Cardiac resynchronization therapy (CRT): clinical trials, guidelines, and target populations.

Over the last 10 years, several large, well-designed clinical trials have firmly established the role of cardiac resynchronization therapy (CRT) as a recommended treatment strategy for moderate-to-severe heart failure (HF). A review of the relevant results from the MUSTIC, MIRACLE, CONAK-CD, and MIRACLE ICD trials reveals that in patients with New York Heart Association (NYHA) class III-IV HF, CRT produces consistent improvements in quality of life, functional status, and exercise capacity while also providing strong evidence for reverse remodeling and diminished functional mitral regurgitation, resulting in reductions in both HF hospitalizations and all-cause morbidity and mortality. In patients with earlier NYHA class I-II HF, the benefit of CRT has been more controversial. The principal ongoing challenges addressed in this article include the substantial 30% of patients who receive a CRT device but fail to respond, the wide variations in how to define "response" vs "nonresponse," and how to identify patients who will benefit from CRT, especially narrow QRS (<120 ms), those with right bundle branch block, and those with mild-to-moderate (NYHA class I-II) HF. An important result of this uncertainty is the lack of a good sense of the optimal rate of CRT implantation, making consideration of the data reviewed in this article crucial for identifying important gaps of knowledge and mechanisms of action that need to be studied in the near future.