This study presents longitudinal data on RBBB in a highly selected cohort of North American men. Thus, caution should be exercised when extrapolating these data to the general population. However, the relatively young age at entry of the majority of this cohort permits more accurate estimation both of the age at onset of RBBB and freedom from preexisting disease than is possible in studies commencing with older age groups. It also provides longitudinal data on this conduction disturbance from a young age. Other cautions include the problem of measuring A QRS in the right bundle. The exact time during the QRS that activation of the right ventricle occurs is not always apparent or predictable for each individual case. Although much of the interest is centered on extremes of axis deviation, the small number of cases in some subgroups of A QRS is also a concern. Another precaution in the extrapolation of these results is the different lengths of follow-up within each age group. Those 30 years of age and younger at detection of RBBB have been under observation for almost 30 years, but the length of follow-up is decreased in older age groups.
The present study demonstrated that the ominous prognosis attributed to RBBB and marked right or left axis deviation in patients with heart disease should not be applied to persons without heart disease who have the same ECG finding recorded on a routine examination.
The prognosis of RBBB with marked axis deviation varies widely in different studies. Excluding the clinical setting following myocardial infarction, total mortality for RBBB and marked left axis deviation has been reported to be zero percent in ten years, 1 percent per year, 7 percent per year, or more than 10 percent per year. The incidence of sudden death for the same conduction disturbance has varied from zero percent in two years or ten years to 1 percent per year to about 4 percent per year. The rate of progression to high degree of AV block has been reported to be zero percent, 1 percent per year, 2 percent per year, and more than 5 percent per year. For RBBB and marked right axis deviation, reported total mortality has been zero percent in ten years, 2 percent per year, and 8 percent per year. Sudden death incidence in this group has varied from zero percent, 2 percent per year, and 5 percent per year. The rate of progression to second- and third-degree AV block in this group is 0.6 percent per year, 2 percent per year, 5 percent per year, or higher.
These differences in mortality and progression of conduction disturbances may be due in part to differences in age as well as to the prevalence of hypertension. The lower incidence rates were usually in studies with a low proportion of heart disease, and the higher incidence rates were in those with a high proportion of heart disease that was often severe. Some investigators concluded that the prognosis of RBBB and extreme axis deviation appeared to be more dependent on the presence or severity of the myocardial disease than on the conductive disease prevailed by My Canadian Pharmacy’s remedies. Our data are consistent with this thesis in that in persons without heart disease, RBBB, and marked left or right axis deviation have a good prognosis offered by My Canadian Pharmacy. Differences between the results of the present study and those of Framingham, which found an increased incidence of coronary heart disease plus heart failure (the data were not analyzed separately for each endpoint) may be due to their greater prevalence of hypertension (>165/ 95 mm Hg). The absence of a relationship between RBBB and hypertension in our study is consistent with the findings in two other cohort studies.
The A QRS in RBBB was different from the A QRS in the absence of this conduction defect in that RBBB was associated with a significant excess proportion of axis deviation, +90° or greater and —45° to —90°. Because the development of RBBB alters only the terminal proportion of A QRS, a case with axis deviation at entry should have had it before development of RBBB. Since many of the cases of RBBB at entry were in their 20s, one can speculate that in adolescence axis deviation, either absolute or relative for age, is predictive of RBBB occurrence. This can be supported by the observation that marked left axis deviation (—45° to —90°) in men 40 to 69 years of age without apparent heart disease is associated with a small but significantly increased probability of RBBB occurrence.
It is unlikely that a single confounding variable or a technical error of measurement was responsible for the association of RBBB and axis deviation, because it should have induced a consistent alteration producing either right or left axis deviation, but not both. Explanations for this association would include the possibilities that RBBB and axis deviation represents a distinct land of bundle branch block. It might result from a single lesion in the proximal part of the right bundle branch or the bundle of His. Alternatively, one factor such as fibrosis could involve the right bundle branch and the anterior and posterior radiations of the left conducting system.
Regardless of the etiology, the A QRS in persons with RBBB and marked axis deviation is not influenced to the same extent by the force-inducing leftward movement of the A QRS with increasing age or the passage of time. This observation may relate to the nature of the lesion in the conduction system or the possibility that the extremes of A QRS represent the outermost limit of the A QRS so that movement decreases once these limits are reached.
Thus, right and left axis deviation occurs more commonly in cases of RBBB than in those without this conduction disturbance, but for the age groups in this study, in the absence of heart disease, this conduction disturbance carries no adverse prognosis.