Alveolar Tidal recruitment/derecruitment and Overdistension During Four Levels of End-Expiratory Pressure with Protective Tidal Volume During Anesthesia in a Murine Lung-Healthy Model.

Purpose: We compared respiratory mechanics between the positive end-expiratory pressure of minimal respiratory system elastance (PEEP) and three levels of PEEP during low-tidal-volume (6 mL/kg) ventilation in rats.

Methods: Twenty-four rats were anesthetized, paralyzed, and mechanically ventilated. Airway pressure (P), flow (F), and volume (V) were fitted by a linear single compartment model (LSCM) P(t) = E × V(t) + R × F(t) + PEEP or a volume- and flow-dependent SCM (VFDSCM) P(t) = (E + E × V(t)) × V(t) + (K + K × |F(t)|) × F(t) + PEEP, where E and R are respiratory system elastance and resistance, respectively; E and E× V are volume-independent and volume-dependent E, respectively; and K and K × F are flow-independent and flow-dependent R, respectively. Animals were ventilated for 1 h at PEEP 0 cmHO (ZEEP); PEEP; 2 cmHO above PEEP (PEEP); or 4 cmHO above PEEP (PEEP). Alveolar tidal recruitment/derecruitment and overdistension were assessed by the index %E = 100 × [(E × V)/(E + |E| × V)], and alveolar stability by the slope of E(t).

Results: %E varied between 0 and 30% at PEEP in most respiratory cycles. Alveolar Tidal recruitment/derecruitment (%E < 0) and overdistension (%E > 30) were predominant in the absence of PEEP and in PEEP levels higher than PEEP, respectively. The slope of E(t) was different from zero in all groups besides PEEP.

Conclusions: PEEP presented the best compromise between alveolar tidal recruitment/derecruitment and overdistension, during 1 h of low-V mechanical ventilation.