Chapter 1 · Counters · Case 10

Zone limits and the VT counter

Patient and episode

Patient

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

The recording

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Rate / interval plot

Medtronic ICD electrogram, segment 1 of 1 — Zone limits and the VT counter

EGM

Medtronic ICD electrogram, segment 1 of 1 — Zone limits and the VT counter
  1. What diagnosis is suggested by the interval plot? The episode was classified as VT-NS by the device (1); the plot shows atrioventricular dissociation with cycles oscillating around the VT zone (2).
  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.

Points to remember

  • This plot shows one of the possible limitations of the VT counter, which requires a programmable number of consecutive cycles classified as TS, and highlights the importance of programming the lower limit of the VT zone.
  • In this patient, the diagnosis of VT is indisputable, with obvious atrioventricular dissociation; however, there is a delay in diagnosis due to specificity in the operation of this counter; the tachycardia oscillates around the lower limit of the VT zone; when a cycle is classified as VS, 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 based on the rate 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 higher for MedtronicTM devices, since the occurrence of a cycle classified as VS has different consequences compared to defibrillators of competing devices.
  • This specificity also means that VT counters are generally not used for tachycardias above 200 beats/minute, as the risk of intermittent undersensing is increased (more frequently with polymorphic tachycardias), resulting in delayed or non-diagnosis and delayed or withheld treatment; the probabilistic VF counter seems more appropriate for this type of tachycardia.
  • In the latest recommendations, there are marked differences in terms of programming the detection zone boundaries depending on the manufacturer; the different operation of the counters explains some of these differences (in primary prevention, MedtronicTM recommends a single VF zone from 188-200 beats/minute, compared with a VT zone up to approximately 230 beats/minute followed by a VF zone for the other manufacturers).

Practice out loud

Just describe the tracing. What do you think is going on? Describing the numbers can help.

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From Medtronic ICD — clinical cases by P. Bordachar, A. Thiyagarajah, L. Fontagne, M. Strik, S. Ploux. Published by Cardiocases. Every numbered marker on a recording is explained in the list beneath it; tap a marker on the trace or an entry in the list.

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