Conventional ventilation modes with small pressure amplitudes and high positive end-expiratory pressure levels optimize surfactant therapy

Objective: High-frequency oscillation studies have shown that ventilation a t high end-expiratory lung volumes combined with small volume cycles at hig h rates best preserves exogenous surfactant and gas exchange in lavaged lun gs. We investigated whether surfactant composition and gas exchange can als o be preserved by conventional modes of mechanical ventilation, which combi ne high levels of positive end-expiratory pressure (PEEP) with small pressu re amplitudes. Design: Prospective, randomized, nonblinded, controlled study. Setting: Research laboratory. Subjects: Thirty male Sprague-Dawley rats. Interventions: Rats were lung-lavaged and treated with exogenous surfactant (100 mg/kg). After 5 mins, four different ventilator settings (FIO2 = 1.0) were applied for 3 hrs in four groups of rats [peak inspiratory pressure ( cm H2O); static PEEP (cm H2O); inspiratory/expiratory ratio; frequency], as follows: 26/2/1:2/30 (group 26/2), 26/6/1:2/30 (group 26/6), 20/10/1:2/30 (group 20/10-static), and 20/6/7:3/130, creating an auto PEEP of 4 cm H2O ( group 20/10-auto). Measurements and Main Results: In all groups, PaO2 increased immediately to prelavage values after surfactant therapy. In group 26/2, PaO2 deteriorate d to postlavage values within 30 mins when PEEP was decreased to 2 cm H2O, whereas PaO2 remained stable for 3 hrs in the other groups, The PaCO2 incre ased in groups 26/2 and 20/10-static; PaCO2 could not be reduced by increas ing ventilation frequency to 130 in group 20/10-static. Groups 26/6 and 20/ 10-auto remained normocapnic. Bronchoalveolar lavage protein concentration was higher in groups 26/2 and 26/6 compared with groups 20/10-static and 20 /10-auto. There was significantly more conversion of surface active large a ggregates into nonactive small aggregates in group 26/2 compared with group s 28/10-static and 20/10-auto. Conclusions: We conclude that exogenous surfactant composition is preserved by conventional modes of mechanical ventilation that use small pressure am plitudes, and adequate oxygenation is maintained by high end-expiratory pre ssure levels. Effective carbon dioxide removal can be achieved by applying a ventilation mode that creates auto PEEP and not by a mode that applies th e same level of PEEP by static PEEP only.