Double counting of the R wave is rare in the latest generation of defibrillators. In a few rare patients with major intraventricular conduction disorders and wide QRS complexes, the ventricular EGM may exceed the ventricular blanking period (post-detection refractory period) and the same signal may be detected twice. Certain drugs (sodium channel blockers, particularly for high heart rates) or certain metabolic disorders (hyperkalemia) can promote this overdetection by lengthening the QRS duration. Double counting of the R wave can also occur in patients with a dual-chamber defibrillator, prolonged PR interval, and loss of right ventricular capture. The defibrillator counts both the stimulated ventricular event and the spontaneous ventricle conducted from the atrium. Similarly, in a patient with a triple-chamber defibrillator and loss of right ventricular capture, the device may count both the stimulated ventricular event and the right ventricular depolarization originating from left ventricular capture.

The appearance of the EGM during a double counting of the R wave is relatively characteristic, with an alternation between two ventricular cycle durations. This alternation is associated with a rail-like appearance on the graph (two distinct lines corresponding to the two cycles R1R2 and R2R1). The second signal (R2) is generally detected at the end of the blanking period (the R1R2 interval is exactly equal to the programmed blanking or within a limit of +20 ms) and always corresponds to the VF zone. The classification of the second cycle (R2R1) depends 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 if the tachycardia zones are programmed low). Double counting can occur on a sinus rhythm, on a ventricular extrasystole, or only on slow VT (often wide QRS; risk of misclassification in the VF zone and unnecessarily aggressive therapies).

Extending ventricular blanking, when this parameter is programmable, generally eliminates this problem of overdetection and should be the first option considered, bearing in mind that excessive extension may lead to an increased risk of underdetection of true VF. 

Decreasing ventricular sensitivity can sometimes solve the problem, but this option may also lead to a risk of VF underdetection. Furthermore, this option is often ineffective because the second ventricular signal may be at least equal in amplitude to the first. Setting a very high VF zone to avoid inappropriate therapies in this context does not seem appropriate either. In rare cases where the blanking period cannot be extended sufficiently, implantation of a new pacing/defibrillation lead may be proposed.

In patients with a very wide QRS complex, it is essential during implantation to carefully analyze the appearance and width of the intracavitary ventricular depolarization and to verify the absence of any ventricular double counting. It is also probably more appropriate in this context to implant a defibrillator with programmable ventricular blanking connected to a dedicated bipolar lead (rather than an integrated bipolar lead, which promotes double counting).