Determination of Intrapulmonary Shunt: Comparison of an Estimated Shunt Equation and a Modified Equation with the Classic Equation

Because the classic shunt equation requires mixed-venous blood gas values, an estimated and a modified equation have been suggested for use when mixed-venous blood is not available. Both these equations assume an arterial-venous oxygen content difference of 3.5 vol%-and the modified shunt equation also assumes a PaO2 of ⩾ 100 torr. We compared the three equations in 13 critically ill patients. Method: All patients had a pulmonary artery catheter and were hemodynamically stable. The classic and estimated shunt equations were used to calculate shunt (Qs/Qt) simultaneously for all data (n = 57). When PaO2 was ⩾ 100 torr, the classic and modified equations were used simultaneously (n = 42). Results: Qs/Qt from the classic shunt equation may be from 6.3 percentage points (units) greater than to 3.3 units less than Qs/Qt from the estimated shunt equation. Qs/Qt from the classic equation may be from 6 units greater than to 4.8 units less than Qs/Qt from the modified equation. Conclusions: The estimated and modified shunt equations provide a reasonable estimate of Qs/Qt in hemody-namically stable patients. However, they should not be used when the arterial-venous content differs significantly from 3.5 vol%, in hemodynamically unstable patients, and when an accurate and precise intrapulmonary shunt value is required.