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_rs) serves as a surrogate of transpulmonary driving pressure (ΔP_lung). Expiratory muscle activity that decreases end-expiratory lung volume may impair the validity of ΔP_rs to reflect ΔP_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 ΔP_lung, (2) the ΔP_rs-ΔP_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, ΔP_lung ≥ 12 cmH₂O 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_lung by ΔP_rs, as suggested by a negative linear relationship between transpulmonary pressure at end-expiration (P_LEE) and ΔP_lung/ΔP_rs. Group A included 17 and Group B 14 patients. As E_lung increased, ΔP_lung increased mainly due to P_LEI increase in Group A, and P_LEE decrease in Group B. Although ΔP_rs had an area receiver operating characteristic curve (AUC) of 0.87 (95% confidence intervals 0.82-0.92, P < 0.001) for ΔP_lung ≥ 12 cmH₂O, this was due exclusively to Group A [0.91 (0.86-0.95), P < 0.001]. In Group B, ΔP_rs showed no predictive capacity for detecting ΔP_lung ≥ 12 cmH₂O [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, ΔP_rs underestimated ΔP_lung. This phenomenon limits the usefulness of ΔP_rs as a surrogate of tidal lung stress, regardless of the mode of support.