VF redetection counter

N° 3
Manufacturer Medtronic Device ICD Field Counter

Patient implanted with a single-chamber ICD (Virtuoso II VR) for hypertrophic cardiomyopathy; syncope during exertion requiring 2 electrical shocks (35 Joules), the second enabling to terminate the arrhythmia; the analyzed tracing allows focusing on the functioning of the redetection counter.

Graph and trace

The graph initially shows a regular tachycardia not corresponding to one of the 3 detection zones; acceleration of ventricular rhythm in the VF zone with initial detection of a VF followed by delivery of the first electrical shock (35 Joules) but unsuccessful; redetection of arrhythmia and second 35 Joules electrical shock; effectiveness of this second shock and termination of the arrhythmia.

  1. the EGM reveals a very fast ventricular arrhythmia detected in the VF zone;
  2. initial detection of a VF episode when the initial counter is full (12/16);
  3. first electrical shock delivered after a confirmation phase; when the shock is delivered, there is a blanking phase of 520 ms after which ventricular sensing resumes;
  4. when the shock is unsuccessful, as in this example, there is a redetection probabilistic counter; this counter is filled after a minimum of 9 out of 12 classified FS intervals (programmable parameter) causing a second charging of the capacitors;
  5. second electrical shock delivered without confirmation phase at the end of the charge;
  6. successful electrical shock and diagnosis of end-of-episode after 8 intervals classified as VS and slower than the slowest programmed detection zone.

This tracing allows highlighting the functioning of the device once the first shock has been delivered. A 520 ms blanking phase, during which no detection is possible, is systematically triggered following the shock to avoid oversensing of the polarization generated by the shock. The device must then differentiate between termination of the arrhythmia episode and inefficiency of the shock with continuation of the arrhythmia, using two different counters: 1) the redetection counter which is programmable; as in the case of the initial counter, this is a probabilistic counter with a ratio of 75% (6/8, 9/12 ... 30/40); the number of required intervals applies to all subsequent shocks (from 2 to 6) during the same episode; it is customary to program the redetection to a lower number of required intervals than for the initial detection, the risk of undersensing increasing 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 intervals 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 intervals is always slower than the programmed lowest detection zone (VF or VT).

Of note, once the redetection counter is filled, the shock will automatically be delivered at the end of the charging period by synchronizing itself on a sensed complex or asynchronously if no interval is detected (committed shock; no confirmation phase at the end of the charging period).