Functional residual capacity and respiratory mechanics as indicators of aeration and collapse in experimental lung injury

Increased functional residual capacity (FRC) and compliance are two desirable, but seldom measured, effects of positive end-expiratory pressure (PEEP) in mechanically ventilated patients. To assess how these variables reflect the morphological lung perturbations during the evolution of acute lung injury and the morphological changes from altered PEEP, we correlated measurements of FRC and respiratory system mechanics to the degree of lung aeration and consolidation on computed tomography (CT).Weused a porcine oleic acid model with FRC determinationsbysulfur hexafluoride washin-washoutandrespiratory system mechanics measured during an inspiratory hold maneuver. Within the first hour, during constant volume-controlled ventilation with PEEP 5 cm H2O, FRC decreased by 45%15% (P0.005) and compliance decreased by 35% 12% (P 0.005). Resistance increased by 60% 62% (P 0.005). Only the FRC changes correlated significantly to the decreased aeration (R20.56;P0.01)andthe increased consolidation (R2 0.43; P 0.04) on CT. When the PEEP was changed to either 10 or 0 cm H2O, there were larger changes in FRC than in compliance. We conclude that, in our model, FRC was a more sensitive indicator of PEEP-induced aeration and recruitment of lung tissue and thatFRCmaybe a useful adjunct to Pao2 monitoring