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Initial counter and redetection counter - Evera XT DR

Patient

Patient implanted with a dual-chamber defibrillator (Evera XT DR) for ischaemic cardiomyopathy; hospitalisation for syncope.


Comments

  1. What diagnosis are you suggesting on this graph?

Three detection zones were programmed; initially, the graph suggested the existence of a VT at the limit of the VT zone (atrioventricular dissociation); in a second phase, the ventricular rhythm accelerated with irregular cycles in the VF zone; a first maximum shock was delivered but did not reduce the arrhythmia; a second maximum shock was delivered; it appeared to be effective; the diagnosis was therefore that of VT degenerating into VF and requiring 2 electric shocks.

 

  1. What is your diagnosis?

The tracing shows a rapid, polymorphic ventricular arrhythmia consistent with VF.

 

  1. How is the initial counter set?

It is set to 30/40; the TF. and FS cycles implement the same counter.

 

  1. How does the device work at the end of this first charge?

The shock is said to be “non-committed”, so there is a confirmation phase; the shock is delivered on the second rapid cycle following the CE marker.

 

  1. What was the effect of the shock?

Shock was ineffective and the arrhythmia persisted.

 

  1. What does the FD marker mean?

The redetection counter programmed on 12/16 is completed; following the shock, 12 cycles classified as FS can be counted for 2 cycles classified as VS.

 

  1. How does the device work at the end of this second charge?

The second shock of the same episode is said to be “commited”, i.e. it will necessarily be delivered if the re-detection counter has been completed; at the end of the charge (EC), the device synchronises to the first QRS complex detected.

 

Take home message

  • This VF trace is used to discuss the operation of the defibrillator once the first shock has been delivered.
  • A blanking phase of 520 ms, in which no detection is possible, occurs systematically following all shocks to avoid any overdetection of the polarisation induced by the shock.
  • The device must then differentiate between reduction of the episode versus ineffectiveness of the shock and continuation of the arrhythmia with competition between 2 counters: 1) the redetection counter which is programmable; like the initial counter, this is a probabilistic counter with a ratio of 75% (6/8, 9/12 … 30/40); the number of cycles required applies to all subsequent shocks (from 2 to 6) during the same episode; it is usual to programme a lower number of cycles required for redetection than for initial detection, as the risk of under-detection increases with the duration of the arrhythmia; 2) the end-of-episode counter which is based on 2 criteria: a) the device diagnoses the end of the episode when 8 consecutive VS or VP cycles slower than the lowest programmed detection zone (VF or VT) are detected; b) it also diagnoses the end of the episode if, for 20 seconds, the median of 12 consecutive cycles is still slower than the lowest programmed detection zone (VF or VT).
  • It should be noted that once the redetection counter has been completed, the shock will be automatically delivered at the end of the charge by synchronising with a detected complex or asynchronously if no cycle is detected (“commited” shock; no confirmation phase at the end of the charge).

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