VT counter

N° 6
Manufacturer Medtronic Device ICD Field Counter

This patient presented multiple episodes of non-sustained ventricular tachycardia stored in the memory of the device and defined by the occurrence of at least 5 consecutive intervals in one of the detection zones without any of the initial counters being filled (VT or VF). For a same patient, the greater the number of required intervals (30/40 versus 9/12), the higher the probability of recording an episode of non-sustained ventricular tachycardia. The programming of this parameter is crucial for the quality of life of the patient but also for his or her prognosis. Indeed, programming 30/40 beats for initial detection not only reduces the number of inappropriate therapies due to lead dysfunction or due to an episode of supraventricular tachycardia, but also reduces the number of appropriate but avoidable therapies. When associated with the programming of relatively high detection zones, this translates into a significant reduction in mortality in primary prevention. Treating an episode of malignant ventricular arrhythmia with electrical shock is the only option for achieving viable hemodynamics. On the other hand, the latest recommendations, based on the most recent studies, suggest the need to avoid treating slower and organized ventricular arrhythmias too prematurely or too aggressively. An electrical shock can save a life but is associated with a deleterious effect on its own and should therefore be avoided whenever possible when spontaneous termination is possible or a less aggressive therapy can be effective. It is therefore advisable 1) not to systematically program treatment zones that are too low for primary prevention; 2) to prolong the initial detection counters in the VT zone but also in the VF area in order to avoid treating spontaneously-terminating arrhythmia episodes (appropriate but avoidable therapies); 3) to promote a first-line treatment with anti-tachycardia pacing even for very fast tachycardias (limit: 230-250 beats/minute).

Graph and trace

The graph shows an initial seemingly normal rhythm followed by a sudden acceleration and demonstration of a regular tachycardia detected in the VT zone and successfullytreated by an anti-tachycardia pacing burst.

  1. on the EGM, there is a sudden acceleration with a regular tachycardia in which all of the intervals are detected in the VT zone (TS);
  2. after 16 consecutive TS intervals, the device diagnoses a VT;
  3. a burst is then delivered;
  4. the burst is successful with a return to sinus rhythm and an end-of-episode diagnosis after 8 consecutive intervals classified as VS (termination).

The specifics of the VT counter must be perfectly integrated in order to allow for an optimal programming. Indeed, for MedtronicTM devices, the counting method differs completely between the VT zone (consecutive intervals) and the VF zone (probabilistic counter), which is not the case for the ICDs of competing manufacturers (except for BiotronikTM devices). An interval classified in the VT zone increments the VT counter by +1; an interval classified in the VF area does not alter the VT counter (no increment, no decrement); a long interval classified as VS resets the VT counter to 0.

This counter has been specifically developed to allow optimal functioning for tachycardias with a rate of less than 200 beats per minute, with a dual objective:

  1. to efficiently detect episodes of monomorphic and regular ventricular tachycardia; since all signals have the same amplitude, the risk of undersensing is limited;
  2. ensure a first level of "discrimination" during an episode of atrial fibrillation.

The VT counter is filled after a programmable number of consecutive intervals classified in the VT zone (tolerance for VF intervals which do not change the counter). Any interval classified as VS resets the VT counter to 0, which has several consequences:

  1. an inappropriate return of this counter to 0 occurs as a result of a ventricular undersensing (common in polymorphic ventricular tachycardia). This counter is therefore probably not suitable for tachycardias greater than 200 beats/minute where some of the arrhythmias are polymorphic with signals of varying amplitude;
  2. this counter allows improving the specificity of the diagnoses, an episode of atrial fibrillation being readily associated with the presence of iterative long intervals (frequent return of the VT counter to 0). 
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