HBF-II LEC 03 Gross Anatomy Middle Mediastinum Heart Notes 2024 Berger PDF

Middle Mediastinum & Heart - Berger Page 3 of 26 Middle Mediastinum A. General Features Anatomical boundaries: Located within the inferior mediastinum – between transverse thoracic plane (level of sternal angle and intervertebral disc of...

Middle Mediastinum & Heart - Berger Page 3 of 26 Middle Mediastinum A. General Features Anatomical boundaries: Located within the inferior mediastinum – between transverse thoracic plane (level of sternal angle and intervertebral disc of T4/T5 vertebra) and diaphragm Contents include: - Pericardium - Heart - Roots of great vessels (ascending aorta, pulmonary trunk, SVC) - Arch of azygos vein - Main bronchi - Phrenic nerves accompanied by pericardiacophrenic vessels Clinical Note: Levels of viscera are dependent on position of pt (supine vs standing). Gravity causes these structures to sag – should be considered during physical and radiological examinations. Figure 1. B. Pericardium 1. Anatomical Features Lies deep to mediastinal parietal pleura Composed of two layers: External sac – fibrous pericardium Outer layer of dense connective tissue Bound to central tendon of diaphragm Fused with tunica adventitia of great vessels (at the level of the sternal angle) Anteriorly attached to sternum by sternopericardial ligaments Anatomy of normal pericardium. (www.clevelandclinicmeded.com) Figure 2. Figure 3. Middle Mediastinum & Heart - Berger Page 4 of 26 Clinical Note: Fibrous pericardium, which is tough and inelastic, protects the heart against abrupt overfilling – pericardial effusion. Internal sac – serous pericardium Single epithelial layer composing two layers (parietal and visceral) Parietal layer lines the internal surface of fibrous pericardium Visceral layer lines the external surface of heart, comprising the epicardium Pericardial cavity is the potential space between the parietal and visceral layers of serous pericardium Serous fluid fills the pericardial cavity to allow the heart to move freely within the sac The reflection of the serous pericardium onto the great vessels results in sinuses: 1) Transverse pericardial sinus – lies posterior to the ascending aorta and pulmonary trunk, anterior to SVC, superior to the left atrium; a passage between two sites of reflected serous pericardium 2) Oblique pericardial sinus – posterior to the left atrium; zone of reflection surrounding the pulmonary veins Figure 4. Middle Mediastinum & Heart - Berger Page 5 of 26 2. Vessels & Nerves Arterial Supply - Pericardiacophrenic artery - Smaller contributions: Internal thoracic and musculophrenic arteries Bronchial, esophageal, superior phrenic arteries (thoracic aorta) Coronary arteries → visceral layer of serous pericardium only Figure 5. Venous Drainage - Pericardiacophrenic veins enter the azygos system and internal thoracic and superior phrenic veins Innervation - Phrenic → primary source of sensory fibers; referred pain to skin of ipsilateral supraclavicular region of the shoulder or lateral neck area (C3-C5 dermatome) - Vagus (CN X) → function is uncertain - Sympathetic trunks → vasomotor Middle Mediastinum & Heart - Berger Page 6 of 26 3. Clinical importance: A surgical clamp or ligature can be placed around the aorta/pulmonary trunk, while tubes of coronary bypass machine are inserted to divert circulation of blood during cardiac surgery (e.g., coronary artery bypass grafting). Pericarditis – inflammation of pericardium as shown in Fig 7. Condition causes chest pain and results in pericardial friction rub. Must be distinguished from myocardial infarction because treatment and prognosis are very different. Figure 6. Pericardial effusion – leaking of fluid from pericardial capillaries into pericardial cavity (Fig 8)→ extensive fluid build-up → prevents the heart from expanding → limits inflow of blood to the ventricles resulting in cardiac tamponade due to an unyielding fibrous sac. Figure 7. Figure 8. Figure 9. Middle Mediastinum & Heart - Berger Page 7 of 26 Cardiomegaly – heart slowly increases in size, which allows for enlargement to occur without compression. Figure 10. www.escardio.org Figure 11. Figure 12. Hemopericardium – blood enters pericardial cavity (e.g., stab wound or weakened area of heart muscle after heart attack); blood accumulates → heart compresses → circulation fails → SVC compresses → veins in the face/neck engorge. (en.wikipedia.org) Pericardiocentesis – drainage of serous fluid from pericardial cavity; relieves cardiac tamponade. Figure 13. Middle Mediastinum & Heart - Berger Page 8 of 26 C. Heart - Basically a two muscular pumps to propel blood to the body - Approximate size of a clenched fist - Right side: receives low O2-blood from SVC/IVC → pulmonary trunk for O2 - Left side: receives high O2-blood from pulmonary veins → aorta for systemic distribution - R/L atria = receiving chambers - R/L ventricles = discharging chambers 1. External Anatomy Anatomical Position Posterior to sternum, costal cartilages, medial ends of ribs 3-5 on left side Sits obliquely, ~2/3 to left & 1/3 right of median plane Apex: left inferolateral part of left ventricle; posterior to left 5th intercostal space (8-9 cm from midsternal line); directed downward and to the left ~45 degrees from median plane; more anteriorly located than the base Base: posterior aspect of heart; mainly composed of left atrium; faces posteriorly toward vertebral bodies of T6-T9; between bifurcation of pulmonary trunk → Figure 14. coronary groove The great veins enter the base of the heart – the pulmonary veins enter the right and left sides of the left atrium and the SVC/IVC at the upper and lower ends of the right atrium, respectively. Borders Right – right atrium, extending between SVC/IVC Inferior – sharp edge formed by right ventricle with small portion of left ventricle Left – obtuse margin formed by left ventricle and left auricle Superior – right and left atria & auricles Figure 15.  Middle Mediastinum & Heart - Berger Page 9 of 26 Surfaces Anterior – sternocostal surface, mainly right ventricle, some right atrium and some left ventricle Right – faces the right lung, consists of right atrium Inferior – diaphragmatic surface, mainly left ventricle, faces inferiorly, rests on the diaphragm, separated from the base of the heart by coronary sinus and extends to the apex, related to central Figure 16. tendon Superior – right and left atria & auricles Left – pulmonary surface as it faces the left lung, mainly left ventricle External sulci Coronary sulcus – separates the atria from the ventricles. As it circles the heart, it includes the right coronary artery, the small cardiac vein, the coronary sinus, and the circumflex branch of the left coronary artery. The two ventricles are separated by anterior and posterior interventricular sulci; the anterior interventricular sulcus includes the great cardiac vein on the anterior surface; the posterior interventricular sulcus contains the posterior interventricular artery & middle cardiac vein on the diaphragmatic surface. Layers of Heart a) Epicardium – visceral layer of serous pericardium b) Myocardium – cardiac muscle c) Endocardium – endothelium & subendothelial connective tissue, covers valves Figure 17. Pericardial layers and layers of the heart. (Marieb & Hoehn, Human anatomy and physiology, 8th ed. Middle Mediastinum & Heart - Berger Page 10 of 26 2. Internal Anatomy Functionally, the heart is made up of two pumps Figure 18. partitioned off from each other. Each pump has an atrium and a ventricle separated by a valve – creating four chambers. On the right side, deoxygenated blood from the body is received, then sent to the lungs. The pump on the left receives oxygenated blood from the lungs and sends it back out to the body. The atria receive incoming blood and as such have thin walls. In contrast, ventricles pump the blood out and have relatively thick, muscular walls to accommodate the required force. Since it takes more force to pump blood throughout the body versus the lungs, the left ventricle has thicker walls than the right. The chambers of the heart are separated by interatrial, interventricular and atrioventricular septa. The internal anatomy of each chamber is critical to its function. Right Atrium - Receives blood from SVC, IVC (body) & coronary sinus (heart walls) - Right auricle: muscular pouch, increases capacity of atrium - Sulcus terminalis cordis (terminal sulcus) is a vertical groove separating two continuous spaces of the atrium and extends from SVC to IVC on the right side; indicated by the crista terminalis internally - Sinus venarum (primordial atrium) – smooth, thin-walled posterior to the crista; where SVC, IVC & coronary sinus empty into - Anterior to the crista, pectinate muscles comprise the rough, muscular wall - Opening of SVC – upper posterior portion of right atrium, level of right 3rd costal cartilage Middle Mediastinum & Heart - Berger Page 11 of 26 - Opening of IVC – lower posterior portion of right atrium, level of ~5th costal cartilage - Coronary sinus opening – between right AV orifice and IVC orifice - Interatrial septum contains the fossa ovalis, important in fetal circulation to allow blood to bypass the nonfunctional lungs before birth. - Openings of the smallest cardiac veins drain the myocardium directly into the atrium Clinical importance: Atrial septal defects - Congenital defect - ~ 15-25% probe-sized patent foramen ovale (< 5mm less likely to cause problems) - Larger ASDs can result in left → right shunt of blood → increases size of right atrium and ventricle → dilation of pulmonary trunk → pulmonary hypertension Figure 19. Blood passes from the right atrium into the right ventricle through the right atrioventricular orifice. It is closed during ventricular contraction by the tricuspid valve and opens facing forward and medially. Blood entering the right ventricle from the right atrium moves in a horizontal and forward direction when in the anatomical position. Middle Mediastinum & Heart - Berger Page 12 of 26 Right Ventricle - Forms largest part of anterior surface and inferior border - Trabeculae carneae – irregular muscular Figure 20. elevations of the walls of the inflow portion - Papillary muscles – attach to ventricular wall on one end, with other end attaches to the tendon-like fibrous cords (chordae tendineae), which connect to the free ends of the cusps of the tricuspid valve. There are three papillary muscles: anterior, posterior and septal. - Septomarginal trabecula – or moderator band, is a single specialized trabeculum and bridges the inferior IV septum → anterior papillary muscle base; shortcut for right bundle of AV bundle to the anterior wall of the right ventricle - Conus arteriosus – smooth-walled, outflow tract to the pulmonary trunk; pulmonary valve is at the apex - Supraventricular crest – separates muscular inflow part from smooth-walled conus arteriosus (outflow) - Right atrioventricular (A/V) orifice – inflow; 4th/5th intercostal level Tricuspid Valve Closes off the right atrioventricular orifice during ventricular contraction. Consisting of three cusps, the base of each cusp is secured to a fibrous ring that helps to maintain the shape of the atrioventricular orifice. Cusps are named based on their relative positions – anterior, septal and posterior. Free margins are attached to chordae tendineae. Middle Mediastinum & Heart - Berger Page 13 of 26 While the right ventricle fills, the cusps project into the ventricle and open the Figure 21. valve. Papillary muscles contract when the ventricular musculature contracts, thus pulling the cusps closed and preventing backflow of blood into the right atrium. Pulmonary Valve Closes the outflow tract between the right ventricle and pulmonary trunk. The valve consists of three semilunar cusps (left, anterior, right), forming a pocket-like sinus. After ventricular contraction, the sinuses fill with blood and forces the cusps closed, thus preventing backflow into the right ventricle. Figure 23. Clinical importance: Pulmonary stenosis - Valve cusps fuse → narrowing - Infundibular pulmonary stenosis → conus arteriosus is underdeveloped → restricts right ventricular outflow Figure 22. Normal and stenotic pulmonary valves. Figure 24. (Adapted from www.heartvalvexpert.com) Middle Mediastinum & Heart - Berger Page 14 of 26 Left Atrium - Similar to right atrium, it is derived embryologically from two structures; smooth wall and muscular portion - Posterior half receives blood from four pulmonary veins (inflow); has smooth walls - Smaller muscular auricle is in the anterior half; but no distinct structure to separate - Interatrial septum is part of the anterior wall; thin area/depression is the valve of foramen ovale (is your cadaver’s probe patent?) Blood passes from the left atrium into the left ventricle through the left atrioventricular orifice. It is closed during ventricular contraction by the bicuspid valve. Blood entering the left ventricle from the left atrium moves in a forward direction to the apex. Figure 25. Clinical importance: Thrombi (immobile blood clots) form on the wall of the left atrium; when they detach, they become emboli (mobile clots) and can occlude an artery. Middle Mediastinum & Heart - Berger Page 15 of 26 Left Ventricle - Most of heart base, longer than right ventricle with thickest layer of myocardium - Lies anterior to the left atrium and forms the apex - Nearly all of left surface/border and diaphragmatic surface - Since arterial pressure is higher in systemic vs. pulmonary circulation, left ventricle performs more work than right ventricle - Aortic vestibule forms the outflow tract with smooth walls - Mitral valve – 4th intercostal cartilage - Walls ~2-3x thicker - Trabeculae carneae are fine and delicate compared to those in the right ventricle - Larger A/P papillary muscles - Intraventricular (IV) septum forms the anterior wall Membranous – upper part, continuous with fibrous skeleton Muscular – thick and bulges into right ventricle because of increased bp in left ventricle; major part of septum Figure 26. Middle Mediastinum & Heart - Berger Page 16 of 26 Clinical importance: Ventricular septal defects Figure 27. - Common site → membranous part of IV septum - ~25% of all forms of congenital heart disease - Defect 1-25 mm - Causes left → right shunt of blood → increases pulmonary blood flow → pulmonary disease Mitral Valve Closes off the left atrioventricular orifice during ventricular contraction. Consisting of two cusps – anterior and posterior, hence bicuspid valve. Similar to the tricuspid, the base of each cusp is secured to a fibrous ring offering structural support. Coordinated action of the papillary muscles and chordae tendineae is as described for the right ventricle, as well. Clinical importance: Mitral valve is the most frequently diseased of heart valves - Nodules on valves → irregular blood flow - Scarring and shortening of valves → mitral insufficiency → blood regurgitates into left atrium when left ventricle contracts → heart murmur Figure 28. Aortic Valve The outflow tract for the left ventricle is continuous with the ascending aorta, which is closed by the aortic valve. Similar in structure to the pulmonary valve, it has three semilunar cusps (right, posterior, left). Left and right coronary arteries arise here, also. Function of the aortic valve is similar to the pulmonary valve. However, blood is forced into the coronary arteries as well as into the ascending aorta. Clinical importance: Aortic insufficiency – defective aortic valve → aortic regurgitation → heart murmur and collapsing pulse Middle Mediastinum & Heart - Berger Page 18 of 26 3. Cardiac Skeleton Figure 30. Figure 31. Middle Mediastinum & Heart - Berger Page 19 of 26 Four rings of dense, fibrous connective tissue (annulus fibrosus) Right and left fibrous trigone – areas of thickened connective tissue between rings Helps to maintain the integrity of the openings it surrounds and provides attachment points for cusps and myocardium Separates atrial from ventricular musculature Serves as a partition of dense connective tissue to electrically isolate the atria from the ventricles, leaving the A/V bundle as the single connection between the two groups of myocardium. 4. Coronary Vasculature Arterial Blood Supply of Heart Right coronary artery  SA nodal branch – supplies SA node  Right marginal branch – supplies right border of heart  AV nodal branch – supplies AV node  Posterior IV artery Typically supplies: Right atrium, most of right ventricle, part of left ventricle, part of IV septum, SA node (60%), AV node (80%) Left coronary artery  SA nodal branch – supplies SA node  Anterior IV artery – supplies ventricle and IV septum  Circumflex branch – supplies posterior surface  Left marginal – branches off of circumflex and supplies left ventricle Typically supplies: Left atrium, most of left ventricle, part of right ventricle, most of IV septum, SA node (40%) Clinical importance: Variations of coronary arteries – - Normally, 50/50 blood supply contribution from LCA & RCA - 15% LCA is dominant  posterior IV branch off of circumflex - Single coronary artery - Circumflex arises directly from right aortic sinus - 4% accessory coronary artery - Branches of coronary arteries are considered end arteries – supply myocardium without functional overlap, BUT, smaller branches do anastomose and functional value of this is apparent in slowly progressive coronary artery disease.