Role of the Pericardium in Diastolic Dysfunction

Introduction

Pericardial diseases are often underdiagnosed and confused with other pathologic entities such as chronic liver diseases, heart failure, and restrictive cardiomyopathies especially in the setting of complicated pericarditis with evolution to constrictive pericarditis. The main reasons for these misunderstandings are related to both the rarity of the diseases and the lack of specific skills and knowledge of pericardial diseases, which are often considered the “Cinderella of cardiovascular diseases.” Nevertheless, the development of first prospective cohort studies, randomized controlled trials (RCTs), international guidelines, and especially the development of multimodality imaging of pericardial diseases has greatly improved the awareness and knowledge of these conditions. This chapter will further contribute to assess the importance of the pericardium in the study of diastolic function of the heart ( Chapter 36 ).

Anatomy and Physiology of the Pericardium

The pericardium (from the Greek πɛρι´, “around” and κa´ρδιον, “heart”) is a double-walled sac containing the heart and the roots of the great vessels. The pericardium is the external layer of the heart providing protection and support to inner structures. It is composed of an external fibroserosal part (parietal pericardium) and an internal serosal part (visceral pericardium) ( Fig. 3.2 ). The internal serosal layer is also named epicardium, which has direct connection with the myocardium. The parietal pericardium has an inner serosal part that is in continuity with the epicardium and an external fibrous part. Between the visceral and parietal pericardial layers, a virtual pericardial cavity is filled by 20 to 30 mL of plasma ultrafiltrate (pericardial fluid) that acts as a lubricant allowing myocardial contraction without irritation of the surrounding anatomic structures. The pericardial fluid is produced by the serosal pericardium, which is conformed by mesothelial cells with microvilla and cilia that further expand the available pericardial surface. The reflection of the visceral pericardium into the parietal pericardium over the great vessels is responsible for the creation of spaces, where pericardial fluid can accumulate and be detected by imaging studies. Greater spaces are called sinuses , while smaller spaces between adjacent anatomic structures are called recesses . The main sinuses include the transverse sinus , located between the aorta and pulmonary trunk anteriorly, the atria and veins posteriorly, and the oblique sinus located behind the left atrium and between the pulmonary veins and the inferior vena cava. Both sinuses may be accessed for electrophysiology purposes for ablation of cardiac arrhythmias. Fat is present under the epicardium (epicardial fat) and in connection with the parietal pericardium (epipericardial fat). Fat tissue provides mechanical, immunologic protection of the heart and serves as a source of fatty acids and thus energy; it may also have endocrine functions by cytokines that act through paracrine mechanisms on myocardial and endocardial cells.

The pericardium is fixed to the surrounding anatomic structures by ligaments: anteriorly to the sternum by the sternopericardial ligaments, posteriorly to the vertebral column, and inferiorly to the diaphragm.

The thickness of the normal pericardium is less than 3 mm by imaging studies and less than 1 to 2 mm on autopsy and surgical specimens.

The pericardium has an extensive network of lymphatics that drain substances from pericardial space and allows the spread of pathologic process from and to the pericardium in case of mediastinal and pulmonary pathology. The anterior part of the pericardium is drained either upwards to anterior mediastinal lymph nodes or downwards to the diaphragm; the posterior part drains into paraesophageal and tracheobronchial lymph nodes; the inferior part drains into prepericardial, lateropericardial, paraesophageal, and tracheobronchial lymph nodes; the lateral part drains into mediastinal, tracheobronchial, lateropericardial, prepericardial, and paraesophageal lymph nodes.

The arterial blood supply of the pericardium is provided by the descending aorta, branches of the mammary arteries, and musculophrenic arteries.

The pericardium has both sympathetic (first dorsal ganglion, stellate ganglion, aortic, and cardiac plexuses) and parasympathetic innervation (vagus, left recurrent laryngeal nerve, esophageal plexus).

The pericardium acts as a relatively inelastic sac because of its high content of collagen fibers, enveloping the heart and providing mechanical protection to the heart, thus allowing movement of cardiac chambers without attrition and limiting their distension. This effect is especially evident on right cardiac chambers.

This effect explains the exaggerated interventricular interdependence that can be observed in pathologic conditions (e.g., cardiac tamponade and constrictive pericarditis), as well as how rapidly accumulating pericardial fluid may be responsible for cardiac tamponade with limited amount of fluid, such as 200 to 300 mL in hemopericardium. On the other hand, slowly accumulating pericardial fluid can allow the generation of large pericardial effusions as 1 to 2 L without the development of cardiac tamponade (e.g., large chronic pericardial effusions). This is well explained by the pressure-volume curve of the pericardium ( Fig. 3.3 ).

The pericardium is not simply a mechanical barrier but also has an active immunologic role that is very relevant in inflammatory conditions such as pericarditis and myocarditis.

Although the pericardium has several important functions ( Box 3.1 ), its absence (total agenesis) or surgical removal (e.g., pericardiectomy) has limited consequences for life, and it is absolutely compatible with a normal life.

Noninvasive Imaging of the Pericardium

Multimodality imaging of the pericardium is an integral part of contemporary management of pericardial diseases that is recommended in all patients with suspected pericardial disease. The main imaging modalities include chest x-ray, echocardiography, computed tomography (CT), and cardiovascular magnetic resonance imaging (cardiac MRI).

In 2013, the American Society of Echocardiography issued a consensus statement on multimodality cardiovascular imaging of pericardial diseases that has been endorsed by the Society of Cardiovascular Magnetic Resonance and Society of Cardiovascular Computed Tomography. In 2014, a European position paper was published by the European Association of Cardiovascular Imaging and European Society of Cardiology (ESC) Working Group on myocardial and pericardial diseases, and multimodality imaging has been revised and recommendations issued in 2015 ESC guidelines on pericardial diseases. A detailed description of the imaging findings in specific pathologic conditions (e.g., cardiac tamponade and constrictive pericarditis) will be provided in subsequent chapters.

X-ray

Chest x-ray is a first-level imaging modality in patients with a suspected pericarditis or pericardial effusion to detect the presence of a cardiomegaly (may suggest a pericardial effusion), pericardial calcifications (chronic and constrictive pericarditis), and/or concomitant pleuropulmonary disease as first screening (e.g., pleural effusion, pneumonia, tuberculosis, lung cancer, and hilar and mediastinal enlargement) ( Fig. 3.4 ).

In a patient with pericarditis and without significant structural heart diseases, the chest x-ray may be absolutely normal. A pericardial effusion greater than 300 mL is able to increase the size of the cardiac silhouette. This one assumes a bottle shape in the presence of large pericardial effusions, especially when chronic and slowly accumulating.

In our patient, pericardial calcifications were detected (see Fig. 3.1 ). Pericardial calcifications have been reported in 30% to 40% of patients with chronic constrictive pericarditis.