What do I do if an ICD keeps discharging?

Most patients who experience a single ICD shock do so for successful conversion of a malignant ventricular arrhythmia. However, it must be remembered that the default programming in an ICD is designed to maximise sensitivity at the expense of specificity. Consequently, a significant number of ICD shocks can be inappropriate.1 For example, multiple shocks in quick succession may indicate inappropriate therapy for an atrial arrhythmia or a problem with the rate sensing lead. For this reason, it is important to retrieve the stored data from the device using the appropriate programmer even after a single shock. Evaluation of events stored in the ICD memory shows intracardiac electrograms, far field electrograms and recorded intervals as well as the onset and stability of the tachycardia to determine appropriate or inappropriate therapy. Frequent episodes of ventricular arrhythmia will require antiarrhythmic drugs for suppression; sotalol is often effective as a first line drug in this situation.

The more common reason for multiple ICD shocks is recurrent ventricular arrhythmia. Patients experiencing “storms” of shocks should be adequately sedated, and monitored in a coronary care setting. Intravenous antiarrhythmic drugs should be used for rapid arrhythmia suppression. Electrolyte abnormalities should be sought and promptly corrected. Myocardial ischaemia has to be a serious consideration when recurrent ventricular fibrillation or polymorphic ventricular tachycardia is responsible for shocks. Most episodes of repetitive ventricular tachycardia respond to intravenous drugs such as lidocaine, procainamide or amiodarone allowing for oral loading with an antiarrhythmic agent in a more controlled fashion.

If it becomes apparent that shocks are being delivered inappropriately (e.g. atrial fibrillation with rapid ventricular rates or shocks with no apparent arrhythmia signifying a lead fracture) suppression of ICD function can be achieved by applying a magnet over the ICD generator. Unless specifically programmed to the contrary, one can temporarily disable the sensing circuit of most ICDs during the period that a magnet is held over the ICD generator and prevent unnecessary shock while awaiting availability of appropriate equipment for definitive ICD programming changes.

Other causes of inappropriate therapy include:

• Sinus tachycardia
• Lead fracture
• Diaphragmatic muscle sensing
• Electromagnetic interference