The pericardium is not simply a covering of the heart. Its reflections define the cardiac hilum, create sinuses and recesses, and determine how catheters, surgical instruments, blood, and fluid can move within the pericardial cavity. A precise understanding of this three-dimensional arrangement is essential for interpreting cross-sectional imaging and for planning epicardial mapping and ablation. The descriptions below use attitudinally appropriate orientation whenever directional terms are required. [1-3]

1  Fibrous and serous pericardium

The pericardium consists of an external fibrous component and an internal serous sac. The fibrous pericardium is a dense connective-tissue envelope that blends superiorly with the adventitia of the great arteries and veins and inferiorly with the central tendon of the diaphragm. It limits acute cardiac distension and provides a mechanically stable interface between the heart and the mediastinum.

The serous pericardium has parietal and visceral layers. The parietal layer lines the inner surface of the fibrous pericardium. At the roots of the great vessels it reflects onto the heart and becomes the visceral layer, or epicardium. The epicardium covers the external cardiac surface and is continuous with the vascular sheaths at the arterial and venous poles. Coronary arteries, veins, nerves, lymphatics, and variable amounts of adipose tissue lie within the subepicardial tissues. [1-3]

2  Pericardial cavity

The pericardial cavity is the potential space between the parietal and visceral layers of the serous pericardium. In normal conditions it contains only a thin film of fluid, allowing near-frictionless cardiac movement. It is therefore better understood as a continuous, deformable space than as a uniformly wide compartment.

Its geometry is determined by the cardiac surfaces, the fibrous sac, the diaphragmatic attachment, and the serous reflections at the cardiac hilum. Around the body of the ventricles the cavity is relatively unrestricted, whereas near the bases of the great vessels it is subdivided into passages and blind recesses. Fluid, air, a guidewire, or a catheter can outline these normally inconspicuous extensions. [1-3]

3  Arterial and venous reflections

The serous pericardium reflects around the arterial and venous poles in two complex sleeves. The arterial reflection surrounds the ascending aorta and pulmonary trunk. Between and behind these arteries, the reflected serosa contributes to the roof and anterior boundary of the transverse pericardial sinus.

The venous reflection is more irregular. It surrounds the caval veins and the pulmonary veins and is moulded around the posterior wall of the left atrium. The right and left pulmonary venous reflections, together with the reflection related to the inferior caval vein, delimit the oblique sinus and its lateral extensions. The precise contours are variable because the lengths of the intrapericardial vessels, the spacing of the pulmonary venous ostia, and the intervening folds differ between individuals. [2,3]

Suggested media: Posterior and superior views of an opened pericardial sac with the arterial and venous reflection lines labelled directly on the specimen.

4  Transverse pericardial sinus

The transverse pericardial sinus is a passage created by the separation of the arterial and venous reflections. It lies posterior to the ascending aorta and pulmonary trunk and anterior to the superior aspect of the atrial mass. Its rightward opening is related to the ascending aorta and superior caval vein, while its leftward extent reaches the region behind the pulmonary trunk and towards the left atrial appendage.

The sinus is important surgically because it permits passage behind the arterial trunks. For electrophysiologists, its shape explains why the superior and posterior surfaces of the great arteries can be approached from a pericardial route, but also why movement becomes constrained near the hilum. The available space changes with cardiac filling and with individual anatomy. [2,3]

5  Oblique pericardial sinus

The oblique pericardial sinus is a blind cul-de-sac posterior to the left atrium. Its lateral boundaries are formed mainly by the reflections surrounding the right and left pulmonary veins. Inferiorly, the reflection related to the inferior caval vein contributes to the boundary of the space. The superior limit merges with the fold separating it from the transverse sinus.

Because the oblique sinus is closed superiorly as a blind recess, a catheter advanced within it cannot pass directly into the transverse sinus. Its depth and width vary with left atrial size and pulmonary venous arrangement. Pericardial fluid within this space may accentuate the posterior contour of the left atrium on cross-sectional imaging. [2,3]

6  Pericardial recesses

Pericardial recesses are extensions of the cavity formed where the serous membrane changes direction around the great vessels. Commonly described arterial recesses include superior and inferior aortic recesses and right and left pulmonic recesses. Venous recesses include the postcaval recess and recesses related to the pulmonary veins. Their terminology is useful, but the recesses form a continuous system rather than isolated chambers. [2,3]

The dimensions and visibility of these spaces vary considerably. When distended by fluid they can resemble mediastinal lymph nodes, cysts, or other masses on CT or magnetic resonance imaging. Identification of their continuity with the pericardial cavity and their predictable relation to vessels is therefore essential. For interventions, narrow recesses can restrict catheter deflection and may place the catheter close to adjacent vascular or atrial structures.

7  Relations with the phrenic nerves

The phrenic nerves descend through the thorax outside the fibrous pericardium, between it and the mediastinal pleura, accompanied by the pericardiophrenic vessels. The right phrenic nerve usually descends along the right side of the superior caval vein, right atrial region, and inferior caval vein before reaching the diaphragm. The left phrenic nerve crosses the left mediastinal surface of the pericardial sac and has a more variable relationship to the left atrial appendage and left ventricular surface. [4,5]

The nerves are separated from an epicardial catheter by the fibrous pericardium and a variable amount of fat, but this does not ensure protection from thermal injury. Their course cannot be inferred reliably from a single external landmark. In regions where ablation is contemplated, functional localization by pacing and integration with imaging or direct visualization are important safeguards. Detailed procedural strategies belong in the dedicated chapter on epicardial ablation. [4,5]

8  Inferior pericardial surface and diaphragmatic attachments

Inferiorly, the fibrous pericardium is firmly attached to the central tendon of the diaphragm. This attachment supports the heart and makes the inferior pericardial surface move with respiration. The inferior cardiac surface, formed predominantly by the left ventricle with a contribution from the right ventricle, rests above this pericardial and diaphragmatic interface.

The attachment is not equivalent to a freely mobile pericardial space. Near the central tendon, connective tissue and fat can limit dissection or catheter movement. More anteriorly and laterally, the muscular diaphragm, pleural reflections, and nearby abdominal viscera become relevant to the trajectory of subxiphoid access. The relation of the liver is particularly important on the right, while the stomach and left hepatic lobe may be relevant more inferiorly and leftward. These relationships vary with body habitus, respiration, and previous surgery. [1,3,6]

9  Clinical relevance for epicardial access and ablation

Percutaneous epicardial access enters a potential space whose usable volume is uneven. The ventricular surfaces provide the broadest working area, whereas reflections, recesses, and the cardiac hilum constrain catheter passage near the base. Epicardial adipose tissue can obscure electrograms, alter contact, and separate an energy source from myocardium. Pericardial adhesions after cardiac surgery, pericarditis, or previous epicardial intervention may partially or completely prevent free catheter movement. [1,6,7]

Anatomical structures at risk include the right ventricle during entry, epicardial coronary arteries and veins during mapping and energy delivery, the phrenic nerves on the lateral surfaces, and the diaphragm and adjacent abdominal organs along inferior access trajectories. Pre-procedural imaging can define cardiac position, pericardial fat, coronary anatomy, and major extracardiac relations, but it does not replace real-time confirmation of catheter position and vulnerable structures.

The purpose of this chapter is to establish the anatomical framework. Needle technique, access variants, mapping strategies, protection of adjacent structures, and management of complications should be developed in the separate chapter devoted to epicardial ablation. [6,7]