An implantable cardioverter-defibrillator also called an implantable defibrillator or ICD is a battery powered device used to monitor for and treat life-threatening heart rhythm abnormalities (arrhythmias) such as ventricular tachycardia (VT) or ventricular fibrillation (VF).
Like a pacemaker implantable defibrillators consists of a generator (which includes a battery) that generates the electrical impulses, and leads or wires that transmit impulse to the myocardium or heart muscle. In addition, the ICD generator also has a high density capacitor that helps generate the charge used to shock the heart. Unlike pacemaker leads the main ICD lead is thicker, incorporating two coils through which the shock is delivered. Dual chamber defibrillators (i.e. defibrillators that sense and pace in both the atrium or upper heart chamber and ventricle or lower heart chamber) use regular pacemaker leads in the atrium.
Patients who have had an episode of ventricular tachycardia, ventricular fibrillation or cardiac arrest may need an ICD. Increasingly, ICD are also being implanted in patients who are at risk for these arrhythmias such as those with coronary artery disease, congestive heart failure, hypertrophic cardiomyopathy or abnormalities electrical system of the heart.
An ICD continuously monitors the heart. If defines an abnormal heart rhythms primarily by the heart rate that it detects. If it detects a serious arrhythmia, it then tries to terminate it by delivering an electric shock to the heart. It can also be programmed to treat the arrhythmias by delivering a fast burst of pulses and pace the heart out of the arrhythmia. After delivering the therapy the device reverts back to monitoring.
Because patients who require ICDs are also prone to slow heart rates and heart block, newer generation of devices also function as a pacemaker with all of regular pacemaker’s features.
The physician sets both the rate at which a shock will occur and the amount of energy that is released to try and terminate a ventricular arrhythmia. If the first shock is unsuccessful, the device delivers progressively stronger shocks.
Implantable defibrillators are very effective in treating life-threatening arrhythmias. It does not however, prevent them. Thus patients who experience frequent ICD shocks or discharges often require anti arrhythmic medications such as amiodarone to prevent or suppress these arrhythmias.
Abraham Salacata, MD, FACC