An Epidemiological Assessment of

THE clinical implications of complete bundle-branch block have long been a subject of dispute. This appears to be attrib-utable to several factors. In the case of left bundle-branch block (LBBB), the nature of the pathological disturbance remains uncer-tain in any individual case. For example, one cannot readily determine from the electro-cardiogram alone whether the conduction dis-turbance is a function of a focal lesion in the main left bundle or the result of a more diffuse myocardial disease producing a type of parietal block. Experimental and patholog-ical-clinical studies1 ' 2 have lent support to both hypotheses but have not assisted in clarifying the clinical dilemma. LBBB, more-over, may preclude the electrocardiographic manifestation of myocardial infarction. And this latter circumstance casts a certain stigma on the electrocardiographic diagnosis of LBBB without contributing to an understanding of its pathogenesis or portent. The pathology of right bundle-branch block (RBBB) is similarly uncertain, " 3-5 (that is, whether it is due to a central and focal, or to a more peripheral and diffuse lesion), and its implications are, if anything, more obscure. It has long been recognized that complete RBBB may, on occasion, be a mani-festation of a congenital anomaly or of pul-monary disease. As a consequence, this dis-turbance has been historically viewed as being of less significance and having a more favor-able prognosis than LBBB."9 An epidemiological approach has been em-ployed in recent years in an attempt to assess the significance of such conduction disturb-ances. From studies of the prevalence of suc