Lower amplitude electrical shock for ventricular fibrillation

N° 2
Manufacturer Abbott Device ICD Field Therapy

This 64-year-old man presenting with ischemic cardiomyopathy and permanent atrial fibrillation (AF) underwent implantation of a Abbott ICD for secondary prevention after an episode of aborted sudden death. He was seen in consultation after a presyncopal episode and an electrical shock.

Graph and trace
  1. spontaneous rhythm (VS);
  2. the patient is in AF with frequent, monomorphic ventricular extrasystoles (VES), at a relatively fixed coupling interval of 320 ms;
  3. another VES of the same morphology and coupling interval, triggering a rapid polymorphic tachyarrhythmia. The first cycles are unclassified (while the interval is in the VF zone, it is non-concordant with the average of the preceding cycles, which are still in the sinus zone);
  4. VVI episode pacing mode (same as the programmed permanent mode) after 4 F cycles;
  5. VF episode detected by the device (after 12 F cycles) triggering the electrograms (EGM) recording and onset of the capacitors charge (*);
  6. end of the capacitors charge (15 J), which lasted 3.6 seconds. The 6 short cycles needed to confirm the arrhythmia were detected during the charge (F underscored);
  7. confirmation at the end of the charge. The charge was completed; at least 6 short cycles were detected since its onset. The shock cannot be delivered on the cycle following the end of charge;
  8. electrical shock synchronized to the second cycle following the end of the charge;
  9. successful shock and termination of the ventricular arrhythmia; the episode lasted 6 to 7 seconds. Probable termination of an atrial arrhythmia and restoration of sinus rhythm (regular ventricular rhythm); return to sinus rhythm diagnosed by the device. This diagnosis by the device does not mean that the rhythm is necessarily sinus (instead of AF for example), but it does mean that the ventricular rate is slower than the slowest rate programmed in the tachycardia zone(s); the post-shock pacing mode is VVI for 30 seconds.

At the end of the ICD implantation procedure, this patient underwent induction of 2 episodes of VF, each successfully terminated by 15-J shocks, which explains the programming of the first shock amplitude. The tracing shows a relatively short charge time and the delivery of a first shock just over 6 to 7 seconds after the onset of arrhythmia, at which time the patient was conscious. A 15-J first shock strength might be programmed with a view to abbreviate the overall duration of therapy, thereby preventing some patients to loose consciousness. The programming of the first shock energy in the VF zone is a compromise: a less than maximum energy shock might terminate VF after a short charge time; however, should it be unsuccessful, the second shock will be delivered after a long overall duration of VF. The programming of an immediately high energy is more likely to terminate VF, though at the cost of a longer initial charge time.

Message to remember

The narrative indicated that the patient had suffered an episode of VF, treated with a 15 J shock (46 Ohms impedance).