Cardiac resynchronization therapy: application of imaging to optimize patient selection and assess response.

Cardiac resynchronization therapy (CRT) has been shown to be effective for advanced heart failure and discoordinate wall motion, although the nonresponse rate for patients meeting standard CRT criteria remains about 30%. Although electrical delay (wide QRS) is commonly used to select candidates, recent data show that direct measures of mechanical dyssynchrony rather than surrogate -electrical measurements improve the identification of potential responders. This observation has stimulated the pursuit of imaging-based methods for characterizing dyssynchrony. Echocardiographic techniques have played a major role in this regard, but their accuracy and reliability have recently been scrutinized in multicenter clinical trials. This has shifted attention toward newer advanced echo methods and cardiac magnetic resonance imaging (CMR). Although CMR has a number of advantages over echocardiography for the assessment of dyssynchrony, these methods are relatively underdeveloped and not used widely clinically. This paper highlights imaging modalities for assessing wall motion and various methods for integrating the output from these complex imaging-based datasets into simple indices of dyssynchrony. Further, we investigate the role of imaging not only to improve selection of CRT responders but also to define CRT response.