['The motion of the interatrial septum primum (septum) is dependent on the interatrial pressure relation, normally with slightly higher pressure in the left atrium and the septum bulging toward the right atrium. The aim of this study was to explore the physiologic mechanisms that reverse interatrial pressures and provoke leftward bulging of septum (LBA). The hypothesis was that both left ventricular unloading with nitroglycerin and sustained Valsalva strain would independently provoke LBA and that their combination would further intensify the effect.', 'Prospectively collected transesophageal echocardiography recordings from 13 patients with obstructive sleep apnea were retrospectively analyzed for the presence or absence of LBA during resting respiration and during Valsalva strain. In each condition, LBA beats were counted at time points before and after nitroglycerin spray had been administered, which enabled a comparison of the independent effects and the combined effect of the nitroglycerin and the Valsalva maneuver. An LBA beat was defined as a heartbeat displaying any LBA during the cardiac cycle.', 'Nitroglycerin increased the proportion of LBA beats significantly during resting respiration, from 21\xa0±\xa027% to 54 ± 43% (P\xa0= .008). During Valsalva strain, the proportion increased with nitroglycerin spray from 48 ± 21% to 80 ± 17% (P\xa0= .001). After nitroglycerin administration, LBA occurred in at least three beats during strain in all Valsalva periods.'] Is leftward bulging of atrial septum provoked by nitroglycerin and by sustained valsalva strain?