Classical and Non-Classical Indications for Cardiac Resynchronization Therapy

Heart failure (HF) is a medical problem of huge socioeconomic importance, mainly due to the increasing life expectancy in our societies and the strides in the treatment of ischemic heart disease which resulted in improved prognosis of our patients. These medical and socioeconomic issues may explain why HF poses a significant financial burden on our health care systems. It is estimated that acute decompensated HF accounts for 2. % of all emergency room visits, its prevalence is steadily increasing in epidemic proportions and in age-dependent manner, reaching an incidence of almost 10% in patients aged >65 years. There has been considerable improvement in our therapeutic armamentarium over the years with significant benefit obtained with use of angiotensin converting enzyme inhibitors or angiotensin receptor blockers and β-blockers. However, there is still a large population of HF patients who remain refractory to current therapeutic approaches and this therapeutic gap has recently been bridged by the newer mode of electrical therapy known as cardiac resynchronization therapy (CRT), effected by biventricular pacing. CRT has been shown to improve symptoms and prognosis of patients with HF. The pathophysiological rationale for the development of CRT was the observation that some patients with advanced HF presented asynchronous contraction of the left ventricle (LV) on echocardiography. Asynchronous contraction due to cardiac conduction abnormalities, often described as cardiac dyssynchrony, reflected by the presence of left bundle branch block (LBBB) on the electrocardiogram (ECG), occurring in 20-30% of patients afflicted by HF, has been documented to adversely affect the function of the failing heart. Furthermore, it has been shown that cardiac dyssynchrony has an unfavorable influence on prognosis in patients with HF. CRT aims to improve the mechanical function of the failing heart. Biventricular pacing is an effective way to achieve CRT and restore electromechanical synchrony by simultaneously pacing at different sites of the heart, classically at the right ventricular apex and the lateral wall of the LV. This is accomplished by inserting the LV pacing lead via the coronary sinus and placing it into a lateral cardiac vein tributary. In a large number of studies, it is a consistent finding that biventricular pacing increases LV ejection fraction (LVEF) and cardiac output and most importantly, improves quality of life, functional class and exercise capacity in the majority of the treated patients. In terms of pathophysiology, CRT has a unique characteristic among other therapies for HF. Its favorable influence on cardiac performance has not been associated with increased oxygen consumption, an issue of profound importance especially in patients with ischemic cardiomyopathy. These striking beneficial effects of CRT, had not been accompanied by any detectable survival benefit in the large number of small randomized trials that were published during the initial period of biventricular pacing. Of course, these studies were not designed to detect a survival benefit and thus, AtHENs cArDIOLOGY UPDAtE 2008