Evaluation of Congenital Heart Defects from Dynamic Tracer Measurements

A novel method for the evaluation of the pulmonary to systemic flow rate ratio in patients with congenital heart defects (shunts) is presented. The evaluation is obtained from the analysis of tracer concentration curves recorded during conventional diagnostic tests. The method is based on a deterministic, time-invariant, linear model of the circulatory system. The impulse response curve of n equal-sized perfect mixers in series is used to derive the first passage time distributions that fit the primary portions of the studied tracer curves. This leads to both "open-loop" (excluding recirculation of tracer) and to "closed-loop" shunt evaluations. The former is based on curve-area calculations and the latter on a simple system of two or three algebraic dynamic tracer material balance equations. The method is demonstrated for three actual cases of unidirectional shunts: one right-to-left and two left-to-right. Good agreement of the fitted model to the recorded measurements is obtained. One of the above cases could be cross evaluated by the direct Fick method as well, resulting in practically the same value.