Fast TV counter

N° 9
Manufacturer Medtronic Device ICD Field Counter

Patient implanted with a single-chamber ICD (Virtuoso II VR) for ischemic cardiomyopathy; 3 zones were programmed (VT between 360 to 300 ms, FVT between 300 and 260 ms, VF encompassing the FVT zone from 300 ms onward); the FVT zone is programmed via VF with an initial counter at 18/24; episode classified as FVT stored in the device memory which allows highlighting the specificities of the functioning of this fast VT zone.

Graph and trace

The graph shows an initial normal rhythm followed by a sudden acceleration with intervals essentially detected in the FVT zone; detection of an episode of FVT treated successfully by a burst.

  1. the EGM shows a regular, monomorphic tachycardia detected in the FVT zone; the dot after the F on the marker indicates that the device is programmed with a FVT via VF zone;
  2. the VF counter is filled with 18 intervals out of 24 classified as FS or TF; the analysis of the 8 intervals preceding the diagnosis reveals only TF intervals without FS interval leading to the diagnosis of FVT;
  3. the first therapy of the FVT zone, a burst, is delivered;
  4. termination of arrhythmia.

Three detection and therapy zones can be programmed on a MedtronicTM 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.

  1. 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.

  1. 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.

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