Prognosis of LBBB
Abstract & Commentary
By Michael H. Crawford, MD, Professor of Medicine, and Chief of Clinical Cardiology, at the University of California, San Francisco. Dr. Crawford is on the speaker's bureau for Pfizer.
Synopsis: Hypertension, ischemic heart disease, left ventricular hypertrophy, ST-T abnormalities, and an increased cardiothoracic ratio were associated with LBBB.
Source: Imanishi R, et al. Significance of Incident Complete Left Bundle Branch Block Observed Over a 40-year Period. Am J Cardiol. 2006;98:644-648.
Left bundle branch block (LBBB) is believed to more often imply cardiac disease than right BBB, but long-term follow-up studies are scant. Thus, this group from Japan studied 17,361 atomic bomb survivors in Hiroshima and Nagasaki biannually since 1958 and present their 40-year follow-up of the 110 who developed LBBB during follow-up. Nine with LBBB at the first exam were excluded. For each LBBB case 5 age- and sex-matched controls were selected. Controls with pacemaker or atrial fibrillation were excluded since none of the LBBB subjects had these conditions at onset of LBBB. The rate of LBBB was 675 in men and 692 in women per 100,000 population (< 1%). Mean age at onset of LBBB was 70 for men and 68 for women and the occurrence of LBBB increased with advancing age. The presence of underlying hypertension, ischemic heart disease, and enlarged heart on chest X-ray were statistically more common with LBBB onset. Surprisingly all cause mortality was not different between the LBBB group and controls, nor was the mean age at death. However, death due to heart failure (16 vs 7%) and myocardial infarction (9 vs 3%) were more common with LBBB as compared to controls. In the Cox multivariate regression analysis, LBBB remained an independent predictive variable for heart failure death. The authors concluded that LBBB is predictive of death due to heart failure, but does not predict all-cause mortality independent of underlying clinical factors.
The strength of this study is the 40-year follow-up of subjects without LBBB at study entry. Although this is an unselected general population of about equal numbers of men and women, they were all exposed to radiation. Nevertheless, the occurrence of LBBB overall was rare, but did increase with advancing age. Also, it was about equal in men and women. Not surprisingly LBBB was strongly associated with ischemic heart disease, hypertension and an enlarged heart on x-ray. With subject evaluations every 6 months, this study provides the clinical diagnosis of the subject at the time of LBBB onset, which is an unique strength of this study. So, the old adage that LBBB means heart disease is present as compared to right BBB where it usually is not seems to be true. Of course this only applies to complete right BBB, as incomplete right BBB is almost always due to right ventricular enlargement.
The weaknesses of this study are those common to large-scare epidemiologic studies in that measurements are parsimonious. For example, there are no echocardiograms, cardiac catheterizations nor autopsies. The data are all from history, physical exam and chest X-ray, and there is certainly death certificate bias. Nevertheless, this is a useful study that points out the high mortality from heart failure in LBBB patients. Perhaps this is due to dyssynchrony and could be reduced by cardiac resynchronization.