New concepts in hypertrophic cardiomyopathies, part II.

Obstructive Hypertrophic Cardiomyopathy: Pathogenesis of the Obstruction In most patients, the hypertrophy develops initially in the septum and extends to the free walls, often giving a picture of concentric hypertrophy. In '25% of patients with familial hypertrophic cardiomyopathy (FHCM), asymmetrical septal hypertrophy leads to a highly variable pressure gradient between the apical left ventricular chamber and the left ventricular outflow tract (LVOT).1 LVOT obstruction with a consecutive increase of left ventricular pressure fuels a vicious circle of further hypertrophy and increased LVOT obstruction. Clinical manifestations of LVOT may be present in early childhood or may develop much later in life.2 In most patients with LVOT obstruction, the hypertrophy is most pronounced in the high intraventricular septum compared with the rest of the left ventricle.3,4 Hypertrophic obstructive cardiomyopathy is characterized by a sphincter-like dynamic midventricular obstruction that creates an outflow tract pressure gradient of varying severity.5–7 The small left ventricular cavity is the main reason for the systolic anterior movement of the mitral valve (SAM), whereby the anterior leaflet may touch the hypertrophied septum.8 Mitral incompetence is almost invariably present with LVOT obstruction.4 Controversy exists regarding the contribution of the Venturi effect secondary to the increased ejection velocity. Most likely, this Venturi effect is the consequence rather than the origin of the LVOT gradient.9 Diastolic left ventricular abnormalities result from loss of myocardial compliance owing to fibrosis and hypertrophy. Whether ischemia is a primary feature or a consequence of the obstruction remains obscure. Because the LVOT obstruction is dynamic, patients in whom the disease is suspected and who do not have a gradient at rest should undergo provocation maneuvers with agents such as dobutamine to determine the severity of the gradient. If pressure gradients of .30 mm Hg at rest are present, the potential for further hypertrophy and deterioration is highly likely. Congestive heart failure is 5 times more likely than in an unaffected population.1