**2. Incidence and importance of functional mitral regurgitation in left ventricular dysfunction**

Functional MR is a common, but often 'silent,' finding in heart failure patients [1–6]. Reported prevalence of functional MR varies depending on a diagnostic method (angiography, color Doppler echocardiography) and heart failure etiology (ischemic, non-ischemic) [1–5]. The incidence of functional MR after myocardial infarction varies from 20 to 50% but exceeds 50% in patients with non-ischemic dilated cardiomyopathy [4–6]. According to general estimates, nearly a half of heart failure patients may have a functional MR of some degree, and approximately one-third of them may suffer from moderate or severe functional MR [1, 3, 7].

Functional MR is an independent predictor of worse prognosis in patients with either ischemic or non-ischemic etiology of heart failure [3]. In patients with non-ischemic LV dysfunction, functional MR is associated with a two- to three-fold increased risk of heart failure episodes and cardiac mortality [4]. After myocardial infarction the presence of at least moderate functional MR is associated with a 3-fold increased risk of heart failure and a 1.6-fold increased risk of death at the 5-year follow-up [2]. Assessment of mitral valve function is included in routine risk stratification after myocardial infarction.

Functional MR is present in a large proportion of patients eligible for cardiac resynchronization therapy, with the incidence varying slightly from population to population and depending on the evaluation method. Significant—that is, at least moderate—functional MR is present in about 40% of patients qualified to CRT [8–10]. Nowadays, functional MR no longer disqualifies patients from resynchronization therapy if such treatment is indicated. Furthermore, a decrease in functional MR severity is a determinant of response to CRT [10–12].
