INTRODUCTION

The heart is surrounded by adipose tissue, which can cover up to 80% of the cardiac contour and constitute up to a quarter of the heart’s weight.¹ Fat around the heart is present in two different anatomic compartments (Figure 13.1). Fat deposits present between the visceral layer of the pericardium and the myocardium are collectively referred to as epicardial fat. In contrast, extrapericardial fat is present outside the parietal layer of the pericardium. These two collections of fat differ in their biochemical properties and vascular supply.² Unlike epicardial fat, which is supplied directly by the coronary arteries, blood for the extrapericardial fat is provided by the pericardiophrenic artery, a branch of the internal thoracic artery and does not affect electrogram interpretation during epicardial mapping.³

EPICARDIAL FAT

Epicardial fat thickness has been shown to correlate with overall cardiac weight, have a direct relationship with total body weight,⁴ and is reported to increase with age.⁵ In one study, women were observed to have 17% more total epicardial fat than men.⁶ As is the case with visceral fat, epicardial fat proportions are reported to be lower in African Americans.⁷

Physiological Role of Epicardial Fat

Epicardial fat is postulated to serve various physiologic functions. It acts as a physical shock absorber, protecting the coronary arteries against the constant commotion of cardiac contractility and arterial pulsation, and an immune barrier that protects myocardial tissue against inflammatory substances.⁸ Epicardial fat also serves as a local source of fatty acids for the myocardium in times of need⁹ and has been reported to have thermogenic effects related to brown adipose tissue.¹⁰ Additionally, epicardial fat might have vasocrine properties that help regulate local blood flow in the myocardium.¹

Figure 13.1 Multidetector computed tomography (MDCT) ventricular short axis illustrating epicardial fat distribution. Panel A: Basal, Panel B: mid-ventricular, and Panel C: apical short-axis view.¹⁰

Distribution of Epicardial Fat

Epicardial fat is unevenly distributed around the myocardium. It is especially concentrated along the length of the coronary arteries, hence being more prominent in the atrioventricular and interventricular grooves.6,11 Sourwine et al. reported epicardial fat thickness averaging 7.8 ± 2.6 mm in perivascular segments as compared to 3.5 ± 0.9 mm in nonperivascular segments (P = 0.001), based on multidetector computed tomography (MDCT) data from 40 patients.¹¹

Epicardial fat is also more preferentially associated with the right ventricle, commonly seen along its right border, anterior surface, and apex.¹² Epicardial fat is 50% thicker along the right ventricular wall as compared to the left ventricular wall (3.9 ± 0.8 mm vs. 2.6 ± 0.6 mm; P = 0.001).¹¹ Fat deposits are also thicker along the base of the heart (6.1 ± 1.7 mm) compared to nonbasal segments of the heart (4.8 ± 1.7 mm, P < 0.001).¹¹ These findings are consistent with those published by Abbara et al.⁶ Mapping epicardial fat in 59 patients using MDCT,⁶ they reported average epicardial fat thickness of 2.8 mm at the LV apex, 12.7 mm at the left AV groove, 14.8 mm at the right AV groove, 4.8 mm at the RV apex, and 7.7 mm at the anterior IV groove.

Epicardial fat thickness also varies with the phases of the cardiac cycle, being significantly thicker during systole. While only 38% of the heart segments demonstrated a fat thickness of > 5 mm during diastole, this number increased to 60% during systole.¹¹

Quantification of Epicardial Fat

Various modalities, including echocardiography, CT, and MRI have been used to assess and quantify epicardial fat. Although MRI has been shown to accurately measure epicardial fat thickness and volume,¹³ MDCT remains the more commonly employed imaging modality for the detection and quantification of epicardial fat.^3,6

Effect of Epicardial Fat on the Electrogram Amplitude

During epicardial ablation of ventricular tachycardias, the ablation catheter comes in direct contact with epicardial fat, which acts as a physical and electric barrier between the catheter electrode and the myocardium. This insulation reduces the signal amplitude recorded by the diagnostic catheter,⁵ and if thick enough, epicardial fat can lead to significant attenuation of bipolar voltage acquired during an electrophysiological study.¹⁴ This may make the differentiation of scar tissue difficult and lead to unnecessary and unsuccessful ablations (Figure 13.2).

In an animal study, Tung et al. performed high-density epicardial mapping in seven porcine hearts, weeks after inducing myocardial infarction. They reported that areas of low voltage (< 1.5 mV) were seen only in regions where the fat thickness was 4 mm or greater.¹⁵

Several studies examined the effect of fat in human hearts. Saba et al. recorded electrograms directly from the epicardium in 10 patients undergoing cardiac surgery.¹⁴

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