An 81-year-old is man admitted to the ICU with staphylococcus endocarditis of his bioprosthetic aortic valve, with large valvular vegetations and cerebral septic emboli:

• Sinus rhythm with Mobitz II second degree AV block (3:2) – Ventricular rate 74/min
• Intermittent right bundle branch block pattern secondary to phase IV block

Comment: This is an example of aberration occurring after a longer R-R interval (phase IV block). The mechanism is most likely a gradual spontaneous reduction of negativity during phase 4 of the action potential within the conduction fibers in a diseased heart, that in this case affects predominantly the right bundle.