Masquerading Bundle Branch Block: A Critical ECG Pattern Hidden in Plain Sight.

Masquerading bundle branch block (MBBB) is a rare and often overlooked electrocardiographic (ECG) pattern characterized by right bundle branch block (RBBB) morphology in the right precordial leads combined with features resembling left bundle branch block in the limb leads and/or the left precordial leads. This pattern reflects bilateral conduction system disease and carries a significant risk of progression to high-grade atrioventricular block (AVB). We report the case of an 84-year-old woman admitted for acute decompensated heart failure with preserved ejection fraction and atrial flutter. Her ECG showed wide QRS complexes with RBBB features in the precordial leads, including an rsR' pattern in leads V1-V2 and slurred S waves in leads V5-V6, along with absent S waves in leads I and aVL, consistent with the standard type of MBBB. Transthoracic echocardiography revealed concentric left ventricular hypertrophy with preserved systolic function and elevated filling pressures. After successful cardioversion of the atrial flutter, the electrophysiological study confirmed trifascicular block with a prolonged His-ventricular (HV) interval at 68 ms. Given the absence of clinical symptoms and an HV interval remaining below the 70-ms threshold, the patient was deemed suitable for discharge. However, in light of the known risk of progression to high-grade AVB in patients with MBBB, a close follow-up was scheduled, including planned reassessment and consideration for pacemaker implantation if indicated. This case underscores the importance of recognizing MBBB, a pattern that is frequently overlooked despite its potential prognostic value. It remains insufficiently addressed in current guidelines. Enhancing awareness of this underrecognized entity is essential to ensure timely diagnosis, appropriate risk stratification, and optimal management.