What are the indications for an Implantable Cardioverter-Defibrillator (ICD)?

Indications for Implantable Cardioverter-Defibrillator (ICD)

ICD implantation is indicated for cardiac arrest survivors due to VT/VF without reversible causes, patients with structural heart disease and spontaneous sustained VT, and for primary prevention in patients with LVEF ≤35% due to prior MI (>40 days) or non-ischemic cardiomyopathy with NYHA Class II-III heart failure. 1, 2

Secondary Prevention Indications (Class I)

These represent the strongest evidence base for ICD therapy, with proven mortality reduction:

• Cardiac arrest survivors resuscitated from VT/VF after excluding completely reversible causes (electrolyte imbalance, acute ischemia, drugs, trauma) 3, 1, 2
• Spontaneous sustained VT in patients with structural heart disease, whether hemodynamically stable or unstable 3, 1, 2
• Syncope of undetermined origin with clinically relevant, hemodynamically significant sustained VT or VF induced at electrophysiological study 3, 1

The secondary prevention trials (AVID, CIDS, CASH) demonstrated consistent mortality reductions of 23-30% relative risk reduction, translating to 6-8% absolute risk reduction over 2-9 years 3. These patients had mean LVEF ranging from 32-46% 3.

Primary Prevention Indications (Class I)

Ischemic Cardiomyopathy

• Post-MI patients (>40 days) with LVEF ≤35% and NYHA Class II or III heart failure 1
• Post-MI patients (>40 days) with LVEF ≤30% and NYHA Class I 1
• Post-MI patients with nonsustained VT, LVEF ≤40%, and inducible VF or sustained VT at electrophysiological study 1, 2

The MADIT II trial showed 28% relative risk reduction (6% absolute) in post-MI patients with LVEF ≤35%, while MADIT I demonstrated even greater benefit (59% relative, 19% absolute) in patients with additional risk markers like nonsustained VT and inducible arrhythmias 3.

Non-Ischemic Cardiomyopathy

• Non-ischemic dilated cardiomyopathy with LVEF ≤35% and NYHA Class II or III 1
• Timing consideration: Generally avoid ICD within first 3 months of diagnosis unless other indications present; can consider at 3-9 months if unlikely to recover LV function 1
• Exception: If sustained or hemodynamically significant ventricular tachyarrhythmias occur <9 months from diagnosis, proceed with ICD 1

The DEFINITE trial showed a trend toward mortality reduction (44% relative, 6% absolute) in non-ischemic patients, while SCD-HeFT demonstrated 23% relative risk reduction (7.2% absolute) across both ischemic and non-ischemic etiologies 3.

Special Populations

Hypertrophic Cardiomyopathy

• ICD reasonable for patients with one or more major risk factors for sudden cardiac death 1
• Risk factors include: family history of sudden death, unexplained syncope, massive LVH, nonsustained VT 3

Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy

• ICD reasonable for patients with one or more risk factors for sudden cardiac death 1
• Patients meeting task force criteria have 73% rate of appropriate ICD therapies over 4.4 years, with 39% in primary prevention and 85% in secondary prevention receiving appropriate shocks 4

Congenital Heart Disease

• Cardiac arrest survivors after excluding reversible causes 3
• Symptomatic sustained VT after hemodynamic and electrophysiological evaluation (catheter ablation or surgical repair may be alternatives) 3
• Recurrent syncope with ventricular dysfunction or inducible ventricular arrhythmias 3
• Do not base decisions solely on ejection fraction; incorporate multiple risk factors 1

Pediatric Patients

Indications mirror adult criteria but require careful consideration of body size limitations, cumulative lifetime risk, and decades of potential device therapy 3. SCD in young patients associates with congenital heart disease, cardiomyopathies, and genetic arrhythmia syndromes 3.

Critical Contraindications and Timing Issues

Absolute Contraindications (Class III)

• Life expectancy <6 months from non-cardiac causes 3, 2
• NYHA Class IV heart failure not eligible for transplantation or cardiac resynchronization therapy 3, 2
• Ventricular tachyarrhythmias due to completely reversible disorders (electrolyte imbalance, drugs, trauma) in absence of structural heart disease 3
• Severe neurological sequelae following cardiac arrest 2

Timing Restrictions

• Do not implant within 40 days of acute MI for primary prevention, as DINAMIT and IRIS trials showed no mortality benefit and potential harm from increased non-arrhythmic deaths 3, 1, 5
• Do not implant within 48 hours of acute MI or during acute ischemia episode 3
• The early post-MI period (6-40 days) shows 67% reduction in arrhythmic death offset completely by increased non-arrhythmic mortality 3

Common Pitfalls to Avoid

Do not withhold ICD from secondary prevention patients based solely on successful revascularization. The arrhythmogenic substrate persists despite revascularization, and AVID trial data showed similar or worse mortality in "correctable cause" patients treated with revascularization alone 2.

Do not assume normal LVEF eliminates need for ICD in secondary prevention. If cardiac arrest was unrelated to acute ischemia and occurred with normal LV function, ICD remains indicated as the substrate persists 2.

Recognize that appropriate ICD shocks carry prognostic significance. In DINAMIT, patients receiving appropriate ICD therapy had >5 times increased risk of death, with 36% mortality versus 13% in ICD patients without shocks 3.

Quality of life considerations matter. ICD therapy carries risk of psychological consequences and QOL decrements, particularly with inappropriate shocks 3. However, this does not override mortality benefit in appropriate candidates.