Zone limits and the VT counter- Evera XT DR

Zone limits and the VT counter - Evera XT DR

Patient

Male implanted with a dual-chamber defibrillator (Evera XT DR) for dilated cardiomyopathy.

Comments

1, 2. What diagnosis do you suggest from this graph?

The episode was classified as VT-NS by the device (1); the graph shows atrioventricular dissociation with cycles oscillating around the VT zone (2).

What is your diagnosis?

The EGM shows VT (ventricular rate > atrial rate); some cycles are classified as TS, others as VS; cycles classified as VS reset the VT counter to 0.

1-2 Quel diagnostic évoquez-vous sur ce graphe ?
L’épisode a été classé TV-NS par le dispositif (1) ; le graphe montre une dissociation auriculo-ventriculaire avec des cycles oscillant autour de la zone de TV (2).

3- Quel est votre diagnostic ?
L’EGM montre une TV (fréquence ventriculaire > fréquence atriale) ; certains cycles sont classés TS d’autres sont classés VS ; les cycles classés VS remettent à 0 le compteur de TV.

Take home message

This plot shows one of the possible limits of the TV counter, which requires a programmable number of consecutive cycles classified as TS, and highlights the importance of programming the lower limit of the TV zone.
In this patient, the diagnosis of VT is indisputable, with obvious atrioventricular dissociation; however, there is a delay in diagnosis due to the specificity of the operation of this counter; the tachycardia oscillates with the lower limit of the VT zone; when a cycle is classified as SV, the VT counter is reset to 0 even if, as in this case, the diagnosis of VT is obvious.
It is therefore essential to program a sufficient margin in relation to the frequency of the clinical tachycardia to avoid this type of problem; it is usual to program the VT zone 10 to 20 beats/minute slower than the clinical VT, this margin should probably be slightly increased for this brand’s devices, as the occurrence of a cycle classified as SV does not have the same consequences as for the defibrillators of competing devices.
This specificity also means that VT meters are not generally used for tachycardias above 200 beats/minute, as the risk of intermittent under-detection is increased (more frequent polymorphic tachycardias), resulting in delayed or non-diagnosis and therefore treatment; the probabilistic VF meter seems more appropriate for this type of tachycardia.
In the latest recommendations, there are marked differences in terms of programming the detection zone terminals depending on the manufacturer; the different operation of the meters explains some of these differences (in primary prevention, Medtronic^TM recommends a single VF zone from 188-200 beats/minute, compared with a VT zone up to around 230 beats/minute and then a VF zone for the other manufacturers).

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