Intracardiac defibrillator (ICD) is useful in which of the following ?
Ref. AHA guidelines
 
ICD Indications (AHA Guidelines)
Class I indications (ie, the benefit greatly outweighs the risk, and the treatment should be administered) are as follows:

Structural heart disease, sustained VT
Syncope of undetermined origin, inducible VT or VF at electrophysiologic study (EPS)
Left ventricular ejection fraction (LVEF) ≤35% due to prior MI, at least 40 days post-MI, NYHA class II or III
LVEF ≤35%, NYHA class II or III
LVEF ≤30% due to prior MI, at least 40 days post-MI
LVEF ≤40% due to prior MI, inducible VT or VF at EPS

 
Class IIa indications (ie, the benefit outweighs the risk and it is reasonable to administer the treatment) are as follows:

Unexplained syncope, significant LV dysfunction, nonischemic cardiomyopathy
Sustained VT, normal or near-normal ventricular function
Hypertrophic cardiomyopathy with 1 or more major risk factors
Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) with 1 or more risk factors for sudden cardiac death (SCD)
Long QT syndrome, syncope or VT while receiving beta-blockers
Nonhospitalized patients awaiting heart transplant
Brugada syndrome, syncope or VT
Catecholaminergic polymorphic VT, syncope or VT while receiving beta-blockers
Cardiac sarcoidosis, giant cell myocarditis, or Chagas disease
Person with Brugada
Person with Arrhythmogenic RV dysplasia
Person after acute MI with CAD
All of the above