Alternating bundle branch block: What is the mechanism?

A 60-year-old gentleman with severe left ventricular systolic dysfunction, multivessel coronary artery disease, and chronic kidney disease presented with New York Heart Association (NYHA) class III breathlessness without any history of syncope or presyncope. The electrocardiogram (ECG) showed sinus rhythm with alternating bundle branch block and a variable PR interval. This was most likely due to consistent conduction delay through the left anterior and posterior fascicles, with intermittent conduction delay or block in the right bundle branch. The patient subsequently underwent dual chamber permanent pacemaker implantation, with the ventricular lead placed at left bundle branch area.