VT counter and atrial fibrillation

N° 7
Manufacturer Medtronic Device ICD Field Counter

Patient implanted with a dual-chamber ICD (Evera XT DR) for ischemic cardiomyopathy with multiple episodes of paroxysmal atrial fibrillation; 2 detection zones were programmed with a VT zone between 350 and 270 ms; the analyzed episode was diagnosed as SVT/AF by the device; the objective of this tracing is to highlight the value of the VT counter to avoid inappropriate therapies during an episode of AF.

Graph and trace

The graph initially shows a very rapid and irregular atrial rhythm associated with a rapid and irregular ventricular rate (alternating intervals detected within or outside the VT zone) compatible with an episode of conducted AF; the ventricular rate is stabilized in a second instance in the VT zone; the VT counter is filled but no therapy is delivered.

  1. on the EGM, there is a very rapid and irregular atrial activity suggesting atrial fibrillation as well as irregular ventricular intervals alternately classified as TS or VS; after 7 consecutive cycles, the VT counter is at +7;
  2. detection of an interval classified as VS resets this counter to 0;
  3. this counter is iteratively reset to 0 as a result of the detection of ventricular intervals classified as VS;
  4. the ventricular rhythm accelerates and stabilizes in the VT zone;
  5. the VT counter is filled after 16 TS cycles and 2 FS cycles (counter programmed to 16, the FS cycles do not modify the VT counter); no therapy is delivered, the PR Logic discrimination algorithm leading to the diagnosis of AF;
  6. end of episode after 8 consecutive cycles classified as VS.

This tracing readily illustrates the value of the MedtronicTM VT counter for avoiding the occurrence of inappropriate therapies during an episode of AF. This patient presented multiple episodes of relatively fast conducted AF but not recorded in the device memory, the iterative presence of long intervals allowing a return of the VT counter to 0, thus preventing this counter from being implemented and the episodes being stored in the device's memory. In rare cases, as on this tracing, the ventricular rhythm stabilized in the VT zone, with PR Logic hence allowing an effective discrimination and avoiding the occurrence of inappropriate therapies.

This specificity is therefore a major advantage of this type of counter. The risk of inappropriate therapies due to atrial fibrillation is significantly reduced, especially since there is a trend suggested by the guidelines for an increase in the number of intervals required for the diagnosis of VT.  In order for an episode of AF, occurring at a rate corresponding to the VT zone, to bring about the occurrence of inappropriate therapies, it must occur in conjunction with:

  1. a number of consecutive intervals in the VT zone (30 in the new guidelines) without any long interval classified as VS which is relatively rare when the lower limit of the VT zone is programmed between 150 and 160 beats/minute; indeed, any interval classified as VS resets the VT counter to 0;
  2. a discrimination error by PR Logic and by Wavelet, which operates in a second instance and can correct a diagnostic error. This explains the relatively low risk of inappropriate therapies due to AF in the VT zone.
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