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Dual-chamber discrimination and atrial detection - Evera XT DR

Patient

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


Trace

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

2- How many detection zones are programmed?
Two detection zones have been programmed with a VT zone of 350 to 270 ms.

3-What diagnosis does the interval plot suggest?
The graph shows a rapid atrial rhythm with occasional slowing; the ventricular response is relatively irregular; 4 bursts and a ramp are delivered.

4- What is your diagnosis?
The tracing showed atrial fibrillation with irregular atrioventricular conduction, supporting the diagnosis of conducted AF.

5-How do you rate the quality of atrial sensing?
The tracing shows intermittent atrial undersensing; over these few seconds, the ventricular rate appears faster than the atrial rate to the device; when the VT counter is full, the defibrillator concludes that the diagnosis is VT (V>A).

Take home message

  • This episode demonstrates one of the limitations of prioritising PR Logic over Wavelet.
  • In this patient, we found multiple episodes of conducted AF that were well discriminated by PR Logic or Wavelet, but in this episode inappropriate therapies were delivered.
  • PR Logic is based on differential analysis between atrial and ventricular rates; one of the basic rules of operation of this algorithm is that if the ventricular rate is judged to be faster than the atrial rate, the device concludes VT without further analysis (no analysis of Wavelet, which would have enabled the diagnosis to be corrected); this therefore exposes the patient to the risk of inappropriate therapies in the presence of atrial undersensing, as in this example (ventricular rate incorrectly classified as higher than the atrial rate).
  • If these episodes recur and there is no programming margin to optimise the quality of atrial sensing, it may be advisable to turn off PR Logic and base discrimination solely on Wavelet analysis.

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