Association of PEEP with two different inflation volumes in ARDS patients: effects on passive lung deflation and alveolar recruitment

Objective: To assess the effects of the association of positive end-expiratory pressure (PEEP) with different inflation volumes (V-T's) on passive lung deflation and alveolar recruitment in ARDS patients. Design: Clinical study using PEEP with two different V-T's and analyzing whether passive lung deflation and alveolar recruitment (Vrec) depend on end-inspired (EILV) or end-expired (EELV) lung volume in mechanically ventilated ARDS patients. Setting: Medical intensive care unit in a university hospital. Patients and participants: Six mechanically ventilated consecutive supine patients with ARDS. Interventions: Time-course of thoracic volume decay during passive expiration and Vrec were investigated in six ARDS patients ventilated on PEEP with baseline V-T (V-T,b) and 0.5V(T) (0.5V(T),b), and on zero PEEP (ZEEP) with V-T,b. Time constants of the fast (tau(1)) and slow (tau(2)) emptying compartments, as well as resistances and elastances were also determined. Measurements and results: (a) the biexponential model best: fitted the volume decay in all instances. The fast compartment was responsible for 84 +/- 7 (0.5V(T),b) and 86 +/- 5% (V-T,b) on PEEP vs 81 +/- 6% (V-T,b) on ZEEP (P:ns) of the exhaled V-T, with tau(1) of 0.50 +/- 0.13 and 0.58 +/- 0.17 s vs 0.35 +/- 0.11 s, respectively; (b) only tau(1) for V-T,b on PEEP differed significantly (P < 0.02) from the one on ZEEP, suggesting a slower initial emptying; (c) for the same PEEP, Vrec was higher with a higher volume (V-T,b) than at a lesser one (0.5V(T),b), reflecting the higher V-T. Conclusions: In mechanically ventilated ARDS patients: (a) the behavior of airway resistance seems to depend on the degree of the prevailing lung distension; (b) alveolar recruitment appears to be more important when higher tidal volumes are used during mechanical ventilation on PEEP; (c) PEEP changes the mechanical properties of the respiratory system fast-emptying compartment.