What is a pericardial fat pad seen on an Echocardiogram (ECHO) indicative of?

Pericardial Fat Pad on Echocardiogram: Clinical Significance

A pericardial fat pad seen on echocardiogram is typically a normal anatomical finding representing epicardial adipose tissue, but when excessive, it may be associated with increased cardiovascular risk factors including metabolic syndrome, coronary artery disease, and atrial fibrillation.

Definition and Identification

Pericardial fat (also called epicardial fat) is a visceral fat deposit located between the heart and the pericardium. On echocardiography, it appears as:

• An echo-free or hypoechoic space between the outer wall of the myocardium and the visceral layer of pericardium 1
• Most commonly visualized and measured on the free wall of the right ventricle
• Typically ranges from 1 mm to 23 mm in thickness 1

It's important to note that epicardial fat pads should not be confused with pericardial effusion, which is a common misinterpretation. As noted in the American College of Emergency Physicians guidelines, "Occasionally, hypoechoic epicardial fat pads may be mistaken for pericardial fluid. Epicardial fat usually demonstrates some internal echoing and is not distributed evenly in the pericardial space." 2

Clinical Significance

Cardiovascular Risk Associations

When epicardial fat thickness is increased (generally >5 mm), it has been associated with:

1. Metabolic syndrome and insulin resistance 1

   • Epicardial fat thickness reflects visceral adiposity rather than general obesity
   • It correlates better with metabolic risk than BMI or other measures of general obesity

2. Coronary artery disease 3

   • Pericardial fat predicts incident coronary heart disease independent of conventional risk factors
   • The Multi-Ethnic Study of Atherosclerosis found that pericardial fat was associated with coronary heart disease risk (relative hazard per 1-SD increment: 1.26; 95% CI: 1.01,1.59) even after adjustment for BMI and other risk factors 3

3. Cardiac structural and functional changes 4

   • Epicardial fat >5 mm has been associated with:
     • Left atrial enlargement
     • Lower ejection fraction
     • Increased left ventricular mass
     • Abnormal diastolic function

4. Atrial fibrillation risk 5

   • Pericardial fat has been linked to AF prevalence, severity, and outcomes after ablation
   • It may promote AF arrhythmogenesis through local inflammatory mechanisms

Pathophysiological Mechanisms

The significance of pericardial fat extends beyond being a simple anatomical finding:

• It is metabolically active tissue that produces inflammatory cytokines 6
• It shares blood supply with the coronary microcirculation 5
• It may exert local paracrine effects on the myocardium and coronary arteries 1

Clinical Implications

1. Diagnostic Considerations

   • Pericardial fat should be distinguished from pericardial effusion
   • It should not be confused with other echo-free spaces adjacent to the heart 2
   • The descending aorta may be mistaken for posterior effusion; this can be resolved by rotating the probe into a transverse plane 2

2. Risk Assessment

   • Echocardiographic measurement of epicardial fat may serve as a simple tool for cardiometabolic risk prediction 1
   • Changes in epicardial fat thickness during weight loss may be a marker of therapeutic effect

Pitfalls and Limitations

• Measurement techniques for epicardial fat are not standardized across studies
• Reference values have not been fully validated for clinical use 6
• The optimal cutoff thickness for defining "excessive" epicardial fat is still debated
• Interpretation should consider other cardiovascular risk factors and not be used in isolation

Conclusion

While a pericardial fat pad is often a normal finding on echocardiography, increased thickness may be a marker of cardiovascular risk. When identified, it should prompt consideration of associated metabolic and cardiovascular conditions, particularly in patients with other risk factors for coronary artery disease or atrial fibrillation.