In the absence of technical or rhythmic problems, the device transmits a periodic 30-second EGM to verify the morphological quality of the endocardial signals. The transmission of a periodic EGM sometimes allows diagnosing asymptomatic oversensing episodes (P wave, T wave, double counting of the R wave), lead fracture (atrial or ventricular) or loss of left ventricular capture in CRT patients which would not otherwise be recorded, given that the VT or VF counters are not full. In this patient, one out of 2 intervals is classified as VS (decrements the VT counter by -1), the other is classified as VF (increments the VT counter by +1). The VT counter remains blocked at 0. Similarly, the probability VF counter is never filled. There is, however, a risk of inappropriate therapies in the presence of sinus tachycardia (the VS interval becomes VT1) or premature ventricular contractions.

In rare patients with severe intra-ventricular conduction disorder and wide QRS, the ventricular EGM can exceed the duration of the post-ventricular refractory period resulting in the same signal being sensed twice. Certain drugs (particularly sodium channel blockers for elevated heart rates) or certain metabolic disorders (hyperkalemia) can favor this oversensing by prolonging the duration of the QRS. The double counting of the R wave can also occur in a patient with a dual-chamber ICD, a prolonged PR interval and loss of right ventricular capture. The defibrillator counts both the paced ventricular event and the spontaneous ventricular activity conducted from the atrium. Similarly, in a patient with a triple-chamber ICD and loss of right ventricular capture, the device can count both the ventricular paced event and the right ventricular depolarization arising from left ventricular capture.

The EGM pattern during a double counting of the R wave is relatively characteristic with alternation between 2 ventricular interval durations. The second signal is usually sensed at the end of the ventricular refractory period (the R1R2 interval is exactly equal to the programmed ventricular refractory period or within a limit of  + 20 ms) and always corresponds to the VF zone. The classification of the second interval (R2R1) is contingent on the programming of the tachycardia zones and the heart rate (higher probability of being in the tachycardia zone if the rate is high and the tachycardia zones are programmed low).

The post-ventricular ventricular refractory period is the parameter incorporated in the various ICDs to solve this oversensing problem without jeopardizing the sensing quality of a VF episode. In older platforms, double counting was relatively common especially when this type of device was connected to an integrated bipolar lead. Two reasons were invoked: a very short post-ventricular ventricular refractory period (in the order of 80 ms) and the large spacing of the 2 sensing electrodes on an integrated bipolar lead favoring the prolongation of the duration of the ventricular EGM.