Relationship between regional lung compliance and ventilation homogeneity in the supine and prone position.

Background The prone position (PP) improves ventilation homogeneity in acute respiratory distress syndrome. The aim of this study was to investigate whether the alleviation of ventilation inhomogeneity in PP was due to changes in regional lung compliance. Methods Ten lung-lavaged piglets were mechanically ventilated in supine position (SP) and in PP. In each position, positive end-expiratory pressure (PEEP) was reduced from 20 to 6 cmH2O in steps of 2 cmH2O every 10 min after full lung recruitment. Respiratory mechanics, blood gas, haemodynamic data and whole-lung computed tomography scans were recorded at each PEEP. The compliances of normally aerated (C normal) and newly recruited (C recruited) lung regions were calculated. Open lung PEEP (OL-PEEP) was defined as the lowest PEEP to maintain full lung recruitment. Results At OL-PEEP, PP significantly increased normally aerated lung regions, decreased poorly aerated and hyperinflated lung regions and decreased tidal recruitment and hyperinflation. C normal was significantly reduced in PP compared with SP (12.8 ± 4.2 ml/cmH2O vs. 20.1 ± 6.2 ml/cmH2O, P < 0.001), whereas C recruited was increased in PP (13.9 ± 3.9 ml/cmH2O vs. 9.4 ± 2.4 ml/cmH2O, P < 0.001). C normal was correlated with hyperinflated lung regions at end-expiration (rho = 0.67) and end-inspiration (rho = 0.56) at OL-PEEP. C recruited was correlated with normally (r2 = 0.36) and poorly aerated lung regions (rho = –0.58) at OL-PEEP. Conclusion This surfactant-depleted model shows that the improvement of ventilation homogeneity in PP is related to an increase in C recruited and a decrease in C normal.