Differential Echocardiographic Response to Cardiac Resynchronization Therapy and Clinical Outcomes According to QRS Morphology and QRS Duration

Study Questions:

What impact does QRS duration and morphology impart to clinical response to cardiac resynchronization therapy (CRT)?


This was a retrospective single-center study of 812 patients undergoing CRT implantation for management of systolic heart failure. Patients were grouped according to preimplant electrocardiogram based on QRS morphology (left bundle branch block [LBBB], right bundle branch block [RBBB], intraventricular conduction delay), and QRS duration (≥150 ms or <150 ms). Endpoints included change in ejection fraction (primary endpoint), echocardiographic measurements, and New York Heart Association (NYHA) class after CRT. A composite endpoint of event-free survival (death, heart transplant, or left ventricular assist device [LVAD] implant) was also examined.


Of the 812 patients reviewed, 496 were included. There were 92 (19%) patients with a non-LBBB and QRS ≥150 ms, 103 (21%) with a non-LBBB and QRS <150 ms, 216 (44%) with an LBBB and QRS ≥150 ms, and 85 (17%) with an LBBB and QRS <150 ms. Paired echocardiograms were available in 313 patients with post-CRT echocardiograms done a mean of 12 months after device implant. Patients with an LBBB and QRS ≥150 ms appeared to have the greatest improvement in echocardiographic and clinical parameters, demonstrating the greatest improvement in ejection fraction, LV dimensions, mitral regurgitation, and NYHA class after CRT. Conversely, patients with a non-LBBB and QRS <150 ms appeared less likely to improve after CRT. After a median follow-up of 5 years, there were 181 (38%) events, of which 171 were deaths. Compared with patients who had an LBBB and QRS ≥150 ms, patients with a non-LBBB and QRS <150 ms had an unadjusted risk (hazard ratio [HR], [95% confidence interval]) of having a cardiac event of 1.8 [1.2-2.6], but this did not remain significant (HR, 1.4 [0.93-2.2]) after adjusting for other predictors. A nonsignificant trend toward worse outcome was noted for those with an LBBB and QRS <150 ms (adjusted HR, 1.5 [0.95-2.4]) versus LBBB and QRS ≥150 ms. When QRS duration and morphology were simultaneously entered into modeling, QRS duration no longer remained a significant predictor of events (p = 0.36).


The authors concluded that QRS morphology is a more important determinant of CRT response than QRS duration.


Predicting response to CRT remains a challenge. In this single-center, nonblinded, and noncontrolled study, it appears that CRT response is greatest in those with an LBBB and QRS ≥150 ms. This is not surprising given that these patients are likely to have the greatest delays in LV activation that are theoretically remediated by LV pacing. In converse, those without an LBBB and with a QRS <150 ms appear to gain least benefit, likely because LV pacing does not address this form of interventricular dyssynchrony. The authors also concluded that QRS morphology is more important than QRS duration in determining response. Others have shown that patients with an RBBB may do worse after CRT than without. A controlled trial of CRT in patients without an LBBB, but morphologically wide QRS, is needed.

Keywords: Follow-Up Studies, Heart-Assist Devices, Mitral Valve Insufficiency, Heart Conduction System, Electrocardiography, New York, Heart Failure, Systolic, Heart Transplantation, Cardiac Resynchronization Therapy, Cardiac Pacing, Artificial, Bundle-Branch Block, Hypertension, Echocardiography

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