Ability of dynamic airway pressure curve profile and elastance for positive end-expiratory pressure titration

Objective: To evaluate the ability of three indices derived from the airway pressure curve for titrating positive end-expiratory pressure (PEEP) to minimize mechanical stress while improving lung aeration assessed by computed tomography (CT). Design: Prospective, experimental study. Setting: University research facilities. Subjects: Twelve pigs. Interventions: Animals were anesthetized and mechanically ventilated with tidal volume of 7 ml kg^-1. In non-injured lungs (n = 6), PEEP was set at 16 cmH₂O and stepwise decreased until zero. Acute lung injury was then induced either with oleic acid (n = 6) or surfactant depletion (n = 6). A recruitment maneuver was performed, the PEEP set at 26 cmH₂O and decreased stepwise until zero. CT scans were obtained at end-expiratory and end-inspiratory pauses. The elastance of the respiratory system (Ers), the stress index and the percentage of volume-dependent elastance (%E₂) were estimated. Measurements and main results: In non-injured and injured lungs, the PEEP at which Ers was lowest (8-4 and 16-12 cmH₂O, respectively) corresponded to the best compromise between recruitment/hyperinflation. In non-injured lungs, stress index and %E₂ correlated with tidal recruitment and hyperinflation. In injured lungs, stress index and %E₂ suggested overdistension at all PEEP levels, whereas the CT scans evidenced tidal recruitment and hyperinflation simultaneously. Conclusion: During ventilation with low tidal volumes, Ers seems to be useful for guiding PEEP titration in non-injured and injured lungs, while stress index and %E₂ are useful in non-injured lungs only. Our results suggest that Ers can be superior to the stress index and %E₂ to guide PEEP titration in focal loss of lung aeration.