Spike on T wave! What went wrong?

Abstract An 80-year-old male, who previously received a dual chamber pacemaker, was referred to our ambulatory for dizziness and fatigue. The ECG obtained showed sinus rhythm, highly variable atrioventricular (AV) interval and alternation between spontaneous and paced ventricular complexes. A spike on the ascending part of the T wave was observed, suggesting ventricular undersensing. However, telemetry-supported pacemaker control showed inconstant atrial undersensing.