Insights From In-Vitro Flow Visualization Into the Mechanism of Systolic Anterior Motion of the Mitral Valve in Hypertrophic Cardiomyopathy Under Steady Flow Conditions

Abstract

Hypertrophic obstructive cardiomyopathy is a heart disease characterized by a thickened interventricular septum which narrows the left ventricular outflow tract, and by systolic anterior motion (SAM) of the mitral valve which can contact the septum and create dynamic subaortic obstruction. The most common explanation for SAM has been the Venturi mechanism which postulates that septal hypertrophy, by narrowing the outflow tract, produces high velocities and thus low pressure between the mitral valve and the septum, causing the valve leaflets to move anteriorly. This hypothesis, however, fails to explain why SAM often begins early in systole, when outflow tract velocities are low or negligible or why it may occur in the absence of septal hypertrophy. The goal of this study was therefore to investigate an alternative hypothesis in which structural abnormalities of the papillary muscles act as a primary cause of SAM by altering valve restraint and thereby changing the geometry of the closed mitral apparatus and its relationship to the surrounding flow field. In order to test this hypothesis, an in vitro model of the left ventricle which included an explanted human mitral valve with intact chords and papillary muscle apparatus was constructed. Flow visualization was used to observe the ventricular flow field and the mitral valve geometry. Displacing the papillary muscles anteriorly and closer to each other, as observed clinically in patients with cardiomyopathy and obstruction produced SAM in the absence of septal hypertrophy. Flow could be seen impacting on the upstream (posterior) surface of the leaflets; such flow is capable of producing form drag forces which can initiate and maintain SAM. In contrast, increasing septal hypertrophy to narrow the outflow tract and create velocities as high as 3.3 m/s did not produce SAM in the absence of papillary muscle displacement, despite an increase in the calculated lift forces. Therefore, primary abnormalities of the papillary muscle-mitral valve apparatus can alter the relationship of the mitral valve to the surrounding flow field in such a way that SAM is generated, whereas the Venturi mechanism, induced by septal hypertrophy alone, is insufficient to do so with a normally positioned and tethered valve.