Echocardiography in Assessment of Cardiac Synchrony

Electromechanical association in a normal heart results in synchronous regional left ventricular (LV) contraction. Differences in the timing of regional contraction may be associated with the failing human heart. Interest in echocardiographic assessment of synchrony began with applications for pacing therapy, in particular cardiac resynchronization therapy (CRT), also known as biventricular pacing. Current clinical practice is to select heart failure patients with reduced ejection fraction by electrocardiographic criteria of QRS complex widening or left bundle branch block morphology. Although CRT often results in improvement in symptoms, LV reverse remodeling, and prolonging life, one-third to one-half of patients do not appear to benefit and are referred to as nonresponders. It was observed that patients with measurable dyssynchrony at baseline before CRT had a much more favorable response to CRT than patients who lacked baseline dyssynchrony. Interest in measuring regional timing of LV contraction with echocardiographic techniques remains high to gain a better understanding of CRT response and prognosis. Tissue Doppler imaging and, in particular, speckle tracking strain measures have been widely utilized. However, the field advanced to reveal that mechanical dyssynchrony was more complicated than originally thought. A more recent understanding of regional myocardial discoordination has emerged as differences in regional contraction from substrates of electrical delay, contractile heterogeneity, and fibrosis or scar. Defining the strain patterns of contraction and stretch appear promising to help identify the electromechanical substrate that is associated with the most favorable outcome after CRT. This chapter will review the progress in understanding of mechanical dyssynchrony, define the current state of the art, and project potential future clinical applications of assessing cardiac synchrony.