How does an ICD work?

Your ICD constantly monitors the heart's electrical activity and recognizes when the heart rate is normal, too slow, or too fast. If the rhythm is abnormal, it automatically delivers an electrical treatment to the heart. The kind of treatment will depend on the way it has been programmed. Your doctor will tell you how your ICD has been programmed.

The most important function of an ICD is to deliver a shock to restore normal rhythm in the case of ventricular tachycardia or fibrillation (defibrillation). However, ICDs can also pace the heart to treat some ventricular tachycardias, or when the natural rhythm is too slow.

Cardioversion / Defibrillation

ICDs can be programmed to deliver low-, intermediate- and high-energy shocks. A sizable proportion of VTs can be terminated by antitachycardia pacing or by low-energy shocks. If antitachycardia pacing or cardioversion fail, a high-energy defibrillation shock is delivered.

If an ICD detects VF, it immediately proceeds to the delivery of a high-energy shock (defibrillation). Since it is delivered directly to the heart, the shock needs only to be approximately 10% of the strength of shocks delivered by external paddles applied to the chest in emergency. The time between onset of VF and delivery of a high-energy shock (the charge time of the capacitors) is usually approximately 10 seconds. It is not uncommon for patients to faint during this charge time.

Antitachycardia pacing

When VT is detected and needs to be treated, the ICD can make an attempt to restore normal rhythm by delivering a burst of small, rapid electrical pulses to interrupt the tachycardia. This therapy, called antitachycardia pacing, is usually imperceptible.

Bradycardia pacing

Your ICD is also a pacemaker, which prevents the heart from beating too slowly. ICDs can sense the activity, as well as pace the right atrium, the right ventricle, or both chambers. This is called single- (VVI) or dual-chamber (DDD) pacing.

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