April 2002 / Volume 47 / Number 4 / Page 469
The Role of Tracheostomy in Weaning from Mechanical Ventilation
IntroductionA common clinical observation is that patients wean more rapidly from mechanical ventilation following tracheotomy. Expected changes in tube resistance and dead space are not adequate to explain this observation in adult patients. Theoretical considerations are too complicated to allow evaluation of expected changes in work of breathing following tracheotomy. The resistance of the upper airway is about the same as breathing quietly through an 8.0 mm endotracheal tube; however, many patients experience a higher work of breathing following extubation. This is not true in infants, in whom the reduction in airway diameter is profound and a marked reduction in resistance is seen following extubation. The other benefits of tracheostomy include better secretion removal, improved oral hygiene, less laryngeal damage, and ability to eat and speak. These should be considered when proposing this procedure. There may be less late ventilator-associated pneumonia following early tracheotomy. The assumed better safety of tracheostomy has been questioned. That patients appear to wean more rapidly is probably accounted for by the variety of factors mentioned above.
Artificial Airway Added Resistance: Theoretical Considerations
Laminar Flow Analysis
Turbulent Flow Considerations
Measurement of Resistance and Work of Breathing
Artificial Airway Resistance: Empirical Data
Reduced Dead Space
Effects of Tracheostomy on Mechanics of Respiration
Safety Concerns in Patients with Artificial Airways
Advances in resuscitation, improvements in clinical care, and changes in the delivery of mechanical ventilation have resulted in a growing population of patients who require prolonged ventilatory support. Many of those patients will benefit from prolonged support programs and will eventually be liberated from mechanical ventilation. At the same time, increased demand coupled with a lack of availability of critical care resources has resulted in search for other venues for caring for ventilator-dependent patients. Early and aggressive weaning attempts have identified a group of intensive care unit (ICU) patients whose primary clinical need is for respiratory support or care, and management of patients with respiratory failure outside the ICU is becoming more common. Airway maintenance and safety issues are paramount in the non-ICU environment. The advent of percutaneous bedside tracheotomy has led to performance of this procedure earlier in the course of weaning. There are many stated advantages to tracheostomy, compared to prolonged translaryngeal intubation, some of which are listed in Table 1.
The timing of tracheotomy in patients requiring mechanical ventilation or airway protection has changed over recent years. In the1980s tracheotomy was considered “early” if it was performed before 21 days of translaryngeal intubation. In the otorhinolaryngology literature, however, tracheotomy to protect the larynx from damage was recommended within 3 days of intubation. This was based on the fact that the visually observed mucosal damage to the larynx and vocal cords was maximal in 3 to 7 days, but if the tube was removed from the larynx at this time, complete healing of these injuries was the rule. The visually assessed damage progresses and healing with scar formation and functional abnormalities (voice changes) occurs with increasing frequency if the endotracheal tube (ETT) is left in place longer than one week. The 3-week time limit of translaryngeal intubation in critically ill patients was based on the belief that the risk ratio (surgical tracheal risk vs laryngeal risk) was somehow exceeded if the ETT was left much longer than a month. There is little objective information to support that belief. Most adult patients requiring tracheotomy were so ill and required such high levels of ventilatory support that it was believed that transport to the operating room and performance of the procedure would frequently result in mortality or substantial morbidity, although there were little data on that issue.
The long-term airway consequences of tracheostomy in children appear to be more important than in adults, so prolonged translaryngeal intubation was usually continued in small children rather than subjecting them to the risks of tracheostomy.
With the increasing demand for ICU beds and the clinical changes mentioned above, tracheotomy is being performed earlier in the course of ventilatory failure. The beneficial effects on patient comfort, airway safety, ability to communicate and eat, and incidence of ventilator-associated pneumonia (VAP), are arguments for tracheotomy. The data supporting these benefits are generally anecdotal or consist of small case series.
The other side of the risk-benefit equation for tracheostomy involves accurate determination of the risk of the procedure. This is not well established for currently employed methods of performing tracheotomy. The surgical risk should include estimation of the mortality and morbidity of treating those patients who develop late complications such as tracheal stenosis at the tracheostomy stoma, the incidence and consequences of which have not been well studied. The newer percutaneous procedures may lower the incidence of stomal stenosis, but the effect on early mortality is debated and may actually be increased over open tracheotomy. Though tracheotomy is very frequently performed, very little systematic information has been accumulated regarding these risk issues.
One of the clinical observations often made is that weaning from mechanical ventilation in difficult patients is hastened by tracheostomy. This is believed to be due to some of the factors listed in Table 1 (eg, lower resistance to breathing, less dead space, better secretion removal, improved patient comfort, less sedation); however, no prospective, systematic study of this issue has been performed and such studies would be inhibited by: inability to blind the investigators to the groupings; bias of the clinicians caring for the patients; differing definitions of respiratory failure and success of weaning; economic concerns (eg, the Medicare payment for respiratory failure is increased if tracheotomy is performed); multiple individuals with different skills performing tracheotomy; and different specialties performing tracheotomy. In order to collect adequate numbers of patients, a multi-institution study or registry would be necessary, which adds problems of data consistency and cost.
|Table 1. Comparison of Prolonged Translaryngeal Intubation with Tracheostomy During Prolonged Ventilatory Support|
Ease and rapidity of initial insertion
Avoids acute surgical complications: bleeding, posterior tracheal wall injury
Avoids late surgical complications: wound infections, recurrent laryngeal nerve injury, stomal stenosis
Lower risk of ventilator associated-pneumonia
Ease of reinsertion if displaced:* allows less skilled nursing care
Reduced laryngeal damage
Better secretion removal with suctioning
Lower incidence of tube obstruction
Less oral injury (tongue, teeth, palate)
Improved patient comfort: less sedation/analgesia required
Better oral hygiene
Improved ability to speak
Preservation of glottic competence: less aspiration risk, lower incidence of ventilator-associated pneumonia
Better preserved swallowing allowing earlier oral feeding
Lower resistance to gas flow
Less tube dead space
Lower work of spontaneous breathing
More rapid weaning from mechanical ventilation
*After tract has matured
**Only considering placement, not cost of maintenance.
Another problem with assessing the effect of tracheostomy on weaning is the confusion concerning the difference between weaning and extubation. While the patient remains intubated with an ETT, the urgency in weaning is driven by a desire to extubate the patient. Once the decision is made to perform a tracheotomy, extubation is no longer the issue. It is easier to remove a patient with a tracheostomy tube from the mechanical ventilator and place him or her on a tracheostomy tube collar than it is to leave a patient with an ETT on a T-piece. Clinicians have been conditioned to believe that an intubated patient cannot tolerate the work of breathing (WOB) through an ETT, so some level of pressure support (5-8 cm H2O), a low intermittent mandatory ventilation rate, or automatic tube compensation must be used to prevent fatigue. The worry about imposed resistance and WOB is often forgotten when the tracheostomy tube is placed. Patients who were able but not allowed to sustain themselves without support are allowed to breathe without support through the tracheostomy tube. They appear to have been “weaned” simply by changing the method of intubation.
The present review considers the effects of tracheostomy on weaning from mechanical ventilation. Theoretical air flow effects of the tubes and reports of actual effects on weaning success are addressed, but this review does not include detailed discussion of surgical techniques, costs, mechanical complications of intubation and tracheostomy, or mechanical properties of tubes and cuffs. Reviews of those aspects of intubation have recently been published and the reader is referred to these for detailed information. The present review also does not address the issue of weaning and extubation criteria.