STUDY OBJECTIVES: To assess the reliability of new and traditional oxygenation measurements in reflecting intrapulmonary shunt. DESIGN: Prospective study. SETTING: Cardiac surgery unit at a university hospital. PATIENTS: Fifty-five patients undergoing coronary artery bypass grafting. Measurements and results: Simultaneous blood samples were collected from an indwelling arterial line and a catheter for determination of blood gases. Standard accepted formulas were utilized to measure a new oxygenation index: PaO(2)/fraction of inspired oxygen (FIO(2)) x mean airway pressure (Paw). The standard formulas used were the oxygenation ratio (PaO(2)/FIO(2)), PaO(2)/alveolar partial oxygen pressure (PAO(2)), alveolar-arterial oxygen tension gradient (P[A-a]O(2)), and intrapulmonary shunt (venous admixture [Qsp/Qt]). There were significant negative (p < 0.05) correlations between the PaO(2)/(FIO(2) x Paw) and Qsp/Qt (r = - 0.85), between the PaO(2)/FIO(2) and Qsp/Qt (r = - 0.74), and between the PaO(2)/PAO(2) and Qsp/Qt (r = - 0.71). There was a significant positive (p < 0.05) correlation between the P(A-a)O(2) gradient and Qsp/Qt (r = 0.66). However, the correlation was strongest between the PaO(2)/(FIO(2) x Paw) and Qsp/Qt. CONCLUSION: In this group of patients, PaO(2)/(FIO(2) x Paw) might be more reliable than other oxygenation measurements in reflecting intrapulmonary shunt.