BACKGROUND: There are many factors such as diffusion abnormality, V/Q mismatch, intrapulmonary shunt, alveolar hypoventilation and FIO2 in reducing arterial hypoxemia. Intrapulmonary shunting can be due to blood going from the right to the left side of the heart without respiring with alveolar gas(true shunt mechanism) or blood that respires but achieves a PaO2 less than the ideal (shunt effect mechanism). Understanding the portion of true shunt in patients with hypoxemia is very important indicator to analyze the effects of oxygen therapy. Several equations are used for calculation of physiologic shunt. The aim of this study was to calculate and compare shunts derived from four shunt equations; classic physiologic, estimated, modified clinical and simple equations. METHOD: After cardiovascular stability following open heart surgery, 40 patients were mechanically ventilated with an FIO2=1.0. Arterial and mixed blood gases were measured. We calculated and compared shunts by classic physiologic [S/T=(CcO2 CaO2)/(CcO2 CO2)], estimated [S/T=(CcO2 CaO2)/ (3.5 CcO2 CO2)], modified clinical [S/T= AaDO2 0.0031/(AaDO2 0.0031 CcO2 CaO2)], and simple equations [S/T=AaDO2/20]/ RESULTS: Shunts by classic physiologic, estimated, and modified clinical shunt equation were 26.9 8.5%, 25.1 7.1%, and 26.3 8.2%, respectively and did not differ one another significantly. Shunts by simple shunt equations was 18.8 6.2% and significantly lower than those by other 3 equations(P<0.05). CONCLUSIONS: It is reasonable to conclude that in post-open heart patients with stable cardiovascular function and mechanically ventilated with an FIO2=1.0, classic physiologic, estimated, and modified clinical shunt equations show a reliable reflection of the physiologic shunt. But simple equation (AaDO2/20) might be used as a simple estimate.