Functional residual capacity as a noninvasive indicator of optimal positive end-expiratory pressure

We hypothesized that functional residual capacity (FRC) could be used as a noninvasive indicator of optimal positive end-expiratory pressure (PEEP), the level of PEEP that results in venous admixture below 15% with an inspired oxygen fraction less than 0.5. We compared several variables for PEEP optimization—oxygen transport, total respiratory system compliance, FRC-based compliance, mixed venous oxygen saturation, end-tidal to arterial carbon dioxide tension difference, and arterial oxygen saturation—by producing four different PEEP levels, 0, 5, 10, and 15 cm H₂O, in 24 mongrel dogs in which pulmonary injury was produced. The data were regressed versus PEEP by using analysis of variance for regression. Venous admixture (F_1,23=149.3;P<0.0001), end-tidal to arterial carbon dioxide tension difference (F_1,23=64.9;P<0.0001), and oxygen transport (F_1,23=95.1;P<0.0001) decreased linearly with PEEP. FRC (F_1,23=248.1;P<0.0001) and arterial oxygen saturation (F_1,23=66.9;P<0.0001) increased linearly with PEEP. Total respiratory system compliance (F_1,23=66.6;P<0.0001) and mixed venous oxygen saturation (F_1,23=12.2;P<0.002) had a quadratic relationship with respect to PEEP with a peak at 5 cm H₂O. FRC-based compliance did not have a significant relationship to PEEP. The maximum values of total respiratory system compliance, FRC-based compliance, mixed venous oxygen saturation, and oxygen transport did not occur at PEEP levels that corresponded to a venous admixture below 15% (optimal PEEP). In this canine oleic acid lung injury model, maximizing these variables would be a poor technique for PEEP titration. FRC and arterial oxygen saturation had a strong relationship to PEEP and venous admixture, and these two would be good noninvasive variables for use in PEEP titration.This work was supported by a grant from Siemens-Elema Ventilator Division, Solna, Sweden.