Surfactant treatment impairs gas exchange in a canine model of acute lung injury

The effectiveness of surfactant (SURF) treatment in acute lung injury in th e adult is controversial. In this study, we tested the effectiveness of ear ly surfactant treatment in a commonly used animal model of acute lung injur y, phorbol-myristate acetate (PMA), to see if it would attenuate the progre ssion of lung injury. We measured the effect on lung compliance and whether positive end-expiratory pressure (PEEP) (10 cm H2O) during SURF administra tion had a synergistic effect. Methods: Four groups of anesthetized dogs were studied: a) normals; b) PMA injury only; c) PMA injury + SURF; and d) PMA + SURF + PEEP. Lung injury wa s induced with 25-30 mu g/kg of PMA. Responses were measured over 7 hrs. Su rfactant was administered in the form of Survanta, 4 x 25 mg/kg doses via t racheal instillation 2.5 hrs after PMA. For the group receiving PEEP, 10 cm H2O PEEP was begun 1.5 hrs after PMA, 1 hr before SURF. Postmortem, the le ft lung was excised and inflated three times to total lung capacity (volume at 30 cm H2O) and expiratory compliance was measured with 25-100 mL volume increments. The trachea was then clamped and trapped volume was determined by water displacement. Results: PMA-induced lung injury significantly reduced expiratory complianc e and total lung capacity (p < .05 from normal). Wet/dry lung weights did n ot differ between groups. SURF without PEEP further decreased lung complian ce as compared with PMA only. Conclusions: SURF administration after PMA injury causes marked reductions in lung compliance when no PEEP is coadministered. However, the loss of sta tic expiratory lung compliance appears partly ameliorated by application of PEEP + SURF. Given that tracheal instillation of SURF is known to acutely elevate lung impedance in the first few hours after administration, coadmin istration of PEEP appears to be critically important in counteracting these early effects of surfactant instillation on the lung.