Scenario: A 56-year-old male complained of intermittent palpitations and fatigue. A 24-hour Holter study was ordered. Below are 2 rhythm strips in lead V1; the top is the patient’s baseline and the lower strip is an arrhythmia.

Top strip: Sinus tachycardia at 100/min with incomplete right bundle branch block (RBBB). Bottom strip: Transient atrial fibrillation (AF) with a rapid ventricular response and aberrant conduction with an RBBB pattern.

During sinus rhythm, the right bundle is conducting the electrical impulse more slowly than normal, resulting in a QRS that is slightly wider than normal but not wide enough to meet criteria for a complete RBBB. During the arrhythmia, the rhythm is irregular, with a variable rapid rate (300/min to 100/min) and an absence of P waves; all supporting the diagnosis of AF. In addition, the QRS is wide and upright, suggesting aberrancy. During aberrant conduction, electrical conduction...

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