100 QUESTIONS IN CARDIOLOGY

90 What do I do if an ICD keeps discharging?

Roy M John and Mark Squirrell

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 speci-

ficity. 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 elec-

trograms 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.^2

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 in-

appropriately (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.