2.5 Thoracic Anatomy: Chest Wall, Intercostal Spaces, Pleura, Mediastinum and Heart

2.5 Thoracic Anatomy: Chest Wall, Intercostal Spaces, Pleura, Mediastinum and Heart

Why Thoracic Anatomy Matters

Anaesthetists and intensivists work on the thorax every day: inserting chest drains, decompressing tension pneumothorax, placing central lines, performing intercostal, paravertebral and serratus blocks, and reading chest radiographs. EDAIC Part I questions test the relationships that make these procedures safe and the landmarks used to find structures without imaging.

The Chest Wall and Intercostal Spaces

The thoracic cage consists of 12 thoracic vertebrae, 12 pairs of ribs and the sternum (manubrium, body and xiphoid process). Ribs 1-7 are true ribs joined directly to the sternum by costal cartilages; ribs 8-10 are false ribs joined to the cartilage above; ribs 11-12 are floating ribs.

Each intercostal space contains three muscle layers:

The Intercostal Neurovascular Bundle

The intercostal vein, artery and nerve (VAN, from above downward) run in the costal groove on the inferior surface of each rib, sheltered by its lower edge. Collateral branches run along the upper border of the rib below.

• Practical rule: Insert needles and drains just above the upper border of the lower rib to avoid the main bundle.
• Posteriorly, near the angle of the rib, the bundle lies more exposed in the middle of the space, which is relevant to the paravertebral and posterior intercostal approaches.
• Intercostal nerves (T1-T11) are the anterior rami of the thoracic spinal nerves; T12 is the subcostal nerve. Each gives a lateral cutaneous branch near the mid-axillary line and an anterior cutaneous branch near the sternum. T2 contributes the intercostobrachial nerve to the medial upper arm.

Pleura and Lungs

The visceral pleura covers the lung; the parietal pleura lines the thoracic wall, diaphragm and mediastinum. The potential space between them normally contains only a thin film of fluid.

The pleura and lung apex rise about 2-3 cm above the medial third of the clavicle into the root of the neck. This explains pneumothorax after supraclavicular brachial plexus block, subclavian and low internal jugular cannulation, and stellate ganglion block. Posteriorly the pleura extends below the 12th rib medially, so it can be breached during posterior renal or paravertebral procedures.

The right lung has three lobes separated by the oblique and horizontal fissures; the left lung has two lobes separated by the oblique fissure, with the lingula as part of the upper lobe. The right main bronchus is shorter, wider and more vertical, so aspirated material and deeply inserted tubes tend to enter it.

Chest Drains and Needle Decompression

SAFE TRIANGLE (lateral chest wall, arm abducted)
  Anterior border:  lateral edge of pectoralis major
  Posterior border: anterior edge of latissimus dorsi
  Inferior border:  horizontal line at the nipple (about 5th intercostal space)
  Apex:             just below the axilla
Insert over the upper border of the rib, blunt dissection, finger sweep before tube.

• Needle decompression in adults: The ATLS 10th edition recommends the 4th or 5th intercostal space just anterior to the mid-axillary line, because the chest wall is thinner there than at the second intercostal space in the mid-clavicular line. A finger thoracostomy is preferred in ventilated trauma patients.
• Hazards: Inserting too low risks liver or spleen injury; inserting medial to the mid-clavicular line in the second space risks the internal thoracic artery, which runs about 1 cm lateral to the sternal edge.

The Mediastinum

The sternal angle (angle of Louis) is the key landmark. A horizontal plane through it reaches the T4-T5 intervertebral disc and marks:

• the articulation of the second costal cartilage (so you count ribs from here);
• the bifurcation of the trachea (carina);
• the beginning and end of the aortic arch;
• the entry of the azygos vein into the superior vena cava;
• the boundary between the superior and inferior mediastinum.

The inferior mediastinum is divided into the anterior (thymus remnants, lymph nodes), middle (heart, pericardium, roots of the great vessels, phrenic nerves) and posterior (descending aorta, oesophagus, thoracic duct, azygos system, vagus nerves, sympathetic trunks) compartments. The thoracic duct ascends to the right of the oesophagus, crosses to the left at about T5 and drains into the junction of the left internal jugular and subclavian veins.

The phrenic nerves (C3-C5) run anterior to the lung roots over the pericardium; the vagus nerves pass posterior to the lung roots. The left recurrent laryngeal nerve hooks under the aortic arch and can be stretched by mediastinal masses or aortic aneurysm, causing hoarseness.

The Heart and Great Vessels

Surface Markings

Coronary Circulation

Dominance is defined by the artery that gives off the posterior descending artery: right-dominant in about 70-80%, left-dominant in about 10%, and co-dominant in the remainder. Venous blood drains mainly through the coronary sinus into the right atrium; small Thebesian veins drain directly into the chambers, contributing to the normal anatomical shunt.

Great Vessels and Central Venous Access

The left brachiocephalic vein crosses anterior to the arch branches to join the right brachiocephalic vein, forming the superior vena cava (SVC) behind the right first costal cartilage. The SVC enters the right atrium at about the level of the right third costal cartilage. For central venous catheters, the tip should lie in the lower SVC near the cavo-atrial junction, which on a chest radiograph is usually just below the level of the carina.

Diaphragm

The diaphragm is supplied by the phrenic nerves (C3-C5), which provide its only motor supply; the lower intercostal nerves supply sensation to the periphery. Its three main openings are: