Unusual atrial response to electrical pacing during ischemia

A 76-year-old woman with a dual chamber pacemaker underwent coronary intervention of the right coronary artery for unstable angina. After the procedure, she had recurrent chest pain, and after the repeat angiography showed reduced coronary flow in the distal vessel, she underwent emergent coronary artery bypass grafting. During this course of events, the stimulation-excitation interval of the atrium was prolonged and paralleled with the severity of the ischemia. Additionally, a transient but unusual electrophysiological phenomenon was observed that the atrium could be captured at a rate of 90 ppm, but not 80 ppm. This phenomenon might be caused by phase 4 depolarization or rate-dependent facilitation of conduction in the ischemic atrium.