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

Background: During control mechanical ventilation (CMV), the driving pressure of the respiratory system (ΔP) serves as a surrogate of transpulmonary driving pressure (ΔP). Expiratory muscle activity that decreases end-expiratory lung volume may impair the validity of ΔP to reflect ΔP. 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 ΔP, (2) the ΔP-ΔP relationship, and (3) whether dynamic transpulmonary pressure (Plung) 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). Patients were divided into Group A, if end-inspiratory transpulmonary pressure (P) increased as E increased, and Group B, which showed a decrease or no change in P with E increase.

Results: In 44,836 occluded breaths, ΔP ≥ 12 cmHO 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 ΔP by ΔP, as suggested by a negative linear relationship between transpulmonary pressure at end-expiration (P) and ΔP/ΔP. Group A included 17 and Group B 14 patients. As E increased, ΔP increased mainly due to P increase in Group A, and P decrease in Group B. Although ΔP had an area receiver operating characteristic curve (AUC) of 0.87 (95% confidence intervals 0.82-0.92, P < 0.001) for ΔP ≥ 12 cmHO, this was due exclusively to Group A [0.91 (0.86-0.95), P < 0.001]. In Group B, ΔP showed no predictive capacity for detecting ΔP ≥ 12 cmHO [0.65 (0.52-0.78), P > 0.05]. Most of the time Plung 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, ΔP underestimated ΔP. This phenomenon limits the usefulness of ΔP as a surrogate of tidal lung stress, regardless of the mode of support.