To ventilate a patient with acute SCI, the peculiarities that exclusively affect these patients must be considered.
- In high cervical and thoracic injuries, ventilation will depend almost exclusively on the functioning of the diaphragm, which will be responsible for providing 90% of the VT.65
- The loss of expiratory musculature function causes an impairment in the ability to produce effective coughing, leading to the subsequent accumulation of secretions.65,71
- The increased production of secretions secondary to autonomic dysfunction, in addition to the above, facilitates the onset of atelectasis.24,65,71
Although patients with acute traumatic SCI can usually be said to have “healthy” lungs,71 up to 60% of the patients may have sustained associated chest trauma.
In this continuing education course we will focus on the management of patients with acute SCI and “healthy” lungs. In contrast to the abundance of the literature on mechanical ventilation in acute pulmonary lesions, the literature on the management of specific complications of acute SCI is very limited and of low quality.
The preservation of diaphragmatic function should be a primary objective in all patients undergoing mechanical ventilation. Diaphragmatic dysfunction is a common cause of weaning failure.73 The consequences of this dysfunction in patients who depend almost exclusively on the diaphragm to maintain effective inspiration are clear.
Although the negative effect of mechanical ventilation on the respiratory musculature has been known for many years, the specific diaphragmatic impairment was recently reported by Vassilakopoulos and given the name “ventilator-induced diaphragmatic dysfunction” (VIDD).73,74
Diaphragmatic atrophy occurs early after only 18 hours of inactivity and appears to be related to an increase in muscle proteolysis.75,76 Diaphragmatic atrophy increases with ventilation time and causes a progressive reduction in diaphragmatic function.
VIDD has been linked to diaphragmatic inactivity caused by controlled ventilation. It has been shown in animal models that assist modes attenuate VIDD.77 These findings have not been confirmed in humans, and no difference has been found in the onset of VIDD between patients ventilated with pressure control and those ventilated with pressure support.78
Patients ventilated with pressure control is often used. Pressure support ventilation (PSV) is often avoided due to the lack of evidence for a better prognosis and the risk of inadequate ventilation and exhaustion in patients with reduced respiratory reserve. The objective is to maintain some level of diaphragmatic contraction, ensuring total respiratory support. Achieving this objective requires adequate interaction between the patient and the respirator and the avoidance of asynchrony.
Asynchrony can occur at any time in the respiratory cycle. With a perfect patient-ventilator interaction, the respirator should trigger in synchrony with the electrical impulses originating in the central nervous system.79 Up to 25% of patients present some type of asynchrony while on mechanical ventilation.80 Most of the ventilators in use today trigger inspiration by a signal measured within the ventilator circuit. The signal may be a fall in the pressure of the airway (pressure trigger) or a variation in the flow signal (flow trigger). Although it was initially believed that the flow trigger produced a better patient-respirator interaction, with current respirators, no differences have been found.81-83
In recent years, a new modality of ventilation has been reported: neutrally adjusted ventilator assist (NAVA).84 In this modality, the signal used by the respirator to deliver assistance is not the flow or the airway pressure, but rather the diaphragmatic electromyogram signal collected from electrodes placed on an esophageal catheter. Despite its promising theoretical advantages, to date there is little evidence of the superiority of NAVA compared with other ventilatory modalities.85
Common practice in some clinical settings is to use the flow trigger. The lowest level possible is used that avoids the auto trigger. In case an ineffective trigger is detected, the presence of auto-PEEP (positive end-expiratory pressure) must be ruled out. Overcoming intrinsic PEEP in a patient with muscular weakness is imperative because intrinsic PEEP is one of the most frequent causes of an ineffective trigger. Lengthening the expiratory time, using bronchodilators, adding external PEEP, and reducing the sensitivity of the trigger are alternatives that can improve synchrony.
Traditionally, the use of tidal volumes between 15 and 20 mL/kg has been recommended, with the goal of avoiding or treating atelectasis. This recommendation is based on the theory that high volumes improve the production of surfactant, prevent the collapse of the airway, promote recruitment, and are better tolerated by the patient.7,24,65,71 The clinical evidence for this recommendation is based on retrospective studies and case series.
One of the most referenced studies to justify this method for ventilating patients with SCI was published in the journal Spinal Cord in 1999.86 Peterson retrospectively reviewed 42 patients with SCI and found that those who were ventilated with >20 mL/kg were weaned 21 days earlier than those ventilated with <20 mL/kg (37.6 days versus 58.7 days). Several limitations in this study include:
- The patients studied were not acute.
- Although the authors indicated that the two patient groups had similar characteristics, they do not explain the reason why they were ventilated with different volumes when their typical practice was to use high volumes.
- The endotracheal cuff was kept to allow for partial leak and to facilitate vocalization. As such, it is difficult to assess which tidal volume was effective.
Only one clinical trial has compared these two approaches.87 The study, reported exclusively as a poster, included only 16 patients ventilated 2 weeks after the injury. There were no differences between ventilating with 10 mL/kg and 20 mL/kg.
To facilitate the removal of secretions, the postural drainage and manually assisted coughing techniques are considered essential. The use of mechanical devices such as intrapulmonary percussive ventilation71 and mechanical insufflation-exsufflation (MIE)24 have not been prospectively assessed in acute patients undergoing mechanical ventilation. Retrospective studies suggest their efficacy for reducing the number of hospitalizations in chronic patients88 and for reducing the weaning time.24 In a survey on the use of MIE, only 49% of the centers that responded acknowledged using the technique routinely.89
Regarding PEEP, the standard recommendation is to use 0 cm H2O (ZEEP). The theory behind this recommendation is that it can increase air trapping in patients with expiratory muscle impairment. Considering that expiration is a passive phenomenon, it is difficult to justify this reasoning. The use of PEEP increases the residual functional capacity and may prevent the collapse and cyclic closure of alveoli, one of the causes directly related to the onset of ventilator-associated lung injury (VALI).90 Lacking evidence to support it, recommending the use of ZEEP no longer seems reasonable for the ventilation of patients with spinal injuries, at least in the acute phase.
Standard practice in some settings with acute SCI patients undergoing mechanical ventilation and who have “healthy” lungs is to use an assisted, pressure-controlled, ventilation mode, adjusted to achieve tidal volumes of 10 to 12 mL/kg with 5 to 7 cm H2O of PEEP, with the pressure plateau always below 30 cm H2O. The goal is to maintain total ventilator support, allowing the patient to initiate most of the cycles and attempting to adjust the inspiratory time to adapt it to the neural inspiratory time. This same ventilation modality is continued once weaning has begun in the resting periods between trials of spontaneous respiration. In all cases of acute pulmonary lesion, the lung protective ventilation strategy is followed.
In the event of recurrent atelectasis that does not resolve despite assisted coughing, bronchodilators, postural drainage, and hydration, the VT can be increased by 100 mL/day up to 15 to 20 mL/kg, as long as the plateau pressure is maintained below 30 cm H2O.
Patients with cervical SCI have compromised respiratory functions and require mechanical ventilation based on the location and degree of the injury, both of which also affect their success in weaning from the respirator, which approaches 40% in patients with cervical injuries above C4, with increasing success in injuries below C5.91 The respiratory modality most commonly used in weaning is Progressive ventilator-free breathing (PVFB or T-tube),92 with noninvasive mechanical ventilation and tracheotomy also playing a role. Other adjuvant treatments include the use of phrenic/diaphragmatic pacemakers in patients who do not have spontaneous breathing and drug treatments whose actual benefit is still untested.
The start of weaning and the strategy to employ are determined by the following three factors:
- Degree of respiratory function at the time weaning is started
- Level of the injury
- Respiratory pathophysiology of the SCI
Therefore, the patient’s respiratory function needs to be assessed before and during weaning.93 The best parameters to use for assessing the patients respiratory function are66:
- Arterial gasometry or capnography
- Effectiveness of cough and diaphragmatic electromyography (which is not very useful in clinical practice)
- Lung function tests especially VC
Before the start of weaning, it is advisable to optimize the patients breathing by94:
- Administering bronchodilators
- Aspirating tracheal secretions
- Positioning the patient in the supine or Trendelenburg position
Once the patient’s breathing has been optimized, the various weaning modalities should be assessed. The three general approaches to weaning are95:
- Progressive ventilator-free breathing (PVFB) or t-tube
Pressure support (PS)Synchronized intermittent mandatory ventilation (SIMV)
- PVFB weaning consists of a respirator-free time that is gradually increased and achieves an increase in muscle force in patients with high and low cervical injuries.
- The procedure is started with a FiO2 of 10% above the respirator baseline and with only 5 minutes of disconnection per hour, which is gradually increased throughout the day depending on the patient’s degree of tolerance, thereby avoiding exhaustion.
- The intervals of connection to the respirator therefore must be sufficient for the diaphragm to recover before the next test (approximately 2 hours).
- The withdrawal of mechanical ventilation can be proposed when the patient tolerates 48 hours without respiratory support.
- Several studies have shown greater success with the T-tube.92,96
- In a recent study performed on patients with prolonged mechanical ventilation, Jubran et al.97 concluded that weaning by PVFB through tracheostomy is faster compared with PS, without affecting 6–12 month survival. When assessing this study in terms of patients with cervical SCI, it is important to consider that one of the exclusion criteria is a bilateral phrenic nerve injury.
- Weaning patients from mechanical ventilation using SIMV takes longer and does not improve the success rate. Currently SIMV is not advocated.96,98
Regardless of the weaning modality, all studies have observed that the time for the withdrawal of the respirator in patients with SCI ranges from weeks to months.99,100
Extubation is considered when, after 48 hours without ventilator support, the patient meets several requirements101:
- A negative inspiratory pressure <−20 cm H2O
- The ability to generate a flow peak of cough > 2.71/s
Checks before extubation include:
- There are no required surgical or X-ray diagnostic procedures close to extubation that require sedation
- Patient is cooperative without sedative drugs
- Patient is afebrile and with stable vital signs
- Saturation > 95% and pCO2 < 40–45 mmHg after >12 h breathing ambient air
- FiO2 no more than 25% and PEEP < 5 cm H2O
- X-rays with no abnormalities or an obvious improvement
- Few bronchial secretions
- Negative inspiratory pressure < −20 cm H2O
- Vital capacity > 10–15 mL/kg of ideal weight
- A normal fluid balance
- No contraindications for performing physical therapy (fractured ribs, etc.) or for the use of noninvasive mechanical ventilation (facial fractures, etc.)
The Role of Noninvasive Mechanical Ventilation (NIMV)
The use of NIMV has been proposed for patients with SCI as respiratory support both in acute conditions and in respirator weaning and as a long-term night support for patients in whom hypoventilation is detected.
Indications for the use of NIMV include:
- The patient must be cooperative
- The patient must have an intact bulbar musculature
- The patient must be medically stable
Bach and Saporito101,102 and Tromans et al.103 describe the use of NIMV in acute conditions:
- To avoid connecting patients to respirators who have a VC < 50% of its normal value
- If the VC falls below 1200 mL, continuous support will be necessary.
- Limitations to these two studies include:
- Both studies were retrospective and had a limited number of cases. Further studies are therefore needed to support the use of NIMV as a substitute for invasive mechanical ventilation.
- NIMV may aid in respirator weaning and prevent postextubation failure, thereby reducing reintubation-related complications.104,105
- If rehabilitation is included with NIMV support, the rate of success increases considerably.
- Chronic SCI with reduced VC may benefit from support from nighttime NIMV in patients in whom nighttime saturation is repeatedly <95% and CO2 > 50 cm H2O, which unequivocally indicates hypoventilation.
- In these patients with chronic hypoventilation, nighttime oximetry monitoring or capnography, indicates the need for nighttime NIMV and the patients who will benefit from respiratory electrostimulation.
NIMV can be used with two ventilator modalities:
- Continuous positive airway pressure (CPAP), where a continuous inspiratory pressure is provided.
- Bilateral Positive Airway Pressure (BPAP), which provides support through two pressures (inspiratory and expiratory).
- BPAP is more advantageous than CPAP because it keeps the alveoli open by providing a minimal PEEP.103
- If, after administering BPAP, the VC presents a reduction >25%, weaning is discontinued.-*
Both modalities may be used with or without supplemental oxygen and with an interface that can be nasal, oral or naso-oral.
Limitations to the use of NIMV include:
- Abdominal distension
- Barotrauma (very uncommon)
- Although the patient may have functionally preserved bulbar musculature innervation, NIMV may not provide sufficient force to hold the nasal or oral piece in place.