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

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Abstract 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 (Plungsw) 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 Plungsw 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.

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