Complete right bundle branch block of unique behavior: what is the mechanism?

A 59-year-old hypertensive man presented to the emergency department with an episode of dizziness without syncope. A surface 12-lead electrocardiogram (ECG) showed sinus rhythm at 82 bpm with a 2:1 complete right bundle branch block (RBBB) pattern (Fig. 1). The patient presented a full spontaneous recovery and was subsequently discharged and referred for ambulatory diagnostic work-up. Echocardiography presented moderate septal hypertrophy and no other structural abnormalities. Continuous 24-h 3-lead Holter monitoring was performed which revealed sinus rhythm with narrow QRS complexes at 42 bpm (Fig. 2A), 2:1 RBBB at 60 bpm (Fig. 2B) and 1:1 RBBB at 38 bpm (Fig. 2C). Following this, an exercise stress test was carried out during which a 3:2 RBBB pattern was observed at the maximum heart rate of 116 bpm (Fig. 3). What is the mechanism explaining the different patterns of transient RBBB observed and how can one relate this to the underlying sinus rate?