Since the publication of European Society of Cardiology guidelines for the diagnosis and treatment of acute and chronic heart failure (HF) in 2016, a new class of HF has been defined, namely HF with mildly reduced ejection fraction (HFmrEF). Although the name was new, there had long been awareness of the existence of a grey area between the two established classes of HF: HF with reduced ejection fraction and HF with preserved ejection fraction. Patients between these two classes were previously either excluded from HF studies or were included in the other groups. With the definition of this new group of patients, a door has opened for researchers to further explore their characteristics, treatment and outcomes. In this article we aim to clarify the existing literature on the clinical characteristics and pathophysiology of this newly-defined group of patients.
Keywords: Acute and chronic heart failure; heart failure with mid-range ejection fraction; heart failure with preserved ejection fraction; heart failure with reduced ejection fraction; left ventricular ejection fraction.