The idea of cardiac resynchronization therapy (CRT), most likely came out of a variety of animal studies performed in the late 1980s and early 1990s. From 1995 - 1997 there were many observational studies that confirmed the initial value of cardiac resynchronization. From 1998 - 1999 we had available randomized studies, placebo controlled, to determine exercise capacity, NYHA, quality of life, and from 2000 we had randomized studies to determine hospitalizations and mortality.
All randomized trials have confirmed a significant alleviation of symptoms and increase in exercise capacity: mean NYHA class decreased by 0.5-0.8 points, distance in 6 min walk test increased by a mean of 20% and peak oxygen consumption during exercise increased by 10-15%. Later, a meta analysis in 2003 found a 30% reduction of hospitalizations for the management of congestive heart failure (CHF), attributable to CRT. Lately, the study COMPANION: CRT with/out implantable cardiac defibrillator (ICD) lowered the combined end point of mortality and re-hospitalization by 35-40% and in the CARE-HF, CRT lowered the proportion of hospitalization for CHF by 52% and unplanned hospitalizations for major cardiovascular events by 39%. CRT reverses left ventricular (LV) remodelling, decreases LV end systolic and end diastolic volumes (By 15%), and increases LV ejection fraction (EF) (By 6%). The REVERSE study has confirmed this aspect. It is often said that about onethird of patients who appear to warrant CRT will not benefit from the therapy. The definition of failure to respond, however, needs to be considered carefully. The failure to implant an effectively functioning system, a poor lead position or failure of pacing stimuli may explain some non responders. Patients whose condition worsened with CRT might have done much worse without CRT, or worsen due to intercurrent illness. In summary there is a large number of patients studied in randomized clinical trials that demonstrate that CRT improves quality of life, exercise capacity, functional capacity, improves cardiac function and structure and, in addition, CRT reduces the risk of mortality.