Epicardial fat in heart failure-Friend, foe, or bystander.

Epicardial adipose tissue (EAT) is a fat depot covering the heart. No physical barrier separates EAT from the myocardium, so EAT can easily affect the underlying cardiac muscle. EAT can participate in the development and progression of heart failure with preserved (HFpEF) and reduced ejection fraction (HFrEF). In healthy humans, excess EAT is associated with impaired cardiac function and worse outcomes. In HFpEF, this trend continues: EAT amount is usually increased, and excess EAT correlates with worse function/outcomes. However, in HFrEF, the opposite is true: reduced EAT amount correlates with worse cardiac function/outcomes. Surprisingly, although EAT has beneficial effects on cardiac function, it aggravates ventricular arrhythmias. Here, we dissect these phenomena, trying to explain these paradoxical findings to find a target for novel heart failure therapies aimed at EAT rather than the myocardium itself. However, the success of this approach depends on a thorough understanding of interactions between EAT and the myocardium.