QRS Morphology and Risk of VT in CRT Recipients

Quick Takes

  • Patients with HFrEF and LBBB QRS morphology derive the greatest clinical benefit from CRT, while patients with non-LBBB may experience harm with CRT.
  • Patients with non-LBBB QRS morphology may be at an increased risk of ventricular arrhythmia with CRT.

Study Questions:

Is there an effect of cardiac resynchronization therapy with a defibrillator (CRT-D) on the risk of life-threatening ventricular tachyarrhythmia in patients with heart failure (HF)?


The authors analyzed pooled clinical variables and outcomes of patients enrolled in MADIT-II, MADIT-CRT, MADIT-RIT, MADIT-RISK, and RAID trials. Patients with QRS duration ≥130 ms were divided into two groups: those implanted with an implantable cardioverter-defibrillator (ICD)-only versus CRT-D. The primary endpoint was fast ventricular tachycardia (VT)/ventricular fibrillation (VF) (defined as VT ≥200 bpm or VF). Secondary endpoints included appropriate shocks, any sustained VT or VF, and the burden of fast VT/VF.


A total of 2,862 patients were implanted with ICDs and CRT-Ds. Among patients with left bundle branch block (LBBB) (n = 1,792), those with CRT-D (n = 1,112) experienced a significant 44% (p < 0.001) reduction in the risk of fast VT/VF compared to ICD-only patients (n = 680), a significantly lower burden of fast VT/VF (hazard ratio [HR], 0.55; p = 0.001), with a reduced burden of appropriate shocks (HR, 0.44; p < 0.001).

In contrast, among patients with non-LBBB (n = 1,070), CRT-D was not associated with reduction in fast VT/VF (HR, 1.33; p = 0.195). Furthermore, non-LBBB patients with CRT-D experienced a statistically significant increase in the burden of fast VT/VF events compared with ICD-only patients (HR, 1.90; p = 0.013).


The authors conclude that a potential proarrhythmic effect of CRT exists among patients with non-LBBB preimplant QRS morphology.


Prior studies have painted an inconsistent picture of the effect of CRT on ventricular arrhythmia in patients with HF and reduced ejection fraction (HFrEF) who have a broad QRS. Some studies demonstrated sudden cardiac death reduction with CRT pacemakers. Others showed that some patients experience proarrhythmia from CRT. It has been established that patients with LBBB treated with CRT-D have a survival benefit, but patients with non-LBBB have more adverse clinical events. The current study suggests that the effect of CRT on proarrhythmia may depend on whether the patient has underlying LBBB or non-LBBB. There are many mechanisms which may explain this, with one hypothesis being that the left ventricular epicardial stimulation is proarrhythmic in the absence of reverse remodeling. The current study provides another confirmation that QRS morphology is key to patient selection for CRT. A prospective randomized clinical trial is needed to further determine the effect of CRT on patients with HFrEF and non-LBBB QRS morphology.

Clinical Topics: Arrhythmias and Clinical EP, Heart Failure and Cardiomyopathies, Implantable Devices

Keywords: Cardiac Resynchronization Therapy, Heart Failure, Reduced Ejection Fraction

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