Alice Grassi

Rationale: Driving pressure (ΔP) is a marker of severity and a possible target for lung protection during controlled ventilation, but its value during assisted ventilation is unknown. Inspiratory holds provide an estimate of ΔP that is quasi-static. Expiratory holds provide an estimate of the inspiratory effort, which is useful to estimate the transpulmonary dynamic ΔP. Objectives: To assess the correlation between ΔPs measured during assisted ventilation and ICU outcomes. Methods: Multicenter prospective observational study. Patients with acute hypoxemic respiratory failure were enrolled within 48 hours of triggering the ventilator. Respiratory mechanics were measured daily, and the variables of interest were averaged over the first 3 days of partial assistance. ICU outcomes were collected until Day 90. Measurements and Main Results: A total of 298 patients from 16 centers were enrolled. Vt, peak airway pressure, positive end-expiratory pressure, and inspiratory effort during the first 3 days of assisted ventilation did not differ between survivors and nonsurvivors. Quasi-static ΔP and transpulmonary dynamic ΔP were higher in nonsurvivors than in survivors (13 [IQR, 11–14] vs. 11 [9–13] cm H2O [P < 0.001]; and 19 [16–23] vs. 16 [13–18] cm H2O [P < 0.001], respectively), whereas compliance normalized to predicted body weight was lower (0.65 [0.54–0.84] vs. 0.79 [0.64–0.97] mL/cm H2O/kg; P < 0.001). Multivariable analysis confirmed the association with outcomes. During the study days, static ΔP significantly diverged between survivors and nonsurvivors. Conclusions: During assisted ventilation, ΔP and normalized compliance are associated with ICU outcome, despite some overlap. Although our study does not allow an estimation of whether ΔP is a marker of severity or a cause of lung injury, it highlights the potential value of monitoring and targeting it during spontaneous assisted breathing.

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 of neuromuscular blockade in acute respiratory distress syndrome (ARDS) is the abolition of expiratory efforts. This may contribute to the 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 the administration of neuromuscular blocking agents (NMBAs), clinical factors associated with this phenomenon, its impact on regional lung ventilation, and any association with changes in pleural pressure. Methods: We included mechanically ventilated patients with ARDS monitored with electrical impedance tomography (EIT) who received NMBAs in one of two centers. We measured changes in EELI, a surrogate for end-expiratory lung volume, before and after NMBA administration. In an additional 10 patients, we investigated the 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 showed an increase in EELI of >10% of the corresponding Vt (46.2%; IQR, 23.9–60.9%). The degree of EELI increase correlated positively with fentanyl dosage and negatively with changes in end-expiratory pleural pressures. This suggests that expiratory muscle activity might exert strong counter-effects against positive end-expiratory pressure that are possibly aggravated by fentanyl. Conclusions: Administration of NMBAs during EIT monitoring revealed activity of expiratory muscles in half of patients with ARDS. The resultant increase in EELI had a dose–response relationship with fentanyl dosage. This suggests a potential side effect of fentanyl during protective ventilation.