Double sensing can be considered as an exceptional finding in current generation ICDs. However, in patients with marked ventricular conduction delay and broad QRS complex, the sensed EGM duration may exceed the ventricular blanking period and the device may detect one QRS twice. R-wave double counting may occur as a result of local ventricular delay in the baseline state or conduction delay caused by drugs (sodium channel blocking particularly at high heart rate) or electrolyte abnormalities (hyperkalemia). It may also occur in patients with a double or triple lead ICD, long PR interval and loss of RV pacing capture. The ICD may count both the paced ventricular event and the spontaneous R wave conducted from the atrium. Finally, another common cause of double counting is loss of RV capture in CRT-Ds. The device counts both the paced ventricular event and the RV depolarization originating from the LV lead.
R-wave double counting results in alternation of 2 ventricular cycle lengths. This alternation produces a characteristic railroad rack pattern on the graph (also observed during P- or T-wave oversensing). The second component of the R wave is usually sensed as soon as the blanking period terminates (the double counted RV-RV interval is exactly equal to the blanking period or within 20 ms) and is always classified in the VF zone. The classification of the first one depends on the programming of the tachy zones and on the heart rate (more likely to be in the VF zone if slow VF zone and high heart rate). The double counting can manifest during sinus rhythm, only during PVCs or during slow VT with a misclassification as VF, the true rate being overestimated and possibly leading to shocks.
Prolongation of the ventricular blanking period from the nominal value corrects ventricular double counting in the majority of cases and must be proposed as the first step when possible, keeping in mind that a common concern is true VF undersensing when the blanking period is over-extended. Similarly, decreasing the programmed ventricular sensitivity may resolve the problem in a certain number of cases but this option requires that reliable sensing of VF is confirmed at the reduced level of sensitivity. Moreover, lowering ventricular sensitivity may be dangerous and useless since the amplitude of the 2 signals may be as high. Programming of very high VF zone to solve the problem seems also inappropriate. When the blanking period cannot be prolonged adequately, a lead revision or addition of a separate pacing/sensing lead in a DF1 system may be proposed.
In a patient with known intrinsic broad QRS, it is mandatory during implantation to thoroughly evaluate the local intra-cardiac electrogram width and morphology and to verify the absence of double counting. It is also probably better to choose an ICD with a programmable blanking period connected to a true bipolar lead rather than an integrated bipolar lead.