Scenario: A 45-year-old male with a medical history of lumbar radiculopathy, coronary artery disease, and myocardial infarction with subsequent bradycardia that required an implantable dual-chamber pacemaker arrived at the emergency department complaining of lower back pain. Because of his cardiac history, a resting 12-lead electrocardiogram (ECG) was obtained, but it was evaluated after his back pain was treated and he was discharged. After a cardiologist read the ECG, the patient was immediately called and encouraged to schedule an urgent appointment with his cardiologist. Why was the patient called?

This ECG shows atrial pacing at 70/min with subtle pacemaker spikes on the ascent of the T wave in leads V2 through V6. This dual chamber pacemaker is malfunctioning because it is undersensing the QRS complex and fails to be inhibited by the intrinsic cardiac rhythm.

The rare phenomenon of R-on-T occurs...

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