Alveolar Tidal recruitment/derecruitment and Overdistension During Four Levels of End-Expiratory Pressure with...

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

Twenty-four rats were anesthetized, paralyzed, and mechanically ventilated. Airway pressure (P_aw), flow (F), and volume (V) were fitted by a linear single compartment model (LSCM) P_aw(t) = E_rs × V(t) + R_rs × F(t) + PEEP or a volume- and flow-dependent SCM (VFDSCM) P_aw(t) = (E₁ + E₂ × V(t)) × V(t) + (K₁ + K₂ × |F(t)|) × F(t) + PEEP, where E_rs and R_rs are respiratory system elastance and resistance, respectively; E₁ and E₂× V are volume-independent and volume-dependent E_rs, respectively; and K₁ and K₂ × F are flow-independent and flow-dependent R_rs, respectively. Animals were ventilated for 1 h at PEEP 0 cmH₂O (ZEEP); PEEP_minErs; 2 cmH₂O above PEEP_minErs (PEEP_minErs+2); or 4 cmH₂O above PEEP_minErs (PEEP_minErs+4). Alveolar tidal recruitment/derecruitment and overdistension were assessed by the index %E₂ = 100 × [(E₂ × V_T)/(E₁ + |E₂| × V_T)], and alveolar stability by the slope of E_rs(t).

%E₂ varied between 0 and 30% at PEEP_minErs in most respiratory cycles. Alveolar Tidal recruitment/derecruitment (%E₂ < 0) and overdistension (%E₂ > 30) were predominant in the absence of PEEP and in PEEP levels higher than PEEP_minErs, respectively. The slope of E_rs(t) was different from zero in all groups besides PEEP_minErs+4.

PEEP_minErs presented the best compromise between alveolar tidal recruitment/derecruitment and overdistension, during 1 h of low-V_T mechanical ventilation.