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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