VT counter

The specifics of the VT counter must be perfectly integrated in order to allow for an optimal programming. Indeed, for MedtronicTM devices, the counting method differs completely between the VT zone (consecutive intervals) and the VF zone (probabilistic counter), which is not the case for the ICDs of competing manufacturers (except for BiotronikTM devices). An interval classified in the VT zone increments the VT counter by +1; an interval classified in the VF area does not alter the VT counter (no increment, no decrement); a long interval classified as VS resets the VT counter to 0.
This counter has been specifically developed to allow optimal functioning for tachycardias with a rate of less than 200 beats per minute, with a dual objective: 1) to efficiently detect episodes of monomorphic and regular ventricular tachycardia; since all signals have the same amplitude, the risk of undersensing is limited; 2) ensure a first level of “discrimination” during an episode of atrial fibrillation.

The VT counter is filled after a programmable number of consecutive intervals classified in the VT zone (tolerance for VF intervals which do not change the counter). Any interval classified as VS resets the VT counter to 0, which has several consequences: 1) an inappropriate return of this counter to 0 occurs as a result of a ventricular undersensing (common in polymorphic ventricular tachycardia). This counter is therefore probably not suitable for tachycardias greater than 200 beats/minute where some of the arrhythmias are polymorphic with signals of varying amplitude; 2) this counter allows improving the specificity of the diagnoses, an episode of atrial fibrillation being readily associated with the presence of iterative long intervals (frequent return of the VT counter to 0).

This specificity is therefore a major advantage of this type of counter. The risk of inappropriate therapies due to atrial fibrillation is significantly reduced, especially since there is a trend suggested by the guidelines for an increase in the number of intervals required for the diagnosis of VT.  In order for an episode of AF, occurring at a rate corresponding to the VT zone, to bring about the occurrence of inappropriate therapies, it must occur in conjunction with: 1) a number of consecutive intervals in the VT zone (30 in the new guidelines) without any long interval classified as VS which is relatively rare when the lower limit of the VT zone is programmed between 150 and 160 beats/minute; indeed, any interval classified as VS resets the VT counter to 0; 2) a discrimination error by PR Logic and by Wavelet, which operates in a second instance and can correct a diagnostic error. This explains the relatively low risk of inappropriate therapies due to AF in the VT zone.

There is, however, a diagnostic delay generated by the functional specificity of this counter. Sometimes, ventricular tachycardia is irregular and oscillates with the lower limit of the VT zone. When an interval is classified as VS, the VT counter is reset to 0 even though the diagnosis is obviousy VT when looking at the EGM. It is therefore essential to program a sufficient margin with regard to the rate of clinical tachycardia in order to avoid this type of problem. It is common to program the VT zone 10 to 20 beats per minute slower than the clinical VT, this margin probably needing to be slightly increased for MedtronicTM devices, the occurrence of a cycle classified as VS not having the same consequences compared to other competing devices. This specificity also implies that the VT counter not be used for tachycardias greater than 200 beats per minute, the risk of intermittent undersensing being increased (polymorphic tachycardias more frequent) and resulting in a delay or absence of diagnosis and therefore of treatment. The probabilistic VF counter appears to be more appropriate for this type of tachycardia.
In the latest international guidelines, there are marked differences in terms of advice for setting the limits of the detection zones according to device manufacturers. The different functioning of the counters explains some of these differences (in primary prevention, MedtronicTM has a single VF zone at 188-200 beats/minute, Boston ScientificTM a VT zone up to 230 beats/followed by a VF zone).

In the Medtronic Encyclopedia (see Bookstore) there are multiple examples shown which clarify the abovementioned specificities.

VF counter

It is a probabilistic counter (fast X/Y intervals) which requires a minimum of 75% fast intervals (programming possibilities: 9/12, 12/16, 18/24, 24/32, 30/40, etc). A ventricular fibrillation is, by definition, an arrhythmia that is fast, disorganized, chaotic, with ventricular signals of low and/or variable amplitude. All of these characteristics increase the risk of undersensing with occasional signals with amplitude below the detection threshold (nominal value of 0.3 mV) and often a high variability in beat-to-beat amplitude, which is problematic given that, the device uses a sensitivity level which adapts to the amplitude of the preceding signal. This 75% ratio between fast and slow intervals was chosen so as to obtain an optimal balance between the proper detection of ventricular fibrillation (necessary tolerance for a certain number of long pseudo-intervals generated by undersensing) and the need to not fill the counters in the presence of an oversensing of T, P or R waves (frequently associated with a 50% ratio of short intervals.


After diagnosis of VF and associated treatment, the device must then differentiate between termination of the arrhythmia episode and inefficiency of the shock with continuation of the arrhythmia, using two different counters: 1) the redetection counter which is programmable; as in the case of the initial counter, this is a probabilistic counter with a ratio of 75% (6/8, 9/12 ... 30/40); the number of required intervals applies to all subsequent shocks (from 2 to 6) during the same episode; it is customary to program the redetection to a lower number of required intervals than for the initial detection, the risk of undersensing increasing with the duration of the arrhythmia; (2) the end-of-episode counter which is based on 2 criteria: a) the device diagnoses the end of the episode when 8 consecutive VS or VP intervals slower than the lowest programmed detection zone (VF or VT) are detected; b) it also diagnoses the end of the episode if, for 20 seconds, the median of 12 consecutive intervals is always slower than the programmed lowest detection zone (VF or VT).
Of note, once the redetection counter is filled, the shock will automatically be delivered at the end of the charging period by synchronizing itself on a sensed complex or asynchronously if no interval is detected (committed shock; no confirmation phase at the end of the charging period).

Fast VT counter

Three detection and therapy zones can be programmed on a Medtronic device (VF, FVT, VT). It is usual and recommended to program a single VF zone from 187 to 200 beats/minute in primary prevention and to add a VT zone 20 beats per minute slower than clinical tachycardia in secondary prevention. In clinical practice, the question arises as to the value of programming a third intermediate zone of FVT. Different questions arise when choosing the number of zones to be programmed and whether or not to program an additional FVT zone (in addition to VF and VT):

1. which of the arrhythmia counters to use?
This question is essential for MedtronicTM devices. Indeed, the functioning of the VT counter and the VF counter differs completely, which is not the case for the Boston ScientificTM, AbbottTM or LivanovaTM devices (hence, this question does not intervene in the choice of the number of zones to be programmed on these devices). In a MedtronicTM device, by contrast, setting the limit of the VT or VF zone also sets the type of counter used (consecutive intervals or probabilistic counter), which will have a decisive influence on the ability to effectively detect a polymorphic ventricular arrhythmia. As explained earlier, the VT counter was designed for monomorphic tachycardias and not for tachycardias> 200 beats/minute, which encompass both monomorphic as well as polymorphic tachycardias. When a FVT zone is programmed, it is possible to select a program via VT or via VF. Accordingly, the VT counter (consecutive intervals) or the VF counter (probabilistic) is used. When the ‘via VF’ option is preferred (desirable) for a zone > 200 beats per minute), choosing the option of the 3 zones (adding a FVT zone) therefore does not modify the counting method comparatively to the option with 2 zones (VT + VF). Indeed, the VF counter is used for all tachycardias greater than 187-200 beats/minute for both options.

2. in which zone is it possible to discriminate the origin of the arrhythmias?
The latest guidelines advocate discriminating the origin of arrhythmias (VT versus SVT) up to very high rates (230 beats/minute). This question is therefore central to the choice of the number of zones and limits of the zones for all of the other manufacturers. Indeed, it is not possible for these latter devices to discriminate in the VF zone. Programming a relatively low VF zone (in the range of 200 beats per minute) would therefore considerably limit the possibility of discriminating the origin of the arrhythmias. Such question does not arise in these terms for MedtronicTM devices. It is indeed possible to discriminate (PR Logic, Wavelet) even in the VF zone. The discrimination limit is programmed independently of the detection zone. The possibility of discriminating therefore does not affect the choice of the option of 2 or 3 zones.

3. should a zone be programmed with intermediate therapies?
Traditionally, it has been customary to program several anti-tachycardia pacing (ATP) sequences (between 3 and 6) in the VT zone before eventually delivering one to several shocks and to deliver electrical shocks without delay as first therapy in the VF zone. It may therefore be useful to add an intermediate FVT zone (for tachycardias between 200 and 230 beats/minute) with the programming of one to two tachycardia pacing sequences before delivering the shocks. Indeed, anti-tachycardia pacing is now considered as first-line treatment for tachycardias less than 230-250 beats/minute. However, the possibility of delivering an ATP during charging or before charging in a VF zone has reduced the interest of programming this intermediate FVT zone. Therefore, its justification is currently only to be able to program more than one anti-tachycardia pacing sequence.
In summary, programming 3 zones for secondary prevention (e.g. VT from 160 to 200, FVT via VF from 200 to 230 and VF over 230 beats/minute) compared to 2 zones (VT from 160 to 200 and VF over 200 beats/minute) does not change the method of counting, nor the discrimination ability, and may simply allow adding 1 or 2 anti-tachycardia pacing sequences to the FVT zone.

Combined counter

In a MedtronicTM ICD, VT and VF counters operate independently of each other, such that an interval classified as FS does not increment the VT counter as opposed to the functioning of certain competitors’ devices. This may therefore theoretically lead to a delay in diagnosis when an arrhythmia fluctuates between the VT zone and the VF zone as observed in this example. The combined counter was designed to resolve this issue and is always active (non-deprogrammable) if a VT zone and VF zone have been programmed. The combined counter is triggered systematically after the detection of 6 intervals classified as FS (essential prerequisite); the combined counter sums the intervals classified as TS and FS and is filled when 7/6 (non-modifiable ratio) of the number of intervals required in initial detection in the VF zone have been detected; in this example, this parameter was programmed to 30/40, the combined counter was therefore filled after 35 intervals classified as TS or FS (7/6 of 30 = 35). When the combined counter is full, the device analyzes the last 8 ventricular intervals; if at least one of these intervals is classified as FS, the device diagnoses a VF and the first therapy of the VF zone is delivered; if the last 8 cycles are classified as TS, the device diagnoses a VT and the first therapy of the VT zone is delivered.
The combined counter therefore prevents a delay in diagnosis when the tachycardia oscillates between the VT zone and the VF zone.