The detection and location of left-to-right shunts is essential in the diagnosis of congenital heart disease. The earliest method for this purpose was analysis of oxygen content or saturation of multiple blood samples from various chambers of the heart and great vessels. This method remains the most widely used, having the advantage of quantitation. However, the amount of blood which is required for this is sometimes dangerously large in infants, and the delay before the results are known may be several hours. In addition, gasometric methods cannot easily be applied in the operating room. The dye-dilution method is excellent for locating right-to-left shunts, but the curves are of no assistance in locating a left-to-right shunt, except using double lumen or double catheter techniques, which are cumbersome and impractical for small children. The method is again limited because of the considerable amount of blood lost, and the accumulation of dye interferes