T wave oversensing and electrical shock

N° 18
Manufacturer Abbott Device ICD Field Sensing

This 27-year-old woman underwent the implantation of an Epic single chamber ICD after she suffered an episode of aborted sudden cardiac death. She was seen after having received an electrical shock while exercising.

Main programmed parameters

  • VF zone at 222 bpm and VT zone at 182 bpm
  • 12 cycles in each the VF and VT zones were needed for the diagnosis
  • Maximum sensitivity programmed at 0.3 mV
  • VF zone: single 20-J shock followed by five 30-J shocks (maximum strength); VT zone: set on monitor
  • Effective discrimination in the VT zone
  • VVI pacing mode at 50 bpm; VVI post-shock pacing mode at 40 bpm
Graph and trace


Episode diagnosed as VF and delivery of 20-J shock. The very short charge time shown in the text (1.2 sec) was the complement to a previous charge that had been interrupted.


1: Diagnosis of VF after 12 F classified cycles that are not visible. The diagnosis of T wave oversensing is evident; beginning of the capacitors charge. At an amplification of 2.4 mm/mV, the T wave amplitude is greater than the R wave;
2: 4 consecutive VS cycles without oversensing: insufficient to fill the return of sinus rhythm counter, which was programmed at 5;
3: The short cycles that follow a VS during the charge are labeled R and are underscored;
4: Diagnosis of return of sinus rhythm after 5 consecutive VS cycles, and interruption of the charge;
5: Further oversensing of T wave;
6: New diagnosis of VF (12 F classified cycles since the previous return of sinus rhythm);
7: End of charge, which was short because the capacitors were already partially charged and the previous charge was incompletely dissipated. At the end of the charge, the return of sinus rhythm counter and the confirmation with a search for 2 short cycles were competing;
8: 4 consecutive or intervening VS with unlabeled intervals: insufficient to fill the return of sinus rhythm counter;
9: Confirmation of arrhythmia and delivery of shock;
10: Absence of oversensing following the shock, as the release of catecholamines caused by the shock probably increased the sensed R waves amplitude. While the amplitude of the T wave was unchanged and stable, the likelihood of T wave oversensing was greater if the preceding R wave was small; return of sinus rhythm diagnosed.


T wave oversensing occurs preferentially during exercise, since effort is often associated with a decrease in the R wave amplitude and an increase in the T wave amplitude, with acceleration of the slew rate, placing the T wave in the bandwidth of sensed ventricular events. The T wave was, thus, mistaken for an R wave, hence the erroneous diagnosis of VF. This episode was associated with the charge of the capacitors without a shock being delivered but followed by redetection, this time with the delivery of an electrical shock. The number of cycles that determines the return of sinus rhythm is a key programming point when confronted with this issue since, in presence of intermittent oversensing, it directly influences the likelihood of inappropriate therapy.