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Choice of discriminators in a single-chamber defibrillator - Visia AF XT VR

Patient


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


Trace

1- What was the diagnosis made by the defibrillator for this episode?
This episode was clasified VT by the device.

2- How many detection zones are programmed?
Three detection zones have been programmed with a VT zone of 370 to 310 ms.

3-What diagnosis does the interval plot suggest?
The graph shows an initially stable rhythm of around 60 bpm with sudden acceleration and cycles recorded in the VT zone; a burst is delivered with normalisation of the heart rate.

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

5-How do you analyse the EGM in tachycardia?
This is a regular, monomorphic tachycardia with wide QRS (shock channel), with a different morphology to the EGM after tachycardia (morphology following the effective burst).

6-What therapy is delivered?
The VT counter is completed after 48 cycles classified as VT; the discrimination criterion indicates a diagnosis of VT and a burst is delivered; this terminates the tachycardia.

7-What do you think of the Wavelet results?
The appearance of Wavelet is clearly different from the reference morphology, with similarity percentages of 0% for the 8 ventricular complexes analysed, leading to a diagnosis of VT.

Take home message

  • In this patient, discrimination was based exclusively on Wavelet; all the QRS complexes analysed were morphologically very different from the reference, the percentage match was very low (0%) and the episode was classified as VT, with a burst was used to interrupt the tachycardia.
  • Wavelet-based discrimination is based on the principle that a ventricular complex conducted during supraventricular tachycardia most often has the same morphology as complexes recorded in the absence of tachycardia; a tachycardia with ventricular complexes of similar morphology to that of a reference complex recorded in sinus rhythm is not considered to be VT by the device and is therefore not treated.
  • In practice, the device compares the morphology of the 8 QRS complexes preceding detection of the tachycardia with a reference morphology; if at least 6 of the last 8 complexes differ from the reference morphology, the device diagnoses an episode of VT; conversely, if at least 3 complexes are judged to be similar, the device diagnoses an episode of SVT and the VT counter is reset to 0; the similarity threshold is programmable (40, 43, …, 70, …, 97%).
  • The device uses EGM2 to define the reference and analyse the morphology during tachycardia; collection of the reference signal is an essential stage in the correct operation of this parameter and can be carried out at the time of each interrogation or automatically and repeatedly by the device; to create the reference complex, the device collects 6 complexes in «normal sinus rhythm» and calculates a first average reference complex followed by an additional complex every 10 seconds for 700 seconds; with automatic collection switched ‘On’ the quality of the reference signal is confirmed with a comparison between the reference complex and a complex collected every 17 minutes.

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