To evaluate the in vivo hemodynamics of smaller-sized (21 mm and 23 mm) Medtronic Hall (MH) valves in the aortic position, we measured the maximal pressure gradients (maxPG) at rest and during dobutamine (DOB) administration by continuous-wave Doppler echocardiography in 20 patients with aortic valve replacement. Their body surface areas (BSA) averaged 1.54 m2 in MH21 (n = 10) and 1.57 m2 in MH23 (n = 10). For comparison, the same study was performed in 18 patients who underwent replacement with 21 mm and 23 mm Björk-Shiley (BS) convexo-concave valves [BSA averaged 1.52 m2 in BS21 (n = 9) and 1.58 m2 in BS23 (n = 9)] and in 10 patients with 19 mm St. Jude Medical (SJM) valves (BSA averaged 1.43 m2). The maxPG was calculated as 4 Vmax2 (V = peak velocity) by means of a simplified Bernoulli equation. Cardiac output (CO) was obtained from a simultaneously recorded M-mode echocardiogram by Gibson's formula. A significant correlation, revealed as a second-degree polynomial regression equation, was found between CO and maxPG at rest and during DOB administration in each type or size of prosthesis.(ABSTRACT TRUNCATED AT 250 WORDS)