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AED, Automatic External Defibrillator

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Author:    David Tilton (RN, BSN)

The Shocking Truth about Cardiac Arrest

Every day, nearly 2,600 Americans die from cardiovascular disease. An average of 1 death every 34 seconds, making heart disease the number 1 killer in the United States (Chronic Disease Notes and Reports, Fall 2004).

Most of these deaths occur outside of a hospital or health care setting. They often occur with no warning. Frequently these deaths are associated with a syndrome referred to as Sudden Cardiac Arrest which is typically the abrupt onset of a deadly heart rhythm known as ventricular fibrillation.

Ventricular fibrillation is a rapid, disorganized heart rhythm that fails to effectively pump blood from the heart to the brain, lungs, and other vital organs. More deaths occur from ventricular fibrillation than from lung cancer, breast cancer, or AIDS.

Ventricular fibrillation is such a profound condition that immediate treatment is necessary in order to avoid sudden cardiac death. Sudden Cardiac Death is simply abrupt death resulting from cardiac arrest. About 85 percent of those experiencing sudden cardiac arrest are unable to obtain the immediate help they need and die. The survivors of sudden cardiac arrest often relate their survival to the quick response of basic life support trained individuals and the near proximity to equipment able to deliver cardiac defibrillation (Heart Center Online, March 18, 2005).

Quick facts (Bocka, 2004), (NCHS, 2005):

  • #1 Cause of Death in the US
  • Number of deaths each year: 696,947
  • Deaths per 100,000 population: 241.7
  • Survival rates for individuals with ventricular fibrillation (VF) treated by AEDs have been reported as high as 31%.
  • Comparatively, the survival rates for performing basic cardiopulmonary resuscitation (CPR) alone are 6% or below.
  • We believe that more lives could be saved if members of the public could obtain early access to and have training in the use of AEDs and CPR.

Death from Heart Disease

Data source: Chronic Disease Notes and Reports, 2004

Shocking News

Unlike the classic Mary Shelley story Frankenstein the introduction of electricity, not even the flashy lightening bolt kind, will not bring life into what is inanimate. In fact the ability to purposely use an electrical current (in a controlled manner) to alter the beating of the complex muscle mass that is the heart is a relatively new skill.

In 1774 a 3-year-old child named Catherine Sophie Greenhill fell from an upper story window onto flagstones and was pronounced dead. A member of the newly formed London based Royal Humane Society arrived on the scene within twenty minutes, and history records, proceeded to give the clinically dead child several shocks through the chest with a portable electrostatic generator. This novel form of intervention led to a palpable pulse and visible respiration. Eventually, after some time in coma, the little girl is reported to have made a full recovery. This may have been the first successful cardiac defibrillation on a human being (Harris, 1990).

Historical Progression (Scheidermayer, 1989):

  1847-Internal heart massage in cats
  1901-Successful internal cardiac massage
  1947-Internal cardiac defibrillation
  1956-External cardiac defibrillation
  1958-Mouth-to-mouth resuscitation

Manual external cardiac defibrillation is a complex skill requiring extensive training and practice in the interpretation of electrocardiogram (ECG) heart rhythm strips. The health professional looking at the strip must be able to recognize which rhythm abnormalities could benefit from, or actually require defibrillation, as well as which rhythms do not. On top of the training and ability to interpret rhythm strips, there must also be an ability to operate the model of manual defibrillator that is present.

Shocking Indications

The electric shock of cardiac defibrillation is used treat sudden cardiac arrest caused by ventricular fibrillation. The application of this shock through the chest wall from special adhesive electrode pads is used to pause the erratic (chaotic) quivering of the muscular ventricles of the heart known as fibrillation or, to be more technical, the shock works to uniformly and simultaneously depolarize a critical mass of the excitable myocardium. However you say it, this shock allows a brief opening for the hearts own intrinsic pacing system to ‘recapture’ muscular contraction into an effective rhythm so that blood can be pumped to the vital organs.

Early AEDs responded primarily to a heart rate greater than 150 electrical complexes per minute and electrocardiographic wave (QRS) amplitude greater than 0.15 mm (Bocka, 2004). Presently, the ECG rhythm is analyzed via a combination of methods. In addition to rate and amplitude, the QRS is analyzed as to its slope, morphology, power spectrum density, and time away from the isoelectric baseline. All measurements are compared against preset levels to determine what is ‘abnormal’. This is all done within 2 to 4 second interval ‘checks’. In general, when abnormal complexes are detected for 3 consecutive checks, the AED will be primed to deliver a shock. Fine VF presents the greatest detection challenge. A trade-off exists between setting the amplitude criterion low enough to detect fine fibrillation, yet high enough to avoid shocking an artifact (static/interference) or asystole (lack of muscular/electrical activity in the heart)The sensitivity of detecting VF by AEDs has been reported at 76-96%. Specificity (correctly identifying non-VF rhythms) is reported to be nearly 100% (Bocka, 2004).

The success achieved with defibrillation varies widely according to the location in which the sudden cardiac arrest occurs. Those experiencing ventricular fibrillation that are in a monitored health setting such as a coronary care unit, for example, have a high survival rate attributed to the ability to rapidly perform the act of cardiac defibrillation. Victims of sudden cardiac death due to ventricular fibrillation who are at home or outside of a hospital setting have a very poor chance of survival unless defibrillation can be performed within the first few minutes following the onset of this lethal heart rhythm.

Roughly speaking, the chance of successfully recapturing an effective pumping action of the heart (resuscitation) following the onset of VF drops by about 10 percent for every minute of time, making successful resuscitation a race against the clock. This limited window of opportunity during which cardiac defibrillation can find success may be somewhat extended by providing a temporary, artificial means of breathing and circulation of the blood in the form of cardiopulmonary resuscitation (CPR).

CPR is not a cure for cardiac arrest! CPR on its own is able to deliver only limited amounts of oxygen to vital organs. Defibrillation is considered to be the only effective means to resuscitate a victim of ventricular fibrillation. Without defibrillation the brain and organs are greatly vulnerable to accrued damage (eMedicine, 2005).

The American Heart Association Chain of Survival is a series of actions highlighted by the American Heart Association as a means for giving the person suffering from Sudden Cardiac Arrest the greatest chance of survival. Each link of this chain increases the probability of overall success and supports every other action, or link. The AED might be the link that ensures survival.

Early Access CPR Early Defibrillation ALS

Early Access: As soon as an emergency is recognized, the first link in the Chain of Survival comes into play. Early activation of local emergency medical services (EMS) by calling 911, or the correct emergency telephone number for that location. Summoning aid early on increases the chance of gaining a good result by getting the right people (those familiar with emergency interventions), with the right equipment (automatic external defibrillator and emergency medical supplies), to the right place early enough to increase the odds of an effective resuscitation!

CPR: Cardiopulmonary resuscitation is the second link in the chain. Effective circulation and rescue breathing provide a better chance of successful resuscitation over a longer period of time.

Early Defibrillation: Perhaps the most critical link in the Chain of Survival for the victim of ventricular fibrillation is early cardiac defibrillation. The growing presence of automatic external defibrillators in workplaces as well as public areas increases the chance of successful resuscitation following sudden cardiac arrest.

ALS: The arrival of trained personnel able to perform advanced life support (ALS) quickly after recognizing the presence of an emergency. This last link of the chain results from the actions taken in the first link, thus creating a living continuum, a true Chain of Survival.

For important reasons external defibrillators were originally available for use only inside of hospitals where specially trained staff were on hand for their safe operation. The 1970’s saw the beginning of an important advance in emergency response health care as the development of more sophisticated portable power sources allowed manual defibrillators to be taken into the field in the hands of carefully trained and supervised paramedics. This proved to be a great stride forward, increasing the chance of survival for those suffering from cardiac arrest outside of a hospital or health care facility.

Paradigm change:
Take the defibrillator to the victim,
Rather then depend on a timely transport
Of the victim to a defibrillator.

Many victims of ventricular fibrillation perish in the first few critical moments of a cardiac arrest. Being able to take the primary treatment for sudden cardiac arrest, defibrillation, to the streets and homes of victims was a tantalizing taste of what might be possible if only more trained users and units could be fielded. However, something more was needed to allow an even quicker access to defibrillation.

Automated Shock

Computers are everywhere in our lives. We often take them for granted. In the mid-1980’s, researchers decided to take them to heart. Advances in microprocessor technology led to a new generation of computer guided external defibrillators called Automated External Defibrillators (AEDs). These computer guided devices allowed the machine to internally compare the electrical pattern being produced by a person’s heart to pre-set guidelines and could, with only minimal assistance from an operator, automatically deliver an electrical shock for the purpose of defibrillation.

The first models of AEDs required an oral/epigastric electrode to be inserted along with placement of a second electrode against the external wall of the chest. Newer technology now allows both electrodes to be placed externally in the form of disposable adhesive pads. One pad is placed along the right sternal border with a second proximal to the cardiac apex. These external pads serve as both a monitor and as an electrode with which to introduce the shock of defibrillation. Newer AEDs even informed the user when poor adhesive pad contact is a problem.

AEDs for the first time allowed emergency service personnel that lacked the extensive training required of paramedics to administer the important life saving shock of defibrillation. This allowed greater numbers of portable defibrillators to be present in the community where they were needed. Suddenly ambulances not staffed by hard to find paramedics could respond with early defibrillator intervention and the survival rates for cardiac arrests outside of hospitals began again to rise.

Yet even with the increase in availability of the new machines, intervention within that critical ten minute window of opportunity remained difficult to achieve. The recognition that other types of emergency workers often beat emergency medical system (EMS) personnel to the scene offered still another means of cutting down the time from sudden cardiac arrest to defibrillator arrival. A new view of cardiac arrest arrived with the addition of the long arm of the law into the chain of survival as many communities and EMS systems began to train and equip police and fire response units to provide early defibrillation at the scene of emergencies using the new AED devices.

The resulting decrease in response time with arrival of a defibrillator, frequently before ambulance or rescue personnel could get to the scene, is contributing significantly to the number of victims successfully resuscitated following sudden cardiac arrest. Yet still the question remains, can we do even better?

Public Shock

As a society we are well into the most recent phase in our race to beat the 10 minute deadline created by sudden cardiac arrest. For this, we are willing to PAD our chances. Public Access Defibrillator (PAD) is the newest step forward in the evolution of automatic external defibrillation.

The extensive use of AEDs by EMS, police, and hospital personnel has established their worth in the effort to provide the victims of ventricular fibrillation with the right intervention in a timely manner. Our use of these devices from their introduction in the 1980’s has shown that AEDs are consistently easy to use. First responders, those persons first on the scene of an out-of-hospital cardiac arrest, can be taught how to use automatic defibrillators in courses such as the American Heart Associations Heartsaver AED course in as little as 4 hours of time. Also, though the desire is to have as many individuals trained in their use as possible in compliance with local or state regulations for PAD programs, the operation of an AED is simple enough that they may be successfully pressed into service in times of emergency even without the benefit of formal training.

AED use is regulated on a State by State basis:

  • Some states restrict the use of AEDs to those who have successfully completed a formal training program
  • Some states require AED users to possess authorization by a licensed physician before use
  • Some states restrict AED use to formal members of trained response teams
  • Some states possess ‘true’ public access defibrillation allowing anyone with the knowledge use of an AED when needed, as it is available
  • Most states agree that any individual using an AED in a good faith attempt to save the life of a cardiac arrest victim will fall under the protection of ‘Good Samaritan’ legislation.

Statutes vary, so take the time to look up your local/state/provincial version.

Simply Shocking

Operation of any Automated External Defibrillator follows a consistent sequence of actions (Brocha, 2004).

  1. The true strength of AEDs is this, no matter what brand of machine you use the only knowledge you must possess, is how to press the on button!
  2. AEDs are constructed so that once the unit is turned on a brief self-check will occur. After that, the unit will speak to you in a computer-generated voice that will guide you through all needed actions.
  3. The first prompt will be to place a set of labeled adhesive pads onto the victim’s bare chest, and to plug the pads (if necessary) into the correct connector on the AED unit. (The adhesive pads consistently show a drawing of the correct placement for that pad to guide the hesitant.)
  4. Once the pads are in place the AED will begin an automatic analysis of that person’s heart rhythm, comparing it to carefully designed patterns. Please follow the machines warning not to touch the victim during the analysis phase, as motion or interference caused by another persons touch may confuse the readings.
  5. Should the AED determine that a shock would be beneficial it will automatically begin to draw power and build a charge for that function. When ready, it will tell you to press the button that will trigger the shock to be delivered (semi-automatic AED). Some (less common) AEDs operate the entire cycle independently once they are applied and turned on. Analyzing, charging, and defibrillating without need of human contact on the keys (automatic AED). With these fully automatic units the only way to prevent a defibrillation once the need is recognized is to turn the unit off or otherwise interrupt the cycle.
  6. After the shock is delivered, or if the AED determines that the heart activity (or lack of activity) would not benefit from a shock, the unit will ask you to check to see if the person has a return of circulation (pulse). If no pulse is present, you will be prompted to initiate CPR. (American Heart Association 2006 Revised Guidelines advocates only a single shock followed by resumption of CPR if a pulse fails to return. CPR will then be continued for two minutes to allow circulation before another attempt to shock is made.)
  7. Leave that machine on! After a preset interval, two minutes (AHA 2006 Revised Guidelines); the AED will prompt you to stop CPR so that it can again check to see if a retrievable (shockable) rhythm is present. This cycle of actions continue until EMS personnel arrive on the scene to initiate advanced life support and transport of the victim.

Shocking Contraindications

Do not shock a person with a pulse! There are good reasons why someone may not be responsive that have nothing to do with heart troubles. The American Heart Association offers some excellent courses for laypersons, first responders, those interested in basic life support and even advanced life support.

Also, whenever the victim of cardiac arrest is in a fluid environment, that is; standing water, pooled vomit, or blood resist temptation to use electric shock! There are many good reasons for this. Perhaps the best one to mention is that electricity is lazy. The presence of copious liquids on, under or soaking the person that the AED is attached to will deflect, or pull-away, the electric defibrillation energy into the fluid, effectively bypassing the heart in its desire to go to ground. First remove the individual from the fluid before seeking to use an AED.

Pediatric Shock

In adults the onset of a primary arrhythmia, often ventricular fibrillation, is common during cardiac arrest. In children, this is not the case. For this reason public access defibrillators are set to deliver the correct amount of electric current that would be needed to treat an adult heart.

There are rare instances in which children can experience ventricular fibrillation:

  • Congenital heart defects, whether previously known or unknown, increase the risk for abnormal heart rhythms such as ventricular fibrillation.
  • Commo Cordis, a syndrome that may be produced by a forceful blow to the chest, can produce a chaotic rhythm disturbance (ventricular fibrillation) if the impact occurs during a specific, very brief, period during each heart beat.

Due to the comparative size of an adult heart to the heart of a child the amount of current delivered by an AED has the capacity to actually damage the heart muscle of the child. The damage caused could prevent the possibility of resuscitation. For this reason the guidelines set forth by the American Heart Association for the use of unmodified AEDs currently limits them to children who are physically the size of a typical eight year old, or larger.

Eyes open people! This is in the process of changing (FDA, 2004):

  • In specialized settings, some clearly marked AEDs are preset for use with children smaller than the ‘rule of eight years’.
  • Several manufacturers have either began, or have announced an intent to begin, distribution of specialty electrode cables or pads able to reduce the current delivered by their standard AEDs so that the unit can be used on a child younger then eight as well as on an adult.
  • AEDs are in development that will have internal processing ability capable of determining just how much of a jolt will be required by the person it is analyzing, and then adjust delivery of current in the defibrillation in accordance.

Shocking Conclusion

So, where does that leave us? During the early days of AEDs use was strictly regulated and vigilance by highly trained medical personnel was the watchword. Gradually area emergency medical service agencies stretched those bonds until today many states have true public access defibrillation programs in place.
With public awareness and the presence of defibrillators in community settings the number of deaths which would have otherwise occurred from sudden cardiac arrest is being noticeably reduced.

The future goal? A chicken in every pot, and an AED wherever you would expect a public emergency fire extinguisher! Don’t laugh. Airports, commercial airliners, shopping malls, golf courses, many supermarkets, police cars and even commercial carrier buses are homes for AEDs. Businesses, institutions and charities may be eligible to receive insurance incentives due to the presence of these devices. Every day more public access defibrillators are placed. Every year the technology improves while costs decrease!

Just remember, should you chance upon the scene of a sudden cardiac arrest, with an AED you simply have to press the on button. The machine will tell you what to do next.


Bocka J. (September 12, 2004.) “Automatic External Defibrillation”. eMedicine. Retrieved from on February 6, 2006.

Centers for Disease Control and Prevention. (2004.) “Heart Healthy and Stroke Free States.” Retrieved from on February 3, 2006.

Chronic Disease Notes and Reports. (Fall 2004.) “Heart Disease and Stroke.” Centers for Disease Control and Prevention. Retrieved from cdnr/pdf/CDNRfall04 on February 2, 2006.

eMedicine. (July 18, 2005.) “Automated External Defibrillators.” Retrieved from on February 3, 2006.

FDA Hearthealth Online. (February 27th, 2004.) “Automatic External Defibrillators”. Retrieved from on February 3, 2006.

Harris, S. (September 1990.) “The Society for the Recovery of Persons Apparently Dead.” Cryonics. Retrieved from on February 3, 2006.

Heart Center Online. (March 18, 2005.) “Cardiac Arrest.” Retrieved from on February 3, 2006.

National Center for Health Statistics. (November 14, 2005.) “Heart Disease.” Fastats.
Retrieved from on February 3, 2006.

Scheidermayer D. (March 1989.) “Comforting Job in the ICU: Ethical Issues in High Technology Medicine.” Perspectives on Science and Christian Faith. Retrieved from on February 3, 2006.