BackgroundWe previously reported that 6 to 12 weeks of dual-chamber (DDD) pacing results in clinical and hemodynamic improvement in obstructive hypertrophic cardiomyopathy (HCM). This study examines the long-term results of DDD pacing in obstructive HCM.Methods and ResultsDDD devices were implanted in 84 patients (mean age, 49 ± 16 years) with obstructive HCM and severe drug-refractory symptoms. At a mean follow-up of 2.3 ± 0.8 years (maximum, 3.5 years), the New York Heart Association (NYHA) functional class had improved significantly (1.6 ± 0.6 versus 3.2 ± 0.5,P< .00001). Symptoms were eliminated in 28 patients (33%), improved in 47 patients (56%), but remained unchanged in 7 patients (8%). Two patients died suddenly (97% cumulative 3-year survival rate). In 74 patients with significant left ventricular outflow tract (LVOT) obstruction at rest, the LVOT gradients were significantly reduced at follow-up (27 ± 31 versus 96 ± 41 mm Hg,P< .00001). Symptoms and provokable LVOT gradients were also reduced in all 10 patients without significant resting but with provokable LVOT obstruction. Persistence of the LVOT obstruction and symptoms was attributed to inability to pre-excite the interventricular septum (n = 8) and onset of atrial fibrillation (n = 7). Fifty patients had two cardiac catheterization evaluations, 3 ± 1 and 16 ± 4 months after implantation of a pacemaker. In this subgroup, the NYHA functional class improved from 3.2 ± 0.5 at baseline to 1.8 ± 0.7 at the initial evaluation (P< .00001), but with a further significant improvement at the second evaluation: 1.4 ± 0.6,P< .001. This symptomatic improvement was associated with progressive reduction of LVOT gradient at the two evaluations: baseline, 100 ± 47 mm Hg; first evaluation, 41 ± 36 mm Hg (P< .0001); and second evaluation, 29 ± 34 mm Hg (P< .01). Despite the presence of left bundle branch block, DDD pacing reduced LVOT obstruction significantly in 15 patients (LVOT gradient, baseline 89 ± 36 mm Hg versus 18 ± 26 mm Hg at follow-up,P< .0001). There was a weak but significant correlation between the reduction in LVOT gradients accomplished by AV pacing before implantation of DDD device and the eventual reduction in LVOT gradients recorded at the follow-up evaluation (r= .38,P= .0017). Echocardiography demonstrated significant thinning of the anterior septum and distal anterior LV wall in the absence of deterioration of LV systolic function.Conclusions(1) Although most of the improvement of symptoms and hemodynamic indexes occurs during the first few months of DDD pacing, further changes are often observed a year later; (2) DDD pacing is associated with an excellent prognosis in a subgroup of severely disabled patients, many of whom present with syncope or presyncope; (3) baseline pacing studies are not essential to identify patients who may benefit from pacing; (4) preexisting left bundle branch block is compatible with severe LVOT obstruction, and DDD pacing is also beneficial in this subgroup; (5) DDD pacing reduces both resting and provokable LVOT obstruction; (6) additional therapy, for example, radiofrequency ablation of the AV node, may be necessary in some patients either to preexcite the interventricular septum or to control atrial fibrillation; and (7) although LV hypertrophy has been considered a primary feature of HCM, pacing appears to reverse LV wall thickness in a significant subset of adult HCM patients.