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AN25.1-9 | Thorax — Part 2

Thoracic Inlet, Outlet, and Imaging (AN25.5, AN25.6)

Thoracic Inlet (Superior Thoracic Aperture):
- Bounded by: T1 body (posterior), first rib (lateral), manubrium (anterior)
- Structures passing through: trachea, oesophagus, subclavian vessels, brachial plexus, thoracic duct, phrenic nerves, vagus nerves, sympathetic chains, internal thoracic arteries
- Thoracic outlet syndrome: compression of brachial plexus (lower trunk, C8–T1) and/or subclavian vessels at the inlet by a cervical rib, tight scalene muscles, or Pancoast tumour

Thoracic Outlet (Inferior Thoracic Aperture):
- Bounded by: T12 (posterior), 12th rib (lateral), costal margin (11th and 12th ribs + costal cartilages), xiphoid (anterior)
- Closed by the diaphragm

Thoracic Imaging (AN25.6) — CXR anatomy:

Key structures to identify on PA CXR:
1. Trachea: midline, slight rightward deviation at aortic arch — widened carina suggests subcarinal nodes
2. Right hilum: lower than left; formed mainly by the right pulmonary artery (upper) and lower lobe pulmonary vein (lower)
3. Left hilum: higher than right (left PA arches over left bronchus)
4. Cardiac silhouette: right border = SVC (upper) + RA (lower); left border = aortic knuckle + PA (upper) + left atrial appendage + LV (lower)
5. Costophrenic angles: should be acute; blunted = effusion (>200–300 mL)
6. Lung markings: bronchovascular markings taper toward periphery
7. Bones: ribs (count by the posterior angle), vertebrae, clavicles, scapulae

Rib counting on CXR: Start from the manubrium (1st rib/costal cartilage junction is at the manubrium upper border); or identify 12th rib posteriorly and count up.

Anomalies, Surface Markings, and Clinical Applications (AN25.7, AN25.8, AN25.9)

Common Thoracic Cage Anomalies (AN25.7):

Anomaly	Description	Clinical Significance
Cervical rib	Extra rib from C7	Thoracic outlet syndrome (brachial plexus + subclavian compression)
Pectus excavatum	Funnel chest — sternum depressed	Cardiopulmonary compromise; cosmetic
Pectus carinatum	Pigeon chest — sternum protrudes	Associated with Marfan syndrome
Bifid rib	Rib divided anteriorly	Usually incidental on CXR
Rib notching	Scalloping of inferior rib borders	Coarctation of aorta (enlarged collateral intercostal arteries)

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Surface Markings of Thoracic Viscera (AN25.8):

Heart borders (on chest wall):
- Upper right: 3rd right costal cartilage, 1.5 cm from sternum
- Lower right: 6th right costal cartilage, 1.5 cm from sternum (RA)
- Lower left: 5th left intercostal space, midclavicular line (apex — LV)
- Upper left: 2nd left costal cartilage, 1.5 cm from sternum (PA trunk)
- Cardiac apex = site of apex beat (maximum impulse) — normally 5th ICS MCL

Lung and pleural borders: (see Lungs module)

Key surface markings:
- Right cardiophrenic angle: overlies inferior vena cava + right phrenic nerve
- Liver dullness: below 5th right ICS (MCL) — if dullness is higher, hepatomegaly; if tympany, pneumoperitoneum
- Spleen dullness: between 9th–11th ribs, posterior axillary line

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Clinical Applications (AN25.9):

1. Flail chest: 3+ consecutive ribs fractured at 2 sites → paradoxical movement (segment moves in on inspiration) → ineffective ventilation → emergency
2. Rib fractures: most common at the rib angle (posterior); complications = pneumothorax, haemothorax, pulmonary contusion
3. Intercostal nerve block: inject 0.5% bupivacaine at costal groove — useful for multiple rib fractures
4. Needle thoracostomy: 2nd ICS MCL for tension pneumothorax (emergency decompression)
5. Heimlich manoeuvre: uses the air trapped behind a foreign body — sharp upward thrust below the costal margin compresses the diaphragm
6. CPR: compressions applied over the lower sternum → compress the heart between sternum and vertebral column