Rapid method for determination of shunt ratio using a thermodilution technique.

A new technique using thermodilution by which shunt ratio can be rapidly and conveniently determined in patients with left-to-right intracardiac shunts is described. A flow-directed balloon-tip thermistor catheter was positioned in the pulmonary artery to measure cardiac output in 20 patients with left-to-right intracardiac shunts. Shunt ratio measured by the Fick technique (Qp:QSF) varied between 1.3:1 and 3.6:1. Early recirculation was seen on the downslope of the thermodilution curves (TC). The first-pass curve before the point of early recirculation was extrapolated to the baseline and the area inscribed by this portion of the TC (A) was measured by planimetry. The area inscribed by the entire TC (A + B) was also measured by planimetry. Shunt size was determined by calculating the ratio (A + B):A. Shunt ratio determined by this technique (Qp:QSTh) correlated well with Qp:QSF (r = 0.89). The thermodilution technique provides a simple, rapid, and accurate method for determining the magnitude of left-to-right intracardiac shunts.