Regular tachycardia despite Wenckebach atrioventricular conduction

We present a 21-year-old woman status post orthotopic heart transplantation initially presenting with a regular narrow complex tachycardia at 159 beats/min. With intravenous diltiazem the rhythm transitioned to a regular tachycardia at 106 beats/min, 2/3rd of the initial heart rate. We demonstrate this to be a novel description of 3:2 second-degree Mobitz type I atrioventricular block (Wenckebach) with the absence of the hallmark regularly irregular (grouped beating) pattern.