IMPROVED DOPPLER ASSESSMENT OF THE BJORK-SHILEY MITRAL PROSTHESIS USING THE CONTINUITY EQUATION

To assess whether derivation of an effective mitral prosthetic valve a rea using the continuity equation provides an improved functional asse ssment of the Bjork-Shiley mitral prosthesis over the pressure half-ti me method, Doppler echocardiographic studies were performed in 43 pati ents 12 +/- 7 months following the valve replacement. Effective valve orifice area used as the standard for comparison was determined by a h ydraulic formula validated in vitro over a wide range of flow rates. A ll patients were clinically stable, without evidence of prosthetic dys function or aortic regurgitation. Prosthetic mitral valve orifice area determined by the hydraulic formula, by the continuity equation and b y pressure half-time method for all prostheses sizes averaged 1.6 +/- 0.46 cm2, 1.83 +/- 0.56 cm2 and 2.34 +/- 0.48 cm2, respectively. Effec tive valve orifice area by the hydraulic formula had a strong correlat ion with that derived by the continuity equation (r = 0.86, P < 0.0001 ; standard error of estimate (S.E.E.), 0.12 cm2), but an insignificant correlation with the area calculated by the pressure half-time method (r = 0.24). Proshetic mitral valve areas determined by the continuity equation and by pressure half-time method also correlated poorly (r = 0.24). Pressure half-time was affected by heart rate, diastolic filli ng period, left ventricular fractional shortening and presence of atri al fibrillation (P < 0.001). Thus, using the standard continuity equat ion to determine the orifice area of the Bjork-Shiley prosthesis in th e mitral position provides improved assessment compared with the press ure half-time method.