What is the difference between implantable cardioverter-defibrillator (ICD) and cardiac resynchronization therapy (CRT) for heart failure treatment?

Difference Between Implantable Cardioverter-Defibrillator (ICD) and Cardiac Resynchronization Therapy (CRT) for Heart Failure Treatment

ICDs and CRT devices serve different primary functions in heart failure treatment, with ICDs preventing sudden cardiac death through defibrillation while CRT improves cardiac synchrony and function by coordinating ventricular contractions.

Primary Functions and Mechanisms

Implantable Cardioverter-Defibrillator (ICD)

• Primary function: Prevents sudden cardiac death by detecting and terminating life-threatening ventricular arrhythmias
• Mechanism: Monitors heart rhythm and delivers electric shocks when dangerous arrhythmias are detected
• Does not: Improve cardiac synchrony or function directly
• Main benefit: Reduction in mortality from sudden cardiac death 1

Cardiac Resynchronization Therapy (CRT)

• Primary function: Improves cardiac function by restoring electromechanical synchrony
• Mechanism: Coordinates ventricular contractions through simultaneous pacing of both ventricles
• Additional benefits: Promotes reverse remodeling, reduces LV volumes, improves LVEF, and reduces mitral regurgitation 1, 2
• Available in two forms:
  • CRT-P: CRT with pacemaker function only
  • CRT-D: CRT combined with defibrillator function 1

Patient Selection Criteria

ICD Indications

• Secondary prevention (patients with prior cardiac arrest or sustained ventricular arrhythmias)
• Primary prevention in patients with:
  • LVEF ≤35% with NYHA class II-III symptoms despite optimal medical therapy
  • LVEF ≤30% with ischemic cardiomyopathy
  • Genetic arrhythmogenic cardiomyopathy with high-risk features and LVEF ≤45% 1

CRT Indications

• LVEF ≤35% with QRS duration ≥120 ms (strongest evidence for QRS ≥150 ms)
• NYHA class II-IV symptoms despite optimal medical therapy
• Particularly beneficial in patients with:
  • Left bundle branch block (LBBB) morphology
  • QRS duration ≥150 ms
  • Non-ischemic cardiomyopathy 1, 2

Clinical Outcomes

ICD Outcomes

• Reduces sudden cardiac death by 50-60%
• Does not improve heart failure symptoms or cardiac function
• No effect on heart failure progression 1

CRT Outcomes

• Reduces all-cause mortality by 22-36% in appropriate patients
• Reduces heart failure hospitalizations by 37-52%
• Improves LVEF by approximately 3-6.9%
• Improves quality of life and functional status (59% of patients improve by ≥1 NYHA class)
• Promotes reverse remodeling with reduction in LV volumes 1, 2, 3

Choosing Between Devices

When to Use ICD Alone

• Patients with LVEF ≤35% and high risk of sudden cardiac death
• QRS duration <120 ms (CRT is contraindicated in these patients)
• No evidence of electrical dyssynchrony 1, 4

When to Use CRT-P

• Patients with significant comorbidities limiting survival
• Patients with contraindications to ICD therapy
• Elderly patients with primary prevention indications 2

When to Use CRT-D

• Patients meeting criteria for both CRT and ICD
• Secondary prevention ICD indications
• Ischemic cardiomyopathy with LVEF ≤30% and QRS ≥150 ms with LBBB pattern
• Reasonable survival expectation >1 year with good functional status 1

Important Considerations and Pitfalls

Key Pitfalls to Avoid

1. Implanting CRT in patients with QRS <120 ms: The EchoCRT trial showed potential harm with increased mortality in these patients 4
2. Ignoring QRS morphology: LBBB pattern predicts better response to CRT than RBBB or nonspecific conduction delay 1, 2
3. Implanting during acute decompensation: Guidelines recommend optimizing medical therapy first and implanting after stabilization 1
4. Inadequate biventricular pacing: Failure to achieve near 100% biventricular pacing reduces CRT effectiveness 1, 2

Special Populations

• Atrial fibrillation patients: CRT can be beneficial if AV nodal ablation or pharmacological rate control allows near 100% ventricular pacing 1
• Ambulatory NYHA class IV: Can benefit from CRT if they haven't had recent HF hospitalizations and have reasonable survival expectation (>6 months) 1

Conclusion

The choice between ICD and CRT depends on patient-specific factors including QRS duration, QRS morphology, LVEF, NYHA class, and etiology of heart failure. In many cases, a CRT-D device may be appropriate to provide both the mortality benefit of defibrillation and the symptomatic improvement from resynchronization therapy.