DDD pacing improves symptoms and relieves LV outflow tract (LVOT) obstruction in most patients with hypertrophic cardiomyopathy (HCM). Notably, when pacing is temporarily discontinued, the beneficial effects of pacing are evident in sinus rhythm. The long term results of this novel therapy are, however, uncertain. We propose (1) to record the hemodynamic changes following >4 years of pacing; and (2) to determine whether DDD pacing continues to be necessary in patients who have had a substantial relief of their LVOT obstruction. Patients who have had >50% reduction in LVOT pressure gradients will be randomized to two pacing modalities: DDD at 70 beats per minute and AAI pacing at 70 beats per minute (DDD switched off), and reevaluated after a six-month period.

Group 1:

HCM patients, aged grater than 20 years of either gender who received DDD pacemakers at the NIH at least 4 years ago, for relief of LVOT obstruction and drug-refractory symptoms.

Group 2:

HCM patients selected from Group 1 who at the follow-up cardiac catheterization are demonstrated to have either (1) no significant LVOT obstruction (less than 30mm HG at rest and less than 50mm HG with Isuprel); or (2) a substantial reduction of the LVOT obstruction (greater than 50% reduction in LVOT gradient).

Patient who have developed a heart block will be excluded from group 2.

Females must have a negative pregnancy test.