Mediastinum, Pericardium, & Heart

Mediastinum

  • Definition: A median fibrous septum (wall) between the two pulmonary cavities (pleural sacs).

    • Extent: From the thoracic inlet to the diaphragm, anteriorly from the sternum to the thoracic vertebral column.

    • Boundaries: Bounded by the mediastinal pleura covering the medial surfaces of the lungs.

  • Contents:

    • Heart

    • Large blood vessels

    • Trachea

    • Esophagus

    • Vagus nerve

    • Phrenic nerve

    • Sympathetic trunk

    • Lymph nodes

    • Thoracic duct

Divisions of the Mediastinum

  • A line drawn from the sternal angle (angle of Louis) to the lower border of the T4 vertebra divides the mediastinum into:

    • Superior Mediastinum: Lies above the line

    • Inferior Mediastinum: Lies below the line

Inferior Mediastinum Subdivisions

  1. Anterior Mediastinum: Lies in front of the pericardium of the heart.

    • Contents:

      • Remains of thymus

      • Right & left brachiocephalic veins

      • Upper half of superior vena cava

      • Arch of aorta and its 3 branches

      • Arch of the azygous vein

      • Vagus nerves

      • Phrenic nerves

      • Left recurrent laryngeal nerve

      • Thoracic duct

      • Trachea

      • Esophagus

      • Lymph nodes

      • Clinical Note: Mediastinal Syndrome: Caused by compression of mediastinal structures by tumors, cysts, or lesions (e.g. Hodgkin’s disease).

  2. Middle Mediastinum: Houses the pericardium and heart

    • Contents:

      • Heart with its pericardium

      • Phrenic nerves

      • Pericardiophrenic vessels

      • Tracheobronchial lymph nodes

      • Bifurcation of trachea into right & left bronchus

      • Ascending aorta

      • Pulmonary trunk & right & left pulmonary arteries

      • Pulmonary veins (right & left)

      • Superior vena cava (lower half)

  3. Posterior Mediastinum: Lies behind the pericardium of the heart.

    • Contents:

      • Esophagus

      • Descending aorta and its branches

      • Thoracic duct

      • Posterior mediastinal lymph nodes

      • Vagus nerve

      • Sympathetic trunk and splanchnic nerves

      • Azygos vein

      • Accessory (superior) hemiazygos vein

      • Inferior hemiazygos vein

Pericardium

  • Definition: A double-walled fibro-serous sac covering the heart and the roots of great vessels in the middle mediastinum.

    • Location:

      • Attached to the diaphragm below

      • Lies behind the body of the sternum and 2nd to 6th costal cartilages

  • Layers:

    1. Outer Fibrous Pericardium: Tough fibrous membrane enclosing the heart and beginning of the great vessels.

      • Fuses with the external coats of the ascending aorta and pulmonary trunk at the sternal angle

      • Fuses with the diaphragm’s upper surface

    2. Inner Serous Pericardium: Divided into:

      • Outer Parietal layer: Attached to the inner surface of the fibrous pericardium

      • Inner Visceral layer: Covers the outer surface of the heart and forms the epicardium.

      • Pericardial Cavity: The space between the parietal and visceral layers, containing around 30-50 ml of pericardial fluid that acts as a lubricant to reduce friction.

Pericardial Cavity Sinuses

  1. Transverse Sinus: A gap between the ascending aorta and pulmonary trunk anteriorly and the superior vena cava and left atrium posteriorly.

    • Applied Anatomy: The transverse sinus may be used to pass a ligature during cardiac surgery (e.g., coronary artery bypass grafting).

  2. Oblique Sinus: A blind sac behind the left atrium, between large veins, leads into the pericardial cavity.

Cardiac Tamponade

  • Definition: Compression of thin-walled atria interfering with the heart’s filling during diastole caused by:

    • Pericardial Effusion: Accumulation of fluid in the pericardial cavity due to pericarditis (inflammation).

      • Large amounts compress the heart, reducing output; symptoms include hypotension and faint heart sounds.

    • Traumatic injuries: stab or gunshot wounds can also cause cardiac tamponade.

Pericardiocentesis

  • Procedure: Drain pericardial fluid using a needle (ultrasound guided), either:

    • Substernal insertion: Below the xiphoid process

    • Parasternal insertion: Through the left 5th or 6th intercostal space, avoiding injury to the internal thoracic artery, left pleura, lung, and right ventricle

    • Insert needle at a 45-degree angle towards the left shoulder.

Heart - General Features

  • Definition: A hollow muscular organ with a conical shape, situated in the middle mediastinum.

  • Function: Pumps deoxygenated blood to the lungs and oxygenated blood to the body.

  • Orientation: 2/3 of the heart lies to the left of the median plane and 1/3 to the right.

  • Size: About the size of a clenched fist.

  • Chambers:

    • Four chambers:

    • Right atrium: Receives deoxygenated blood from the body

    • Right ventricle: Pumps blood to the lungs

    • Left atrium: Receives oxygenated blood from the lungs

    • Left ventricle: Pumps blood to the body

External Features of the Heart

  • Apex: Formed by the left ventricle, located 9 cm from the median plane in the left 5th intercostal space (at the midclavicular line).

  • Base (Posterior Surface): Directed upwards and posteriorly, mainly formed by the left atrium. Related posteriorly to the esophagus, descending aorta. Clinical Note: A distended left atrium in mitral stenosis can cause dysphagia.

  • Anterior Surface (Sternocostal Surface): Mainly formed by the right ventricle (right 2/3) and partly by the left ventricle (left 1/3). Includes:

    • Anterior interventricular groove: Between right and left ventricles, containing the anterior interventricular artery (LAD) and the great cardiac vein.

  • Inferior Surface (Diaphragmatic Surface): Mainly by the left ventricle (left 2/3) and partly by the right ventricle (right 1/3). Contains the posterior interventricular groove with the posterior interventricular artery and middle cardiac vein.

  • Borders:

    • Superior border: 2-3 formed by the roots of the great vessels

    • Right border: 3-6 formed by the right atrium

    • Inferior border: 6-5 formed by the right ventricle and the apex

    • Left border: 5-2 formed by the left ventricle

Thorax X-Ray (P-A View)

  • Displays the anatomy and blood vessels relative to the heart, including apex, right and left vents, atria, and major arteries.

Layers of the Heart Wall

  1. Epicardium: Formed by the visceral layer of the serous pericardium.

    • Subepicardium: Contains the SA node, connective tissue, coronary vessels, and nerves.

  2. Myocardium: A thick layer of cardiac muscle responsible for contraction.

  3. Endocardium: Composed of simple squamous epithelium; the subendocardial layer lies between the endocardium and myocardium containing the AV node, AV bundle, and Purkinje fibers.

Fetal Circulation

  • Shunts:

    1. Ductus Venosus: Shunts oxygenated blood from umbilical vein to IVC (bypassing liver).

    2. Ductus Arteriosus: Shunts deoxygenated blood from pulmonary artery to arch of aorta (bypassing lungs).

    3. Foramen Ovale: Shunts oxygenated blood from right atrium to left atrium (bypassing lungs).

  • Changes After Birth:

    1. Closure of ductus venosus -> ligamentum venosum

    2. Closure of ductus arteriosus -> ligamentum arteriosum

    3. Closure of foramen ovale -> fossa ovalis

    4. Closure of umbilical vessels.

Septa of the Heart

  • Interatrial Septum: Between right and left atria.

  • Interventricular Septum: Between right and left ventricles.

  • Development of Interatrial Septum:

    • Septum Primum: Grows from primitive atrium towards endocardial cushion, forming foramen primum.

    • Before closure, foramen secundum develops above, leading to formation of foramen ovale when septum secundum grows.

    • At birth, increased left atrial pressure fuses septa, preventing direct blood flow.

    • Fetal Remnants:

      • Fossa Ovalis: Represents septum primum

      • Limbus Fossa Ovalis: Represents free margin of septum secundum.

Interatrial Septal Defects

  • Patent Foramen Ovale: Failure of fusion between septum primum and secundum, causing left-to-right shunt (non-cyanotic).

  • Ostium Secundum Defect: Caused by increased resorption of upper part of septum primum.

  • Ostium Primum Defect: Result of failure of fusion of septum primum with endocardial cushion.

Interventricular Septum

  • Definition: Partition between right and left ventricles, having:

    • Thin Membranous Part: Upper

    • Thick Muscular Part: Lower

  • Development:

    • Membranous part develops from aorticopulmonary septum; muscular part from a muscular ridge.

  • Defects: Ventricular Septal Defect results from failure of fusion, leading to right ventricular hypertrophy (non-cyanotic).

Fibrous Skeleton of the Heart

  • Structure: Four fibrous rings around aortic, pulmonary, and AV openings interconnected by fibrous tissue.

    • Functions: Support valves, prevent stretching, and divide atria from ventricles except at AV bundle.

Right Atrium

  • Description: Venous chamber with main chamber and right auricle; sulcus terminalis along right border.

  • Interior:

    • Crista Terminalis: Divides cavity into anterior rough (musculi pectinati) and posterior smooth parts.

  • Openings:

    1. Superior Vena Cava: Located superior.

    2. Inferior Vena Cava: Inferiorly guarded by a rudimentary valve.

    3. Coronary Sinus: Opens between inferior vena cava and right atrioventricular orifice, guarded by a semilunar valve.

    4. Anterior Cardiac Veins:

    5. Right Atrioventricular Opening: Guarded by the tricuspid valve.

Right Ventricle

  • Function: Pumps deoxygenated blood to the lungs.

  • Features:

    • Forms right 2/3 of anterior surface; inflow and outflow tracts with valves and muscular projections.

  • Inflow Part: Receives blood through right atrioventricular orifice guarded by the tricuspid valve. Contains trabeculae carneae and moderator band transmitting the right branch of the AV bundle.

  • Outflow Part: Funnel-shaped infundibulum separated by supraventricular crest leading to pulmonary trunk.

Left Atrium

  • Description: Receives oxygenated blood via 4 pulmonary veins.

  • Form: Forms base of the heart; left auricle connected to left ventricle by the mitral orifice.

  • Interior: Musculi pectinati only in left auricle; smooth part develops from absorbed pulmonary veins.

  • Responsibilities: Directs blood into left ventricle through a guarded orifice.

Left Ventricle

  • Function: Pumps oxygenated blood into the ascending aorta, generating systolic blood pressure.

  • Size: Wall thickness is three times that of the right ventricle and forms the apex of the heart.

  • Inflow and Outflow Tracts: Similar to right ventricle but contains no moderator band.

    • Valves: Left atrioventricular orifice is guarded by bicuspid/mitral valve, with outflow to aortic vestibule leading to aortic valve.

Tetralogy of Fallot

  • Definition: Cyanotic right-to-left shunt congenital defect involving four components:

    1. Pulmonary Stenosis

    2. Overriding Aorta: Receives blood from both ventricles.

    3. Membranous Interventricular Septal Defect

    4. Right Ventricular Hypertrophy: Boot-shaped heart appearance on X-ray.

Heart Sounds

  • First Heart Sound (Lub): Closure of AV valves (tricuspid and mitral) at beginning of systole.

  • Second Heart Sound (Dub): Closure of semilunar valves (aortic and pulmonary) at end of systole.

  • Heart Murmur: Extra sound caused by turbulent blood flow; may indicate valvular heart disease (insufficiency or stenosis).

Auscultatory Areas of the Heart Valves

  • Pulmonary Area: Second left intercostal space near the sternum

  • Aortic Area: Second right interspace near the sternum

  • Tricuspid Area: Lower end of the sternum near left 5th intercostal space

  • Mitral Area: Cardiac apex

Conducting System of the Heart

  • Components: Specialized myocardial cells for cardiac impulse generation.

    • SA Node: Pacemaker located near superior vena cava opening in the right atrium.

    • AV Node: Lower end of interatrial septum just above coronary sinus opening.

    • Bundle of His: Only muscular connection between atrium and ventricle, divided into right and left branches.

    • Purkinje Fibers: Terminal branches of conduction system.

Heart Contractions and Rhythm

  • Normal Rhythm: 70-90 beats per minute.

    • Systole: Contracts atria first, then ventricles.

    • Diastole: Chambers relax to allow coronary artery filling.

  • Arrhythmias: Irregular heartbeats due to faulty electrical signaling, classified as tachycardia, bradycardia, or arrhythmic.

    • ECG Phases: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).

Arterial Supply of the Heart

  • Coronary Arteries: Right and left; branch from ascending aorta at aortic sinuses.

    • Function: Supply myocardium during diastole; inadequate functional anastomoses can lead to myocardial ischemia or infarction.

Right Coronary Artery

  • Path: Runs in coronary sulcus to crux of heart; supplies right atrium, right ventricle, and parts of left ventricle through branches.

  • Origin: Arises from the anterior (right) aortic sinus of the ascending aorta.

  • Course:

    • Passes anteriorly between the pulmonary trunk and the right auricle.

    • Descends in the right coronary sulcus (atrioventricular groove) between the right atrium and right ventricle.

    • Winds around the inferior border of the heart to reach the posterior surface and the crux (the junction of the interatrial and interventricular septa).

  • Major Branches:

    • Sinuatrial (SA) Nodal Branch: Supplies the SA node in approximately 60%60% of individuals.

    • Right Marginal Branch: Runs along the inferior border of the heart toward the apex; supplies the right ventricle.

    • Atrioventricular (AV) Nodal Branch: Arises near the crux and supplies the AV node in 80%80% of individuals.

    • Posterior Interventricular Artery (PDA): Runs in the posterior interventricular groove. In right-dominant hearts (90%), this arises from the RCA.

  • Supply Areas: Right atrium, most of the right ventricle, part of the left ventricle (diaphragmatic surface), posterior 1/3 of the interventricular septum, and the conducting nodes (SA and AV).

Left Coronary Artery

  • Path: Supplies larger volume of myocardium; includes anterior interventricular artery (LAD) and circumflex artery.

  • Origin: Arises from the left posterior aortic sinus of the ascending aorta.

  • Course:

    • Passes between the pulmonary trunk and the left auricle to enter the coronary sulcus.

    • Usually a short trunk that quickly divides into its two primary terminal branches.

  • Major Branches:

    • Anterior Interventricular Artery (Left Anterior Descending - LAD):

      • Passes along the anterior interventricular groove to the apex.

      • Supplies the anterior parts of both ventricles and the anterior 2/3 of the interventricular septum.

    • Circumflex Artery (Cx):

      • Follows the coronary sulcus around the left border to the posterior surface.

      • Left Marginal Artery: A branch of the circumflex that supplies the left border of the heart.

  • Supply Areas: Left atrium, most of the left ventricle, part of the right ventricle, the anterior 2/3 of the interventricular septum, and the SA node in about 40% of individuals.

Right and Left Dominance

  • Right Dominance: Posterior interventricular artery arises from RCA (90% cases).

  • Left Dominance: Posterior ventricular artery arises from LCA (10% cases).

  • Balanced Arterial Supply: Arises from both right and left coronary arteries.

Angina Pectoris

  • Description: Pain radiating along ulnar border of left arm indicating partial coronary artery obstruction; conveyed via T1 spinal segment.

Venous Drainage of the Heart

  • Overview: 60% of coronary venous blood drains into the right atrium via the coronary sinus, while 40% drains into heart chambers via venae cordis minimae.

  • Coronary Sinus: Situated in the coronary sulcus, opens into right atrium to left of IVC opening.

Tributaries of the Coronary Sinus

  1. Great Cardiac Vein: Accompanies LAD in anterior interventricular groove.

  2. Middle Cardiac Vein: Accompanies RCA in the posterior interventricular groove.

  3. Small Cardiac Vein: Accompanies main trunk of RCA in coronary sulcus.

Nerve Supply of the Heart

  • Components: Autonomic nerves form superficial & deep cardiac plexuses; divided into sympathetic, parasympathetic, and sensory fibers.

Sympathetic Nerve Supply

  • Preganglionic Fibers: Originate from T1-T4 lateral horns; end in upper thoracic sympathetic ganglia.

    • Postganglionic Fibers: Supply coronary arteries, nodes, and muscle fibers, leading to dilation and increased heart rate.

Parasympathetic Nerve Supply

  • Preganglionic Fibers: Originate from dorsal motor nucleus of vagus nerve in medulla, reach heart terminating in terminal ganglia.

    • Postganglionic Fibers: Supply coronary vessels, nodes, and muscle fibers causing decreased heart rates.

Sensory Nerve Supply

  • Pain Fibers: Originate as afferent visceral pain fibers; lead to referred pain patterns in conditions like angina pectoris.