CRT Guidelines 2026: Indications, QRS Criteria & CRT-P vs CRT-D

 

Cardiac Resynchronization Therapy (CRT): Indications, Guidelines and Clinical Decision-Making

A practical, guideline-based guide for cardiologists, electrophysiologists, postgraduate trainees and medical students.

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Introduction

Cardiac resynchronization therapy (CRT) is a device-based treatment for selected patients with heart failure and ventricular electrical dyssynchrony. By coordinating right and left ventricular activation, CRT can improve left ventricular systolic function, reduce heart failure hospitalizations, improve functional status and, in appropriately selected patients, reduce mortality.

Patient selection is critical. The greatest benefit is generally observed in patients with heart failure with reduced ejection fraction (HFrEF), left bundle branch block (LBBB), and a markedly prolonged QRS duration.

Key clinical message

The classic CRT candidate has LVEF ≤35%, persistent symptoms despite guideline-directed medical therapy (GDMT), sinus rhythm, LBBB morphology and QRS duration ≥150 ms.

What Is CRT?

CRT uses coordinated pacing of the right and left ventricles, usually through a right ventricular lead and a left ventricular lead positioned through the coronary sinus into an appropriate cardiac vein. A CRT system may also include a right atrial lead when indicated.

• CRT-P: Cardiac resynchronization therapy with a pacemaker.
• CRT-D: Cardiac resynchronization therapy combined with an implantable cardioverter-defibrillator (ICD).
• Conduction system pacing (CSP): His bundle pacing (HBP) or left bundle branch area pacing (LBBAP), which may provide an alternative or rescue strategy in selected patients when conventional biventricular pacing is unsuitable or ineffective.

Major Guidelines for CRT

Clinical decisions should be guided by the 2021 ESC pacing and CRT guideline, the 2022 AHA/ACC/HFSA heart failure guideline and the 2023 HRS/APHRS/LAHRS guideline on cardiac physiologic pacing.

• ESC: Emphasizes LVEF, QRS morphology, QRS duration and clinical symptoms.
• AHA/ACC/HFSA: Recommends CRT in appropriately selected patients with symptomatic HFrEF and electrical dyssynchrony, with additional indications involving AV block and atrial fibrillation.
• HRS/APHRS/LAHRS: Provides recommendations for conventional CRT, prevention of pacing-induced cardiomyopathy and selected use of conduction system pacing.

1. Core Indications for CRT

The following table summarizes the principal adult CRT indications. Recommendation classes and evidence levels may differ between guidelines; the table is a practical synthesis rather than a verbatim reproduction of one guideline.

Clinical profile	CRT recommendation
LVEF ≤35%, NYHA II–IV symptoms, LBBB, QRS ≥150 ms, despite GDMT	Strong indication; Class I in major guidelines.
LVEF ≤35%, NYHA II–IV, LBBB, QRS 120–149 ms	CRT is reasonable or should be considered; benefit may be greater in selected patients, including women.
LVEF ≤35%, NYHA II–IV, non-LBBB, QRS ≥150 ms	CRT may be useful; recommendations are generally weaker than for LBBB.
LVEF ≤35%, non-LBBB, QRS 120–149 ms	Benefit is less certain and depends on symptoms and the guideline applied.
LVEF ≤35%, QRS <120 ms	Routine CRT is not recommended in the absence of another specific pacing indication.
High-grade AV block requiring ventricular pacing with LVEF 36–50%	CRT is reasonable under the AHA/ACC/HFSA guideline; physiologic pacing strategies should be considered.
HFrEF with an indication for substantial ventricular pacing	Prefer CRT or an appropriate physiologic pacing strategy over routine right ventricular pacing in suitable patients.

Important: The traditional 2021 ESC CRT recommendation for LBBB with QRS 130–149 ms differs in wording and threshold from US guidance and the 2023 HRS guideline. The HRS guideline supports CRT in selected patients with LVEF ≤35%, LBBB and QRS 120–149 ms. Do not apply one QRS threshold without considering the specific guideline and clinical context.

2. CRT According to QRS Duration and Morphology

A. LBBB with QRS ≥150 ms

This is the group with the most consistent and strongest evidence for CRT.

• LVEF ≤35%.
• Persistent NYHA class II, III or ambulatory class IV symptoms despite GDMT.
• LBBB morphology on a properly interpreted 12-lead ECG.
• QRS duration ≥150 ms.

CRT reduces heart failure morbidity and mortality and can improve symptoms, LVEF and reverse ventricular remodeling.

B. LBBB with QRS 120–149 ms

CRT can benefit selected patients with LBBB and reduced LVEF, although the average benefit is less consistent than in patients with QRS ≥150 ms. Patient characteristics, including sex, body size, symptom burden and the certainty of LBBB diagnosis, may influence the expected response.

The 2021 ESC guideline uses a QRS threshold of 130 ms for its principal CRT recommendations in patients without a separate pacing indication. The 2023 HRS guideline supports CRT in selected patients with LBBB and QRS 120–149 ms.

C. Non-LBBB with QRS ≥150 ms

Patients with non-LBBB patterns, including right bundle branch block (RBBB) and nonspecific intraventricular conduction delay, may be considered for CRT when LVEF is ≤35% and symptoms persist despite GDMT. However, benefit is less predictable than in LBBB. In particular, evidence for benefit in isolated RBBB is limited, so morphology should be assessed carefully.

D. Non-LBBB with QRS <150 ms

Routine CRT is generally not recommended for patients with NYHA class I–II symptoms and non-LBBB morphology with QRS duration below 150 ms. For more symptomatic patients, the benefit remains uncertain and individualized specialist assessment is appropriate.

3. Role of Left Ventricular Ejection Fraction

LVEF is a central component of conventional CRT eligibility. For the standard heart failure indication, LVEF ≤35% is the usual threshold when symptoms persist despite appropriate medical treatment.

• LVEF ≤35%: Assess symptoms, QRS morphology and duration, rhythm and GDMT to determine eligibility.
• LVEF 36–50% with high-grade AV block: Consider CRT or another physiologic pacing strategy when substantial ventricular pacing is expected.
• LVEF 36–50% with LBBB and QRS ≥150 ms: The 2023 HRS guideline states that cardiac physiologic pacing may be considered in selected symptomatic patients to maintain or improve LVEF, particularly when a pacing device is otherwise being implanted.
• Preserved LVEF: Conventional CRT is not routinely indicated solely for a wide QRS in the absence of another relevant pacing indication.

4. CRT and Guideline-Directed Medical Therapy

Before elective CRT implantation for HFrEF, clinicians should optimize GDMT and reassess symptoms and ventricular function when clinically appropriate. The aim is to avoid implanting a device before potentially reversible LV dysfunction has improved.

Foundational HFrEF therapy generally includes four medication classes, as tolerated and indicated:

1. ARNI, or an ACE inhibitor/ARB when appropriate.
2. Evidence-based beta-blocker.
3. Mineralocorticoid receptor antagonist (MRA).
4. SGLT2 inhibitor.

Diuretics are used to control congestion. Other therapies are selected according to the underlying cause, blood pressure, renal function, potassium level, comorbidities and contraindications.

Practical point

There is no universal fixed waiting period that fits every patient. Reassessment after GDMT depends on clinical stability, duration of LV dysfunction, reversible causes, recent myocardial infarction or revascularization, and whether a separate pacing indication requires earlier intervention.

5. CRT in Atrial Fibrillation

Atrial fibrillation (AF) does not automatically exclude a patient from CRT. However, the treatment is effective only when a sufficiently high proportion of ventricular beats are effectively resynchronized.

Consider CRT in patients with AF who meet conventional CRT criteria or require ventricular pacing, particularly when:

• LVEF is ≤35% and other CRT criteria are satisfied.
• There is an indication for AV junction ablation or ventricular pacing.
• Rate control or AV junction ablation can achieve near-complete effective biventricular capture.

When conducted AF prevents adequate biventricular pacing, AV junction ablation may be required in selected patients. Device counters alone can overestimate effective capture because fusion and pseudofusion beats may be counted as paced events.

6. CRT in Patients Requiring Frequent Ventricular Pacing

Chronic right ventricular pacing can produce ventricular dyssynchrony and contribute to pacing-induced cardiomyopathy in susceptible patients.

In patients with reduced LVEF who require ventricular pacing because of high-grade AV block, CRT is often preferable to conventional RV pacing. The 2022 AHA/ACC/HFSA guideline considers CRT reasonable in patients with high-degree or complete heart block and LVEF 36–50%.

The 2023 HRS/APHRS/LAHRS guideline also addresses cardiac physiologic pacing, including CRT, His bundle pacing and left bundle branch area pacing, to prevent or mitigate pacing-induced heart failure. The choice depends on the anticipated pacing burden, LVEF, anatomy, operator expertise and patient-specific factors.

7. CRT-P Versus CRT-D

Feature	CRT-P	CRT-D
Resynchronization	Yes	Yes
Defibrillation for ventricular tachyarrhythmias	No	Yes
Typical consideration	Patients who need resynchronization but do not have a separate indication for an ICD, or in whom ICD benefit is not expected to outweigh risks.	Patients who meet criteria for both CRT and primary or secondary prevention ICD therapy.

The choice between CRT-P and CRT-D should be individualized. Assess ischemic versus nonischemic etiology, arrhythmic history, age, comorbidities, competing non-arrhythmic mortality, expected survival, patient preferences and local guidelines. CRT eligibility alone does not automatically establish an ICD indication.

8. Pre-Implantation Assessment

A structured evaluation helps confirm eligibility and identify factors that may affect the response to CRT.

• Clinical assessment: NYHA class, symptom duration, heart failure admissions, volume status, comorbidities and expected survival.
• ECG: QRS duration, LBBB versus non-LBBB morphology, rhythm and existing pacing pattern.
• Echocardiography: LVEF, LV dimensions and volumes, RV function, mitral regurgitation and relevant valve disease.
• Medical therapy: Review adherence, tolerability and optimization of HFrEF treatment.
• Rhythm assessment: Evaluate AF burden, ventricular rate and the likelihood of achieving effective biventricular capture.
• Procedural planning: Review venous access, prior devices, infection risk, renal function and other factors affecting implantation.

9. How to Assess CRT Response

Response to CRT should be assessed using clinical, electrical and imaging parameters rather than a single measurement.

• Improvement in dyspnea, exercise tolerance and NYHA functional class.
• Reduction in heart failure hospitalization and congestion.
• Improvement in LVEF and reduction in LV end-systolic volume.
• Effective biventricular pacing, ideally as close to 100% as possible.
• Assessment of lead function, arrhythmias, battery status and device programming.

A commonly used research definition of echocardiographic response is a reduction in LV end-systolic volume of at least 15%, but definitions vary between studies. Clinical benefit and reverse remodeling do not always occur together.

10. Causes of CRT Non-Response

Some patients fail to achieve the expected clinical or echocardiographic improvement. Before labeling a patient a non-responder, systematically assess:

• Incorrect patient selection or an uncertain diagnosis of LBBB.
• Inadequate biventricular pacing due to AF, frequent PVCs or other arrhythmias.
• Suboptimal LV lead position or ineffective electrical capture.
• Scar tissue at the pacing site, particularly in ischemic cardiomyopathy.
• Suboptimal AV or VV timing in selected cases.
• Persistent severe mitral regurgitation or other structural disease.
• Inadequately treated heart failure, ischemia or comorbidities.
• Progression of the underlying cardiomyopathy.

Management may include device interrogation, ECG review, optimization of medical therapy, treatment of arrhythmias, assessment of lead position and selected programming or lead revision procedures.

11. Practical CRT Decision Algorithm

Patient with heart failure or a pacing indication

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Assess LVEF, NYHA class, 12-lead ECG and QRS duration

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LVEF ≤35% + LBBB + QRS ≥150 ms + persistent NYHA II–IV symptoms?

Yes → Strong indication for CRT, after confirming clinical suitability and appropriate medical therapy.

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If no: Evaluate other guideline-supported groups, including LBBB with shorter QRS duration, non-LBBB with QRS ≥150 ms, and patients requiring substantial ventricular pacing.

If AF is present: Determine whether near-complete effective biventricular capture can be achieved, with AV junction ablation considered when appropriate.

Before implantation: Individualize device selection, procedural strategy, expected benefit and patient preferences.

12. Important Clinical Pearls

• The strongest conventional CRT indication is LVEF ≤35%, symptomatic HFrEF, LBBB and QRS ≥150 ms despite GDMT.
• QRS morphology matters: patients with LBBB generally benefit more than patients with RBBB.
• Do not use QRS duration alone to select patients; integrate LVEF, symptoms, rhythm and pacing requirements.
• In AF, effective biventricular capture is essential for a meaningful response.
• When high-grade AV block requires substantial ventricular pacing, avoid unnecessary chronic RV pacing in patients at risk of pacing-induced cardiomyopathy.
• CRT-P and CRT-D are not interchangeable: assess the indication for defibrillator therapy separately.
• Conduction system pacing is an evolving option and may be appropriate in selected patients, but it is not automatically interchangeable with conventional CRT in every clinical situation.

Conclusion

CRT is a cornerstone device therapy for appropriately selected patients with symptomatic HFrEF and electrical dyssynchrony. The best-established candidates have LVEF ≤35%, LBBB and QRS duration ≥150 ms despite optimized medical treatment. Other patients may benefit depending on QRS morphology, symptom severity, atrial fibrillation, AV block and anticipated ventricular pacing burden.

Accurate ECG interpretation, echocardiographic assessment, optimization of GDMT and careful device selection are essential to maximize benefit and minimize avoidable procedures.

References and Further Reading

1. Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy.