Distinction of "fat around the heart".

We were greatly interested by the study by Wong et al. (1) on pericardial fat and atrial fibrillation (AF) outcomes. This study further highlights the association of pericardial fat with cardiac disease. The authors remark that pericardial fat may predict the severity and symptomatic burden of AF and recurrence of AF after conventional AF ablation techniques. The authors have used the term pericardial fat in this study to denote the fat present between the myoepicardium and the pericardium. Others have described this structure more aptly, however, as epicardial fat, thus distinguishing it from paracardial fat that is present outside the parietal pericardium (2). This differentiation is necessary since these 2 types of fat deposits have different embryological origins (3). Epicardial fat, having similar origin as that of omental and mesenteric fat (3), correlates better with body mass index and metabolic syndrome and is likely to be a better surrogate for cardiac disease (4). Unlike the paracardial fat, the epicardial fat shares a common blood supply with the myocardium and is believed to have paracrine properties that are implicated as a cause of myocardial inflammation due to its proximity to the myocardium. Indeed, autopsy studies have shown that epicardial fat extends into the apposing myocardium (5). Prior studies have used the term pericardial fat to represent both paracardial and epicardial fat (6). It appears that in this study, the authors have used pericardial fat interchangeably for epicardial fat. Furthermore, they have not clearly mentioned whether magnetic resonance imaging is sufficiently sensitive to separate the epicardial fat from the paracardial fat given the temporal motion artifact in magnetic resonance images. Interestingly enough, the authors have calculated epicardial fat in end diastole. A recent review by Iacobellis et al. (2) suggests that epicardial fat should be assessed in end systole during echocardiography. Fat being a compressible structure is better visualized in systole, as there is more space for it to expand. Nevertheless, the authors have made a significant contribution to our knowledge of the association between pericardial fat and cardiac disease, and should be congratulated on their effort. Future insights into defining the role of epicardial fat in cardiac disease may usher development of interventions to curb the growing epidemic of cardiac disease across its spectrum.