Left anterior fascicular block is associated with non-ischemic myocardial scar and proportionately decreased ejection fraction

Abstract Background Left anterior fascicular block (LAFB) has been associated with increased mortality, but the underlying causes are unknown. Objectives To determine whether LAFB is associated with increased left ventricular (LV) scar burden and reduced LV ejection fraction (LVEF). Methods LAFB patients (n=51) and matched control patients (n=600) were retrospectively enrolled. Both groups had been referred for cardiovascular magnetic resonance imaging (CMR) and electrocardiography (ECG). They were compared regarding size and location of LV scar, LVEF, and a dysfunction index describing the difference between measured LVEF and expected LVEF based on scar size. Results Patients with LAFB had on average a larger LV scar (median [interquartile range] 0.7 [0.0-6.6] vs 0.0 [0.0-1.5] % LV mass, p<0.001). LAFB was associated with a higher prevalence of any scar (59% vs 33%, p<0.001). The groups had similar prevalence of ischemic scar (29% vs 23%, p=0.40) but LAFB patients a higher prevalence of non-ischemic scar (29% vs 10%, p=0.001) most frequently located in the basal and mid inferoseptal segments and the anterior and lateral apical LV segments. LVEF was lower in LAFB than in controls (58 [43-60] vs 60 [55-60] %, p=0.02). There was no difference in dysfunction index (24.0 [17.8-25.5] vs 24.0 [19.0-27.8] %-points of LVEF, p=0.32). Conclusions In a matched cohort, LAFB was associated with a small decrease in LVEF that was proportionate to the increased LV scar burden, which was more commonly of non-ischemic etiology and not infarction, and not more commonly located near the expected course of the left anterior fascicle.