VF counter

N° 1
Manufacturer Medtronic Device ICD Field Counter

Primary implantation of a dual-chamber ICD (Evera XT DR) in a patient with ischemic cardiomyopathy; at the end of the procedure, induction of ventricular arrhythmia by low amplitude electrical shock on the T wave; a single VF zone with a 270 ms limit was programmed (initial detection counter: 12/16); this tracing corresponds to the induction procedure and allows focusing on the functioning of the VF probabilistic counter.

Graph and trace

The graph shows a sinus rhythm followed by rapid ventricular pacing (8 stimuli at 150 beats/minute); the low-amplitude electrical shock delivered on the T wave cannot be visualized on the graph; induction of a probable rapid ventricular arrhythmia detected in the VF zone; highlighting of a few intervals falling outside the VF zone suggesting ventricular undersensing; detection of VF followed by electrical shock of 25 Joules terminating the arrhythmia.

  1. the EGM initially shows atrial and ventricular pacing;
  2. rapid ventricular pacing;
  3. low-amplitude electrical shock delivered on the T wave (320 ms coupling interval); following the shock, blanking period with no possible sensing for 520 ms;
  4. induction of a rapid and polymorphic ventricular arrhythmia;
  5. undersensing of a secondary ventricular interval with high variability in signal amplitude; the ensuing interval is classified as VS (long pseudo-interval);
  6. new undersensing;
  7.  the VF probabilistic counter is filled (programmed at 12/16) with detection of a VF episode and start of the capacitor charge;
  8. during the charging period, all intervals are classified as VS even when fast and corresponding to the VF zone;
  9. end of charge and confirmation phase;
  10. delivered electrical shock; successful electrical shock and end of episode after 8 consecutive intervals classified as VS or VP.

This tracing allows detailing the functional operation of the VF counter in MedtronicTM devices. It is a probabilistic counter (fast X/Y intervals) which requires a minimum of 75% fast intervals (programming possibilities: 9/12, 12/16, 18/24, 24/32, 30/40, etc). A ventricular fibrillation is, by definition, an arrhythmia that is fast, disorganized, chaotic, with ventricular signals of low and/or variable amplitude. All of these characteristics increase the risk of undersensing with occasional signals with amplitude below the detection threshold (nominal value of 0.3 mV) and often a high variability in beat-to-beat amplitude, which is problematic given that, the device uses a sensitivity level which adapts to the amplitude of the preceding signal. This 75% ratio between fast and slow intervals was chosen so as to obtain an optimal balance between the proper detection of ventricular fibrillation (necessary tolerance for a certain number of long pseudo-intervals generated by undersensing) and the need to not fill the counters in the presence of an oversensing of T, P or R waves (frequently associated with a 50% ratio of short intervals.

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