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THE PRESENCE of complete heart block offers a good opportunity to study cardiovascular functions, since one of the major variables of cardiac activity, i.e., heart rate, is maintained constant.113 In addition, if a cardiac pacemaker is implanted in a heart for treatment of complete heart block,14-20 total control of the cardiac rate is achieved, thereby providing an excellent opportunity for studying cardiovascular hemodvnamics under a variety of fixed rates.The present study was unidertaken (1) to attempt to establish the most appropriate heart rate for permanent pacing; (2) to ascertain the hemodynamic consequences of complete heart block at a fixed slow heart rate, and the effect of increasing heart rate;(3) to determine the effects of exercise and nitroglycerin on the cardiac dynamics at various fixed heart rates. A patient with complete heart block resulting from coronary artery disease was studied.Case Report R. W. (no. 61-1624), a 45-year-old white man, a diabetic, had an uncomplicated ivyocardial infaretion in 1956. He did well until June 1962, when he experienced a single, painless, syncopal episode, with transient loss of consciousness followed by apparent complete recovery. He was hospitalized 3 months later and found to be in congestive heart failure with complete atrioventricular block and an idioventricular rate of 30 beats per minute.The blood pressure was 160/65 iimm. Hg. The pulse rate varied from 22 to 36 and was irregular. The neck veins were distended and showed "cannon"' waves. Auscultation of the heart revealed

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