Several studies have shown that specialized pacemaking devices (DDD pacing) can improve the symptoms associated with hypertrophic cardiomyopathy (HCM) in adults. In addition, studies have also shown that specialized pacemaking devices (DDD pacing) can improve conditions of HCM in children. However, growth of the body and organs, including the heart, is very rapid during childhood. Therefore the long-term effects of DDD pacing in children are unknown.