Expiratory Muscle Activity Counteracts Positive End-Expiratory Pressure and Is Associated with Fentanyl Dose in Patients with Acute Respiratory Distress Syndrome

Rationale: Hypoxemia during mechanical ventilation might be worsened by expiratory muscle activity, which reduces end-expiratory lung volume through lung collapse. A proposed mechanism of benefit from neuromuscular blockade (NMBA) in acute respiratory distress syndrome (ARDS) is abolition of expiratory efforts. This may contribute to restoration of lung volumes. The prevalence of this phenomenon, however, is unknown. Objectives: To investigate the incidence and amount of end-expiratory lung impedance (EELI) increase after NMBA, clinical factors associated with this phenomenon, its impact on regional lung ventilation, and association with changes in pleural pressure. Methods: We included mechanically ventilated ARDS patients monitored with electrical impedance tomography (EIT) who received NMBA in two centers. We measured changes in EELI, a surrogate for end-expiratory lung volume, before and after NMBA. In additional 10 patients, we investigated characteristic signatures of expiratory muscle activity depicted by EIT and esophageal catheters simultaneously. Clinical factors associated with EELI changes were assessed. Measurements and Main Results: We included 46 patients, half of whom increased EELI by more than 10% of the corresponding tidal volume (46.2% [23.9–60.9%]). Degree of EELI increase correlated positively with fentanyl dosage and negatively with changes in end-expiratory pleural pressures. This suggests expiratory muscle activity might exert strong counter-effects against PEEP, which are possibly aggravated by fentanyl. Conclusions: Administration of NMBA during EIT monitoring disclosed activity of expiratory muscles in half of ARDS patients. The resultant increase in EELI had a dose-response relationship with fentanyl dosage. This suggests a potential side-effect of fentanyl during protective ventilation.