Right Bundle Branch Block is Associated with Large Anterior Scar
Abstract & Commentary
By Andrew J. Boyle, MBBS, PhD
Assistant Professor of Medicine, Interventional Cardiology, University of California, San Francisco
Source:Strauss DG, et al. Right, but not left, bundle branch block is associated with large anteroseptal scar. J Am Coll Cardiol 2013;62:959-967.
The left bundle branch supplies a coordinated electrical impulse to the left ventricle. In patients with acute chest pain, left bundle branch block (LBBB) that is new or presumably new is treated as an equivalent of ST elevation. However, the left bundle usually has dual blood supply from both the left anterior descending (LAD) and right coronary artery (RCA). Therefore, an acute myocardial infarction (MI) due to LAD occlusion should be less likely to cause LBBB and more likely to cause right bundle branch block (RBBB). To test this hypothesis, Strauss and colleagues studied the electrocardiograms (ECG) and magnetic resonance imaging (MRI) with late gadolinium enhancement (LGE) to assess the relationship between anterior wall scar and LBBB. They studied 233 patients with left ventricular ejection fraction (LVEF) < 35% receiving implantable cardioverter defibrillators (ICDs) for primary prevention. In addition, they studied 20 patients undergoing alcohol septal ablation for hypertrophic obstructive cardiomyopathy (HOCM) as a model for acute MI involving the proximal septum. All underwent MRI and ECG after the procedure.
Their cohort of 233 had a mean age of 57 years, was 77% male, a mean LVEF of 27%, and 56% were ischemic in etiology. Forty-five patients had LBBB and 19 had RBBB. There were no differences between the LBBB and RBBB groups in terms of age, gender, ethnicity, heart failure class, or LVEF. On MRI with LGE, patients with RBBB had larger scars than those with LBBB (24.0% vs 6.5% of the LV; P < 0.0001) and patients with non-specific left ventricular conduction delay or QRS < 120 ms had intermediate scar size (12.9% and 14.4%, respectively). In addition, those with RBBB (compared with LBBB) were more likely to have ischemic heart disease (79% vs 29%; P < 0.0001). When considering only the ischemic cardiomyopathy patients, a larger scar size persisted among those with RBBB vs those with LBBB (28.5% vs 14.7%; P = 0.006). Among non-ischemic patients, none of the RBBB patients had absence of detectable scar, whereas 20 of 32 (63%) LBBB patients had no detectable scar (P = 0.017). In the alcohol septal ablation cohort, 15 of 20 patients (75%) developed RBBB, but none developed LBBB. The authors conclude that in patients with LVEF < 35%, RBBB is associated with significantly larger scar size than LBBB, and occlusion of a proximal LAD septal perforator causes RBBB. In contrast, LBBB is most commonly caused by non-ischemic pathologies.
Commentary
This study examines the relationship between extensive scar in the LV and bundle branch block. The major finding is that LBBB is more often related to non-ischemic cardiomyopathies and RBBB is associated with ischemic cardiomyopathy with large anteroseptal scar. It is LBBB, not RBBB, that is included as a guideline to activate the cath lab during acute chest pain. This study is consistent with prior pathological studies that show RBBB is much more commonly associated with anterior MI. It is important to emphasize that the authors studied patients with chronic LV dysfunction and their results may not directly relate to patients with acute MI. There may have been significant bias in their study, with patients suffering larger infarcts dying before they could receive their ICD. In addition, their acute study of alcohol septal ablation involved occlusion of a septal perforator only and does not accurately represent the clinical presentation of anterior MI. It should be considered proof of principle that proximal septal ischemia more often results in RBBB and does not rule out the possibility of acute anterior MI existing with LBBB. How should the clinician incorporate these data into their daily management of patients? This should increase our index of suspicion for acute anterior MI when a patient with chest pain presents with a new (or presumably new) RBBB on their ECG. We should not abandon treating LBBB as potentially ischemic, because patients with chronic LBBB can subsequently develop acute MI.