In order to evaluate the magnitude of pulmonary "systolic runoff", we studied the pulmonary vein blood flow velocity waveform by positioning a catheter-tip velocity-pressure transducer into the extraparenchymal pulmonary vein just distal to the left atrium. We recorded blood flow velocity and pressure simultaneously, and subsequently identified the zero blood flow velocity with blood flow velocity level of the pulmonary artery in diastole. Patients with atrial septal defect were used as subjects because of the technical ease although the altered hemodynamics were present. Two kinds of flow velocity waveforms were consistently demonstrated. One was a waveform of two peaks with the first peak in late systole and the second peak in early diastole (n = 9). The other was a waveform of one peak with a summit near the end of systole (n = 5). On the assumption that the blood flow velocity waveform obtained with this method is roughly equivalent to the flow volume waveform, we initiated the second study. The area encompassed between the actual flow velocity waveform and the line of zero flow velocity was divided into two compartments, i.e., ventricular systole (S) and diastole (D). The ratios of the area in systole to the sum of the areas in systole and diastole, i.e., (S)/[S) + (D], which are analogous to the pulmonary "systolic runoff", were 0.45 +/- 0.07 (mean +/- SD, n = 13). This suggests that about 40% of the right ventricular stroke volume flows into the pulmonary veins, the left atrium and a portion of the right atrium through the atrial septal defect during ventricular systole.