Background and objectives: Left ventricular (LV) early filling deceleration time (DT) is determined by the sum of atrial and ventricular stiffnesses (K_la+K_lv). However, if K_lv, and not K_la, predominantly determines DT, it is possible to determine diastolic ventricular (passive) properties from noninvasively acquired Doppler mitral flow velocity tracings. It has been assumed that the effect of K_la on DT is negligible, because during early LV filling the atrium behaves mostly as a conduit, maintaining relatively constant volume and pressure. However, this assumption has not been directly evaluated. The purpose of this study was to measure the left atrial and ventricular volumes and to quantify K_la during DT. Methods: In 15 patients undergoing open-heart surgery, with a wide range of DT, no or mild mitral (MV) insufficiency and regular rhythm, K_lv was assessed, immediately after cardiopulmonary bypass from E-wave DT, as derived from MV velocity tracings obtained by transesophageal echocardiography according to the formula: K_lv=[(70 ms/DT-20 ms)]². In each patient a LA volume curve was also obtained combining MV and pulmonary vein (PV) cumulative flow, plus LA volume measured at end-diastole by 2-D echocardiography. PV flow was computed integrating cumulative left PV velocity * PV cross-sectional area, where PV area = [(MV velocity integral * MV area)/PV velocity integral] calibrated to 2-D echocardiographic LV stroke volume. Time-adjusted LA pressure was measured, simultaneously with Doppler data, in 12 patients by large bore fluid-filled catheters and in 3 patients by micromanometers introduced into the atrial cavity through the upper right PV. K_la was then calculated during the ascending limb of the V-loop and computed as the change in LA pressure from the time of minimal to maximal systolic pressure by the change in LA volume during this period. K_la was also assessed during DT. Results: Left atrial (LA) volume decreased by 7.3±6.5 ml/sqm during the first 4/6 of mitral DT, while ventricular volume increased 9.4±8.4 ml/sqm (both p<0.001). There was a small but definite amount of blood coming from the PV during the same time interval, cumulative flow averaging 3.2±2.4 ml/sqm (p<0.001). Mean LA pressure was 10.0±5.1 mmHg and it did not change during 4/6 DT (from 7.8±4.3 to 8.0±4.3 mmHg, NS), making K_la, which averaged 0.46±0.39 mmHg/ml during the V-loop, close to zero during DT [K_la(DT): from -0.002±0.08 to -0.001±0.031 mmHg/ml, NS]. K_lv, as assessed noninvasively from DT, averaged 0.25±0.32 mmHg/ml. onclusion: Notwithstanding the significant decrement in LA volume, K_la does not change and can be considered not different from zero during DT. Thus K_la does not affect the estimation of K_lv from Doppler parameters.