In Biotronik the VF counter 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).
The specifics of the VT counter must be perfectly integrated in order to allow optimal programming. Indeed, for BiotronikTM devices, the counting method differs completely between the VT zone (up and down counter) and the VF zone (probabilistic counter), which is not the case for the ICDs of other manufacturers (with the exception of MedtronicTM ICDs).
The various ventricular events are classified according to the RR intervals in the paced ventricle (PV), sinus zone (VS), VT1, VT2 or VF. The low frequency of each zone is programmable, the different zones being continuous. It is therefore possible to program a single sensing zone (VF), 2 zones (VF + VT1) or 3 zones (VF + VT2 + VT1). An interval sensed in the VF zone increments the VT2 and VT1 counter (+1) as well as an interval in the VT2 zone for the VT1 zone. Indeed, the sensing of an interval in a tachycardia zone increments the counter of the slower tachycardia zones by 1. Conversely, an interval in the sinus zone decrements the VT1 and VT2 counter(s) (-1) as well as an interval in the VT1 zone for the VT2 zone. It is therefore an “up and down” counter. For the Hyperia platform, the VT1 counter is programmable between 10 and 100 intervals, the VT2 counter between 10 and 80 intervals. A counter reaching the required number of intervals necessitates diagnosis and associated therapies.
The “up and down” counter accurately detects monomorphic and regular ventricular tachycardias between 150 and 200 beats/minute with consecutive intervals sensed in the same tachycardia zone. This counter can also function efficiently for faster tachycardias (between 200 and 230 beats/minute), which are more likely to be polymorphic and therefore at increased risk of undersensing. Indeed, an interval classified as VS in conjunction with an undersensing does not reset the VT counter to 0 (difference with MedtronicTM ICDs) but decrements the latter only by 1; an intermittent undersensing thus delays but does not prevent the detection.
In clinical practice, different questions arise when choosing the number of zones to program and whether or not to program one or two VT zones (in addition to the VF zone):
1. which arrhythmia counter does one wish to use?
This issue is important for BiotronikTM devices. Indeed, the operation of the VT counter and the VF counter differs completely, which is not the case for the Boston ScientificTM, AbbottTM or LivanovaTM devices (this issue therefore does not intervene in the choice of the number of zones to be programmed on these devices). For a BiotronikTM ICD, in contrast, setting the limit of the VT or VF zone also sets the type of counter used (“up and down” counter or probabilistic counter) which can have an influence on the ability to effectively detect polymorphic ventricular arrhythmia.
2. in which zone is it possible to discriminate the origin of the arrhythmias?
The latest guidelines recommend discriminating the origin of the arrhythmias (VT versus SVT) up to very high rates (230 beats/minute). This issue is therefore central to the choice of the number of zones and zone limits for BiotronikTM devices. Indeed, it is not possible for these devices to discriminate in the VF zone. Programming a relatively low VF zone (in the order of 200 beats/minute) would therefore considerably limit the possibility of discriminating the origin of the arrhythmias.
3. should different therapies be programmed according to the frequency of the tachycardias?
Traditionally, it was customary to program several antitachycardia pacing sequences (between 3 and 6) for tachycardias between 150 and 200 beats/minute before eventually delivering one to several shocks and to deliver electric shocks without delay as first line treatment for faster tachycardias. Today, antitachycardia pacing is considered the first-line therapy for tachycardias up to rates of 230-250 beats/minute. It could thus be useful to program a second VT zone (VT2) between 200 and 230 beats/minute to allow initially treating these tachycardias by one or more antitachycardia pacing sequences.