This patient developed several episodes of T wave oversensing during exercise, prompting the delivery of shocks or of interrupted charges of the capacitors. The systematic oversensing of a supernumerary cardiac signal causes the sensing of 2 signals of different morphology for a single cardiac cycle with alternation between a short and a longer cycle. This form of oversensing is facilitated by exercise, as effort may be associated with a decrease in the R wave and an increase in the T wave amplitudes. T wave oversensing after a spontaneous ventricular event is more likely to occur in presence of a low-amplitude R wave, as in this patient’s case. The sensitivity and the gain are automatically set, based on the amplitude of the previously sensed R wave: when that amplitude is low, the likelihood of rapidly reaching the following level of maximum sensitivity is high. This limits the risk of rectifying this oversensing by a decrease in the ventricular sensitivity that might cause undersensing of VF. These difficult situations may result in programming compromises. The various settings can be modified separately (these settings can be accessed by a code known by the staff of Biotronik) or one can program an enhanced suppression of the T wave, which, compared with standard programming, increases the upper threshold to 75% of the R wave amplitude (instead of 50% with standard programming) and increases the high-pass filter to 32 Hz to filter the T wave, which is a lower-frequency signal than the R wave. After this had been chosen for this patient, the proper detection of VF was verified by an induction procedure.

This tracing also illustrates the potential danger represented by the delivery of inappropriate shocks, which resulted in a run of VT. In other instances, VF can be triggered by a shock delivered in the vulnerable period of a spontaneous QRS.