Indications for Cardiac Resynchronization Therapy
CRT is indicated for patients with heart failure on optimal medical therapy who have LVEF ≤35%, sinus rhythm, LBBB pattern with QRS ≥150 ms, and NYHA class II-IV symptoms, as this combination demonstrates the strongest mortality and morbidity reduction. 1
Class I Indications (Strongest Evidence)
NYHA Class II-IV with LBBB
- LVEF ≤35%, sinus rhythm, LBBB pattern, QRS ≥150 ms, and NYHA class II, III, or ambulatory class IV symptoms on guideline-directed medical therapy (GDMT) 1
- This represents the most robust evidence base, with significant reductions in death and heart failure hospitalization (HR: 0.58, p<0.00001) 1
- The LBBB morphology is critical—patients with LBBB show 36% reduction in adverse events (RR: 0.64), while non-LBBB patterns show no benefit (RR: 0.97) 1
Non-LBBB Pattern (More Restrictive)
- LVEF ≤35%, sinus rhythm, non-LBBB pattern with QRS ≥150 ms, and NYHA class III/ambulatory class IV symptoms on GDMT 1
- Evidence is weaker for non-LBBB patterns; benefit only demonstrated with QRS ≥150 ms in severely symptomatic patients 1
Class IIa Indications (Reasonable to Provide)
Atrial Fibrillation with High Ventricular Pacing Burden
- LVEF ≤35% with atrial fibrillation when: 1
- Patient requires ventricular pacing or otherwise meets CRT criteria, AND
- AV nodal ablation or pharmacologic rate control will achieve near 100% ventricular pacing with CRT
- This prevents right ventricular pacing-induced cardiomyopathy 1
Anticipated High Ventricular Pacing Requirement
- LVEF ≤35% undergoing new or replacement device placement with anticipated ≥40% ventricular pacing requirement 1
- Prevents pacing-induced cardiomyopathy from conventional right ventricular pacing 1
Class IIb Indications (May Be Considered)
Mildly Symptomatic Ischemic Cardiomyopathy
- LVEF ≤30%, ischemic etiology, sinus rhythm, LBBB with QRS ≥150 ms, and NYHA class I symptoms on GDMT 1
- May prevent disease progression in early-stage heart failure 1
Non-LBBB with Intermediate QRS Duration
- LVEF ≤35%, sinus rhythm, non-LBBB pattern with QRS 120-149 ms, and NYHA class III/ambulatory class IV on GDMT 1
- Evidence is equivocal; benefit not reliably demonstrated in this subgroup 1
Non-LBBB with Wide QRS, Mild Symptoms
- LVEF ≤35%, sinus rhythm, non-LBBB pattern with QRS ≥150 ms, and NYHA class II symptoms on GDMT 1
Class III: No Benefit (Do Not Provide)
Contraindications
- NYHA class I or II symptoms with non-LBBB pattern and QRS <150 ms 1
- No mortality or morbidity benefit demonstrated; may cause harm 1
- Comorbidities and/or frailty limiting survival with good functional capacity to <1 year 1
- Device implantation risks outweigh potential benefits in this population 1
Critical Prerequisites
Mandatory Requirements Before CRT Consideration
- Optimal medical therapy for ≥3 months (or ≥40 days post-MI if applicable) 1
- Documented LVEF ≤35% on echocardiography 1
- Life expectancy >1 year with acceptable functional capacity 1
Key Clinical Nuances
QRS Duration Thresholds
- QRS ≥150 ms shows clear benefit across all NYHA classes with LBBB 1
- QRS 120-149 ms has uncertain benefit—only consider in NYHA class III-IV with LBBB pattern 1
- QRS duration <150 ms is a risk factor for CRT non-response 1
QRS Morphology Matters More Than Previously Recognized
- LBBB pattern is essential for Class I indications 1
- Right bundle branch block shows no benefit (RR: 0.91, p=0.49) 1
- Nonspecific intraventricular conduction delay shows no benefit (RR: 1.19, p=0.28) 1
Common Pitfalls to Avoid
- Do not implant CRT based solely on echocardiographic dyssynchrony in narrow QRS (<120 ms) patients—clinical trials have not supported this approach 1
- Ensure adequate trial of GDMT before device implantation—premature CRT may miss opportunity for medical optimization 1
- Non-response rate remains 30-40% even in optimal candidates; set realistic expectations 2, 3
- In atrial fibrillation, ensure commitment to rate control or AV nodal ablation—inadequate ventricular pacing capture negates CRT benefit 1