Driving pressure of respiratory system and lung stress in mechanically ventilated patients with active breathing

Abstract Background During control mechanical ventilation (CMV), the driving pressure of the respiratory system (Δ P rs ) serves as a surrogate of transpulmonary driving pressure (Δ P lung ). Expiratory muscle activity that decreases end-expiratory lung volume may impair the validity of Δ P rs to reflect Δ P lung . This prospective observational study in patients with acute respiratory distress syndrome (ARDS) ventilated with proportional assist ventilation (PAV+), aimed to investigate: (1) the prevalence of elevated Δ P lung , (2) the Δ P rs -Δ P lung relationship, and (3) whether dynamic transpulmonary pressure (Plung sw ) and effort indices (transdiaphragmatic and respiratory muscle pressure swings) remain within safe limits. Methods Thirty-one patients instrumented with esophageal and gastric catheters ( n = 22) were switched from CMV to PAV+ and respiratory variables were recorded, over a maximum of 24 h. To decrease the contribution of random breaths with irregular characteristics, a 7-breath moving average technique was applied. In each patient, measurements were also analyzed per deciles of increasing lung elastance ( E lung ). Patients were divided into Group A, if end-inspiratory transpulmonary pressure ( P LEI ) increased as E lung increased, and Group B, which showed a decrease or no change in P LEI with E lung increase. Results In 44,836 occluded breaths, Δ P lung ≥ 12 cmH 2 O was infrequently observed [0.0% (0.0–16.9%) of measurements]. End-expiratory lung volume decrease, due to active expiration, was associated with underestimation of Δ P lung by Δ P rs , as suggested by a negative linear relationship between transpulmonary pressure at end-expiration ( P LEE ) and Δ P lung /Δ P rs . Group A included 17 and Group B 14 patients. As E lung increased, Δ P lung increased mainly due to P LEI increase in Group A, and P LEE decrease in Group B. Although Δ P rs had an area receiver operating characteristic curve (AUC) of 0.87 (95% confidence intervals 0.82–0.92, P < 0.001) for Δ P lung ≥ 12 cmH 2 O, this was due exclusively to Group A [0.91 (0.86–0.95), P < 0.001]. In Group B, Δ P rs showed no predictive capacity for detecting Δ P lung ≥ 12 cmH 2 O [0.65 (0.52–0.78), P > 0.05]. Most of the time Plung sw and effort indices remained within safe range. Conclusion In patients with ARDS ventilated with PAV+, injurious tidal lung stress and effort were infrequent. In the presence of expiratory muscle activity, Δ P rs underestimated Δ P lung . This phenomenon limits the usefulness of Δ P rs as a surrogate of tidal lung stress, regardless of the mode of support.