Surfactant impairment after mechanical ventilation with large alveolar surface area changes and effects of positive end-expiratory pressure

We have assessed the effects of overinflation on surfactant function and composition in rats undergoing ventilation for 20 min with 100% oxygen at a peak inspiratory pressure of 45 cm H2O, with or without PEEP 10 cm H2O (groups 45/10 and 45/0, respectively). Mean tidal volumes were 48.4 (SEM 0.3) ml kg-1 in group 45/0 and 18.3 (0.1) ml kg- 1 in group 45/10. Arterial oxygenation in group 45/0 was reduced after 20 min compared with group 45/10 (305 (71) vs 564 (10) mm Hg); maximal compliance of the P-V curve was decreased (2.09 (0.13) vs 4.16 (0.35) ml cm H2O-1 kg-1); total lung volume at a transpulmonary pressure of 5 cm H2O was reduced (6.5 (1.0) vs 18.8 (1.4) ml kg-1) and the Gruenwald index was less (0.22 (0.02) vs 0.40 (0.05)). Bronchoalveolar lavage fluid from the group of animals who underwent ventilation without PEEP had a greater protein concentration (2.18 (0.11) vs 0.76 (0.22) mg ml- 1) and a greater minimal surface tension (37.2 (6.3) vs 24.5 (2.8) mN m- 1) than in those who underwent ventilation with PEEP. Group 45/0 had an increase in non-active to active total phosphorus compared with nonventilated controls (0.90 (0.16) vs 0.30 (0.07)). We conclude that ventilation in healthy rats with peak inspiratory pressures of 45 cm H2O without PEEP for 20 min caused severe impairment of pulmonary surfactant composition and function which can be prevented by the use of PEEP 10 cm H2O.