Color flow Doppler determination of transmitral flow and orifice area in mitral stenosis: Experimental evaluation of the proximal flow-convergence method

To evaluate the in vivo accuracy of color Doppler flow-convergence methods for determining transmitral flow volumes and effective orifice areas in mitral stenosis, we studied two models for flow-convergence surface geometry, a hemispheric (HS) model and an oblate hemispheroid (OH) model in a chronic animal model with quantifiable mitral flows. Color Doppler flow mapping of the proximal flow-convergence region has been reported to be useful for evaluation of intracardiac flows. Flow-convergence methods in patients with mitral stenosis that use HS assumption for the isovelocity surface have resulted in underestimation of actual flows. Chronic mitral stenosis was created surgically in six sheep with annuloplasty rings (group 1) and in 11 sheep with bioprosthetic porcine valves (group 2). Hemodynamic and echocardiographic/Doppler studies (n = 18 in group 1; n = 21 in group 2) were performed 20 to 34 weeks later. Left ventricular inflow obstruction was of varied severity, with mean transmitral valve gradients in group 1 ranging from 1.3 to 18 mm Hg and in group 2 ranging from 6.3 to 25.6 mm Hg. Although transmitral flows derived by both geometric flow convergence models showed significant correlations with reference cardiac outputs, the correlations for the OH model were better than those for the HS model (group 1, r = 0.86 for the OH model vs r = 0.72 for the HS model; group 2; r = 0.84 for the OH model vs r = 0.62 for the HS model). The OH model was also superior to the HS model in determining effective orifice areas compared to reference orifice areas determined by postmortem planimetry of anatomic orifices (group 1 only, r = 0.64 for OH vs 0.58 for HS), by the Gorlin and Gorlin formula (group 1, r = 0.63 for OH vs 0.72 for HS; group 2, r = 0.82 for OH vs 0.76 for HS), and by the Doppler pressure half-time method (group 1, r = 0.76 for OH vs 0.69 for HS; group 2, r = 0.84 for OH vs 0.62 for HS). The percentage differences between the reference values and calculated data with the OH model were significantly smaller for transmitral flows (-5.8% vs -59% in group 1 and -13% vs -63% in group 2) and for effective orifice areas in both groups, p < 0.0001 (-21% vs -56% for mitral orifice area by postmortem planimetry in group 1; -28% vs -61% in group 1 and -24% vs -68% in group 2 for mitral orifice area by the Gorlin and Gorlin formula; and -18% vs -57% in group 1 and -27% vs -71% in group 2 for the pressure half-time method, p < 0.0001 for each comparison). These studies demonstrate that flow convergence principles applied to color Doppler flow mapping permit estimation of transvalvular flow volumes and orifice areas in mitral stenosis. However, in the presence of orifices that are not infinitesimally small and when it is not clinically feasible to sample at substantial distances proximal to the orifice, determination of flows and areas with flow convergence principles results in underestimation relative to reference standards. This underestimation can be minimized by applying an OH geometric model to the flow convergence surface area rather than a strictly HS model.