Beat-to-Beat Alternating Bundle-Branch Block

A 67-year-old man was admitted to the primary healthcare emergency department due to dizziness. During the physical examination, an irregular heart rhythm was detected, prompting a request for a 12-lead electrocardiogram (ECG). The ECG () revealed a beat-to-beat alternating pattern of right and left bundle branch blocks (RBBB and LBBB) with Mobitz I second-degree atrioventricular (AV) block.

The patient was transferred to the hospital emergency department, where two consecutive ECGs were performed. One ECG () showed persistent LBBB with first-degree AV block (PR interval of 320ms), and the subsequent ECG () indicated persistent RBBB with left anterior fascicular block and fixed 2:1 second-degree AV block.

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