The purpose of this study is to look at the acute effect of changes in heart rate on exercise performance. This will differentiate exercise effects induced by the change in heart rate on contractility from the acute effects of changing heart rate. Twenty four hours following randomization of patients to the first heart rate, they will have an exercise tolerance test. As previously indicated, the effect of a chronic change in heart rate ill be evaluated following each 2 month period. Subsequently, when patients are crossed over to another heart rate, they will have an exercise tolerance test 24 hours later. This will follow the chronic exercise test of the previous period by at least 24 hours.