BACKGROUND: Respiratory failure secondary to cardiopulmonary bypass (CPB) remains a major complication after cardiac surgery. We tested the hypothesis that post-CPB lung function impairment can be prevented by continuous positive airway pressure (CPAP) applied during the CPB. METHODS: In 6 pigs, CPAP with 5 cmH2O pressure was applied during CPB. Six other pigs served as control, i.e. the lungs were open to the atmosphere during CPB. After median sternotomy, the right atrial appendage as well as the ascending aorta were cannulated. The total CPB duration was 90 min with 45 min cardioplegic arrest. Ventilation-perfusion distribution was measured with the multiple inert gas elimination technique and atelectasis by CT-scanning. RESULTS: Large atelectasis appeared after CPB, corresponding to 14.5% +/- 5.5 (percent of the total lung area) in the CPAP group and 18.7% +/- 5.2 in the controls (P = 0.20). Intrapulmonary shunt increased and PaO2 decreased after the CPB in both groups. CONCLUSIONS: We conclude that in this pig model post-CPB atelectasis is not effectively prevented by CPAP applied during CPB.