In a patient with third-degree atrioventricular block, left ventricular ejection fraction 20% and no left‑bundle‑branch block on ECG, which device is indicated—dual-chamber pacemaker, cardiac resynchronization therapy (CRT) pacemaker, or CRT with defibrillator (CRT‑D)?

Device Selection for Complete Heart Block with LVEF 20% and No LBBB

A CRT-D (cardiac resynchronization therapy with defibrillator) is the recommended device for this patient with complete heart block, severely reduced LVEF of 20%, and no LBBB on ECG. 1

Rationale for CRT-D Over Standard Pacemaker

Pacing Indication is Clear

• Permanent pacing is absolutely indicated for third-degree AV block regardless of symptoms (Class I recommendation), as this patient has complete heart block requiring ventricular pacing. 1

Why CRT Over Conventional Pacing

• In patients with LVEF ≤35% who require permanent pacing, consideration should be given to CRT-P or CRT-D when improvement in LVEF is not anticipated. 1 This patient's LVEF of 20% falls well below this threshold.

• The 2019 ACC/AHA/HRS guidelines specifically recommend techniques that provide more physiologic ventricular activation (CRT or His bundle pacing) over right ventricular pacing in patients with LVEF 36-50% who are expected to require ventricular pacing >40% of the time. 1 With complete heart block, this patient will require 100% ventricular pacing, and the LVEF is even lower at 20%.

• Right ventricular pacing burden >20% is strongly associated with pacing-induced cardiomyopathy, with a hazard ratio of 6.76 for developing further LV dysfunction. 2 This patient will have 100% RV pacing burden with a conventional pacemaker, making CRT strongly preferable.

• The BLOCK HF trial demonstrated that biventricular pacing was superior to conventional right ventricular pacing in patients with AV block and LVEF ≤50%, reducing the composite endpoint of death, urgent heart failure care, or 15% increase in LV end-systolic volume (hazard ratio 0.74). 3

Why CRT-D Over CRT-P (Adding the Defibrillator)

• With LVEF of 20%, this patient has a primary prevention indication for an ICD based on standard heart failure guidelines, assuming they have reasonable life expectancy and functional status. 1

• Patients with severely reduced LVEF have high risk of sudden cardiac death from ventricular arrhythmias, and the defibrillator component addresses both bradycardia (via pacing) and tachyarrhythmia (via defibrillation).

Absence of LBBB: Important Consideration

Does Lack of LBBB Matter?

• Traditional CRT indications prioritize LBBB with QRS ≥150ms as the strongest predictor of CRT response. 4 However, this patient's situation is different—they require pacing for complete heart block, not elective CRT for dyssynchrony.

• When pacing is mandatory (as with complete heart block), the goal shifts from correcting existing dyssynchrony to preventing pacing-induced dyssynchrony. 2, 3

• The absence of LBBB does not contraindicate CRT when the alternative is 100% right ventricular pacing in a patient with severely reduced LVEF. 1

Alternative Considerations

His Bundle or Left Bundle Branch Area Pacing

• Conduction system pacing (His bundle or left bundle branch pacing) represents an emerging alternative that may provide more physiologic activation than conventional CRT. 5, 6

• Current European guidelines give only low-level recommendations for conduction system pacing due to limited randomized outcome data, though feasibility is high (85-98% success rates). 5, 6

• If conduction system pacing expertise is available, it may be considered, particularly given the absence of LBBB, but CRT-D remains the guideline-supported standard. 6

Clinical Algorithm

1. Confirm complete heart block is persistent and not due to reversible causes (medications, electrolytes, Lyme disease, etc.). 1

2. Assess life expectancy and functional status to determine if ICD component is appropriate (generally requires >1 year expected survival). 1

3. If reasonable life expectancy with LVEF 20%:

   • Primary choice: CRT-D 1, 3
   • Alternative if CRT-D unavailable or patient refuses ICD: CRT-P 1
   • Consider conduction system pacing if local expertise available 6

4. If poor life expectancy or patient declines ICD:

   • CRT-P preferred over conventional dual-chamber pacemaker to prevent further LV dysfunction 2, 3

Common Pitfalls

• Do not implant a standard dual-chamber pacemaker in this patient—with 100% anticipated RV pacing and LVEF 20%, this will likely worsen heart failure and potentially cause further LV dysfunction. 2, 3

• Do not delay device implantation waiting for "LVEF improvement"—complete heart block requires prompt pacing, and the severely reduced LVEF makes CRT the appropriate initial choice. 1

• Ensure optimal guideline-directed medical therapy for heart failure is initiated or optimized alongside device implantation. 4

• CRT response rates are lower in non-LBBB patients (approximately 60-70% vs 80-90% with LBBB), but preventing pacing-induced cardiomyopathy remains crucial. 4, 2