BACKGROUND: Prone position improves oxygenation in patients with ARDS probably by reducing shunt Reduction of shunt in prone position is thought to be effected by lowering of the critical opening pressure (COP) of the dorsal lung because the pleural pressure becomes less positive in prone position compared to supine position. It can then be assumed that prone position would bring about greater improvement in oxygenation when PEEP applied in supine position is just beneath COP than when PEEP is above COP. Hemodynamically, prone position is expected to attenuate the lifting of cardiac fossa induced by PEEP. Based on these backgrounds, we investigated whether the effect of prone position on oxygenation differs in magnitude according to the level of PEEP applied in supine position, and whether impaired cardiac output in supine position by PEEP can be restored in prone position. METHOD: In seven mongrel dogs, PaO2/FIO2(P/F) was measured in supine position and at prone position 30min. Cardiac output (CO), stroke volume (SV), pulse rate (PR), and pulmonary artery occlusion pressure (PAOP) were measured in supine position, at prone position 5 mm, and at prone position 30 mm. After ARDS was established with warmed saline lavage(P/F ratio 134+/-72 mmHg), inflection point was measured by constant flow method(6.6+/-1.4 cm H2O), and the above variables were measured in supine and prone positions under the application of Low PEEP(5.0+/-1.2 cm H2O), and Optimal PEEP(9.0+/-1.2 cm H2O) (2cm H2O below and above the inflection point, respectively) consecutively. RESULTS: P/F ratio in supine position was 195+/-112 mmHg at Low PEEP and 466+/-63 mmHg at Optimal PEEP(p=0.003). Net increase of P/F ratio at prone position 30 mm, however, was far greater at Low PEEP (205+/-90 mmHg) than at Optimal PEEP(33+/-33 mmHg) (p=0.009). Compared to CO in supine position at Optimal PEEP(2.4+/-0.5 L/min), CO in prone improved to 3.4+/-0.6 L/min at prone position 5 mm (p=0.0180) and 3.6+/-0.7 L/min at prone position 30 mm (p=0.0180). Improvement in CO was attributable to the increase in SV : 14+/-2ml in supine position, 20+/-2ml at prone position 5 mm (p=0.0l80), and 21+/-2ml at prone position 30 mm (p= 0.0180), but not to change in PR or PAOP. When the dogs were turned to supine position again, MAP (92+/-23 mmHg, p=0.009), CO (2.4+/-0.5 L/min, p=0.0277) and SV (14+/-1ml, p=0.0277) were all decreased compared to prone position 30 min. CONCLUSION: Prone position in a dog with saline-lavaged acute lung injury appeared to augment the effect of relatively low PEEP on oxygenation, and also attenuate the adverse hemodynamic effect of relatively high PEEP. These findings suggest that a PEEP lower than Optimal PEEP can be adopted in prone position to achieve the goal of alveolar recruitment in ARDS avoiding the hemodynamic complications of a higher PEEP at the same time.