Induction of VF by T-wave shock

N° 1
Manufacturer Boston Scientific Device ICD Field Implantation

This 80-year-old man underwent implantation of a Boston Science Teligen dual chamber defibrillator for secondary prevention after an episode of VT causing syncope, in the context of severe ischemic cardiomyopathy and a 20% LVEF. A lead was implanted at the RV apex and another in the right atrium; VF was induced at end of the procedure under general anesthesia; the device was programmed specifically for this type of procedure, with a ventricular sensitivity set at 1.0 mV, a single detection zone (VF at 200 bpm), no discrimination, and a first shock at 31 J followed by 7 shocks at the maximum amplitude of 41 J. 

Graph and trace


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).


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).


The induction of VF at the end of an implant procedure has 3 main objectives: 1) verify the integrity of the defibrillation circuit and connection(s) between lead(s) and pulse generator, 2) confirm a reliable detection of VF by the device, a step that is a) critical for the proper function of the device, and b) imperfectly confirmed by measurements of the R wave during sinus rhythm, 3) verify successful termination of VF by a shock delivered by the device.

In this patient, VF was induced by a shock delivered on the T wave. The arrhythmia was accurately detected despite the programming of a 1.0-mV sensitivity. We observed a short period of ventricular undersensing at the beginning of the capacitors’ charge, which seemed acceptable and associated with a sufficient safety margin with respect to the long-term programming of 0.6 mV. The induction of VF was repeated a second time 5 minutes after this episode. The sensing was correct and the 31 J shock was efficient. Therefore, the first electrical shock of 31 J terminated the ventricular arrhythmia twice, guaranteeing a sufficient safety margin with respect to the maximum 41 J output of the device. The 40-Ohm shock impedance was satisfactory for a single coil lead and suggested that the high-voltage circuit was intact.

Message to remember

Episode summary

The ventricular Tachy mode was on “Monitor + Therapy”, meaning that the entire range of detection and treatment options had been activated.

The summary indicates that this episode corresponded to an arrhythmia induced at the end of the implantation procedure and was classified in the VF zone.

Onset of VF event: the ventricular rate (314 bpm) is the average of 4 cycles preceding the V-Epsd marker; the atrial rate is non-recorded (NR) because of the sensing of an insufficient number of atrial EGM during that period; rate only indicates that, at the onset of the event, heart rate was analyzed without additional discrimination.

During Detect-V, the atrial (42 bpm) and the ventricular (314 bpm) rates were calculated by averaging the 4 cycles preceding the V-Detect marker. The episode was always detected in the VF zone. In that zone, no other discriminator is used besides rate. However, various parameters are analyzed, including stability, V>A rate, A Fib, correlated Rhythm ID even if they do not interfere with the decision to treat the episode. Neither the Sustained Rate Duration nor the ATP limit intervened in the VF zone.

31 J V shock attempt: the first programmed and delivered therapy in the VF zone was a 31 J shock. The charge duration of the biphasic shock waveform was 5.6 sec, with a lead impedance of 40 Ohms. The initial, nominal lead polarity corresponded to a cathodal shock (on a single coil lead, the RV coil was the cathode and the pulse generator the anode for the first phase, and inversely for the second phase).