Induction of ventricular fibrillation with prominent undersensing

Patient

A 37-year-old man presenting with hypertrophic cardiomyopathy and an episode of sustained VT, underwent implantation of a Current single chamber ICD and an induction procedure.

Main device programming

Single VF zone for rates >187 bpm
12 cycles in the VF zone were needed for the diagnosis
Sensitivity programmed at 0.5 mV
First shock at 25 J, second shock at 36 J (maximum strength).

Trace

Narrative (trace 5a)

The narrative indicated the occurrence of an 8-sec, self-terminated VF episode. Therapy was cancelled when return to sinus rhythm was diagnosed (425-ms coupling interval, however). The episode was diagnosed after 6 sec, which was relatively long and suggested undersensing. It is noteworthy that the programmed sensitivity was 0.5 mV.

Trace

Spontaneous rhythm (VS);
Induction of VF with DC delivered for 2 sec;
600-ms post-induction blanking period;
Ventricular arrhythmia with first sensing failure, explaining the presence of a 523-ms cycle marked (-);
First F classified cycle after 2 sec of arrhythmia;
Because of multiple sensing failures, the on-going and averaged cycles were longer than the VF cycles, causing cycles marked VS;
Delayed (beyond 12 F) detection of the episode (the diagnosis lasted 6 sec) because of sensing defects; capacitors charge;
Persistence of undersensing;
Interruption of the capacitors charge prompted by an erroneous diagnosis of return to sinus rhythm (3 consecutive VS or intervening unclassified intervals) after multiple sensing failures;
Repeat detection of VF with charge of the capacitors (undersensing re-occurs and the charge is interrupted).

Narrative (trace 5b)

This episode was recorded in the wake of the first episode. VF was re-detected with, this time, the delivery of a 25-J shock associated with a 45-Ohm impedance. The shock was apparently successful as the post-shock coupling interval was 805 ms.

Trace

Sensing remains unreliable;
Detection of a new VF episode and charge of the capacitors;
Re-interruption of the charge because of erroneous diagnosis of return to sinus rhythm;
Re-detection of VF and charge of the capacitors;
A 25-J shock was delivered after a relatively short charge time, because the capacitors were already partially charged;
Successful shock and return to sinus rhythm.

Comments

An accurate detection of the arrhythmia is indispensable for a successful shock therapy of VF. The occurrence of undersensing was evident despite the programming of sensing at a maximum of 0.5 mV for this induction procedure. Because the detection of a spontaneous, even long-lasting episode of VF could not be guaranteed, the lead position had to be changed. Three choices were available to remedy the underdetection of VF during the induction procedure: 1) wait for an accurate detection of VF by the device and delivery of the shock as programmed. Because of the repetitive sensing failures and aborted capacitors charges, however, this choice seemed uncertain and potentially risky, 2) forced delivery of an automatic shock by the ICD, which did not necessarily require a reliable sensing, and 3) delivery of a shock by an external defibrillator, which must be ready before each VF induction procedure.

Pacing & Defibrillation

ECG

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