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The unexpected behaviour of an algorithm - Cobalt VR

Patient


Male implanted with a single-chamber defibrillator (Cobalt VR) for primary prevention of ischaemic cardiomyopathy.


Trace

1-How was this episode classified?
This episode was classified as supraventricular tachycardia following Wavelet analysis.

2- What is your diagnosis?
This is an episode of T wave oversensing.

3- Has therapy been provided?
When the VT counter is filled after 36 consecutive cycles classified as VS (TS marker), discrimination based on Wavelet analysis classifies the episode as SVT and therefore inhibits therapies; analysis then continues cycle by cycle with inhibition (WV marker).

4-How do you analyse the Wavelet analysis?
In the presence of T wave oversensing, the defibrillator senses the R wave and T wave alternately; as expected, 50% of the complexes (the R waves) are judged to be similar to the reference morphology with a percentage match higher than the threshold value (here between 85 and 88%), the other 50% of complexes (the T waves) are judged to be different with a percentage match of 0%.

Take home message

  • To avoid inappropriate therapies related to T wave oversensing, it is necessary to : 1. understand the reason for oversensing in order to optimise programming (for example, change the sensing polarity in the presence of a small amplitude R wave), 2. program one or more algorithms enabling therapies to be avoided if oversensing occurs.
  • Wavelet analysis was initially designed to differentiate between VT and SVT; this tracing shows that this algorithm can also be used favourably in a completely different context; here the defibrillator «makes the wrong diagnosis», diagnosing the episode as SVT, but this still avoids inappropriate therapies.
  • When the T wave is oversensed, one out of two cycles is considered similar to the sinus rhythm template, which explains the diagnosis of SVT; in fact, at least 3 cycles out of 8 must be correlated; this algorithm therefore constitutes a first line of defence against inappropriate therapies when the T wave is oversensed.
  • However, there are certain limitations: this algorithm only works if the frequency does not exceed the SVT limit (260 ms is the nominal cycle length); similarly, on a dual or triple chamber defibrillator, when PR Logic is programmed simultaneously with Wavelet, the latter is not applied if PR Logic considers the ventricular rhythm to be faster than the atrial rhythm (which is the case when the T wave is oversensed); in these 2 cases, the Wavelet algorithm will not be implemented and another algorithm is then required to avoid inappropriate therapies.

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