The availability of implantable cardiac pacemakers has increased the importance of knowledge of the etiology, clinical features, and course of complete heart block as a basis for formulating indications for use of such pacemakers. At present the implantation of a cardiac pacemaker is considered in those forms of high-grade or complete heart block in which failure of an idioventricular pacemaker or severe bradycardia results in potentially fatal cardiac arrest (Adams-Stokes syndrome) or intractable heart failure. We are therefore not concerned here with first degree heart block or Wenckebach periods with occasional dropped beats. But Adams-Stokes syndrome with or without transient complete heart block may occur in patients whose electrocardiograms between attacks disclose 2: 1 or higher grade heart block-Mobitz block,' bundle branch block, and occasionally even sinus rhythm2 Adams-Stokes syndrome is not used synonymously with syncope of various etiologies, other than heart block or severe bradycardias, such as syncope due to reflexes resulting in vagal stimulation producing cardiac standstill. The etiology, course, outlook, and management of such cases are different from those of Adams-Stokes syndrome.
EtiologyThe etiology in 100 successive cases of Adams-Stokes syndrome due to heart block, observed at The Mount Sinai H~s p i t a l ,~ is indicated in TABLE 1. The predominant underlying or associated disease is coronary heart disease, either in the form of chronic coronary disease alone, or in association with hypertension, or characterized by acute myocardial infarction. Attention is directed to the relatively high incidence (28 per cent) of cases in which there was no specific related disease. This list of possible etiologic diseases is compared with the etiology reported in several large series of cases of high-grade or complete heart block with or without AdamS-Stokes syndrome (TABLE 2) .3-7 In all there is a predominance or high incidence of chronic coronary and hypertensive disease. Acute myocardial infarction was responsible for heart block in 10 to 25 per cent of the cases in the various series and will be discussed in more detail. The variable frequency of cases attributed to digitalis is difficult to interpret since it is uncertain whether it was actually responsible or whether it was blamed when there was no other apparent cause. Although none of these series lists as high an incidence (28 per cent) of unknown etiology as our series, in the recent series reported by Curd et aL7 advanced heart block was classified as of unknown etiology in 14 per cent, the second most frequent category after acute myocardial infarction.We are seeing increasing numbers of patients with advanced heart block and Adams-Stokes syndrome beyond the age of 60 and even beyond the age of 80. It is probable such patients have been classified in many series as having coronary or arteriosclerotic heart disease purely on the basis of age, even in the absence of angina pectoris or of clinical or electrocardiographic evidence of recent o...