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

The effectiveness of surfactant (SURF) treatment in acute lung injury in the adult is controversial. In this study, we tested the effectiveness of early surfactant treatment in a commonly used animal model of acute lung injury, phorbol-myristate acetate (PMA), to see if it would attenuate the progression of lung injury. We measured the effect on lung compliance and whether positive end-expiratory pressure (PEEP) (10 cm H2O) during SURF administration 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 was induced with 25-30 mu g/kg of PMA. Responses were measured over 7 hrs. Surfactant was administered in the form of Survanta, 4 x 25 mg/kg doses via tracheal 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 left 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 compliance and total lung capacity (p < .05 from normal). Wet/dry lung weights did not differ between groups. SURF without PEEP further decreased lung compliance 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 static 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, coadministration of PEEP appears to be critically important in counteracting these early effects of surfactant instillation on the lung.