++
The spontaneous breathing trial (SBT) is the best way to determine readiness for ventilator liberation. Patients who tolerate an SBT for 30 to 120 minutes should be considered liberated from ventilatory support and candidates for extubation. The traditional way of performing an SBT is with a T-piece connected to the endotracheal tube, providing humidified oxygen. Many clinicians use the ventilator for SBTs, which has the advantage of maintaining a precise Fio2 and patient monitoring during the SBT. Moreover, if the SBT fails, ventilatory support can be quickly reestablished. If the SBT is to simulate a T-piece trial, the pressure support and PEEP should both be set at 0.
++
It is the practice of some clinicians to perform the SBT using low levels of pressure support (5-10 cm H2O), PEEP (5 cm H2O), or tube compensation. Most patients do equally well on T-piece trials, pressure support and PEEP set at 0, or low levels of pressure support and PEEP. The intent of using a low level of pressure support or tube compensation is to overcome the resistive load of the endotracheal tube. However, the resistive load of the endotracheal tube is usually not excessive unless the tube size is small. Patients with small endotracheal tubes or nasal intubation may benefit from the application of low levels of pressure support (5-10 cm H2O). Otherwise, breathing through the endotracheal tube with no support from the ventilator closely simulates the resistance through the upper airway after extubation.
++
Use of low levels of PEEP during the SBT is discouraged. In patients with chronic obstructive pulmonary disease (COPD), the application of 5 cm H2O PEEP during the SBT may counterbalance auto-PEEP. In patients with poor cardiac function, a low level of PEEP during the SBT may be sufficient to keep the patient out of failure. In patients with COPD or poor cardiac failure, the use of PEEP during the SBT might predict success, only to have the patient develop respiratory failure soon after extubation.
++
Patients successfully completing an SBT of 30 to 120 minutes are considered for extubation (Table 16-3). If the patient does not tolerate the SBT, the ventilator is set to provide a comfortable level of support. Once a comfortable level of support is provided, evidence is lacking for reducing the level of support before the next SBT. Before the next SBT, usually on the following day, an attempt should be made to identify and correct all potential causes of the failed SBT.
++
+++
Approaches to a Failed Spontaneous Breathing Trial
++
A common reason for a failed SBT is an imbalance between the capacity of the respiratory muscles (weakness) and the load that is placed on them (Table 16-4). Causes of respiratory muscles weakness include critical illness weakness, electrolyte imbalance, malnutrition, and primary neuromuscular disease. An excessive respiratory muscle load can be the result of high airways resistance, as in the patient with COPD, or a low compliance, as in a patient with pneumonia or pulmonary edema. Auto-PEEP and a high minute ventilation requirement also increase the load on the respiratory muscles.
++
++
The maximal inspiratory pressure (Pimax) is used to measure respiratory muscle strength. To ensure the best results, measurement of Pimax should be performed at residual volume. To achieve this, a one-way valve as illustrated in Figure 16-1 is used. This allows exhalation but not inspiration. Thus the lung volume at which Pimax is measured decreases with each breathing attempt. The Pimax measurement should be performed for about 20 seconds provided no arrhythmias or desaturation occurs.
++
++
There is increasing evidence supporting the role of early mobility of critically ill mechanically ventilated patients. This improves skeletal muscle conditioning, including the respiratory muscles. Early mobility has also been associated with a lower incidence of delirium, which might also result in earlier liberation from mechanical ventilation.
++
Iatrogenic causes also contribute to a failed SBT. Examples included a partially obstructed endotracheal tube, dead space in the ventilator circuit, or increased airways resistance while breathing aerosolized water from the T-piece.