Long-term Outcomes of Left Anterior Fascicular Block in the Absence of Overt Cardiovascular Disease

To the Editor: Left anterior fascicular block (LAFB) is considered a benign electrocardiographic (ECG) finding, but its long-term consequences have not been comprehensively studied. Conduction blocks occur due to conduction system fibrosis and are associated with myocardial fibrosis, even in the absence of other cardiovascular disease. Therefore, LAFB may be an immediately accessible marker of left heart fibrosis, a substrate for atrial fibrillation (AF) and congestive heart failure (CHF). We investigated the long-term outcomes of participants with LAFB in the absence of manifest cardiovascular disease in the Cardiovascular Health Study (CHS). Methods. Established in 1989, CHS is a prospective cohort study of individuals aged 65 years or older sampled from Medicare eligibility lists nationwide. Institutional review board approval was waived for the current study. Participants provided written consent and were followed up semiannually. We excluded participants with a history of diabetes, hypertension, coronary disease, myocardial infarction (MI), AF, and CHF. We defined LAFB as a QRS axis between 45 and 90 and QRS duration of less than 0.12 seconds in the absence of left ventricular hypertrophy, inferior MI, and ventricular preexcitation on the baseline ECG. Outcome ascertainment for this study ended on June 30, 2008. We identified AF by study visit ECGs or hospital discharge diagnosis, and CHF was identified by physician diagnosis plus either pharmacological therapy or clinical findings. Deaths were identified from obituaries, the Social Security Death Index, Medicare claims, and participant proxies. Outcomes were compared using Kaplan-Meier estimates and Cox proportional hazards models. Adjusted comparative incidence rates (IRs) obtained from Poisson regressions are reported per 100 person-years. Covariates were chosen a priori or when associated with the predictor and outcome using a P value of less than .10 in the predic-