Evaluation of mitral regurgitation using a digitally determined color Doppler flow convergence 'centerline' acceleration method: Studies in an animal model with quantified mitral regurgitation

Background: The imaging and measurement of the proximal flow convergence region in the left ventricle have been reported to be useful for identifying the site of mitral regurgitation (MR) and for evaluating its severity. However, the application of this method has not gained general acceptance. There have been few in vivo studies with quantified reference standards for determining regurgitant volume, and those that have been reported used spectral Doppler standards and/or nonsimultaneously performed contrast ventriculography. The purpose of the present study was to evaluate the proximal flow convergence centerline velocity-distance profile method applied to chronic MR resulting from flail mitral leaflets in an animal model in which regurgitant flow rates and regurgitant volumes were determined simultaneously with electromagnetic flow probes and flowmeters. Methods and Results: In six sheep, a total of 18 hemodynamically different states were obtained when the animals were restudied 6 months after surgical induction of MR produced by severing chordae tendineae to the anterior (three sheep) or posterior (three sheep) mitral leaflet. Echocardiographic studies with a Vingmed 750 were performed to obtain complete proximal axial flow acceleration velocity-distance profiles for each hemodynamic state. The color Doppler velocity data were directly transferred in digital format from the ultrasound instrumentation to a microcomputer. The severity of MR was assessed by the magnitude of the mitral regurgitant fraction determined using both mitral and aortic electromagnetic flow probes balanced against each other to yield regurgitant volume. MR was classified as grade I when the regurgitant fraction was <20%, as grade II when it was 20% to 35%, and as grade III to IV when it was >35%. Thus, of the 18 hemodynamic states, 4 (from two sheep) were grade I, 7 (from five sheep) were grade II, and 7 (from three sheep) were grade III to IV. All of the velocity-distance acceleration curves showed organized acceleration fields with highly significant correlations using multiplicative regression fits (y=a · x^-b, r=.90 to .99, all P<.01). Grade III to IV MR resulted in rightward and upward shifts of the velocity- distance profile curves compared with those produced by grade II and grade I MR. All of the centerline velocity-distance profiles for grade III or IV regurgitation resided in a domain encompassed by velocities >0.5 m/s at distances from the orifice >0.6 cm; the profiles for grade I regurgitation resided in a domain encompassed by velocities <0.3 m/s at distances from the orifice of <0.45 cm. The profiles for grade II regurgitations resided in a domain between them. Regression analysis for the distance at which a velocity of 0.5 m/s was first reached bore a close relation to regurgitant fraction (r=.92, P<.0001) and peak regurgitant flow rate (r=.89, P<.0001). In addition, an equation for quantitatively correlating both a and b (coefficients from the multiplicative regression fits) with the peak regurgitant flow rate (Q(peak) in L/min) was derived from stepwise regression analysis: Q(peak)=12a+2.7b-2.4 (r=.96, P<.0001, SEE=.45 L/min). Conclusions: In this study, using quantified MR volume, we demonstrate that the proximal flow convergence axial centerline velocity-distance profile method can be used for evaluating the severity of MR without any assumption about isovelocity surface shape geometry.