What causes ineffective cardioversion in implantable cardioverter-defibrillators (ICD)?

Updated: Oct 11, 2019
  • Author: Daniel M Beyerbach, MD, PhD; Chief Editor: Jeffrey N Rottman, MD  more...
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Ineffective cardioversion may result from inadequate energy output, rise in defibrillation threshold (possibly due to an antiarrhythmic medication, such as amiodarone, flecainide, or phenytoin), myocardial infarction at the lead site, lead fracture, insulation breakage, scarring at the lead implantation site, and lead dislodgment.

Many ICDs deliver a programmed set of therapies per dysrhythmic episode. The number of therapies per episode is programming specific. If a delivered therapy does not terminate the arrhythmia, the device proceeds to the next programmed therapy. For example, a total of 6 attempts at defibrillation are attempted per episode of ventricular fibrillation. The device attempts defibrillation and then reevaluates the cardiac rhythm. If the arrhythmia persists, it delivers therapy number 2 and so on until all 6 attempts have been delivered. Once this occurs, the device does not deliver therapy until a new episode is declared. Initial therapy for VT may be antitachycardia pacing (also known as overdrive pacing) rather than cardioversion.

ICDs do not prevent all sudden deaths, and acknowledging that cardiac arrest is not necessarily an ICD malfunction is important. The device may have properly delivered the required shocks for the triggering rhythm but still have been ineffective in resolving it.

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