Thorax Anatomy & Diaphragm

Diaphragm: openings, innervation, and referred pain

Three openings, three levels. Each diaphragmatic opening sits at a fixed vertebral level and carries predictable companions. The IVC passes through the central tendon at T8 with the right phrenic nerve; the esophagus passes through the right crus at T10 with the anterior and posterior vagal trunks; and the aorta passes at T12 with the thoracic duct and azygos vein. Strictly, the aorta never pierces muscle: it runs behind the diaphragm, under the median arcuate ligament.

Phrenic nerve. "C3, 4, 5 keep the diaphragm alive": the phrenic nerve (C3–C5) is the sole motor supply to the diaphragm and carries sensation from the central diaphragm and the pericardium (plus mediastinal pleura). It descends anterior to the root of the lung, whereas the vagus passes posterior to it. The cervical origin reflects development: the diaphragm begins in the neck region (septum transversum) and descends, bringing its nerve along. Phrenic injury leaves the ipsilateral hemidiaphragm elevated and moving paradoxically upward on a sniff test, and cord injury above C3 abolishes spontaneous breathing.

Referred pain. Irritation of the central diaphragm (subphrenic abscess, splenic rupture, free blood, gallbladder disease) is felt at the ipsilateral shoulder tip, because phrenic afferents enter the cord with the C3–C5 dermatomes. Left shoulder pain after splenic rupture is the Kehr sign. The peripheral diaphragm is supplied by intercostal nerves (T5–T11), so irritation there refers to the adjacent thoracoabdominal wall instead.

Hernias. A sliding hiatal hernia (most common) carries the gastroesophageal junction up through the esophageal hiatus; a congenital Bochdalek hernia is posterolateral, usually left-sided, and causes pulmonary hypoplasia.

Mediastinum, landmarks, and great vessels

Compartments. A plane from the sternal angle to the T4/T5 disc separates the superior from the inferior mediastinum; the inferior part is divided into anterior, middle, and posterior compartments. The middle mediastinum is essentially the heart.

Sternal angle of Louis. Palpable where the second rib joins the sternum, it marks the T4/T5 level and several events: the tracheal bifurcation (carina), the start and end of the aortic arch, the azygos vein draining into the SVC, and the superior–inferior mediastinal boundary.

Aortic arch and recurrent laryngeal nerves. The arch branches are the brachiocephalic trunk, left common carotid, and left subclavian arteries. The left recurrent laryngeal nerve loops under the arch at the ligamentum arteriosum (the ductus arteriosus remnant); the right loops under the right subclavian artery. These nerves supply every intrinsic laryngeal muscle except the cricothyroid, so stretch or compression produces hoarseness.

Ortner syndrome: an aortic arch aneurysm or a dilated left atrium (classically mitral stenosis) stretches the left recurrent laryngeal nerve and causes hoarseness. The left atrium is the most posterior chamber and can also indent the esophagus (dysphagia); the right ventricle is the most anterior chamber and the one most exposed to a parasternal stab wound.

Coronary arteries and dominance

Dominance. Dominance is defined by the artery that gives rise to the posterior descending artery. The PDA comes from the RCA in about 85% of people (right dominant); in the remainder it arises from the LCX (left dominant) or from both (codominant).

Conduction consequences. Because the RCA supplies the SA node in ~60% and the AV node in ~90%, an inferior MI can cause sinus bradycardia and AV block. This block is usually nodal, often transient, and atropine-responsive, unlike the infranodal block of a large anterior MI.

The posteromedial papillary muscle depends on the PDA alone, whereas the anterolateral muscle has dual supply (LAD and LCX); papillary muscle rupture with acute mitral regurgitation therefore favors inferior MI. LV perfusion occurs mainly in diastole, so tachycardia worsens ischemia.

Lungs, airways, and pleura

Lobes and bronchi. The right lung has 3 lobes divided by horizontal and oblique fissures; the left has 2 lobes plus the lingula (its counterpart of the middle lobe) and a cardiac notch. The right main bronchus is wider, shorter, and more vertical than the narrower, longer, more horizontal left bronchus, so aspirated objects and material usually go right. At the hilum the pulmonary artery lies anterior to the bronchus on the right and superior to it on the left (RALS).

Aspiration by position. Gravity decides the exact segment:

• Upright: posterobasal segment of the right lower lobe
• Supine: superior segment of the right lower lobe (the posterior segment of the right upper lobe is the other dependent site)
• Right lateral decubitus: right middle and upper lobes

Pleural recesses. The parietal pleura reaches about two ribs lower than the lung edge (lung at ribs 6, 8, and 10; pleura at ribs 8, 10, and 12 in the midclavicular, midaxillary, and paravertebral lines). The resulting costodiaphragmatic recess is where pleural fluid collects and where thoracentesis is performed.

Insert the thoracentesis needle just above (superior to) the rib. The intercostal vein, artery, and nerve (VAN, top to bottom) run in the costal groove along the inferior margin of the rib above.

The thoracic apex: outlet syndrome and Pancoast tumor

Thoracic outlet syndrome. The lower brachial plexus (C8–T1, lower trunk) and/or the subclavian vessels are compressed between the scalenes and the first rib; the plexus trunks and subclavian artery pass between the anterior and middle scalenes, while the subclavian vein runs anterior to the anterior scalene. A cervical rib or a Pancoast tumor is the classic cause. Symptoms follow the ulnar distribution, with medial forearm and hand paresthesias and atrophy of the intrinsic hand muscles; vascular compression adds arm pallor, coolness, or swelling.

Pancoast (superior sulcus) tumor. An apical lung cancer, usually non-small cell, invades the structures crowded at the thoracic inlet: