When is an implantable cardioverter‑defibrillator (ICD) indicated for primary or secondary prevention in adults (≥18 years) with heart failure (LVEF ≤ 35% on optimal guideline‑directed therapy, NYHA class II‑III, ischemic or non‑ischemic cardiomyopathy) or in survivors of ventricular tachyarrhythmia?

ICD Indications in Heart Failure and Ventricular Arrhythmia Survivors

ICD therapy is indicated for both primary prevention in patients with LVEF ≤35% on optimal guideline-directed medical therapy (GDMT) for ≥3 months and secondary prevention in survivors of cardiac arrest or sustained ventricular tachyarrhythmias, provided patients have >1 year expected survival with good functional status. 1, 2

Secondary Prevention (Strongest Indication)

ICD implantation is mandatory in survivors of cardiac arrest due to ventricular fibrillation (VF) or hemodynamically unstable sustained ventricular tachycardia (VT) after excluding completely reversible causes. 1, 3

• This applies to any patient with structural heart disease and spontaneous sustained VT, whether hemodynamically stable or unstable 1, 3
• For post-MI patients, if sustained VT/VF occurs >48 hours after STEMI and is not due to transient ischemia, reinfarction, or metabolic abnormalities, ICD placement before discharge is indicated 1
• Patients with VT and elevated cardiac biomarkers should be treated identically to those without biomarker elevation 1

Primary Prevention in Ischemic Cardiomyopathy

NYHA Class II-III Patients

For patients ≥40 days post-MI with LVEF ≤35% and NYHA Class II or III symptoms on chronic GDMT, ICD therapy is recommended. 1, 2

NYHA Class I Patients

For patients ≥40 days post-MI with LVEF ≤30% and NYHA Class I symptoms on GDMT, ICD therapy is recommended. 1, 3

Critical Timing Considerations

• Do not implant ICDs within 40 days of MI for primary prevention - the DINAMIT trial showed a 58% reduction in arrhythmic deaths was completely offset by increased non-arrhythmic deaths 1, 3
• Wait at least 90 days post-revascularization before ICD implantation 1
• The CABG-Patch trial demonstrated no survival benefit and increased infection rates when ICDs were placed at the time of bypass surgery 1

Special Ischemic Scenarios

• Patients with prior MI, LVEF <40%, nonsustained VT, and inducible sustained VT at electrophysiologic study qualify for ICD therapy 1
• For clinically relevant ventricular arrhythmias occurring >48 hours but within 40 days post-MI, ICD implantation is reasonable to improve survival 1

Primary Prevention in Non-Ischemic Dilated Cardiomyopathy

ICD therapy is recommended for patients with non-ischemic DCM who have LVEF ≤35% and NYHA Class II or III symptoms on chronic GDMT. 1, 2, 3

Mandatory Prerequisites Before Implantation

• Optimize medical therapy for at least 3 months before ICD consideration - ventricular function may improve substantially with ACE inhibitors/ARBs, beta-blockers, mineralocorticoid receptor antagonists, and SGLT2 inhibitors 2
• Reassess LVEF after optimization; if improved to >35%, ICD may not be indicated 2
• NYHA Class I patients with non-ischemic cardiomyopathy have only a Class IIb indication 2
• NYHA Class IV patients who are not candidates for transplantation or cardiac resynchronization therapy should not receive an ICD 2

Supporting Evidence

• The SCD-HeFT trial enrolled 2,521 patients with LVEF ≤35% and demonstrated significant mortality reduction with ICD therapy regardless of cardiomyopathy etiology 2
• The DEFINITE trial showed ICD therapy reduced mortality from 14.1% to 7.9% over 2 years in non-ischemic cardiomyopathy 2

Universal Exclusion Criteria

Do not implant an ICD if life expectancy is <1 year with acceptable functional status - this is a Class III contraindication. 1, 2, 3

• This applies to all indications, both primary and secondary prevention 1, 2, 3
• Patients must have reasonable expectation of meaningful survival with good functional status 1

Common Pitfalls to Avoid

Timing Errors

• Never implant within 40 days of acute MI for primary prevention - this increases non-arrhythmic mortality despite reducing arrhythmic deaths 1, 3
• Do not rush to ICD implantation in non-ischemic cardiomyopathy without adequate trial of GDMT for ≥3 months 2

Misidentifying Reversible Causes

• Always exclude acute myocardial ischemia as the immediate precipitant of VF - if present, coronary revascularization is the primary therapy, not ICD 1
• Exclude transient or reversible causes (electrolyte abnormalities, drug toxicity, acute ischemia) before labeling as secondary prevention indication 1, 3

Inappropriate Patient Selection

• Do not implant in patients with limited life expectancy from non-cardiac causes 1, 2, 3
• Avoid ICD placement in NYHA Class IV patients unless they are candidates for advanced therapies 2

Algorithm for Decision-Making

Step 1: Determine Prevention Category

• Secondary prevention: History of cardiac arrest, sustained VT/VF → Proceed to ICD if reversible causes excluded 1, 3
• Primary prevention: No prior sustained arrhythmia → Continue to Step 2

Step 2: Assess Cardiomyopathy Etiology and Timing

• Ischemic: Must be ≥40 days post-MI and ≥90 days post-revascularization 1
• Non-ischemic: Must have ≥3 months of optimal GDMT with reassessment of LVEF 2

Step 3: Verify LVEF and NYHA Class Criteria

• LVEF ≤30%: NYHA Class I, II, or III qualify 1, 3
• LVEF 31-35%: Only NYHA Class II or III qualify 1
• LVEF ≤40%: Only if inducible VT at EP study (ischemic only) 1

Step 4: Confirm Life Expectancy

• Expected survival >1 year with acceptable functional status required 1, 2, 3

Step 5: Proceed with Implantation

• If all criteria met, ICD therapy is indicated 1, 2, 3