What are the primary indicators for implantable defibrillator?

AICD: The primary indications for an aicd are a left ventricular ejection fraction (lvef) of <35% from any cause, for ventricular tachycardia or fibrillation (vt/vf) >6 weeks after heart attack, lvef < 30% with life expectancy > 1 year, patients in whom medical therapy doesn't suppress vt/vf, or cardiac arrest or sustained VT with defibrillation.

Cardiac Arrest risk: When the risk for cardiac arrest is high enough, a physician may recommend a patient get an icd. This might be for patients with a weak heart, genetic disease (like hocm, long qt, arvd), or someone who has survived a cardiac arrest already.