Immediate positive pressure ventilation with positive end-expiratory pressure (PEEP) improves survival in ovine smoke inhalation injury

Background: The purpose of this study was to compare the effects of immediate initiation of positive pressure ventilation (PPV) with positive end-expiratory pressure (PEEP) versus the initiation of PPV with PEEP only after hypoxemia ensued following severe smoke inhalation injury. Methods: We prospectively evaluated chronically instrumented adult sheep treated with immediate versus delayed PPV with PEEP and compared oxygen requirements, hemodynamics, pleural fluid formation, postinjury survival, and tracheobronchial pathologic processes among groups. The immediate group (group I; smoke, n = 6; sham, n = 2) underwent tracheostomy and bilateral chest tube placement before they received inhalation injury. They were then immediately placed on PPV with PEEP (12 cm H₂O). The animals in the delayed group (group D) (n = 6) were placed on PPV with PEEP when arterial hypoxemia (Pao₂ < 80 mm Hg {11.2 kPa~ on 0.4 FIo₂) or respiratory distress developed. Results: Groups were matched for smoke exposure and peak carboxyhemoglobin. Both groups developed a characteristic decrease in PaO₂/FIO₂ ratio. Initiation of PPV + PEEP improved Pao₂ in the delayed group (69 ± 7 to 126 ± 21 mm Hg {9.2 ± 0.9 to 16.7 ± 2.8 kPa~). Pleural fluid output was greater in the immediate group compared with the delayed group (1559 ± 415 vs. 426 ± 236 mL). At 96 hours after injury five of six animals in the delayed group had died. In contrast, six of six animals in the immediate smoke group survived 96 hours (p < 0.05 versus delayed group). The immediate group had fewer and less extensive tracheobronchial casts at necropsy. Conclusions: Immediate PPV + PEEP did not prevent the development of hypoxia and was associated with increased pleural fluid formation. Death within 96 hours in the delayed group was the result of respiratory failure aggravated by bronchial cast formation despite vigorous pulmonary toilet. Early positive pressure ventilation with PEEP, preferably initiated immediately after the inhalation insult, significantly increases short-term survival and is associated with decreased tracheobronchial cast formation in this ovine model of severe smoke inhalation injury.