This first tracing allows detailing the operation of the VF counter used in Biotronik^TM ICDs. It is a probabilistic counter (fast X/Y intervals) with various programming options depending on the age of the device (for the Iperia 6/8, 8/12, 10/14, 12/16, 16/20, 18/24, 20/26, 22/30, 24/30, 30/40 ICD platforms). Ventricular fibrillation is, by definition, a rapid, disorganized, chaotic arrhythmia, with low and/or variable amplitude ventricular signals. All of these characteristics favor the risk of undersensing with sometimes amplitude signals below the sensing threshold (nominal value of 0.8 mV) and often a high beat-to-beat variability in amplitude which may detract the sensing circuit given the use by the ICD of a sensitivity level which adapts according to the amplitude of the preceding signal. The different counter values ​​were chosen so as to obtain an optimal balance between the accurate sensing of ventricular fibrillation (necessary tolerance of a certain number of long pseudo-intervals generated by undersensing) and the need not to fill the counters in the presence of T wave, P wave or R wave oversensing (frequently associated with a 50% short interval ratio).

In this patient, the VF counter was programmed at 18/24 (75% ratio) and the tracing showed 2 intervals classified as VS during the initial sensing without any significant impact on the sensing of the episode, the probabilistic counter tolerating a maximum ratio of 25% of long intervals.  The first VS interval was related to an intermittent undersensing, the second with a slower interval than the limit of the lower zone of the programmed VF zone. The detection quality of a ventricular arrhythmia is multiparametric and is contingent on:

1. the filters and the amplification which vary according to the manufacturers;
2. the amplitude of the ventricular signals with a limit set by the programmed sensitivity (nominal value 0.8 mV for Biotronik^TM devices);
3. the variability in signal amplitude: certain characteristics of the adjustment of the sensitivity level during the cardiac cycle (adjustment percentage and adjustment delay) are independently programmable;
4. the number of intervals required to fill the counter which is programmable; as seen previously, the percentage of fast intervals (X/Y ratio) also varies slightly depending on the programming (66 to 80%);
5. the limit of the programmed zones and the type of counter used; one of the features of the devices of this manufacturer is that the counting method differs completely between the VT zone and the VF zone.