Tuesday, 30 January 2018

The Association of Epicardial Fat Thickness to Cardiovascular Clinical Outcomes

Epicardial fat is a visceral fat deposit which is located between the heart and the pericardium sharing many of the patho-physiological properties of other visceral fat deposits. There is recognition of three functional types of adipose tissue. The first type, the white adipose tissue consists of large unilocular adipocytes whose primary function is to store energy in the form of triglyceride. The second type, the brown adipose tissue which contains multilocular adipocytes with large numbers of mitochondria, this is most commonly found in young mammals and rodents. Its primary function is to generate heat via uncoupled oxidative phosphorylation. Third, the beige adipose tissue is form of brown adipocytes that arises within the white adipose depots and also has thermogenic capacity. 

It is important to differentiate between the adipose tissue located on the outer surface of the fibrous pericardium (paracardial fat) from the one in the inner surface of the visceral pericardium (epicardial fat) which is in direct contact with the myocardium and the epicardial vessels, since they differ in their biochemical, molecular and vascular nutrition properties. The paracardial fat is nourished by the pericardiophrenic artery, a branch of the internal thoracic artery, while the epicardial fat is nourished by the coronary arteries. The epicardial fat is more prominent in the atrioventricular and interventricular grooves and right ventricular lateral wall. Adipocyte infiltration into the myocardium wall as well as triglyceride infiltration into myocytes may also occur. The paracardial fat has been also called intrathoracic, mediastinal or pericardial. In addition, some other groups treat these different fat deposits as a single compartment, calling it pericardial fat. Since several studies have observed a moderate association between EFT and cardiovascular clinical outcomes, it is important to analyze this relationship at the light of medicine based evidence.

Epicardial fat thickness (EFT) can be measured by different imaging modalities. Magnetic resonance imaging (MRI) is considered the gold standard for the assessment of total body fat and reference modality for the analysis of ventricular volumes and mass, thus making it a natural choice for the detection and quantification of EFT. For purposes of cardiovascular risk stratification, measurement of EFT using echocardiography has generally been the study of choice, due to its lesser cost, ease of use, and absence of radiation. By echocardiography, measurements of the right ventricular free wall from both parasternal longitudinal and transverse parasternal views should be performed using the mean of three consecutive beats. These echocardiographic measurements show good correlation with the values found on MRI (r = 0.91, p = 0.001). There are some controversial issues in the EFT measurements by echocardiography. For example, there are some inconsistencies in the site of measurement due to spatial variations of the echocardiographic window, especially along the great vessels and the right ventricle. In addition, it is uncertain yet which moment of the cardiac cycle is the most suitable for measuring EFT by echocardiography. Some recommend the measurement during systole to prevent possible deformation by compression of the epicardial fat during diastole. On the other hand, other researchers prefer measurements in diastole to coincide with measurements of other imaging modalities like CT scans and MRI. 

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