Induction of VF by T-wave shock

Tracing

For dual or triple chamber defibrillators, 3 channels are available in nominal: 1) the bipolar atrial (A) sensing channel, 2) the bipolar ventricular (V) sensing channel, and 3) the high-voltage (shock) channel between the distal coil and the pulse generator. This latter channel yields a morphology relatively similar to that of an electrocardiographic lead. Markers and time intervals are also available.

The calibration available on the tracing measures the amplitude of the various signals. On this tracing, the 0.5-mm/mV calibration was identical on the 3 channels. It is automatically set by the device (auto-calibration), though can be reprogrammed by the programmer to simplify the analysis of an episode.

The recording of the EGM of an episode begins 5 seconds before the shock delivery on the T wave.

1. Chrg: this marker corresponds to the end of the charge needed to deliver the shock on the T wave;
2. VS-Hy: this marker indicates that, in dual chamber devices, a hysteresis of the AV delay can be programmed to minimize deleterious RV pacing; 
3. the induction procedure began by a train of 8 ventricular stimuli (VP) at 150 bpm (400 ms between each stimulus in nominal setting);
4. a 1.1-J electrical shock (nominal value) was delivered in the ventricular vulnerable period, at a 310-ms coupling interval (nominal value) from the last paced ventricular cycle; the polarity of the shock on T wave was the same as that of the first shock programmed in the VF zone;
5. this shock is followed by a 500-ms refractory period, divided between a) an interval during which neither chamber is able to sense, and b) an interval during which a sensed ventricle event is marked between brackets;
6. during the detection of an arrhythmia, “PVP” indicates that the PVARP has been lengthened to prevent ventricular pacing after the sensing of spontaneous atrial activity. The pacing mode remains what was programmed (DDD in this case) until fulfillment of the 8 out of 10 criterion;
7. (VS): the device sensed this ventricular event at the end of the refractory period;
8. after the shock and the end of the refractory period, the first ventricular then atrial complexes are not counted, explaining the two “--" markers;
9. a rapid ventricular arrhythmia (more ventricular than atrial events) was induced with markers in the VF zone; the device suspected the presence of an arrhythmia after having counted 3 consecutive short cycles; thereafter, it incremented the episode number, stored the historical data and the EGM, and began the initial detection window;
10. the detection window was fulfilled when 8 out of 10 short cycles were counted (detection of an episode, V-Epsd marker); in this case 8 consecutive cycles were counted in the VF zone, when the defibrillator switched to the VDI mode (mode VTR: VT response);
11. from this marker onward, the initial Duration (programmed here at 1 sec), which began at the end of the initial detection window, ended; this clock verified that the arrhythmia was sustained. If, for this duration, ≥6 out of 10 cycles remain short, the arrhythmia is considered sustained, a V-Detect marker appears and the capacitors begin to charge (Chrg);
12. during the charge, an undersensed ventricular EGM caused the occurrence of a VS marker; as long as ≥6 out of 10 cycles are in the VF zone, the charge continued uninterrupted;
13. end of the capacitors’ charge (Chrg); a 135-ms refractory period followed the end of the charge; the first ventricular cycle following the end of charge was systematically excluded (--);
14. to be delivered, if the shock was confirmed, 2 out of 3 short cycles were needed following the end of charge; when a single VF zone has been programmed (as in this example), a cycle is short if it is in the VF zone; if one or two VT zones are programmed, a cycle is short if it is at least as short as the longest VT zone cycle; a 500-ms window follows the end of the charge, during which the shock cannot be delivered, to give a chance to divert the shock with a programmer (divert window); in this example, the second VF cycle fell in that window, explaining the delay of the shock to the end of that 500 ms refractory period;
15. electrical shock delivered at the end of the 500-ms delay;
16. the next atrial and ventricular cycles following that refractory period are not counted, explaining the 2 “--" markers; a 2-sec interval followed the shock, during which the defibrillator could not pace;
17. the shock was successful and sinus rhythm was restored;
18. atrial pacing with atrial signal sensed in the ventricular channel in the ventricular noise window following atrial stimulation (VS); the AV delay was unchanged with effective ventricular pacing  (VP).

Graph

The graph offers an overall display of the episode, including the pacing train followed by an electrical shock (21), the ventricular arrhythmia detected in the VF zone (22), and the termination of the arrhythmia after the electrical shock (23).