Physiological Consequences of Breathing Effort According to the Mode of Ventilation During Acute Hypoxemic Respiratory Failure

Rationale: Excessive stress (distending pressure), strain (volume deformation), and drop in inspiratory alveolar pressure are proposed mechanisms for patient self-inflicted lung injury. Objectives: To dissect the influence of inspiratory effort, respiratory mechanics, and ventilation mode on lung stress, strain, and drop in inspiratory alveolar pressure; and explore their impact on oxygenation and lung compliance. Methods: International cohort study analyzing respiratory recordings (esophageal pressure) of patients with acute hypoxemic respiratory failure. Association between muscular pressure (Pmus), surrogates of stress (driving trans-alveolar pressure), strain (tidal volume), and inspiratory alveolar pressure relative to PEEP were explored with mixed-models, including interactions for ventilation mode, respiratory system elastance, and synchrony. Association between these and changes in oxygenation and lung compliance were explored. Measurements and main results: 60 patients from 15 centers represented 528 recordings (339,796 breaths). For each cmH2O Pmus increase there was an increase in driving trans-alveolar pressure (median[CI 95%] 0.28[0.27-0.29]cmH2O) and tidal volume (0.16[0.16-0.17]ml/kg of predicted body weight) and decrease in alveolar pressure (-0.25[0.24-0.6]cmH2O, p<0.001). Volume-control ventilation showed less increase in stress and strain surrogates than pressure-targeted modes, but more drop in alveolar pressure (p<0.001, Pmus:mode interaction). Breath-stacking was infrequent and associated with higher stress. Lower inspiratory alveolar pressure relative to PEEP was associated with subsequent worsening oxygenation (p=0.04) and higher stress with worsening lung compliance (p=0.023). Conclusion: Strong efforts are associated with high surrogates for lung stress, strain, and lower inspiratory alveolar pressure relative to PEEP, differently according to the mode of ventilation, being associated with subsequent worsening oxygenation and lung compliance.