What is Cardiac Resynchronization Therapy Device (CRTD)?

What is Cardiac Resynchronization Therapy Device (CRTD)?

CRTD is a specialized implantable device that combines biventricular pacing to correct ventricular dyssynchrony with an integrated defibrillator to prevent sudden cardiac death in heart failure patients with reduced ejection fraction and prolonged QRS duration. 1

Device Components and Mechanism

CRTD consists of three pacing leads: one in the right atrium, one in the right ventricle, and one positioned via the coronary sinus to pace the left ventricle, plus defibrillation capability for life-threatening arrhythmias. 1

The device works by:

• Restoring synchronized contraction between the right and left ventricles (biventricular pacing or "cardiac resynchronization therapy") 1
• Correcting interventricular and intraventricular conduction delays caused by bundle branch blocks, particularly left bundle branch block (LBBB) 1
• Providing backup defibrillation to terminate ventricular tachycardia or fibrillation 1

Physiologic Benefits

CRTD addresses the mechanical consequences of electrical dyssynchrony by:

• Improving ventricular systolic function and increasing left ventricular ejection fraction by 5-10% 1
• Reducing functional mitral regurgitation caused by temporal delays in ventricular activation 1
• Promoting reverse remodeling with reduction in left ventricular end-diastolic and end-systolic volumes 1
• Decreasing metabolic costs of cardiac contraction 1

Clinical Outcomes

The mortality and morbidity benefits are substantial:

• 36% reduction in all-cause mortality compared to medical therapy alone in NYHA class III-IV patients 1
• 30% decrease in heart failure hospitalizations across multiple trials 1
• 20% reduction in the composite endpoint of death or hospitalization 1
• Improved functional capacity: 50-70 meter increase in 6-minute walk distance and 1-2 mL/kg/min increase in peak oxygen consumption 1
• Enhanced quality of life: 10-point or greater reduction on the Minnesota Living with Heart Failure scale 1

CRTD vs CRT-Pacemaker

The key distinction is the defibrillator component:

• CRTD (CRT-D) includes both resynchronization therapy AND defibrillation capability for sudden cardiac death prevention 1
• CRT-Pacemaker (CRT-P) provides only resynchronization therapy without defibrillation backup 1

In the COMPANION trial, CRT-D showed a 36% mortality reduction while CRT-P showed a non-significant trend toward mortality benefit, though both reduced the composite endpoint of death or hospitalization by approximately 20%. 1

Ideal Candidates for CRTD

The strongest indication (Class I) is for patients with:

• LVEF ≤35% on guideline-directed medical therapy 1, 2
• Sinus rhythm (not atrial fibrillation) 1, 2
• LBBB pattern with QRS ≥150 ms 1, 2
• NYHA class II, III, or ambulatory class IV symptoms 1, 2

LBBB morphology is critical—patients with LBBB show 36% reduction in adverse events, while non-LBBB patterns show no benefit. 2 The prevalence of mechanical dyssynchrony is 70% among patients with QRS ≥150 ms compared to only 40% with QRS 120-149 ms. 1

Important Caveats

CRTD is contraindicated in patients with:

• QRS duration <120 ms, even with echocardiographic dyssynchrony—multiple trials (RethinQ, ESTEEM-CRT, LESSER-EARTH) showed no benefit or potential harm 1
• NYHA class IV with refractory symptoms requiring continuous intravenous inotropes (except selected transplant candidates) 1
• Life expectancy <1 year due to non-cardiac comorbidities 1, 2

Common pitfall: Approximately 30-40% of patients are "non-responders" who fail to show clinical improvement despite meeting standard criteria. 3, 4 Response rates are highest with QRS ≥150 ms and LBBB morphology, and lowest with right bundle branch block or anterior myocardial scar. 1

Device selection consideration: For NYHA class II patients, CRTD is typically preferred over CRT-P given the demonstrated mortality benefit in trials like MADIT-CRT and RAFT, unless clinical factors or patient preferences favor CRT-P alone. 1