Spontaneous breathing trials: should we use automatic tube compensation?

Weaning from mechanical ventilation is a 2-step process. First, objective criteria are used to determine whether sufficient recovery from acute respiratory failure has occurred to allow the patient to breathe independently. This “readiness test” is followed by a spontaneous breathing trial (SBT) to assess whether the patient still requires mechanical ventilatory support. When the patient tolerates the SBT, the clinician addresses the separate question of whether the endotracheal tube (ETT) is still required; that is, can the patient be safely extubated? There has been considerable interest in determining the best approach for conducting the SBT and how long it should be. The ideal SBT would accurately mimic the work of breathing (WOB) done without ventilatory support and without the ETT in place. Numerous investigations have compared T-piece, CPAP, low-level pressure support ventilation (PSV), and more recently, automatic tube compensation (ATC). Interest in providing some level of support stems from the belief that, in some patients, the resistive WOB imposed by the ETT may prevent successful unsupported breathing, causing “iatrogenic” (ie, ETTinduced) weaning failure. A PSV level of 7 cm H2O was considered sufficient to compensate for the resistance imposed by the ETT, when compared to T-piece, in a large randomized controlled trial.1 However, the group weaned with T-piece had a higher rate of unsuccessful SBT (22%) than the PSV group (14%) (P .03), whereas the percentage of patients successfully extubated after 48 h was similar in both groups (T-piece 63%, PSV 70%, P .14). In a non-randomized study of patients who failed a 30-min T-piece SBT, immediate conversion to PSV of 7 cm H2O for an additional 30 min led to weaning success in 21 of 31 patients, which suggests that the ETT can contribute to iatrogenic weaning failure.2 Unfortunately, it is challenging to predict the exact PSV level required to overcome this imposed WOB in a given patient. Too little support may result in SBT failure because the imposed WOB from the ETT is insufficiently negated. This may lead to repeated SBT failure and unnecessarily prolonged mechanical ventilation. On the other hand, too much support may allow the patient to succeed the SBT without accurately gauging whether the patient is truly capable of unsupported breathing. This may lead to premature extubation and re-intubation. Therefore, methods to accurately compensate for the added ETT resistance have the potential to improve the weaning process.3 ATC, a relatively new commercially available built-in ventilation mode, is designed to compensate for the nonlinear pressure drop across the ETT during spontaneous breathing. ATC provides dynamic support for each spontaneous breath, to deliver the exact pressure needed to overcome the ETT resistance. Theoretically, conducting the SBT with ATC simulates spontaneous breathing without the added resistance of the ETT. Studies comparing ATC to PSV suggested that ATC is more effective in overcoming the WOB,4 and is more comfortable,5 with less patient-ventilator asynchrony.6 Haberthür et al4 found that ATC decreased the total WOB, in comparison to PSV and continuous positive airway pressure (CPAP), in 10 ventilator-dependent patients ventilated through tracheostomy. Similarly, Fabry et al7 found that the advantage of ATC in relieving additional WOB depended on the patient’s pulmonary condition. In intubated spontaneously breathing patients, Fabry et al found that in postoperative patients ATC and PSV of 10 cm H2O or 15 cm H2O were sufficient to compensate for the added WOB. In contrast, only ATC was able to compensate for the added WOB in patients with lung injury. Despite these encouraging findings, a task force on weaning from mechanical ventilation3 concluded that there is a lack of controlled trials to make any meaningful recommendations about ATC.