Development and determinants of dynamic obstruction to left ventricular outflow in young patients with hypertrophic cardiomyopathy.

BACKGROUND. To study the development of dynamic subaortic obstruction in young patients with hypertrophic cardiomyopathy (HCM), serial echocardiograms were retrospectively analyzed in a group of 26 consecutive children with this disease who showed no evidence of dynamic outflow obstruction at their initial evaluation (age, 11 +/- 3 years). METHODS AND RESULTS. After a follow-up of 3-12 years (mean, 7 +/- 3 years), seven of the 26 patients (27%) developed echocardiographic evidence of subaortic obstruction, i.e., marked systolic anterior motion (SAM) of the mitral valve with mitral-septal apposition and increased left ventricular outflow tract systolic velocities (3.8 +/- 0.3 m/sec; range, 3.1-4.5). Patients who developed SAM had smaller transverse dimension of the left ventricular outflow tract and more anteriorly displaced mitral valve when initially evaluated than did patients without development of SAM (outflow tract dimension, 19.1 +/- 4 versus 24.6 +/- 4 mm; mitral valve position index, 1.07 +/- 0.2 versus 0.73 +/- 0.3; each p less than 0.02). In patients with development of SAM, the already reduced outflow tract dimension decreased further during follow-up, and the mitral valve became even more anteriorly displaced within the left ventricular cavity. These developmental alterations in outflow tract size were associated with increases in left ventricular wall thickness, particularly of the basal anterior septum (11.0 +/- 8 mm; 72 +/- 33%) compared with control patients with HCM who did not develop SAM (3.0 +/- 3 mm; 17 +/- 10%; p less than 0.05). CONCLUSIONS. Development of subaortic obstruction in young patients with HCM results from a process of dynamic remodeling of left ventricular geometry over several years and is characterized by progressive narrowing of the outflow tract with anterior displacement of the mitral valve and disproportionate thickening of the basal anterior ventricular septum.