C ineangiographic studies in patients with ventricular septal defect (VSD) have occasionally demonstrated that part of the blood across the defect is ejected immediately into the pulmonary artery (PA) passing through the outflow tract of the right ventricle (RV), but without being trapped in it. We attempted to make a quantitative evaluation of the flow of a partial shunt pathway (a direct VSD-PA pathway) that drains that part of the blood from the defect. Our method depended on a thermal dilution technique to obtain the ejection fraction of the RV and to observe a simultaneous pair of dilution curves at the roots of the aorta and PA after introduction of tracer into the left atrium. An analytical process was specially designed by incorporating a stable one-pass deconvolution technique. The method was applied to eight anesthetized dogs with acutely produced experimental VSD on the entrance of the outflow tract of the RV. The flow through the direct VSD-PA pathway was, in most cases, greater than 50 and up to 85% (mean of the eight, 57 +/- 5% SE) of the total left-to-right shunt flow. This would imply that less than 50%, and down to as little as 15%, of the total amount of shunt flow contributed to extra work of the RV in these cases. In addition, the impact on the pulmonary vasculature due to such a large amount of pulsatile flow through the direct VSD-PA pathway may accelerate the development of hypertrophy of the pulmonary vessel wall.