Hypertrophic Cardiomyopathy: Pathophysiology, Functional Features, and Treatment of Outflow Tract Obstruction

Pathophysiology Of Left Ventricular Outflow Tract Obstruction

The combination of (1) multiple structural abnormalities and (2) hydrodynamic forces causes systolic anterior motion (SAM) of the mitral valve and the development of dynamic LVOTO in patients with HCM ( Fig. 59.1 and Video 59.1A, Video 59.1B, Video 59.1C ). Morphologic characteristics that promote LVOTO include septal hypertrophy, narrowing of the outflow tract, intrinsic abnormalities of the mitral leaflets, ^, anterior displacement of the mitral valve apparatus, ^, and anterior malposition of the papillary muscles. Consequently, the mitral leaflets coapt in the body of the mitral leaflets rather than at the leaflet tip. ^, The initiation of SAM is facilitated by malposition of the papillary muscle and mitral leaflet elongation, which increase chordal-leaflet laxity. During rapid ventricular ejection, the tip of the anterior mitral leaflet, distal to its point of coaptation with the posterior mitral leaflet, is susceptible to hydrodynamic forces. The phenomenon of SAM was first identified by echocardiography in the late 1960s, and is considered a hallmark of patients with HCM and LVOTO ( Fig. 59.1B ). Multiple echocardiographic observations have helped to elucidate the role of Venturi and drag forces ^{, ,} in drawing the anterior mitral leaflet into the outflow tract and toward the ventricular septum. In typical SAM, sharp anterior and superior angulation of the anterior mitral leaflet occurs, which results in contact of the anterior mitral leaflet with the septum in early to midsystole. ^,

Video 59.1 . Mechanisms and functional features of obstructive hypertrophic cardiomyopathy (HCM). A , Parasternal long-axis view (systolic frame) in a young patient with obstructive HCM shows the asymmetric septal hypertrophy (septal thickness of 24 mm) and anterior and superior motion of the anterior mitral leaflet toward the septum during systole. B , Corresponding parasternal long-axis M-mode image depicts the septal hypertrophy, severe systolic anterior motion (SAM), and prolonged anterior leaflet–septal contact. C , Apical three-chamber view (systolic frame) provides detailed visualization of mitral leaflet elongation and morphology and leaflet motion: SAM of the anterior leaflet with septal contact in systole and, in this particular case, SAM of the posterior mitral leaflet toward the basal septum as well. The interleaflet gap between the mitral leaflets is observed. D , Apical three-chamber view with color Doppler imaging, which outlines the color turbulence in the left ventricular outflow tract, caused by SAM–septal contact causing left ventricular outflow tract obstruction, and the mitral interleaflet gap leading to a posteriorly directed jet of mitral regurgitation.