Quantitation of Bidirectional Shunt from A Single Dye-Dilution Curve

A method for quantitating intracardiac bidirectional shunt and estimating its blood flow is presented. Three curves, defined as "right-to-left shunt curve", "normal pathway curve", and "left-to-right shunt curve" were separately drawn from the original downstream dye-dilution curve, and their areas ?A'', A, A', respectively, were calculated in the conventional manner. A'/A showed left-to-right shunt ratio (QL-R%) even in the presence of right-to-left shunt. However, right-to-left shunt ratio (QR-L%) was given by (1-A'/A)/(A/A'' + 1--A'/A), and equalled A''/(A + A'') only when left-to-right shunt was absent (A' equals O). A''/(A + A) should become increasingly higher than QR-L% with rise in QL-R% (equals A'/A). The relationship between A''/(A + A'') and QR-L% for various A/A isopleths is shown by figure. A good agreement (r equals 0.848, p less than 0.001) was found between QL-R% values by the present method and those calculated by the oxygen method. In contrast, QR-L% values by the present method were consistently lower than those by the oxygen method; the difference was exaggerated especially in cases with history of cardiac failure, and these cases tended to show lower systemic flow. The average difference in QR-L% between the two methods, found in cases free from failure, was 7 per cent. This was thought to be ascribed to intrapulmonary physiological shunt.