Implantable cardioverter defibrillator (ICD) is a device which can automatically detect and treat cardiac arrhythmias. It has improved a lot since the first introduction thirty years back. Initial devices were just ‘shock boxes’ which could deliver an internal direct current shock for treating ventricular fibrillation. Modern day devices are much more sophisticated and have various zones of therapy like VT, fast VT and VF (ventricular fibrillation). Then can deliver overdrive pacing for suppression of ventricular tachycardia (VT) as a ‘painless therapy’. Different overdrive pacing protocols can be programmed depending on the rate of the detected arrhythmia. Atrial arrhythmia detection avoids unnecessary or inappropriate shocks. Back up bradycardia pacing is useful for managing spontaneous and post shock bradycardias. The fact that an ICD is a life saving device is now well established and its use for secondary prevention following a resuscitated cardiac arrest is the standard of care. Being an expensive device requiring an invasive procedure, implantation is limited those individuals with a reasonable predicted survival of more than one year. This means that those with terminal illness due to other comorbidities like renal failure or malignancy may not be considered for an ICD implantation.
Regarding the role of ICD in primary prevention of cardiac arrest, the studies have shown the it is most useful in those with left ventricular dysfunction regardless of the presence of inducible VT or VF. In NYHA Class II and III heart failure, the predominant mode of death is arrhythmic sudden cardiac death rather than death due to pump failure. In Class II and III, roughly two thirds of deaths are sudden while in Class IV heart failure, it contributes to one third of the mortality and pump failure predominates as the cause of death. Hence implanting an ICD for protection against sudden cardiac death in those with severe left ventricular dysfunction is an important preventive strategy in those with a projected longevity of more than one year. Certain analyses have shown that the benefit of an ICD increases with the square of the duration of use, in terms of life years saved.