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

Abstract

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 (τ1) and slow (τ2) emptying compartments, as well as resistances and elastances were also determined. Measurements and results: (a) the bi-exponential 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 τ1 of 0.50 ± 0.13 and 0.58 ± 0.17 s vs 0.35 ± 0.11 s, respectively; (b) only τ1 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.