What are Cardiac Resynchronization Therapy (CRT) studies in cardiology?

From the Guidelines

Cardiac Resynchronization Therapy (CRT) is a highly effective treatment for patients with moderate to severe heart failure, particularly those with left bundle branch block (LBBB) and a QRS duration ≥150 ms, as it has been shown to reduce mortality and hospitalization rates, and improve quality of life, as demonstrated in studies such as COMPANION and CARE-HF 1, 2.

Key Findings

• CRT has been shown to reduce the risk of death from any cause and hospital admission for worsening heart failure in patients with moderate to severely symptomatic heart failure (NYHA class III or IV) 1.
• The benefits of CRT are greatest among patients with a QRS duration ≥150 ms, with a significant reduction in the primary endpoint of death or hospitalization for heart failure (HR: 0.58; 95% CI: 0.50 to 0.68; p<0.00001) 2.
• CRT also improves symptoms, quality of life, and ventricular function, and has been shown to reduce mitral regurgitation and promote reverse remodeling of the left ventricle 1, 3.
• The scientific basis for CRT lies in correcting ventricular dyssynchrony, which improves cardiac output and reduces metabolic costs 4.

Patient Selection

• Patients with LBBB and a QRS duration ≥150 ms are most likely to benefit from CRT, with a Class I recommendation for CRT in these patients 2, 4.
• Patients with non-LBBB conduction abnormalities may not benefit from CRT, with no significant reduction in composite adverse clinical events (RR: 0.97; 95% CI: 0.82 to 1.15; p=0.75) 2.
• The use of biomarkers, such as circulating miRNAs, may help identify patients who are most likely to benefit from CRT, with plasma miR-30d identified as a predictor of therapeutic response in multivariate models 5.

Clinical Implications

• CRT should be considered as a standard treatment option for patients with moderate to severe heart failure, particularly those with LBBB and a QRS duration ≥150 ms.
• Patient selection for CRT should be based on the presence of LBBB and a QRS duration ≥150 ms, as well as other clinical factors such as symptoms and ventricular function.
• Further research is needed to identify biomarkers that can help predict response to CRT and optimize patient selection.

From the Research

Cardiac Resynchronization Therapy (CRT) Studies

The following are key points about Cardiac Resynchronization Therapy (CRT) studies in cardiology:

• CRT is a therapy of choice for patients with symptomatic systolic heart failure (HF) and left bundle branch block (LBBB), despite optimal medical therapy (OMT) 6.
• The 2021 European Society of Cardiology (ESC) Guidelines highlight the importance of CRT on top of OMT in HF patients with left ventricular ejection fraction (LVEF) ≤ 35%, sinus rhythm, and typical LBBB with QRS duration ≥ 150 ms 6.
• CRT may be considered in cases when increased pacing of the right ventricle is not desirable, and alternative pacing sites and strategies are available if CRT is not feasible and effective in patients 6.
• Strategies targeting "multi-sides" or using "multi-leads" have shown superiority over classic CRT, and conduction system pacing seems to be a promising technique 6.
• A real-world registry study found that 58.9% of patients improved, 20.1% stabilized, and 21.0% worsened after CRT, with several patient characteristics associated with a lower likelihood of response to CRT, including older age, ischemic aetiology, renal dysfunction, AF, non-LBBB morphology, and diabetes 7.
• Patients with AF had particularly worse clinical outcomes, higher HF hospitalization, and mortality rates, and lower percentages of ventricular pacing 7.
• CRT has emerged as a valued nonpharmacologic therapy in patients with heart failure, reduced ejection fraction (EF), and ventricular dyssynchrony manifest as left bundle branch block, with mechanisms of benefit including remodeling of the left ventricle and decrease in the severity of mitral regurgitation 8.
• Device therapy in heart failure with reduced ejection fraction (HFrEF) includes implantable cardioverter-defibrillators and CRT, with CRT being the therapy of choice in symptomatic patients with HFrEF and a broad QRS complex with a left bundle branch block (LBBB) morphology 9.
• Alternative electrical therapies such as baroreflex activation therapy (BAT) and cardiac contractility modulation (CCM) may be considered for patients who are not candidates for CRT 9.
• CRT may be beneficial in heart failure patients with LVEF ≤35% and electrical dyssynchrony, and its effects among patients with less severe LV dysfunction have not been established, although recent post-hoc analyses suggest that CRT benefit may be present in patients with LVEF >35% 10.

Key Findings

• CRT is effective in improving symptoms and reducing mortality in patients with heart failure and left bundle branch block 6, 7, 8.
• Patient characteristics such as older age, ischemic aetiology, renal dysfunction, AF, non-LBBB morphology, and diabetes are associated with a lower likelihood of response to CRT 7.
• Alternative pacing sites and strategies, as well as conduction system pacing, may be considered for patients who are not candidates for CRT or have failed CRT 6, 9.
• Further research is needed to establish the effects of CRT in patients with less severe LV dysfunction and to determine the optimal patient selection criteria for CRT 10.

Patient Selection

• Patients with symptomatic heart failure and left bundle branch block are ideal candidates for CRT 6, 8.
• Patients with AF, non-LBBB morphology, and other comorbidities may require individualized consideration for CRT 7, 9.
• Patients with less severe LV dysfunction may benefit from CRT, although further research is needed to establish the effects of CRT in this population 10.

Future Directions

• Further research is needed to determine the optimal patient selection criteria for CRT and to establish the effects of CRT in patients with less severe LV dysfunction 10.
• Alternative pacing sites and strategies, as well as conduction system pacing, may be considered for patients who are not candidates for CRT or have failed CRT 6, 9.
• Large, global registry studies are needed to provide new insight into factors influencing response to CRT and to determine the clinical outcomes of patients with different characteristics 7.