Doppler echocardiographic assessment with the continuity equation of St. Jude medical mechanical prostheses in the mitral valve position

Abstract

Evaluation of the St. Jude Medical (SJM) valve in the mitral position with Doppler echocardiography has usually involved the use of gradients across the valve and the application of the pressure half-time (PHT) method to derive a mitral valve area. The purpose of this study was, first, to determine the normal values of effective orifice areas for the SJM valve in the mural position using the continuity equation, and second, to evaluate whether this parameter provides an improved assessment of valve function. Accordingly, Doppler echocardiography was performed in 40 patients within 6 weeks after valve replacement. All patients were clinically stable, without evidence of valvular dysfunction or aortic insufficiency. Valve size ranged from 23 to 33 mm and ventricular ejection fraction averaged 54 ± 13%. Effective orifice area was derived by the continuity equation using stroke volume measured in the ventricular outflow tract, divided by the time-velocity integral of the SJM valve jet, and by PHT. Doppler-derived SJM valve mean gradient averaged 4 ± 2 mm Hg. Effective area by the continuity equation averaged 1.82 ± 0.36 cm 2 (range 1.03 cm 2 for a 23 mm valve to 2.63 cm 2 for a 31 mm valve) and was smaller than by PHT (mean 3.10 ± 0.65 cm 2, p = 0.0001; range 1.38 to 4.78 cm 2). Areas by both methods were smaller than the actual valve orifice area provided b the manufacturer (4.53 ± 0.80 cm 2, p = 0.0001). With use of analysis of variance, effective SJM valve area by the continuity equation differentiated various valve sizes (p = 0.0002) better than did mean gradient (p = 0.013) or PHT (p = NS). Effective area by the continuity equation also correlated better with actual valve orifice area (r = 0.68, p <0.0001) than the area derived by PHT (r = 0.31, p = 0.06). Derivation of effective orifice area of SJM valves in the mitral position with the continuity equation allows better differentiation among valve sizes than mean gradients and PHT, and provides an additional index for assessing prosthetic mitral valve function.