Background The effective aortic regurgitant orifice area varies with aortic pressure in animal models of acute aortic regurgitation.The purpose of this study was to determine whether the aortic regurgitant orifice area changes during diastole in patients with chronic aortic regurgitation.Methods and Results Two-dimensional and Doppler echocardiography were performed immediately before and after magnetic resonance velocity mapping using a cine phase contrast sequence in 17 patients with chronic aortic regurgitation. ECG-gated continuous-wave Doppler velocity time integrals and magnetic resonance flow rates were measured 16 times per cardiac cycle. The mean aortic regurgitant orifice area (centimeters squared) was calculated by the continuity equation. The regurgitant orifice area was also determined for each diastolic acquisition interval. Changes in the regurgitant orifice area during diastole were modeled using an asymptotic exponential decay model to determine the static and dynamic components of the orifice. The regurgitant orifice area increased directly with regurgitant fraction (y(cm (2))=0.0072(cm2/%)*x(%)-0.0409(cm2); r=.86, P<.0001). In 15 of 17 (88%) patients, the regurgitant orifice area decreased during diastole. The dynamic component of the regurgitant orifice area decreased with increasing regurgitant fraction (y(%)=-0.98x(%)+96.9(%); r=-.90, P<.0001). There were no significant differences in heart rate, systolic or diastolic blood pressures, or continuous-wave Doppler velocity time integrals measured before or after the magnetic resonance examination.Conclusions The effective regurgitant orifice area decreases during diastole in patients with chronic aortic regurgitation.This phenomenon should be considered when evaluating aortic regurgitant severity. (Circulation. 1994;89:2085-2092.)