibrillation as soon as possible when indicated.
      • Shorter defibrillation response intervals correlate with greater survival to hospital discharge.
    • Automated external defibrillators
      • The use of AEDs by early responders is another approach to more rapid resuscitation.
      • In most but not all studies, AEDs have improved survival after out-of-hospital cardiac arrest.
  • Several observational studies evaluating compression-only CPR versus standard CPR, including rescue breathing, reported no significant differences in survival or long-term neurologic function between the two groups, suggesting that compression-only CPR could be safely delivered (as long as the arrest is not a respiratory arrest).
    • It is recommended that if a sole bystander is present or multiple bystanders are reluctant to perform mouth-to-mouth ventilation, CPR performance using chest compressions should only be performed.
  • The induction of mild to moderate hypothermia (target temperature of 32 to 34ºC for 24 hours) may be beneficial in patients successfully resuscitated after a cardiac arrest. Improved neurologic outcome and reduced mortality have been demonstrated in a series of patients with VF arrest in whom spontaneous circulation was restored, even when the patient remains comatose after resuscitation.

Implicit Bias Statement

CEUFast, Inc. is committed to furthering diversity, equity, and inclusion (DEI). While reflecting on this course content, CEUFast, Inc. would like you to consider your individual perspective and question your own biases. Remember, implicit bias is a form of bias that impacts our practice as healthcare professionals. Implicit bias occurs when we have automatic prejudices, judgments, and/or a general attitude towards a person or a group of people based on associated stereotypes we have formed over time. These automatic thoughts occur without our conscious knowledge and without our intentional desire to discriminate. The concern with implicit bias is that this can impact our actions and decisions with our workplace leadership, colleagues, and even our patients. While it is our universal goal to treat everyone equally, our implicit biases can influence our interactions, assessments, communication, prioritization, and decision-making concerning patients, which can ultimately adversely impact health outcomes. It is important to keep this in mind in order to intentionally work to self-identify our own risk areas where our implicit biases might influence our behaviors. Together, we can cease perpetuating stereotypes and remind each other to remain mindful to help avoid reacting according to biases that are contrary to our conscious beliefs and values.


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