Transient high-grade atrioventricular block in an orthotopic cardiac transplant recipient

A 56-year-old male with a history of orthotopic cardiac transplantation secondary to cardiac sarcoidosis presented with recurrent episodes of syncope preceded by dizziness. While on telemetry, he had transient episodes of high-grade atrioventricular (AV) block that reproduced his prodrome. After excluding allograft rejection, ischemia, recurrent cardiac sarcoidosis, and vagally mediated block as a cause of high-grade AV block, adenosine testing was done which reproduced the spontaneous high-grade AV block. We concluded that hypersensitivity to endogenously released adenosine was the likely mechanism of AV block in our patient. This is the first reported case of adenosine testing in a cardiac transplant patient for diagnosing episodic high-degree AV block of unclear etiology.