|Arash Aryana, MD, PhD; André d’Avila, MD, PhD|
Percutaneous epicardial catheter ablation is being increasingly utilized to treat patients with cardiac arrhythmias, particularly ventricular arrhythmias. But the human heart bears a significant layer of adipose tissue, which has the potential to impede epicardial mapping and ablation. The adipose tissue surrounding the heart can be classified as either pericardial fat, which refers to the adipose layer found outside of the pericardial sac attached to the fibrous pericardium, or as epicardial fat, which lies beneath the visceral epicardium. Adipose tissue can elicit poor electrical conductivity and serve as insulation to catheter ablation. Thus, knowledge about details and distribution of epicardial fat is essential when considering an epicardial catheter mapping and ablation procedure. This chapter will provide an overview of epicardial fat as it pertains to epicardial catheter mapping and ablation.
The epicardial fat (sometimes also called subepicardial fat) is a layer of adipose tissue located underneath the visceral epicardium, in which the coronary vessels and their main branches are embedded.1,2 It is concentrated heavily along the coronary sulcus and the interventricular groove. However, there are varying reports on the precise distribution of epicardial fat relative to specific anatomical locations of the ventricular myocardium.3 In one study,4 it was reported that as much as 80% (range: 56–100%) of the ventricular myocardium is covered by an epicardial fat pad. Other studies have shown that epicardial fat occurs more frequently on the right as opposed to the left ventricle.5,6 Having said that, the region of the diaphragmatic wall along the septum is frequently devoid of fat, whereas fat over the left ventricle tends to be largely confined to the location of the coronary vessels.
There have been several investigations relative to a possible correlation between epicardial fat mass versus patient age, gender, and body mass index.1,2 While the thickness can vary significantly from individual to individual, epicardial fat typically increases in mass with advanced age.7 Women generally also exhibit thicker epicardial fat overlying the right ventricle than men, with a ratio 1.65:1. Although a positive correlation seems to exist between the thickness of subcutaneous and epicardial fat, this relationship is not very robust.3,4,6,7 Therefore, even very slim patients can present with a large amount of epicardial fat. In a histological study, Schejbal et al.7 found that the thickness of epicardial fat overlying the surface of the right ventricle may range anywhere from 0 to 13.6 mm (mean: 2.19). Furthermore, these authors found that the epicardial fat thickness varied by cardiac region. For instance, the mean thickness within the mid-region of the intraventricular septum was only around 0.8 mm as compared to 4.12 mm at the base of the heart, whereas a thicker fat layer was found along the ventrolateral edge of the right ventricle.8,9 On the other hand, the quantity of subepicardial fat is most extensive over the anterior ventricular wall. Abbara et al.10 assessed the distribution and thickness of epicardial fat in 59 patients who underwent multidetector computed tomography (MDCT) and found that although the acute margin and the right ventricular anterior wall exhibited greater epicardial fat and the right ventricular diaphragmatic and the left ventricular lateral walls showed little to no fat, once again there were significant variations among individuals (Table 39.1). It is also important to note that the area between and around the pulmonary veins is covered by minimal epicardial fat. Recent studies have also linked epicardial fat with progression and severity of coronary artery disease6,11 as well as a higher incidence of atrial arrhythmias, particularly atrial fibrillation.12
Table 39.1 Fat Thickness and Distribution in Various Cardiac Segments Stratified by Gender and Age, Based on the Analysis of Data in 59 Patients Who Underwent Multidetector Computed Tomography for Coronary Artery Assessment Using a 16-Slice Scanner
Table is reproduced from Abbara et al. Eur J Radiol. 2006;57:417-422. Abbreviations: IV, inter-ventricular; RV, right ventricular; diaphrag, diaphragmatic; ant, anterior; AV, atrio-ventricular.
While the fundamentals of endocardial and epicardial electroanatomic mapping and ablation are the same, presence of epicardial fat can in certain instances confound catheter mapping and ablation within the pericardial space. Moreover, identification of epicardial fat during electrophysiologic mapping is not always straightforward. d’Avila et al. examined bipolar epicardial electrogram characteristics and epicardial ventricular stimulation thresholds obtained using a standard 4-mm tip radiofrequency ablation catheter in an open-chest model.13 The authors recorded epicardial electrograms in 45 areas with and 44 without overlying epicardial fat distributed randomly along the free walls of the left and right ventricular outflow tract in 10 patients undergoing coronary artery bypass graft surgery. They found that the presence of epicardial fat measuring < 5 mm in thickness did not significantly impact the bipolar stimulation threshold in areas with versus without fat (4.8 ± 1.6 vs. 4.6 ± 1.8 mA, P = NS). Similarly, bipolar peak-to-peak voltage amplitude (40 ± 5 mm vs. 43 ± 4 mm) and bipolar electrogram duration (44 ± 5 ms vs. 43 ± 6 ms) were virtually unaffected by the presence or absence of epicardial fat measuring < 5 mm in thickness (Figure 39.1). However, the authors found that in areas exhibiting a fat layer measuring > 5 mm in thickness, high-output ventricular pacing capture was no longer possible at even 10 mA.14
In another study, Saba et al.15 sought to establish reference values for bipolar electrogram voltage over non-scarred ventricular myocardium with overlying epicardial fat of varying thickness in 10 patients undergoing open-chest cardiac surgery. These authors found that bipolar epicardial electrogram amplitude varied between the left and the right ventricles and also by the thickness of epicardial fat, such that thick fat (≥ 5 mm) typically located at the basal right and left ventricular regions was associated with significantly attenuated electrogram amplitudes. However, no significant differences in the electrogram amplitude were noted in areas with overlying thin epicardial fat (< 5 mm). As also noted by d’Avila et al., the authors showed that presence or absence of thin or thick fat did not in any way impact the electrogram duration. Tung et al.16 reconfirmed these findings in a porcine infarct model created by occlusion of the circumflex artery using an angioplasty balloon. After 4 to 12 weeks of infarct healing, these investigators performed in vivo epicardial mapping. Areas with low-voltage bipolar electrograms (< 1.5 mV) were then analyzed. The authors found that areas bearing epicardial fat > 4 mm in thickness registered low-voltage bipolar electrograms. They also found that although the mean bipolar electrogram amplitude was similar in areas with overlying “thick” fat vsersus scar (0.77 ± 0.34 mV vs. 0.75 ± 0.38 mV; P = NS), the mean electrogram duration was significantly longer in areas of scar but not in areas with overlying fat (68.8 ± 18.9 ms vs. 50.1 ± 11.6 ms; P < 0.0001). The authors further reported that scar exhibited more fractionation (8.5 ± 3.1 deflections vs. 4.7 ± 1.8 deflections; P < 0.0001) and that manifestation of late potentials was 99% specific for presence of scar. Therefore, while thick epicardial fat can potentially yield low-amplitude electrograms, it may be distinguished from scar on the basis of the electrogram morphology. That is, epicardial fat does not yield fractionated, late, or split potentials that are characteristic of scar tissue (Figure 39.2). Similar findings were reported by Desjardins et al.17 who studied epicardial electroanatomic maps in consecutive patients referred for catheter ablation. The authors found that a bipolar voltage ≥ 1.5 mV correlated best with the absence of epicardial fat as assessed by cardiac computed tomography, whereas an epicardial fat thickness ≥ 2.8 mm yielded attenuations in bipolar voltage and best distinguished areas of low voltage (< 1.5 mV) vs. normal voltage (≥ 1.5 mV) with high sensitivity (81%) and specificity (81%).
Figure 39.1 The figure shows a series of bipolar recordings recorded in an open-chest model, in presence (top panels) and absence (bottom panels