An integrated mechanism for systolic anterior motion of the mitral valve in hypertrophic cardiomyopathy based on echocardiographic observations

Although many mechanisms have been proposed to explain systolic anterior motion (SAM) of the mitral valve in hypertrophic cardiomyopathy, the precise mechanism of its onset and cessation remain undefined. The Venturi theory, based on increased flow velocity in a narrowed outflow tract, is widely accepted but fails to explain several important characteristics of SAM. It also neglects the potential role of drag forces generated by interposition of the leaflets into the path of ejection and of factors that would decrease the effectiveness of papillary muscle restraint. In order to obtain further insight into the mechanism of SAM, a detailed geometric study of the left ventricle and mitral apparatus was performed with cross-sectional echocardiography in three equal-sized groups of patients with hypertrophic cardiomyopathy and SAM, patients with hypertrophy and no anterior motion, and normal control subjects. A salient finding was that SAM began prior to ejection in patients with hypertrophic cardiomyopathy, which cannot be explained by the Venturi theory. Further, SAM began and was most prominent in the central portion of the leaflet as opposed to its lateral edges; this finding is not predicted by the Venturi mechanism. In addition to outflow tract narrowing, other structural changes unique to patients with SAM included anterior and inward displacement of the papillary muscles, anterior displacement of the mitral leaflets, and elongation of the mitral leaflets, which were, on the average, 1.5 to 1.7 cm longer than in the other subjects (p less than 0.0001). On the basis of these observations, an integrated mechanism for the initiation and resolution of SAM is proposed that would explain observed features such as onset before ejection and central prominence. This mechanism combines the effects of outflow tract narrowing with those of papillary muscle displacement. In particular, anterior and inward displacement of the papillary muscles can be predicted to alter the effectiveness of chordal support so that the central leaflet portions become relatively slack and are more readily displaced anteriorly. The altered distribution of chordal tension can also be predicted to orient the distal leaflets upward into the outflow tract at the onset of systole, prior to aortic valve opening, so that ventricular ejection will actually drag the interposed leaflets anteriorly. The resolution of SAM can be understood in terms of a reverse Venturi effect created by mitral regurgitation, as well as continued traction of the centrally displaced papillary muscles on the lateral leaflet margins.(ABSTRACT TRUNCATED AT 400 WORDS)

Variable effects of changes in flow rate through the aortic, pulmonary and mitral valves on valve area and flow velocity: impact on quantitative Doppler flow calculations

Doppler echocardiographic methods for measuring volumetric flow through the aortic, pulmonary and mitral valves provide the cardiologist with several potentially interchangeable noninvasive methods for determining cardiac output. In addition, comparison of flow differences through individual valves offers the potential to quantitate shunt flow and regurgitant volumes. To date, however, no study has compared the relative accuracies of each of these flow measurements in a controlled experimental setting. Therefore, in this study, Doppler echocardiography was used to measure aortic, pulmonary and mitral valve flows in seven open chest dogs on right atrial bypass where forward cardiac output was precisely controlled with a roller pump. Correlations with roller pump output were better for Doppler measurements of aortic (r = 0.98, SD = 0.3) and mitral (r = 0.97, SD = 0.3) than for pulmonary (r = 0.93, SD = 0.5) valve flow. Interobserver reproducibility was also better for aortic (r = 0.94) and mitral (r = 0.97) than for pulmonary (r = 0.88) valve flow measurements. All valves showed flow-related increases in cross-sectional area, but the slope of this response was variable: 0.05, 0.16 and 0.21 for the aortic, the pulmonary and the mitral valve, respectively. Increased forward flow through the aortic valve, therefore, was manifested primarily by an increase in velocity, whereas increasing flow through the pulmonary and mitral valves produced more significant area changes with correspondingly smaller increases in the velocity component. Recalculation of Doppler-determined outputs, assuming a fixed valve area for the entire range of flows, resulted in a decreased correlation with roller pump output. Both velocity and valve area should be measured at each flow rate for greatest accuracy in volumetric flow calculations.

A Doppler-two-dimensional echocardiographic method for quantitation of mitral regurgitation

A noninvasive method to accurately quantitate the severity of mitral insufficiency would be of major clinical value. In theory, in the absence of confounding variables, regurgitant mitral flow should represent the difference between forward mitral blood flow and aortic blood flow. Since Doppler-two-dimensional echocardiographic (D2DE) methods for measuring transvalvular mitral and aortic flow have been validated, it should be possible to use mitral and aortic flows derived by this method to calculate regurgitant mitral flow. To assess the validity and accuracy of this combined approach for quantitation of regurgitant flow, we developed an open-chest canine preparation in which we could simulate, vary, and accurately measure degrees of mitral regurgitation. Seven animals were anesthetized and prepared to allow controlled right heart output. Mitral regurgitation was than simulated by placing a flexible conduit incorporating a one-way valve and electromagnetic flowmeter between the left ventricular apex and left atrium. Flow through the tube (effective mitral regurgitation) was varied between 0.2 and 1.8 liters/min and forward cardiac output ranged between 0.5 and 4 liters/min. Transmitral and transaortic flows were calculated by previously reported Doppler methods. Doppler-derived estimates of forward flow through the aortic valve correlated well with the flow measured by flowmeter (r = .92), and regurgitant flow and regurgitant fraction calculated by the D2DE approach also compared well with those measured by flowmeter (r = .84 and .83, respectively). This study demonstrates that mitral regurgitant flow and regurgitant fraction calculated by the D2DE method provide an acceptable measure of both absolute regurgitant flow and the regurgitant fraction in the experimental setting.