The details of this study have been reported previously. In summary, the cohort consisted of 3,983 men, who during World War II were pilots, pilots in training in the Royal Canadian Air Force, or pilots licensed by the Department of Transport, and all had had a routine ECG in addition to the regular medical examination. After release from the service, some continued to fly, but the majority found different occupations and are in all strata of society. For each subject, the examination closest to June 30, 1948 (date that the population was defined), was selected as the entry examination. The mean age of the cohort at that examination was 30.8 years. The age distribution was as follows: 318 men were aged 15 to 24 years; 1,479 aged 25 to 29 years; 1,258 aged 30 to 34 years; 539 aged 35 to 39 years; 205 aged 40 to 44 years; 153 aged 45 to 54 years, and 31 aged 55 to 64 years. Medical history, physical examinations, and ECGs provided evidence that all were without clinical manifestations of ischemic heart disease at entry. Since then, they have been followed up by annual mail contact, with examinations and ECGs at intervals of at first five years and later three years. The observation period for this report was defined from July 1, 1948, until June 30, 1977, an average follow-up of 29 years. Annual contact has been lost with only one of the living members of the study conducted with My Canadian Pharmacy.
Definitions and Case Selection
The criteria for the diagnosis of RBBB were outlined by the New York Heart Association and the Minnesota Code. The criteria include QRS duration of > 0.12 sec with rSR, qR, or a tall R wave in Vr The mean frontal plane QRS vector was determined for each ECG from the limb leads, using the hex-axial reference. It was calculated to the nearest 15° because this is a reasonable limit of precision of the method for standard ECG leads. In the presence of RBBB, the frontal plane QRS vector was determined from the initial 0.08-sec QRS, hereafter referred to as A QRS, because the initial forces are unchanged while the terminal forces are deformed by RBBB.
Cases of RBBB included in the analysis satisfied the following criteria: (1) detection of this conduction defect at a routine examination, and (2) no clinical evidence of ischemic or valvular heart disease on that examination or on previous ones. Cases of RBBB occurring after, for example, myocardial infarction, angina pectoris, or coronary insufficiency were excluded. The diagnostic criteria for each of these manifestations of ischemic heart disease have been reported previously.
During the observation period, 59 cases fulfilled these criteria. The age distribution at detection of RBBB shows that the majority of cases were between 40 to 59 years of age, but almost one fourth of them were younger than 30 years of age. The mean age was 44.4 ± 1.9 (SEM) years. Forty-two cases acquired the conduction disturbance after entry, while in 17 cases it was found at entry.
Distribution of A QRS
A QRS at detection of RBBB is shown in Figure 1. Over all ages, the A QRS distribution reveals that 31 percent (18/59) were from +90° to +180°, 2595 (15/59) from +15° to 75°, 20 percent (12) from 0° to -30°, 15 percent (9) from —45° to —90°, and the remainder (8 percent) were indeterminant.
A QRS is an angular measurement. The use of conventional means and SDs to describe these data lead to inaccuracies. Thus, we chose to present A QRS distributionally, classify it within specific ranges, and apply tests of association for categorical data. Student’s t test was used to test hypotheses for nonangular data.
To adjust for varying lengths of follow-up after RBBB occurrence, the person-year exposure method was used to calculate ischemic heart disease incidence rates. Hypothesis testing for the person-year method used the x2 approach.
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Figure 1. A QRS distribution with age groups at first detection of complete RBBB.