LAYERS OF THE PERICARDIUM
The pericardium is located behind the sternum and r2-6 costal cartilages. It is separated into an outer and inner layer:
- Outer layer: fibrous pericardium. This is the strong fibrous part of the sac. This layer is continuous into the sheaths of the vessels which pierce it (aorta, pulmonary trunk, superior and inferior vena cava and the pulmonary veins).
- Inner layer: serous pericardium. The serous pericardium is divided into parietal and visceral layers.
Parietal layer of the serous pericardium: lines the fibrous pericardium. It is reflected around the roots of the great vessels to become continuous with the visceral layer.
Visceral layer of the serous pericardium (aka epicardium): it is closely applied to the heart.
Pericardial cavity: lies between the parietal and visceral layers. Contains the pericardial fluid that acts as a lubricant to facilitate movement of the heart.
Marchand (1951) found at the anterior origin of the aorta the fibrous and serous layers of the parietal pericardium are separated by loose connective tissue. Over the ventricles, however, the two layers become firmly adherent to each other.
Fascial sheaths derived from the fibrous pericardium invest the great vessels in their extrapericardial course: the ascending aorta, the pulmonary trunk (branches into the right and left pulmonary arteries), the pulmonary veins and the superior vena cava. Note the fascial sheets from the fibrous pericardium don’t invest the inferior vena cava.
The fascial sheaths of the pulmonary arteries and veins are prolonged around these vessels into the substance of the lung.
At the hilum of the lung this fascia is more loosely attached to the pulmonary arteries and veins but it becomes more adherent blending with the outer coat of the vessels as they begin to branch along their progression through the lung.
LIGAMENTS OF THE PERICARDIUM
- Superior (sterno-costo-pericardiacum) ligament: consists of two groups of fibres. Superior end of the sternum --> anterior wall of the pericardium below the large vessels of the heart. Suspends the pericardium in both vertical and horizontal positions.
- Inferior (xipho-pericardiacum) ligament: xiphoid process --> ascends towards the anterior wall of the pericardium. Suspends the pericardium in a horizontal position.
- Ligamentum sterno-pericardiacum medium: sometimes another group of ligaments extends from the middle part of the sternum and the neighbouring costal cartilages --> anterior wall of the pericardium.
Anterior ligamenta phreno-pericardiaca
The anterior ligamenta phreno-pericardiaca is the connective tissue that attaches the the top of the central tendon of the diaphragm to the bottom of the fibrous pericardial sac. These attachments span from the left side (near the apex of the heart) to the right side (anterior to the inferior vena cava).
These fibers can be followed superiorly into the adventitial layer of the aorta and pulmonary artery.
The attachments of this ligament must be considered together with the fibres which pass from the various large blood vessels of the heart into the anterior wall of the pericardium.
During inspiration when the diaphragm contracts and moves downward, the pericardial sac and the heart are pulled down and elongated. The phrenopericardial ligament is the fulcrum around which the diaphragm is supported when it comes to distribute its contractile tension laterally (Bordoni and Zanier 2013).
Ligamentum vertebro-pericardiacum (ligament of Beraud or ligamentum superius pericardii)
Popa & Lucinescu (1932) believed this ligament does not exist but did acknowledge some longitudinal fibres placed on the posterior aspect of the pericardium which might be taken for such a ligament (refer to 'fibers of the pericardium' 1b).
Vertebro-pericardiacum: C6-T1 --> pericardium: base of the posterior aspect of the pericardium.
Fibers in the pericardium
Popa & Lucinescu (1932) distinguish an outer and inner layer of fibres which are thin in the anterior wall of the pericardium and very thick in the posterior wall.
- Outer layer of fibers in the pericardium.
1a. Anterior wall of pericardium.
- Fasciculus xipho-anonymus: xiphoid process --> along the anterior wall of the pericardium --> brachiocephalic artery (arteria anonyma). Its fibres intermingle with those of fasciculus phreno-aorticus dexter.
- Fasciculus phreno-aorticus dexter: diaphragm: central tendon (right side) --> arch of the aorta. With some other fibres which continue farther on fasciculus phreno-anonymus (diaphragm: central tendon --> brachiocephalic artery).
- Fasciculus aortico-apicalis: arch of aorta --> descends to the ventral side of the pericardium --> apex of the heart.
- Ligamentum phreno-apicale: diaphragm: central tendon --> apex of the heart (pericardium).
- Fasciculus phreno-aorticus sinister: diaphragm: central tendon --> arch of the aorta.
- Phreno-pulmonalis anterior: diaphragm: central tendon --> pulmonary artery.
- Fasciculus aortico-cardiacus: ascending aorta --> left edge of the pericardium.
- Fasciculus cavo-anonymus: inferior vena cava --> brachiocephalic artery.
- Fasciculus cavo-pulmonalis: inferior vena cava --> pulmonary vein.
- Fibrae transversae anteriores & fibrae xipho-apicales: these are horizontal fibers connecting longitudional fibers on the left side (fasciculus aortico-cardiacus) and on the right side (fasciculus cavo-anonymus & fasciculus cavo-pulmonalis).
- Starting from each pulmonary vein, bundles of fibres spread underneath and between all the above described bundles.
- Ligamentum cardio-aorticum: a thick and large bundle of fibres completely blended with the fibrous ductus arteriosus (foetal blood vessel: pulmonary artery --> descending aorta). Anterior wall of the pericardium --> arch of the aorta.
- Systems of circular fibres around every pulmonary vein producing inextensible rings around every venous orifice.
1b. Posterior wall of the pericardium.
The collagen fibres are extremely thick and grouped in large bundles generally directed longitudinally.
Nearly all the bundles spread from the central tendon of the diaphragm --> small area lying behind and a little to the right of the inferior vena cava.
Popa & Lucinescu (1932) failed to find the vertebro-pericardiac ligament attributing its existence to a long bundle of one of these ligaments (especially Teutleben's left ligament).
These authors found long tracts of fibres ascending to various levels and producing loops around the pulmonary veins and arteries. The longest of these fibres pass up to the arch of the aorta and the bronchi.
Fasciculus centro-atrialis sinister (& dexter): central tendon of diaphragm --> atrium: left (& right).
Fasciculus centro-aorticus: central tendon of diaphragm --> arch of the aorta.
Fasciculus centrotrachealis: central tendon of diaphragm --> trachea.
Fasciculus centro-cardiacus I & II: central tendon of the diaphragm --> pericardium: posterior and left wall.
2. Inner layer of fibers in the pericardium.
2a. Anterior wall of the pericardium.
Generally it is noticed that, whereas on the anterior aspect of the outer wall of the pericardium the fibres are orientated chiefly in a longitudinal direction, on the inner layer they almost run transversely.
In this place there are very few fibres, and almost all are mere extensions from the posterior and lateral walls of the pericardium.
- Fasciculus phreno-trachealis: a long bundle of fibres. Trachea --> posterior surface of the anterior wall of the pericardium.
- Fibrae arciformes posteriors: some fibres running more or less transversely and coming from the left side of the pericardium.
2b. Posterior wall of the pericardium.
Three categories of bundles can be defined: longitudinal, transverse and circular.
i. Longitudinal fibres.
- Fasciculus phreno-trachealis: a long tract of fibres descends from the trachea --> pericardium: through the diaphragmatic portion of the pericardium up to its anterior wall.
- Annulus periatrialis (fibrae dextrae): right pulmonary veins --> descends --> inferior vena cava: near the inferior vena cava changes its direction to become transverse --> pericardium: surrounds the whole pericardium in front of the limit between atria and ventricles.
- Fasciculus broncho-apicalis: large fasciculus. Both bronchi --> pericardium: lying over the apex cordis (rounded end of the left ventricle).
- Fibrae verticals: large off shoot of fasciculus broncho-bronchi: both bronchi --> in front of the inferior vena cava.
- Fibrae cavo-pulmonales superiores; fibrae cavo-hilares et bronchiales dextrae; fibrae cavo-pulmonales inferiors: on the posterior side of the inferior vena cava bundles of fibres ascend in front of or behind various components of the right hilum of the lung.
- Fibrae intercavae: inferior vena cava --> superior vena cava.
- Fasciculus cavo-dorsalis, annulus periatrialis (fibrae dextrae and fibrae sinistrae), fasciculus aortico-periatrialis: several bundles, more or less vertical, become horizontal at their caudal end and form together large rings. These rings are called periatrial because of their position just between the atria and ventricles of the heart.
ii. Transverse fibres.
- Fibrae transversae interhilares posteriors: right root of lung --> left root of lung.
- Fibrae transversae infrahilares: just below left root of the lung --> just below right root of the lung.
- Fibrae cavo hilares sinistrae, et dextrae: transversely from the inferior vena cava --> lung root.
- Fibrae transversae interhilares anteriores: on an anterior plane. Right lung root --> left lung root.
- Fibrae transversae arciformes: root of lungs --> pericardium: lateral walls of the pericardium.
- Fibrae phreno-pulmonales dextrae and sinistrae: pulmonary artery: bifurcation of the pulmonary artery --> pericardium: posterior wall of the pericardium.
- Fibrae phreno-aorticae: arch of the aorta --> pericardium: lateral wall of the pericardium. In this transversely orientated group of fibres terminate many oblique or vertical fibres which contribute to the formation of the periatrial rings.
iii. Circular fibres.
- Pulmonary vein and arteries: strong rings are around these vessels.
- Annulus perihilaris: long fibres round the roots of the lungs.
- Annulus periatrialis: long fibres are grouped in a very large and thick rings which surrounds the whole pericardium in front of the limit between atria and ventricles.
THE MECHANICAL FACTORS WHICH MIGHT EXPLAIN THE ORIENTATION OF FIBRES IN THE PERICARDIUM
Extrinsic mechanical factors
Respiratory movements exert a direct line of pull on the pericardium. These mechanical tensions, from inspiration, exerted on the pericardium, is generated from antagonistic lines of pull, in multiple directions, from the pericardial ligaments:
- Trachea and larynx: the trachea and larynx descend in inspiration (Harris 1959). At the level of the bifurcation of the trachea (carina) it maybe lowered by as much as two vertebrae.
- Sternum and ribs pull on the anterior wall of the pericardium during inspiration.
- Diaphragm pulls the pericardium downward during inspiration.
- Blood vessels and bronchi: superiorly and laterally the pericardium is fixed to the blood vessels: aorta, pulmonary artery and vena cava superior; and laterally it is also fixed to the pulmonary veins and bronchi. During inspiration the bronchi elongate (as much as 25% during deep inspiration) and dilate stretching their connective tissue. All these points oppose the downwards pull of the diaphragm.
- Lung apex: during inspiration the lung apex can’t expand superiorly due to sibson’s fascia and the scalenes and only expands limitedly in an A-P direction. This results in the lungs being pulled inferiorly by the diaphragm which intern drags the bronchi with it (Harris 1959).
- Roots of the lungs: on inspiration the roots of the lungs descend with the diaphragm and are pulled laterally (following the expanding thoracic wall) which laterally pulls on the pericardium. As the roots of the lung are posterior to the pericardium, during inspiration, the pericardium is simultaneously pulled posteriorly.
In summary the pericardium is strongly fixed between four opposing points: transversely between the roots of the lungs, vertically between the diaphragm, the blood vessels, and, again, the roots of the lungs. The pericardium in such conditions becomes absolutely rigid and appears like a hard bony membrane.
Therefore the greatest stress exerted upon the pericardium coincides with a combination of trunk extension and forced inspiration. This tenses all the longitudinal ligaments causing the shape of the pericardium to become more elongated and quite narrow, especially in its upper outline.
Conversley the greatest relaxation of the pericardium coincides with a combination of trunk flexion with forced expiration. This relaxes the longitudinal bundles of the pericardium to a certain degree. In this case the shape of the pericardium becomes short and large, and its upper outline is thick.
As a result of such changes in the shape and position of the pericardium, the heart can move only laterally and vertically inside the pericardium.
Effects of the sternum and ribs on pericardial movement
Sternal and rib ligaments:
- Ligamentum sterno-pericardiacum superius: extended by the manubrium sterni at every inspiratory movement. Pulls the upper part of the pericardium upwards.
- Ligamentum sterno-pericardiacum inferius: during inspiration the xiphoid process tenses the ligamentum sterno-pericardiacum inferius. Pulls the lower part of the pericardium downward.
- Ligamentum sterno-pericardiacum medium: when present this ligament pulls the pericardium directly forwards.
The antagonistic nature of these ligaments in a cranio-caudal direction produce tension in the anterior wall of the pericardium; this tension is directed longitudinally with a slight divergence backwards.
These ligaments also work synergistically when they transmit the traction of the sterno-costal wall during inspiration to the pericardium. During inspiration the sternum moves anteriorly attempting to drag the pericardium with it. This anterior movement is resisted the posterior connections of the pericardium with the diaphragm around the inferior vena cava (< fasciculus centro-cardiacus, fasciculus centro-atrialis dexter, and ligamentum phreno-pericardiacum anterius dextrum). The trajectorial lines of forces in this case pass from the upper left side of the pericardium to the right inferior side and posteriorly, crossing the anterior wall of the pericardium diagonally. This line of force corresponds to the fibrae transversae anteriores.
Effects of the diaphragm on pericardial movement
The descent of the diaphragm causes a still greater effect upon the pericardium after the forward and upward movement of the anterior wall of the thorax is accomplished. When the ribs and sternum are fully stretched after the initial phase on inspiration further contraction of the diaphragm has a more concentrated pull upon the pericardium.
The pull of the diaphragm is orientated in this case vertically from the diaphragm towards the arch of the aorta and brachiocephalic artery (refer to 'effects of the blood vessels on pericardial movement'), its greatest power being exerted on the posterior side of the pericardium. Particularly important is the posterior longitudinal bundles which fix a permanent distance between the central tendon of the diaphragm and the bronchi (and the roots of the lungs as a unit) via the fasciculus centro-aorticus, fasciculus centro-trachealis, and fasciculus centro-atrialis dexter et sinister.
Effects of the blood vessels on pericardial movement
During inspiration the trajectory of force is directed from the diaphragm through the pericardium up to the arch of the aorta and brachiocephalic artery.
These trajectories are grouped especially along four principal lines:
- Brachiocephalic artery --> inferior vena cava.
- Aorta and pulmonary artery --> left side of the central tendon of the diaphragm.
- Superior vena cava --> inferior vena cava.
- Posteriorly: central tendon of the diaphragm --> arch of the aorta and other large blood vessels.
These longitudinal sheets of resistance provide an excellent support for the transverse bundles, and together they form a strong membrane well planned for resistance to every direction of pull.
Effect of movement of the roots of the lung on pericardial movement
The roots of the lungs on inspiration pull the pericardium laterally (following the expanding thoracic wall) and backwards (as the roots of the lung are posterior to the pericardium). The descent of the roots of the lungs during inspiration will exert a downward pull on the pericardium.
This fact explains the presence of several strong bundles of fibres: fibrae transversae interhilares, fibrae transversae arciformes, fibrae transversae arciformes and fibrae transversae interhilares anteriores.
This same mechanism explains the presence of transverse bundles of fibres which connect the pulmonary vessels to the pericardium: pulmonary artery (fibrae phreno-pulmonales sinistrae et dextrae) and the pulmonary vein (fasciculus transversus pericardiacus anterior).
The strong ligaments of the central tendon of the diaphragm and the bronchi/roots of the lungs is defined by their inextensile nature. This causes the roots of the lungs and the central tendon to ascended and descended together at every movement of respiration like two structures firmly fixed to each other.
Intrinsic mechanical factors
Movement of the heart
Because of the relations of the heart with the pericardium only the ventricles can move inside the pericardial cavity. Thus, the change of position that takes place is a certain rotation of the heart round its longitudinal axis, associated with the movement of the tip of the heart from left to right. Popa & Lucinescu (1932) believe that there is a relationship between these rotatory movements of the heart and the direction of the ligamentum cardio-aorticum.
The heart as an active organ contains a variable amount of blood which causes it to vary in weight.
In systole blood ejects from the heart and it becomes smaller and diminishes in weight; conversley in diastole blood enters the heart and it becomes larger and heavier.
It is around the auricle that the pericardium is strongest and the fasciculi of fibres are best developed.
Fasciculi periatriales dexter et sinister: large inextensible rings which encircle the thin walls of the auricles at their ventricular extremities. This ring is prolonged backwards by the thick wall of the pericardium. It plays the same role for the atria as the blood vessel rings for the pulmonary veins in that it assures a constant space for the atria. By its inextensibility it protects the atria against over dilatation and compression from the increase volume of the lungs during inspiration.
As well as traction being produced by the moving roots of the lungs, a great compressive power is set up by the expanding mass of the lungs acting upon the pericardium and heart.
As the roots of the lungs separate strong perihilar rings hold together the pulmonary artery, pulmonary vein and bronchus. In addition, every pulmonary vein and pulmonary artery has a special ring which assures a constant opening at every moment of function. These rings protect the blood vessels, with their thin walls, against possible blood engorgement, and tendency to closure by external pressure.
During respiration the pericardium also plays an important mechanical role in the redistribution of blood by changing the size of large arteries merely by traction exerted upon the arch of the aorta, brachiocephalic artery and carotid artery.
As the diaphragm descends during inspiration the thoracic inlet (with the arteries) is raised. This increases the distance between these two points. The pericardium, which is is fixed to diaphragm inferiorly, and blood vessels superiorly, exerts a mechanical tension on the arch of the aorta, brachiocephalic artery and carotid artery. This is more notable when accompanied by trunk extension, a typical movement associated with inspiration. The effects of this mechanical tension on the blood vessels results in the ligaments pulling the blood vessels tight to restrict blood flow; or the ligaments relaxing to allow the blood vessels to open up and permit more blood flow:
On inspiration/trunk extension the brachiocephalic artery, left >right carotid and left subclavian arteries decrease in diameter and a smaller quantity of blood is allowed to pass through. The surplus blood is passed on through the arch of the aorta and descending aorta into the abdominal organs. As a result bpm increases during inspiration/trunk extension.
On expiration/trunk flexion the brachiocephalic artery, left>right carotid and left subclavian arteries increase in diameter and more blood can pass through. This will result in less blood surplus to pass through the arch of the aorta and descending aorta into the abdominal organs. As a result bpm decreases during expiration/trunk flexion.
Therefore the pericardium by pulling on the arch of the aorta and large arterial blood vessels plays an important role as a mechanical device for the redistribution of blood between the head and upper limbs and the abdominal organs and lower limbs.
Systolic/diastolic pressure in the arteries
During systole the force of the blood pushes vigorously against an arched tube, the arch of the aorta. Under such conditions there is a tendency for this arched tube to straighten out.
If this was to be permitted it would:
- Endanger the patency of openings in the brachiocephalic artery, left carotid and left subclavian artery.
- Increase the distance between the left subclavian and descending aorta.
As the pericardium is fixed to the arch of the aorta (fasciculus aortico-periatrialis), this arch remains almost fixed in systole. The arch of the aorta goes up and down as a unit and does not distend itself.
The same effect upon the arch of the aorta is produced also by the ligamentum cardio-aorticum, fibrae aortico-subelaviales (descending aorta and left subclavian artery).
Natale et al (2015) identified the fascia pretrachealis as part of the endocervical (visceral) fascia. The endocervical fascia corresponds to the endothoracic (subpleural) fascia in the thorax, to the endoabdominal (transversalis/subperitoneal) fascia in the abdomen, and to the endopelvic (subperitoneal) fascia in the pelvis. The continuity of these connective fascia is only interrupted by Sibson’s fascia (neck–thorax) and by the diaphragm (thorax–abdomen).
Details of the fascia pretrachealis is outlined here as its attachments to the trachea, bronchi, pericardium and blood vessles match those of the ligaments of the pericardium. Natale et al (2015) found a reference to the prevertebral fascia, as it descends in the posterior mediastinum, sending attachments to the pericardium, although this is not widely recognised.
The following work on the fascia pretrachealis is based on that of Cavdar et al (1995).
The fascia pretrachealis is superiorly continuous with the buccopharyngeal aponeurosis. It is thin and translucent at its upper end and considerably thicker at its lower end. It is attached to:
- Thyroid: attaches superiorly to the upper brim of the thyroid cartilage and laterally to its oblique line. Ensheaths the thyroid gland very firmly.
- Trachea (neck): continues down over the trachea attaching laterally to the edge of the cartilagenous part of the trachea on both sides.
- Retrosternal area: relates to the vessels of the retrosternal area.
- Blood vessels and pericardium: attaches to the brachiocephalic vein and brachiocephalic artery. It then terminally fuses with the adventitia of the posterior aspect of the arch of the aorta and with the posterior aspect of pulmonary artery and the pericardium.
- Trachea (thorax): laterally, at the bifurcation of the trachea, the fascia pretrachealis covers the angle between the trachea and main bronchus on either side. At the lower end of the trachea it forms a ring like structure formed by the fusion of the fascia pretrachealis to the brachiocephalic artery.
- Bronchus: Marchand (1951) found the perivisceral fascia* (the part of this fascia that is superficial to the trachea is the fascia pretrachialis) invests the left and right bronchi. Extrapulmonary the fascia is readily separated from the bronchi as it contains the bronchial artery, lymphatic vessels, and nodes. Intrapulmonary, after the bronchus enters the hilum of the lung the fascia becomes thinner and more adherent the further the lung is penetrated. Marchand (1951) identified it as the possible fascia that is prolonged around the bronchioles, and perhaps forms the alveolar septa.
- Superior vena cava and pulmonary artery: after spanning the angle between the trachea and bronchus the fascia pretrachealis fuses with the vascular sheath around the superior vena cava on the right and with the sheath of the pulmonary artery on the left.
The lungs house fascial attachments from the perivisceral fascia (including the fascia pretrachealis) and fibrous pericardium. The perivisceral fascia tightly adheres to the bronchi as they advance through the lung just as the fibrous pericardium attaches to the pulmonary artery and vein as they advance through the lung.
The space between the fascia pretrachealis and the trachea is termed the "axial mediastinum".
*: the perivisceral fascia includes the fascia pretrachealis. The anterior section of the perivisceral fascia superficial to and in front of the trachea is called the fascia pretrachealis. The posterior section of the perivisceral fascia that encloses the trachea and oesophagus is called the perivisceral fascia proper. Enclosing the trachea and oesophagus collectively the perivisceral fascia descends through the mediastinum, attaching to the bronchi, and then, following the oesophagus blends with the phrenoesophageal ligament to terminate in the outer coat of the stomach. Superiorly the perivisceral fascia attaches to the hyoid bone and runs continuous with the buccopharyngeal fascia. Posteriorly it is continuous with the prevertebral fascia.
Fascia pretrachealis is important in allowing the gliding movement of the trachea during swallowing.
Perivisceral fascial attachments may have a mechanical and physiological function in the thorax. This maybe determined by the relationship of head position with the perivisceral fascial connections to the trachea, bronchi, pericardium, brachiocephalic arteries and arch of the aorta.
Harris (1959) explained the association with cervical spine posture and the perivisceral fascia attachments to the trachea and bronchi. Cervical spine extension stretches the infrahyoid respiratory passage and trachea at its upper end. By pulling the trachea tight at its upper end this inhibits the descent at its lower end (carnia) during inspiration. The author hypothesised that in this posture if the lung root doesn't descend then this might compromise expansion at the lung apex. One may also assume this would also compromise the inferior displacement of the bronchi.
Popa and Lucinescu (1932) illustrated the association of posture with the perivisceral fascia attachments to the brachiocephalic artery, arch of the aorta and pericardium. These authors found on inspiration/trunk extension the brachiocephalic artery, left >right carotid and left subclavian arteries decrease in diameter and a smaller quantity of blood is allowed to pass through. The surplus blood is passed on through the arch of the aorta and descending aorta into the abdominal organs. As a result bpm increases during inspiration/trunk extension.
If trunk extension accompanies neck extension then in this posture the perivisceral fascia may have a complex mechanical role in mediating and integrating (1) the changes in tracheal and bronchial movement noted by Harris (1959) and (2) vasoconstriction and dilation of the subclavian artery and arch of the aorta as well as pericardial movement (bpm) noted by Popa and Lucinescu (1932).
Anatomic connections of the diaphragm: influence of respiration on the body system (2013). Bruno Bordoni and Emiliano Zanier
The Anatomy of Lamina Pretrachealis Fasciae Cervicalis (1995). S. CAVDAR, F. KRAUSE, H. DALCIK and Y. ARIFOGLU
THE ANATOMY AND APPLIED ANATOMY OF THE MEDIASTINAL FASCIA (1951) BY PAUL MARCHAND
Is the cervical fascia an anatomical proteus? (2015). Natale G, Condino S, Stecco A, Soldani P, Belmonte MM, Gesi M
TRACHEAL EXTENSION IN RESPIRATION (1959) BY R. S. HARRIS