Quadricuspid aortic valve and aortic regurgitation diagnosed by Doppler echocardiography: report of two cases and review of the literature. Two cases of patients with quadricuspid aortic valve diagnosed by Doppler echocardiography are presented. Aortic regurgitation was detected by pulsed Doppler echocardiography in both patients. Because they were young patients, this aortic regurgitation indicated that aortic regurgitation is not necessarily a late-developing event, as has been thought previously. 