A small study followed up an older cohort of patients with isolated left anterior fascicular block on their ECG (isolated left axis deviation), without clinically manifest cardiovascular disease.
LAFB was associated with an increased risk of atrial fibrillation, heart failure and death. LAFB is caused by conduction tissue fibrosis, and is a marker of other left heart fibrosis. The patients did not go on to develop left bundle branch block, and only 2 of 39 required pacing in 10 years, suggesting these outcomes were not due to progression of conduction disease.
Long-term Outcomes of Left Anterior Fascicular Block in the Absence of Overt Cardiovascular Disease
JAMA. 2013 Apr 17;309(15):1587-8