Electrical shock for ventricular fibrillation

Tracing
N° 26
Manufacturer Abbott Device ICD Field Therapy
Patient

This 69-year-old recipient of an Atlas single chamber ICD, implanted after an episode of aborted sudden cardiac death, was seen in consultation for evaluation of syncope followed by an electrical shock. The patient was in atrioventricular (AV) block and chronic AF.

Main programmed parameters

  • Single VF zone at 222 bpm
  • 16 cycles in the VF zone were needed for the diagnosis
  • Sensitivity programmed at 0.2 mV
  • Programmed shock characteristics: cathodal, single-coil, fixed waveform duration
  • VVI episode pacing mode at 80 bpm; VVI post-shock pacing mode at 60 bpm 
Graph and trace

Tracing

  1. Ventricular paced rhythm at 80 bpm (the back-up rate); VES;
  2. Ventricular pair;
  3. Same sequence, ventricular paced rhythm and ventricular pair with onset of VF; the first few cycles are unclassified (while they are in the VF zone, they are non-concordant with the average of the preceding cycles, which were still in the sinus zone);
  4. VVI pacing mode of the episode (same as the programmed permanent pacing mode) after 4 F cycles;
  5. Failure of sensing low-amplitude ventricular signal (autogain 1.4 mm/mV) delaying the diagnosis of the episode;
  6. Detection of a VF episode after 16 F cycles, which trigger the recording of EGM and the onset of the capacitors charge (*); confirmation of the arrhythmia during the charge (underscored F);
  7. Capacitors charge with persistence of sensing failure; the first cycles are unclassified (the analyzed cycles are in the sinus zone, while the 4 preceding cycles were in the VF zone);
  8. Persistent undersensing and VS cycle (concordance between the analyzed cycle and the average of the preceding cycles);
  9. During the charge, following this VS, the cycles in the VF zone are labelled underscored R instead of underscored F;
  10. Electrical shock delivered at the end of charge; it is noteworthy that the second cycle following the end of charge is unclassified; consequently, instead of delivering a shock on this cycle, the device waited for the next short cycle (instantaneous duration at 172 ms in the VF zone and averaged cycle also in the VF zone) before delivering it;
  11. successful shock terminating the arrhythmia and diagnosis of restoration of sinus rhythm (redetection of sinus rhythm set on “rapid: 3 intervals”); post-shock pacing mode: VVI 60 bpm.
Comments

This tracing, like the previous, shows the main role played by ICD, which consists of preventing sudden death. The arrhythmia is, at its very onset, extremely rapid, polymorphic and disorganized. Any attempt at terminating this type of arrhythmia by antitachycardia pacing (ATP) seems futile, whereas electrical shocks remain the treatment of choice. The origin of the arrhythmia is not scrutinized in this range of heart rates. In a first stage, electrical shock therapy of VF mandates flawless sensing of the rapid, often low-amplitude ventricular EGM, which requires the programming of short refractory periods as well as a high and adaptive sensitivity. In this patient, sensing was of mediocre quality even if, ultimately, therapy was successfully delivered. A prominent cycle-by-cycle variability is often more important than the absolute signal amplitude, and may be the cause of undersensing. The shock was successful and associated with a high impedance. In such situation, it seems appropriate to optimize the shock duration instead of programming a fixed tilt. It is noteworthy that the early Saint Jude Medical ICDs were set with a cathodal nominal shock polarity (cathode in the ventricle, anodal pulse generator). However, the nominal programming of shocks is now anodal (anode in the ventricle, cathodal pulse generator).

Message to remember

The narrative reported the delivery of a 24.5-J shock for an episode of VF, associated with a high shock impedance (130 Ohms) for a single-coil lead.