Echocardiographic Assessment of Dyssynchrony for Predicting a Favorable Response to Cardiac Resynchronization Therapy

Echocardiographic Assessment of Dyssynchrony for Predicting a Favorable Response to Cardiac Resynchronization Therapy

J M Prutkin
US Cardiovascular Disease 2006
Published: June 2006
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Numerous clinical trials involving patients with severe, symptomatic heart failure and a wide QRS complex have shown benefits from cardiac resynchronization therapy (CRT). CRT can increase left ventricular (LV) ejection fraction, decrease left ventricular volume and mitral regurgitation, improve symptoms caused by heart failure, and may also improve mortality. Current American College of Cardiology/American Heart Association (ACC/AHA) guidelines suggest CRT implantation for patients with left ventricular ejection fraction (LVEF) <35%, New York Heart Association (NYHA) class III or ambulatory IV heart failure, and dyssynchrony defined by a QRS>120ms. However, among patients who receive a CRT based on this criteria, one-third of patients will fail to respond to CRT therapy.Many non-responders do not exhibit intra-ventricular mechanical dyssynchrony at baseline, and it may be that CRT most benefits those with underlying dyssynchrony.

Due to its ease of use and wide availability, echocardiography has emerged as the preferred modality to assess dyssynchrony. There are several echocardiographic techniques under investigation including tissue Doppler imaging, realtime three dimensional (3-D) echocardiography, and speckle tracking, although currently there is no clearly accepted standard.


What is Dyssynchrony?
Patients who have LV systolic dysfunction and dilatation frequently have a prolonged QRS complex, often in an left bundle-branch block (LBBB) pattern. QRS prolongation is generally associated with delayed electrical activation of the left ventricle, leading to uncoordinated ventricular motion, decreased stroke volume, and mitral regurgitation. Because of this association, QRS duration has been used as a surrogate marker for ventricular dyssynchrony. However, several studies have demonstrated that not all patients with a wide QRS complex exhibit have evidence of mechanical dyssynchrony. Furthermore, up to 30-50% of patients with a narrow QRS complex may have mechanical dyssynchrony measured by echocardiography. Some of these patients with narrow QRS may benefit from CRT.


The goal of CRT is to synchronize LV contraction.This is accomplished by pre-emptively pacing the latest activated segment in the LV, resulting in improved coordination of each region of the LV's contribution to systole. This results in an increase in stroke volume, improvement in myocardial efficiency, an increase in the diastolic filling time, and reduction of mitral regurgitation by synchronizing papillary muscle activation. CRT may also improve atrioventricular (AV) timing, which may improve diastolic filling and reduce mitral regurgitation. Interventricular timing may also be enhanced through CRT, though the significance of this is currently not known. In addition to the positive hemodynamic effects mentioned above, CRT exerts a favorable effect on cardiac autonomic control that results in less dependence on sympathetic activation. This can lead to smaller ventricular size and improved cardiac function in patients with severely symptomatic heart failure, known as reverse remodeling.

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