PR Interval Alternans and the Mystery of Inverse Decremental Conduction.

CASE SUMMARY: A 73-year-old man with severe aortic stenosis and preexisting right bundle branch block (RBBB) underwent transcatheter aortic valve replacement. Postprocedural day 4 mobile cardiac outpatient telemetry revealed intermittent atrioventricular block. A 12-lead electrocardiogram obtained in the emergency department revealed PR interval alternans with RBBB, with longer PR following longer preceding RP intervals, and shorter PR following shorter RP intervals. He suddenly experienced cardiac arrest due to proximal atrioventricular block, requiring cardiopulmonary resuscitation.

TAKE-HOME MESSAGES: Inverse decremental conduction, known as Yan conduction, demonstrates an inverse relationship between conduction time and frequency of upstream stimulation. Inverse decremental conduction within the left bundle branch in the setting of RBBB may manifest as PR alternans, heralding proximal atrioventricular block.