SecureSense™ algorithm during a VF episode

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Tracing
N° 5
Manufacturer Abbott Device ICD Field Securesence
Patient

This 80-year-old man presenting with an ischemic cardiomyopathy and a 35% left ventricular ejection fraction had undergone implantation of a Abbott Fortify Assura™ VR ICD. He was seen after an episode of syncope during which he received an electric shock; the SecureSense™ algorithm was programmed ON with a RV coil to can configuration; 2 zones of detection were programmed i.e. VF at 222 bpm (270 ms cycle length) and VT at 166 bpm (360 ms cycle length).

 
Graph and trace
  1. slow and irregular baseline rhythm consistent with AF and a slow ventricular response;
  2. premature ventricular contraction triggering an episode of fast ventricular arrhythmia;
  3. ventricular sensing with non-classified (-) cycle because of discordance between the instantaneous interval (short cycle consistent with the VF zone) and the average of 4 cycles (consistent with the sinus zone);
  4. second, short, F-classified cycle (concordance between instantaneous interval and the average of 4 cycles); after the detection of this second short cycle on the bipolar channel, activation of the noise counter on the discrimination channel (begins after an additional warm-up of 350 ms);
  5. T-classified cycle without VS2 marker on the discrimination channel despite a visually tall amplitude, because the signal fell in the warm-up phase; 
  6. persistence of the ventricular arrhythmia; first VS2 cycle on the discrimination channel;
  7. signal sensed on the bipolar and not on the discrimination channel (intermittent undersensing on this channel);
  8. resumption of sensing on both channels;
  9. each short cycle on the bipolar channel increases the noise counter; however, this noise counter is reset to 0 after 2 short cycles on the discrimination channel (on this channel, a cycle is short if <360+30=390 ms);
  10. systematic resetting of the noise counter to 0 after 2 short cycles on the discrimination channel;
  11. the VF counter is filled (12); since the noise counter is <10, the therapies are not inhibited by the algorithm;
  12. ATP during the charge; 8 stimuli are delivered at a fixed rate while the capacitors are charging;
  13. unsuccessful ATP and continuation of fast, polymorphous VT;
  14. end of charge (36 J);
  15. electric shock delivered;
  16. 1-second post-shock blanking;
  17. successful electric shock and interruption of the tachycardia; it is noteworthy that, for this cycle, the discrimination channel oversenses, illustrating the limitations of this channel after the delivery of a shock (distortion of the signal), explaining the interruption of the algorithm after a first therapy;
  18. return to sinus rhythm diagnosed after 3 consecutive VS-classified cycles.
Comments

This tracing illustrates characteristic aspects of the SecureSense™ algorithm function during a ventricular arrhythmia: 

  1. the noise counter is not constantly activated. When it is inactive, the VS2 markers are missing on the discrimination channel. The activation of this counter requires 2 instantaneous cycles out of 3 corresponding to the VT or VF zone. The second cycle is followed by a 350-ms interval during which this channel cannot sense. This explains, during this episode, the absence of VS2 marker on the discrimination channel corresponding to the T cycle following the second short cycle.;
  2. during an episode of ventricular arrhythmia, the occurrence of short cycles on the discrimination channel prevents increases in the noise counter, which is reset to 0 each time 2 short cycles are detected, even if they are not consecutive. The definition of a short cycle on the bipolar versus on the discrimination channel is different. On the latter, it depends on the number of zones of tachycardia programmed and their rate limits (30 ms above the limit of the lowest zone, if 2 or 3 zones have been programmed). In this patient, the threshold defining a short cycle on the discrimination channel is, therefore, 360 + 30 ms = 390 ms;
  3. in the beginning of the tracing, a cycle was not detected on the discrimination channel, which has no effect on the final analysis of the episode. A moderate amount of undersensing interferes minimally with the function of the algorithm, since the number of short cycles remains high and reinitializes often the counter;
  4. in this example of fast, polymorphous VT, multiple short cycles are detected on the discrimination channel, and the noise counter is constantly reset to zero. During an episode of VT or VF, if the discrimination channel detects accurately, the device observes a concordance between the 2 analyzed channels and detects short cycles consistent with an arrhythmia. When the VF counter is filled, the SecureSense™ counter is <10 and the therapies are delivered. The next tracings show a protection against undersensing, which limits the risk of inappropriate inhibition of the therapies for ventricular arrhythmia;
  5. after an electric shock, oversensing may appear on the discrimination channel because of a poor quality of the signal. After delivery of the therapy the SecureSense™ algorithm remains inactive until the next return to sinus rhythm has been diagnosed.
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