Thoracic Wall, Diaphragm and Mechanics of Breathing

• Intercostal neurovascular bundle runs in the costal groove on the inferior border of each rib, ordered vein, artery, nerve from superior to inferior (VAN). Needles and chest tubes are therefore passed over the superior border of the rib below the target space.
• Thoracentesis is performed in the mid-scapular or posterior axillary line, typically the 7th–9th intercostal space, above the rib. Chest tubes are usually placed in the "safe triangle" of the 4th–5th intercostal space at the anterior axillary line. Needle decompression of a tension pneumothorax is classically 2nd intercostal space mid-clavicular line, with the 4th–5th lateral approach increasingly preferred in adults.
• The diaphragm is innervated by the phrenic nerve (C3, C4, C5 — "keeps the diaphragm alive"), which also carries sensation from the central diaphragm and pericardium. Irritation there refers to the shoulder tip — the mechanism of Kehr sign in splenic rupture and of shoulder pain after laparoscopy.
• Diaphragmatic openings: T8 inferior vena cava (in the central tendon), T10 oesophagus with the vagal trunks, T12 aorta with the thoracic duct and azygos vein — "I ate ten eggs at twelve".
• Congenital diaphragmatic hernia is usually a left posterolateral (Bochdalek) defect, causing pulmonary hypoplasia; the Morgagni hernia is anterior and rarer.

Structural framework

• Ribs and joints: ribs 1–7 (true) articulate with the sternum via their own costal cartilage; 8–10 (false) join the cartilage above; 11–12 (floating) end in muscle. Each typical rib articulates posteriorly at costovertebral and costotransverse joints, which act as the hinge for respiratory motion.
• Two axes of rib motion: upper ribs rotate about a transverse axis and lift the sternum forward (pump handle), increasing the anteroposterior diameter; lower ribs swing laterally about an anteroposterior axis (bucket handle), increasing the transverse diameter.
• Neurovascular plane: the intercostal bundle lies between the internal and innermost intercostal muscles, in the costal groove (VAN order, as above). Posterior intercostal arteries arise from the aorta; anterior intercostals arise from the internal thoracic artery, a branch of the subclavian.

Generating flow

• Diaphragm is the principal inspiratory muscle and supplies most of resting tidal volume. Phrenic (C3–C5) firing shortens the crural and costal fibers, the dome descends, and the lower ribs are levered outward against the abdominal contents (zone of apposition).
• Pressure sequence: descent enlarges the thorax → pleural pressure becomes more negative → transpulmonary pressure (alveolar minus pleural) rises → alveoli expand → alveolar pressure falls below atmospheric → air flows in (Boyle's law).
• Quiet expiration is passive, driven by stored elastic recoil of lung and chest wall; the system rests at FRC, where inward lung recoil exactly balances outward chest wall recoil.
• Recruitment order: external intercostals and interchondral fibers assist inspiration; scalenes and sternocleidomastoid are accessory inspiratory muscles. Forced expiration recruits abdominal wall muscles and internal intercostals.
• Rate-limiting points: neural drive from the medullary respiratory centers, phrenic conduction, diaphragmatic length–tension (flattening shortens fibers and cripples force generation), and chest wall/lung compliance.

Phrenic nerve and diaphragm

• Unilateral phrenic palsy: from Pancoast (superior sulcus) tumor, mediastinal malignancy, cold cardioplegia injury during cardiac surgery, or interscalene block. Presents as an elevated hemidiaphragm on chest radiograph with **paradoxical upward motion on the fluoroscopic *sniff test***; often asymptomatic at rest, dyspneic when supine.
• Cord level matters: lesions above C3 abolish diaphragmatic drive and cause apnea requiring ventilation; lesions below C5 spare the diaphragm but paralyze intercostals, producing paradoxical chest wall motion and a reduced vital capacity.
• Neuromuscular respiratory failure (Guillain–Barré, myasthenic crisis, ALS): follow bedside forced vital capacity and negative inspiratory force serially; do not wait for hypercapnia on ABG, since CO2 retention is a late finding.

Wall and pleural space

• Flail chest: three or more contiguous ribs fractured in two places, giving a paradoxically inward-moving segment; the real morbidity is the underlying pulmonary contusion. ATLS teaching (American College of Surgeons Committee on Trauma) emphasizes analgesia, oxygenation, and judicious fluids over strapping.
• First rib or scapular fracture implies high-energy trauma — screen for great-vessel and brachial plexus injury.
• Thoracic outlet syndrome: a cervical rib or fibrous band compresses the lower trunk (C8–T1), causing intrinsic hand wasting and medial forearm numbness; venous compression gives effort thrombosis (Paget–Schroetter).
• Procedural injury: entering below the rib lacerates the intercostal bundle. Intercostal neuralgia and post-thoracotomy pain follow nerve entrapment; herpes zoster mimics it in a strict dermatomal band.

Diaphragmatic defects

• Traumatic rupture is usually left-sided (liver shields the right); a nasogastric tube coiled in the left hemithorax is diagnostic.
• Hyperinflation in COPD flattens the diaphragm, shortening fibers and abolishing the zone of apposition — hence Hoover sign, inward lower rib movement on inspiration.
• Ankylosing spondylitis fuses costovertebral joints, producing restrictive physiology with preserved diaphragmatic function.

• "C3, 4, 5 keeps the diaphragm alive": any stem with shoulder-tip pain plus an abdominal catastrophe (splenic rupture, perforated viscus, subphrenic abscess, post-laparoscopic CO2) is central diaphragmatic irritation referred through the phrenic nerve — Kehr sign.
• The distractor: only the central diaphragm refers to the shoulder. The peripheral rim is supplied by the lower intercostal nerves, so peripheral irritation refers to the lower chest wall and upper abdominal wall, not C4 dermatome.
• Always over the superior border of the rib below — the bundle hugs the inferior groove. If a stem describes brisk bleeding after thoracentesis, the intercostal artery was struck by entering too high in the space.
• Best next step in suspected tension pneumothorax is immediate needle decompression before imaging; ATLS (American College of Surgeons Committee on Trauma) treats it as a clinical diagnosis, followed by tube thoracostomy.
• Openings mnemonic: I 8 (T8) ten (T10) eggs at twelve (T12) — IVC pierces the central tendon, esophagus the muscular right crus, aorta passes behind the diaphragm (hence unaffected by contraction).
• Elevated hemidiaphragm + orthopnea → sniff test; paradoxical ascent on inspiration confirms phrenic palsy, and a lung apex mass on the same side points to Pancoast tumor.
• Inferior rib notching on radiograph = dilated collateral intercostal arteries in coarctation of the aorta, fed via the internal thoracic artery.
• Internal thoracic (mammary) artery is a subclavian branch and the preferred LIMA-to-LAD conduit in CABG, per ACC/AHA–SCAI coronary revascularization guidance.
• Dermatomal landmarks for procedures and referred pain: nipple T4, xiphoid T6, umbilicus T10, inguinal region L1.