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Real-time cross-sectional echocardiography was performed to record the interatrial septal echogram by right atrium-interatrial septum-left atrium (ASA) direction of the ultrasound beam by positioning the transducer to the right of the sternum. The configuration of the interatrial septum (IAS) and the change of the configuration through each cardiac cycle were studied in 10 normal subjects and 29 patients with left or right atrial overloading.In normal subjects the IAS was slightly convex toward the right atrium (RA) in end-systole and slightly convex toward the left atrium (LA) in end-diastole, respectively. In patients with mitral stenosis, the IAS protruded archwise toward the RA both in end-systole and in end-diastole, and showed only minimum difference through each cardiac cycle. In patients with acute mitral regurgitation, the IAS was markedly convex toward the RA in end-systole and slightly convex toward the LA in end-diastole; thus, the difference of the configuration of the IAS was increased. In patients with chronic mitral regurgitation, the IAS was moderately convex toward the RA in end systole and flat or slightly convex toward the RA in end-diastole. In patients with tricuspid regurgitation, the interatrial septal echogram showed several patterns. However, the change in the configuration of the IAS throughout the cardiac cycle showed a characteristic pattern, i.e., it was more convex toward the LA or less convex toward the RA in end-systole than in end-diastole.The observation of the change in the interatrial septal configuration may be useful in the diagnosis of right or left atrial overloading. The mechanism by which the configuration is altered seems to be the interatrial pressure gradient through each cardiac cycle.