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Single-chamber defibrillator and SVT V. Limit

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Male implanted with a primary prevention single-chamber defibrillator (Visia AF XT VR) for ischaemic cardiomyopathy.

Single-chamber defibrillator and SVT V. Limit: Medtronic ICD tracing (EGM)


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Trace

What was the diagnosis made by the defibrillator for this episode? This episode has been classified as VF by the system.

How many detection zones are programmed? A single zone (VF 310 ms) has been programmed.

What diagnosis does the interval plot suggest? The graph shows an initially stable rhythm of around 60 bpm with sudden acceleration and very rapid cycles recorded in the VF zone; a maximum output shock is delivered with a return to the initial heart rate.

How many discrimination parameters are programmed? Wavelet is the only discrimination criterion programmed (70% similarity); the stability and sudden onset criteria are programmed ‘Off’.

How do you analyse the EGM in tachycardia? This is clearly ventricular fibrillation (very rapid, irregular and polymorphic tachycardia).

What therapy is delivered? The VF counter is completed after 30 cycles classified as FS; a shock is delivered after the capacitors are charged; this terminates the tachycardia.

What do you think of the Wavelet results? Wavelet was not used because the 8 intervals analysed were too fast.

Comments

Points to remember

  • The SVT V. Limit parameter is specific to MedtronicTM devices; in fact, the SVT V. Limit is the maximum heart rate up to which discrimination based on Wavelet applies and is independent of the programmed detection zones; it can therefore be applied in the FVT or VF zone; on the other hand, the sudden onset and stability criteria are limited to the VT or FVT via VT zone; in devices from other manufacturers, no discrimination is possible in the VF zone; in this example, the VF zone starts at intervals of 310 ms and the SVT V. Limit is programmed at 260 ms; discrimination therefore applies in the VF zone for intervals between 310 and 260 ms.
  • When the median cycle length of the 12 most recently detected consecutive intervals is less than this programmed limit, discrimination no longer applies; it is preferable not to program a discrimination limit that is too fast (> 230 bpm) in the majority of patients; in fact, when the heart rate exceeds this limit, the probability that the arrhythmia is supraventricular is low; moreover, incorrectly inhibiting therapies on a potentially lethal ventricular arrhythmia could prove catastrophic.

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