Dissociation between airway and transpulmonary pressures during external chest wall restriction: a prospective self-controlled physiological study.

BACKGROUND: External chest wall restriction can alter how airway pressure is transmitted to the lung. This may make airway pressure an unreliable surrogate for lung stress during mechanical ventilation. We aimed to determine whether short-term chest strapping dissociates airway pressure from transpulmonary pressure by increasing chest wall elastance in mechanically ventilated postoperative intensive care unit (ICU) patients. METHODS: This was a single-center, prospective, self-controlled physiological study. Postoperative ICU patients receiving invasive mechanical ventilation were screened for eligibility. Ventilator settings were kept constant. A chest strap was tightened for 30 minutes as a model of acute external chest wall restriction. Airway and esophageal pressures were recorded before and during strapping, and transpulmonary pressure (PL) was derived. This post hoc re-analysis was restricted to patients without chronic obstructive pulmonary disease (COPD) or an acute pulmonary diagnosis at ICU admission (n=21). RESULTS: During strapping, gas exchange and hemodynamics remained stable, whereas peak and plateau airway pressures, along with airway driving pressure (ΔP), increased. End-inspiratory and end-expiratory esophageal pressures rose, while the corresponding PL decreased. Transpulmonary driving pressure (ΔPL) decreased numerically but did not reach statistical significance. Respiratory system compliance (Crs) decreased, driven mainly by reduced chest wall compliance (Ccw). The elastance ratio (Elung/Ers) declined. Mechanical power increased, with a significant increase in the elastic component but no significant change in the resistive part. CONCLUSIONS: In sedated, volume-controlled postoperative ICU patients undergoing short-term external chest wall restriction, chest strapping increased airway pressures mainly through chest wall loading. Higher airway pressures did not correspond to higher PL, and ΔPL did not increase significantly. Esophageal pressure monitoring may help interpret airway pressures when Ccw is externally restricted.