Mediastinum and Great Vessels

Structural Organization and Function

• The mediastinum is divided into superior mediastinum (above thoracic inlet at T1) and inferior mediastinum (below thoracic inlet), with the inferior further subdivided into anterior, middle, and posterior compartments by the pericardium and pleural reflections
• The middle mediastinum contains the pericardium (fibrous and serous layers), heart, and proximal great vessels; the anterior mediastinum contains lymph nodes and connective tissue; the posterior mediastinum contains the esophagus, thoracic aorta, azygos system, and thoracic duct

Great Vessel Anatomy and Relationships

• The ascending aorta (Valsalva sinuses, coronary ostia, sinotubular junction) arises from the left ventricle and curves to become the aortic arch, which gives three major branches: brachiocephalic trunk (right common carotid and subclavian), left common carotid, and left subclavian
• The descending thoracic aorta passes through the diaphragm at T12 (via aortic hiatus) and gives intercostal arteries, esophageal branches, bronchial arteries, and subcostal arteries
• The superior vena cava (formed by union of right and left brachiocephalic veins at T2-3) drains the head, neck, upper extremities, and upper thorax; the inferior vena cava enters the right atrium at T8
• The pulmonary trunk arises from the right ventricle, passes behind the ascending aorta, and bifurcates at approximately T5 into right and left pulmonary arteries
• The pulmonary veins (typically four: right upper/lower, left upper/lower) drain into the left atrium; the left pulmonary veins pass posterior to the left main bronchus, while right pulmonary veins pass posterior to the right main bronchus

Pressure Relationships and Hemodynamic Principles

• Venous return to the right atrium is determined by cardiac output, venous capacitance, and intrathoracic pressure; the vena cava is a thin-walled, highly compliant structure vulnerable to pressure changes
• Aortic compliance decreases with age due to collagen and elastin changes, leading to increased pulse pressure and wall stress
• Mediastinal pressures affect great vessel distention and function; increased intrapericardial pressure (as in tamponade) compresses the vena cava and right atrium, reducing venous return
• The pericardium normally contains 15-50 mL of serous fluid; acute accumulation of fluid increases pressure disproportionately due to the pericardium's limited compliance (inelastic properties)

Aortic Pathology (Dissection, Aneurysm, Atherosclerosis)

• Sudden, severe chest pain radiating to the back (between scapulae) is classic for aortic dissection; pain may be tearing or ripping in quality; blood pressure differential between arms (≥20 mmHg systolic) suggests subclavian involvement
• Syncope or neurologic deficits occur with carotid artery involvement or disruption of spinal arteries (anterior spinal artery syndrome)
• Acute aortic regurgitation produces an early diastolic murmur (best heard at left sternal border with patient leaning forward)
• Contained rupture may present with hemodynamic stability initially, but sudden expansion causes cardiovascular collapse

Superior Vena Cava Syndrome

• Progressive facial and upper extremity swelling, prominent chest wall veins, and venous distention
• Cyanosis and plethora (reddish discoloration) of the face and neck
• Dyspnea and stridor if tracheal compression occurs

Cardiac Tamponade

• Beck's triad: hypotension, elevated jugular venous pressure (JVP), and muffled heart sounds
• Pulsus paradoxus (>10 mmHg drop in systolic BP during inspiration) is a key finding, though not pathognomonic
• Dyspnea, chest discomfort, and anxiety; symptoms worsen in supine position and with inspiration

Pulmonary Embolism (PE) with Great Vessel Involvement

• Acute dyspnea, pleuritic chest pain, and hemoptysis (with pulmonary infarction)
• Syncope if massive PE obstructs pulmonary outflow; acute cor pulmonale presents with right heart strain

Mediastinitis

• Fever, chest pain, and hemodynamic instability following cardiac surgery or esophageal perforation
• Subcutaneous emphysema (crepitus) may be palpable in the neck and upper chest
• Severe presentation with septic shock in fulminant cases

Imaging Modalities

• Chest X-ray: evaluates mediastinal width (>8 cm at T2 may indicate pathology), heart size (cardiothoracic ratio >0.5 suggests cardiomegaly), and gross abnormalities; limited sensitivity for acute aortic dissection
• CT angiography (CTA) with intravenous contrast: gold standard for aortic dissection (sensitivity/specificity >95%), evaluates extent of dissection, branch vessel involvement, and pericardial effusion; also used for aortic aneurysm sizing and PE diagnosis
• Transesophageal echocardiography (TEE): superior to TTE for visualizing the ascending aorta and aortic arch; excellent for detecting intimal flaps and aortic regurgitation in acute dissection; used intraoperatively
• Transthoracic echocardiography (TTE): assesses pericardial effusion, cardiac function, and tamponade physiology (RA/RV collapse); useful for bedside evaluation but limited field of view
• MRI and MRA: excellent for chronic aortic pathology and branch vessel involvement but impractical in acute, unstable settings

Laboratory and Clinical Tests

• ECG: may show nonspecific ST changes, ischemia (if ostial MI), or signs of right heart strain in PE; dissection itself does not typically cause diagnostic ECG changes
• Troponin and BNP: elevated in acute aortic dissection (type A) due to coronary involvement or acute MI; also elevated in PE and heart failure
• D-dimer: sensitive but nonspecific for PE; rarely useful in aortic dissection workup
• Blood cultures and inflammatory markers (CRP, procalcitonin, WBC): indicated for mediastinitis
• Pericardiocentesis: diagnostic and therapeutic in tamponade; send fluid for cell count, protein, glucose, LDH, cultures, and cytology to differentiate causes (infectious, malignant, autoimmune)

Diagnostic Criteria

• Aortic dissection: Stanford Type A involves ascending aorta (surgical emergency); Type B is descending aorta only (medical management initially)
• Tamponade: clinical diagnosis confirmed by hemodynamic findings and imaging; pulsus paradoxus + elevated JVP + muffled heart sounds + evidence of pericardial effusion on echo
• Superior vena cava syndrome: clinical presentation + imaging confirmation (CTA or venography) showing obstruction

Compartment localization is the whole question

• **Anterior mediastinum — the *4 T's*: Thymoma, Teratoma/germ cell tumor, Thyroid (retrosternal goiter), T**errible lymphoma. Thymoma is the association examiners test: myasthenia gravis (anti-AChR), and less commonly thymoma-associated pure red cell aplasia or hypogammaglobulinemia (Good syndrome). A young man with an anterior mass plus elevated AFP/β-hCG is a nonseminomatous germ cell tumor, not lymphoma.
• Posterior mediastinum: neurogenic tumors — neuroblastoma in young children, schwannoma/neurofibroma in adults — plus esophageal and descending aortic pathology. Middle mediastinum: lymphadenopathy (sarcoidosis, lymphoma), bronchogenic and pericardial cysts.

Nerve relations that generate the stem's one physical finding

• Left recurrent laryngeal nerve loops under the aortic arch at the ligamentum arteriosum, so hoarseness signals arch aneurysm, left hilar tumor, or massive left atrial enlargement (Ortner cardiovocal syndrome). The right nerve loops under the right subclavian and is spared by arch disease.
• Phrenic nerve (C3–C5) runs anterior to the hilum; vagus runs posterior — the most common distractor in hilar-relations questions. Phrenic involvement gives an elevated hemidiaphragm.

Thoracic duct and lymph

• The duct enters the thorax through the aortic hiatus (T12) with the aorta and azygos, ascends in the posterior mediastinum, crosses to the left in the mid-thorax, and empties at the left subclavian–internal jugular junction. It drains everything except the right upper quadrant of the body. Injury during left-sided thoracic or esophageal surgery → chylothorax with milky, triglyceride-rich pleural fluid.
• Diaphragmatic apertures: I 8 (T8) IVC, 10 (T10) eats — esophagus with vagal trunks, 12 (T12) aorta.

Vascular pearls and next steps

• Sternal angle (T4/T5) marks arch beginning and end, tracheal bifurcation, and azygos drainage into the SVC — the azygos is the key SVC-to-IVC collateral in SVC syndrome, whose leading causes are small cell lung cancer and non-Hodgkin lymphoma.
• Blunt deceleration injury ruptures the aortic isthmus just distal to the left subclavian (tethered by the ligamentum arteriosum); a widened mediastinum on chest radiograph mandates CT angiography as the next step per the 2022 ACC/AHA aortic disease guideline and ACR Appropriateness Criteria — not observation.