[Doppler sonography determination of flow ratios Qp/Qs and resistance ratios Rp/Rs in isolated ventricular septal defect].

We present a model based on an electric analogon that describes the haemodynamic situation of an isolated ventricular septal defect. The model shows a way to estimate the ratio of pulmonary to systemic vascular resistance by Doppler sonographic measurements of pulmonary to systemic flow ratio (Qp/Qs) and the ratio between the size of the defect and the cross-sectional area of the aorta (Av/Aa). The ratio of pulmonary to systemic vascular resistance is theoretically given by: Rp/Rs = Qs/Qp + log (Av/Aa) * (1-Qs/Qp). Systemic to pulmonary flow ratio can be measured by Doppler echocardiography as well as Av/Aa; therefore, Rp/Rs can be estimated noninvasively by the equation given above. The values estimated noninvasively by Doppler echocardiography showed a good correlation with the haemodynamic data obtained at cardiac catheterisation.