What are the indications for implantable cardioverter defibrillator (ICD) implantation and what are the survival benefits?

Studies in the early 1980s showed that recurrence rates were high for patients presenting with a malignant arrhythmia unrelated to myocardial ischaemia or infarction. Survivors of cardiac arrest, those presenting with sustained monomorphic VT and unexplained syncope in the presence of heart disease clearly are patients at high risk for sudden cardiac death. A series of clinical trials completed in the recent past have confirmed the uniform survival benefit from ICD therapy in such patients (AVID, CASH, CIDS) when compared to therapy with amiodarone or sotalol. In the largest prospective randomised trial (Antiarrhythmics versus Implantable Defibrillators Trial – AVID trial), the ICD reduced mortality by 39% at 1 year and 31% at 3 years. Most patients randomised to the antiarrhythmic arm of the trial were treated with amiodarone.

With remarkable improvements in ICD technology allowing easier implantation, the ICD is being embraced increasingly and earlier in the course of cardiac disease. Attention has now turned to primary prevention of sudden death. For patients with asymptomatic non-sustained VT, there appears to be a clear survival benefit from ICD in the presence of a remote myocardial infarction, LVEF <40%, and inducible VT at electrophysiological study (MADIT, MUSTT). Interestingly, this benefit cannot be extrapolated to patients without nonsustained VT or inducible VT. The CABG patch trial that randomised patients with LVEF <36% and positive signal averaged ECG to ICD or not during elective bypass surgery failed to show a survival benefit. The role of the ICD in primary prevention of sudden death in non-ischaemic dilated cardiomyopathy is also unclear at this time. Clinical trials are in progress.

The benefit from an ICD appears to be greatest for patients with severe LV function and additive to conventional therapy with ACE inhibitors and beta adrenergic blockers. In the AVID trial for example, survival benefit with ICD was observed only when LVEF was less than 35%. Similarly, in the primary prevention trials, the mean LVEF was 30%. One could advance the argument that the ICD should be reserved for those with the worst LV function. Unfortunately, such patients have competing causes for mortality such as pump failure and electromechanical dissociation that are responsible for 50% of deaths. On the other hand, patients with little or no impairment of LV function and a single tachyarrhythmic event usually have late and rare recurrence leading to sudden death. An ICD can potentially restore them to near normal life expectancy in the absence of ongoing myopathic process. The long term studies requiring more than one life span of an ICD are not available to define the true value of
ICD therapy in such patients.

Although the ability of the implantable cardioverter defibrillator (ICD) to terminate potentially lethal ventricular arrhythmias is well acknowledged there is less consensus as to whom should receive an ICD. A good place to start is the American College of Cardiology/American Heart Association Practice Guidelines for Arrhythmia Devices.1 There are three classes of indications: class one, where there is evidence and/or general agreement that the treatment is beneficial, useful and effective; class two, where there is conflicting evidence or a divergence of opinion; and class three, where there is evidence and general agreement that a treatment is not useful or effective.

The class one indications for ICD implantation are:

1. Cardiac arrest due to VF or VT not due to a transient or reversible cause.
2. Spontaneous sustained VT.
3. Syncope of undetermined origin with clinically relevant, haemodynamically significant sustained VT or VF induced at electrophysiological study when drug therapy is ineffective, not tolerated or not preferred.
4. Non-sustained VT with coronary disease, prior MI, LV dysfunction, and inducible VF or sustained VT at electrophysiological study that is not suppressible by a class I antiarrhythmic drug.

The class two indications for ICD implantation are:

1. Cardiac arrest presumed to be due to VF when electrophysiological testing is precluded by other medical conditions.
2. Severe symptoms attributable to sustained ventricular arrhythmias while awaiting cardiac transplantation.
3. Familial or inherited conditions with a high risk for lifethreatening ventricular tachyarrhythmia such as long QT syndrome or hypertrophic cardiomyopathy.
4. Non-sustained VT with coronary artery disease, prior MI, and LV dysfunction, and inducible sustained VT or VF at electrophysiological study.
5. Recurrent syncope of undetermined aetiology in the presence of ventricular dysfunction and inducible ventricular arrhythmias at electrophysiological study when other causes of syncope have been excluded.