BACKGROUND One possible mechanism of impaired oxygenation in cardiac surgery with extracorporeal circulation (ECC) is the accumulation of extravascular lung water (EVLW). Intrathoracic blood volume (ITBV) and pulmonary blood volume (PBV) also may increase after separation from ECC, which can influence both cardiac performance and pulmonary capillary fluid filtration. This study tested whether there were any relationships between lung fluid accumulation and pulmonary gas exchange during the perioperative period of cardiac surgery and ECC. METHODS Ten patients undergoing myocardial revascularization were studied. ITBV, PBV, and EVLW were determined from the mean transit times and decay times of the dye and thermal indicator curves obtained simultaneously in the descending aorta. Gas exchange was assessed by arterial and mixed venous partial pressure of oxygen (PO2) and carbon dioxide (PCO2), and calculation of alveolo-arterial PO2 gradient (PA-aO2) and venous admixture (QVA/QT). Recordings were made after induction of anesthesia, after sternotomy, 15 min after separation from ECC, and 4 and 20 h postoperatively. RESULTS After induction of anesthesia, EVLW (6.0 +/- 1.0 ml/kg, mean +/- SD), PBV (3.6 +/- 1.3 ml/kg), and ITBV (18.4 +/- 2.7 ml/kg) were within normal ranges. Oxygenation was moderately impaired, as indicated by an increased PA-aO2 (144 +/- 46 mmHg) and QVA/QT (11 +/- 4%). After separation from ECC, EVLW had increased to 9.1 +/- 2.6 ml/kg, which was accompanied by an increase of ITBV (26.0 +/- 4.4 ml/kg) and PBV (5.6 +/- 1.9 ml/kg). PAa-O2 (396 +/- 116 mmHg) and QVA/QT (29 +/- 7%) also were increased. ITBV and PBV remained increased 4 and 20 h postoperatively, but EVLW decreased to presurgery values. No correlations were found between thoracic intravascular and extravascular fluid volumes and gas exchange. CONCLUSIONS Cardiac surgery with the use of ECC induces alterations of thoracic intravascular and extravascular fluid volumes. Postoperatively, increased ITBV and PBV need not be associated with higher EVLW. Thus, sufficient mechanisms protecting against lung edema formation or providing resolution of EVLW probably are maintained after ECC. Since oxygenation is impaired during and after cardiac surgery, it is concluded that mechanisms other than or in addition to changes of ITBV, PBV, and EVLW predominantly influence gas exchange.