Plateau airway pressure understimates end-inspiratory alveolar pressure during mechanical ventilation

Introduction: Plateau airway pressure is thought to reflect end-inspiratory alveolar pressure. If this is true, Peak Expiratory Flow (PEF) following an inspiratory pause (when plateau airway pressure is the driving force) will be equal to PEF during on-going mechanical ventilation (when end-inspiratory alveolar pressure is the driving force). Methods: Five healthy anesthetized piglets were ventilated with 24 different combinations of Tidal Volume (300, 400, 500, 600, 700 and 800 ml) and Inspiratory Flow (300, 600, 900 and 1200 ml/sec). Following 5 minutes of ventilation with each combination, PEF was measured after a 5-sec inspiratory pause (PEFstat) and during on-going mechanical ventilation (PEFdyn, Fig. 1). Results: PEF recorded after an end-inspiratory pause was significantly lower than PEF recorded during on-going ventilation (49±12 L/sec vs. 55±15 L/sec, p<0.001; paired t-test). This discrepancy increased with Tidal Volume and Inspiratory Flow (p<0.005 for both; 2-way repeated measures analysis of variance). Conclusions: PEF driven by plateau airway pressure (PEFstat) is significantly lower than PEF driven by end-inspiratory alveolar pressure (PEFdyn). Based on this finding, one can conclude that plateau airway pressure is significantly lower than end-inspiratory alveolar pressure (especially if Tidal Volume).