INTRODUCTION

Understanding the details and complexities of pericardial anatomy is essential when considering a percutaneous, subxiphoid epicardial procedure. The Nomina Anatomica recognizes two compartments within the pericardial cavity: the transverse and the oblique sinuses.¹ However, a series of irregular and topographically complex interconnections are formed by the subcompartments of these sinuses giving rise to four recesses (the post-caval recess, the right and left pulmonary vein recesses and the inferior aortic recess) as well as the superior pericardial recess, which is sometimes also referred to as the superior sinus, particularly by invasive Cardiologists. The pericardial sinuses and recesses are located within the basal and the posterior aspects of the heart. Knowledge of their anatomical relationships is critical when performing catheter mapping and ablation within the pericardial space. This chapter will provide a general overview of the pericardial anatomy as well as certain anomalies relevant for the interventional cardiologist and the cardiac electrophysiologist.

PERICARDIAL SAC

The normal pericardium consists of a double-layered, flask-shaped sac comprised of an outer fibrous envelope and an inner serous sac that is invaginated by the heart itself.² The serous pericardium can be divided into a visceral layer, an epicardial layer that overlies the heart and the great vessels, and a parietal layer that lines the fibrous pericardium.² The epicardium is reflected from the heart onto the parietal pericardium along the great vessels in tube-like extensions that include the aorta, the pulmonary artery and its primary branches, the proximal pulmonary veins, and the vena cavae.^3,4 The thickness of the parietal pericardium varies from 0.8–2.5 mm. The epicardial cavity is a virtual space that lies between the two serous pericardial layers.^3,4 At the point of pericardial reflections and at the posterior walls between the great vessels, the pericardial space is apportioned into a contiguous network of recesses and sinuses. The pericardium typically contains 15–25 mL of physiologic fluid within this space.^3,4 Upon imaging in the supine position, this fluid can sometimes be detected in the superior and the transverse sinuses. Of note, all pericardial reflections are located basally in relation to the great vessels. Thus, there are no obstacles during epicardial catheter manipulation along the anterior or apical ventricular surfaces of the heart.³

The fibrous pericardium is attached to the central tendon of the diaphragm by loose fibroareolar tissue, except for fusion over a small area of the central tendon and the pericardium. The pericardium is also attached to the posterior sternal surface by superior and inferior sterno-pericardial ligaments that anchor the fibrous pericardium and maintain the general position of the heart inside the thorax.⁴

PERICARDIAL SINUSES

As mentioned, in the human, invasive cardiologists recognize three sinuses within the pericardial space: the superior sinus, the transverse sinus, and the oblique sinus (Figure 2.1).^5–7

The transverse sinus lies posterior to the ascending aorta and the main pulmonary artery. Its posteroinferior boundary is formed by a pericardial reflection extending transversely between the right and the left superior pulmonary veins at its junction with the left atrium (Figure 2.2). Therefore, the dome of the left atrium forms the anterior boundary of the transverse sinus. It is important to also note that the right pulmonary artery is essentially a transverse sinus structure. As it courses laterally toward the right hilum, the right pulmonary artery protrudes into the transverse sinus. As such, exploration of the transverse sinus allows access to segments of the left atrium relevant to the pathogenesis of atrial fibrillation, including the posterior aspect of the left atrium. The transverse sinus also contains several recesses that extend as diverticulae between the major vessels: the superior aortic (also designated as the superior sinus), the inferior aortic, the right pulmonary vein and the left pulmonary vein recesses. Most relevant to interventional cardiac electrophysiology is the inferior aortic recess which affords epicardial access to the noncoronary aortic cusp and also the inferior aspect of the right coronary cusp (Figure 2.2).

Some investigators have proposed that the transverse sinus has a 3D configuration and may be divided into three different segments: anterior vertical, middle horizontal, and posterior vertical segments. Knowledge of these anatomic details can be helpful during interpretation of computed tomography/magnetic resonance scans as presence of pericardial effusion or cysts within these spaces can rarely simulate mediastinal or intracardiac lesions.^8–10 Nonetheless, exploring these details is beyond the scope of this review as they are unlikely to markedly impact intrapericardial cardiac electrophysiologic therapies.

Figure 2.2 The anatomy of the transverse sinus and its inferior extension. The inferior aortic recess allows access to the epicardial portions of the ascending aorta related to the noncoronary cusp and the inferior aspect of the right coronary cusp of the aorta. Illustrations modified with permission from: d’Avila A, et al. J Cardiovasc Electrophysiol. 2003;14:422–430.

Oblique Sinus

The oblique sinus extends behind the atria, particularly the left atrium, in the region of the four pulmonary veins (Figure 2.1). It is bounded on the right by the inferior vena cava and to the left by the pericardial reflection connecting the two left pulmonary veins. Superiorly, the oblique sinus is separated from the transverse sinus by the double reflection of serous pericardium transversely connecting the left and the right superior pulmonary veins.^5–7 The shape of the oblique sinus is variable and depends on the depth of the left and the right pulmonary recesses. It is rectangular when the pulmonary recesses are attenuated and triangular when they are prominent. Interestingly, regardless of its shape, the oblique sinus generally maintains a similar width and depth.

PERICARDIAL RECESSES

Excluding the superior aortic recess which is often recognized as the superior sinus among cardiac electrophysiologists, there are three recesses found within the pericardial cavity: the post-caval recesses, the left pulmonary vein recesses, and the right pulmonary vein recesses.^5–7 Their shape varies according to their depth and width and the ability to position a mapping/ ablation catheter around the pulmonary veins largely depends on these two variables.⁷

Post-caval Recess

The post-caval recess (also sometimes designated as the retro-caval recess) extends around the superior vena cava as a lateral extension of the superior sinus (Figure 2.1

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