Cardiac resynchronization therapy: the issue of non-response.

Cardiac resynchronization therapy reduces mortality and morbidity in heart failure patients with wide QRS and severe impairment of left ventricular systolic function, who are symptomatic despite optimal medical therapy. However, a high percentage of patients fail to show clinical or echocardiographic response to this treatment. Beyond current selection criteria, other elements, such as QRS duration and morphology, concomitant medical therapy, degree of right ventricle dysfunction, myocardial viability, presence of left ventricular dyssynchrony, and associated renal dysfunction, play a crucial role in modulating the response to cardiac resynchronization. Consequently, they should be part of the standard pre-implant evaluation, as they could be used to identify patients who are very unlikely to be responders.