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

Middle Mediastinum & Heart - Berger Page 1 of 26 Learning Objectives I. Course Objective: Describe the anatomy of the region under study. Session Objectives:  Describe the anatomy of the middle mediastinum including boundaries an...

Middle Mediastinum & Heart - Berger Page 1 of 26 Learning Objectives I. Course Objective: Describe the anatomy of the region under study. Session Objectives:  Describe the anatomy of the middle mediastinum including boundaries and contents.  Describe the anatomy of the pericardium including anatomical features, sinuses, blood supply & venous drainage, and innervation.  Describe the external anatomy of the heart including its layers, anatomical position, borders and surfaces.  Describe the internal anatomy of the heart including right atrium, right ventricle, tricuspid valve, pulmonary valve, left atrium, left ventricle, mitral valve, and aortic valve.  Describe the cardiac skeleton.  Describe the coronary vasculature including arteries, veins, and lymphatics.  Describe the conducting system of the heart.  Describe the innervation of the heart. II. Course Objective: Relate the anatomy of structures to their function(s). Session Objectives:  Relate the anatomy of the serous and fibrous pericardium to their functions.  Relate the external and internal anatomy of the heart to their functions including the cardiac cycle.  Relate the anatomy of the cardiac skeleton to its function.  Relate the anatomy of the coronary vessels to their blood supply and drainage functions.  Relate the anatomy of the conducting system to its functions.  Relate the anatomy of nerves to their functional innervation of the heart III. Course Objective: Apply knowledge of anatomy to evaluate clinically relevant problems. Session Objectives:  Apply the anatomy of the pericardium to pericarditis, pericardial effusion, cardiomegaly, hemopericardium, and pericardiocentesis.  Apply the anatomy of the heart to the problems of pulmonary stenosis, thrombi/emboli, ventricular septal defects, heart valve diseases, aortic insufficiency, myocardial infarction, and referred pain. Middle Mediastinum & Heart - Berger Page 2 of 26 Lecture Content Outline Middle Mediastinum A. General Features B. Pericardium 1. Anatomical Features 2. Vessels & Nerves a. Arterial Supply b. Venous Drainage c. Innervation 3. Clinical Importance C. Heart 1. External Anatomy a. Layers of the Heart b. Anatomical Position c. Borders d. Surfaces 2. Internal Anatomy a. Right Atrium b. Right Ventricle c. Tricuspid Valve d. Pulmonary Valve e. Left Atrium f. Left Ventricle g. Mitral Valve h. Aortic Valve 3. Cardiac Skeleton 4. Coronary Vasculature a. Arterial Blood Supply of Heart b. Venous Drainage of Heart c. Lymphatic Drainage of Heart 5. Conducting System of Heart 6. Innervation 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 17 of 26 Figure 29. 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. Middle Mediastinum & Heart - Berger Page 20 of 26 Figure 32. Clinical importance: Myocardial infarction (MI) – major artery is suddenly occluded by an embolus, myocardium in that region becomes infarcted. MI is an area of myocardium that is necrotic. Atherosclerosis of coronary arteries → coronary insufficiency → ischemic heart disease; often characterized by angina pectoris (pain in substernal region and down medial side of the left arm and forearm). Figure 33. Middle Mediastinum & Heart - Berger Page 21 of 26 Venous Drainage of Heart - Heart drains via coronary sinus (1°) and anterior cardiac veins → right atrium (2°) - Coronary sinus receives great cardiac vein, middle and small cardiac veins - Left posterior vein and left marginal vein → coronary sinus - Smallest cardiac veins – capillary beds of myocardium → atria (provide collateral circulation to musculature) Lymphatic Drainage of Heart - Myocardium and subepicardial connective tissue → subepicardial lymphatic plexus Figure 34. - Travel along coronary groove → ends in inferior tracheobronchial lymph nodes Figure 35. Middle Mediastinum & Heart - Berger Page 22 of 26 5. Conducting System of Heart: - Impulse-conducting system coordinates the cardiac cycle - Consists of cardiac muscle cells & highly specialized conducting fibers for initializing impulses and conducting them through the heart - Nodal tissue initiates heartbeat & coordinates contractions 1. SA node: Impulse 70x/min Pacemaker of heart Located just deep to epicardium, junction of SVC & right atrium 2. AV node: Smaller collection of nodal tissue Just posteroinferior of interatrial septum, near coronary sinus opening Figure 36. Middle Mediastinum & Heart - Berger Page 23 of 26 Figure 37. Figure 38. Cardiac cycle – complete movement of the heart; period from the beginning of one heartbeat to the beginning of the next. Cycle consists of diastole (ventricular filling) and systole (ventricular emptying). 1. Beginning of diastole upon closure of aortic and pulmonary valves 2. Opening of atrio-ventricular valves during early moments of diastole 3. Atrial contraction during final moments of diastole 4. Closure of atrioventricular valves (tricuspid and mitral) very soon after systole begins 5. Opening of aortic and pulmonary valves during systole Figure 39. Summary of cardiac cycle. (Adapted from www.studyblue.com) Middle Mediastinum & Heart - Berger Page 24 of 26 6. Innervation of the Heart: Cardiac plexuses - Anterior to bifurcation of the trachea, posterior to the ascending aorta, superior to the bifurcation of the pulmonary trunk - Autonomic division of the PNS is only in the atria to any specific degree and very little in ventricles; as a result, directly responsible for regulating heart rate and force of atrial contraction and no direct effect on ventricles - Sympathetic stimulation of the nodal tissue increases heart rate and the force of cardiac contractions - Parasympathetic stimulation slows the heart rate, saving energy - Branches from both parasympathetic and sympathetic contribute to the cardiac plexus - Visceral afferents from the heart are also part of the cardiac plexus Figure 40. Middle Mediastinum & Heart - Berger Page 25 of 26 Figure 41. Figure 42. Figure 43. Referred cardiac pain. (Reproduced from https://drsvenkatesan.com/tag/referred-pain/) Clinical importance: The heart is insensitive to touch and temperature; however, ischemia can stimulate pain endings in the myocardium. The axons of afferent pain fibers enter spinal cord segments T1 – T4/5, especially on the left side. Cardiac referred pain is a phenomenon whereby stimuli originating in the heart is perceived as pain arising from a superficial body part – e.g., skin on the medial aspect of left upper limb. Middle Mediastinum & Heart - Berger Page 26 of 26 Auscultatory areas of the heart are shown in Figure 44. Four traditional valve areas are indicated – note that these are not the anatomical positions of the valves, but where they are best heart based on sound projections related to blood flow. Figure 44. Aortic valve area – 2nd right intercostal space Pulmonic valve area – 2nd left intercostal space Mitral valve area – 5th intercostal space Tricuspid valve area – left lower sternal border 7. Great Vessels The pulmonary trunk is contained within the pericardial sac, along with the ascending aorta. The pulmonary trunk arises from the conus arteriosus and divides into left and right pulmonary arteries between TV5-TV6 Ascending aorta originates at the aortic orifice and moves superiorly to the level of the 2nd right costal cartilage, where it enters the superior mediastinum and becomes the arch of the aorta. Figure 45.