Surface and Applied Thoracic Anatomy

Bony and cartilaginous landmarks:

• Sternal angle (Angle of Louis): palpable ridge at the manubriosternal joint; the single most important thoracic landmark (see Revision Points below for its many correlates).
• Jugular (suprasternal) notch: superior border of the manubrium, palpable between the two sternoclavicular joints; corresponds roughly to the T2/T3 vertebral level.
• Clavicles: palpable along their entire length, form the superior border of the anterior chest wall.
• Ribs and costal margins: palpable and countable, used for level identification (the 2nd costal cartilage articulates at the sternal angle, providing the starting point for rib counting).
• Xiphisternal joint: junction of the sternal body and xiphoid process, corresponds to the T9 vertebral level; marks the inferior limit of the central chest wall and the approximate location of the cardiac apex's deep relations.
• Intercostal spaces: numbered by the rib above; used to describe the location of cardiac auscultation points, needle insertion sites, and pathology.
• Apex beat: normally palpable at the 5th intercostal space, midclavicular line; displacement suggests cardiomegaly or mediastinal shift.

Important surface planes:

• Sternal angle plane (T4/T5 transverse plane): The single most valuable thoracic reference plane, corresponding to: the manubriosternal joint anteriorly; the T4/T5 intervertebral disc posteriorly; the tracheal bifurcation/carina; the beginning and end of the aortic arch; the 2nd costal cartilage (rib-counting start point); the azygos vein's termination into the SVC; the superior/inferior mediastinal boundary; and the point where the thoracic duct crosses from right to left (approximately, at T5).
• T4/T5 level (posterior correlate): same plane, described from the vertebral column, useful when working from a posterior/radiographic reference.

Surface projection of the lungs:

• Apex: 2–3 cm above the medial third of the clavicle.
• Anterior border: Descends from the apex to the sternal angle, then close to the midline down to the 4th costal cartilage (right lung continues to the 6th; left lung deviates laterally at the cardiac notch, from the 4th to 6th costal cartilage).
• Inferior border: Crosses the 6th rib in the midclavicular line, the 8th rib in the midaxillary line, and reaches the 10th rib (or T10 spinous process) posteriorly, importantly, the lung's inferior border sits two ribs higher than the corresponding pleural reflection at each of these points, the intervening costodiaphragmatic recess.

Surface projection of the pleura:

• Extends approximately 2–3 cm above the clavicle (same as the lung apex, cervical pleural dome).
• Inferior reflection: 8th rib in the midclavicular line, 10th rib in the midaxillary line, 12th rib (or T12 spinous process) posteriorly: consistently two ribs lower than the corresponding lung border at each of the same reference lines, defining the extent of the costodiaphragmatic recess.

Surface projection of the heart:

• Roughly trapezoidal outline: upper right border: 3rd right costal cartilage to the 2nd right costal cartilage (approximating the SVC/right atrium); lower right border: 2nd right to 6th right costal cartilage (right atrium); inferior border: 6th right costal cartilage to the cardiac apex (right ventricle, with left ventricle apex); left border: apex up to the 2nd left costal cartilage (left ventricle, then pulmonary trunk/left auricle); upper left border: 2nd left costal cartilage to the 3rd right costal cartilage (great vessels).
• Apex: 5th left intercostal space, midclavicular line.

Surface projection of the cardiac valves:

• All four valves project close together near the sternum, but their auscultation points are deliberately displaced along the direction of blood flow to where each valve's sound transmits most clearly, and do NOT correspond directly to their anatomical surface projection:
• Aortic valve: anatomically behind the sternum at the 3rd left costal cartilage; auscultated at the 2nd right intercostal space, parasternal.
• Pulmonary valve: anatomically behind the 3rd left costal cartilage; auscultated at the 2nd left intercostal space, parasternal.
• Mitral valve: anatomically behind the 4th left costal cartilage; auscultated at the apex (5th left intercostal space, midclavicular line).
• Tricuspid valve: anatomically behind the midline, roughly the 4th-5th intercostal space; auscultated at the left lower sternal border, 4th-5th intercostal space.

Surface projection of the great vessels:

• Ascending aorta: Behind the sternum, from the 3rd left intercostal space upward to the level of the sternal angle (2nd right costal cartilage).
• Aortic arch: Behind the manubrium, from the sternal angle level, arching to the left, reaching as high as the mid-manubrium before descending.
• SVC: Right side of the sternum, from the 1st to 3rd right costal cartilages.
• Pulmonary trunk: Behind the 3rd left costal cartilage/left sternal border, bifurcating just below the sternal angle level.

Rib Fracture

• Most commonly involves the middle ribs (4–9), the region of greatest mechanical exposure.
• Lower rib fractures (9–12) raise concern for splenic (left) or hepatic (right) injury; upper rib fractures (1–3) suggest substantial force and possible great vessel/brachial plexus injury.

Flail Chest

• Requires three or more consecutive ribs, each fractured in two or more places, creating a free-floating segment that moves paradoxically (inward on inspiration).
• A significant thoracic trauma emergency, often with associated pulmonary contusion impairing ventilation.

Pneumothorax

• Air within the pleural cavity, disrupting negative intrapleural pressure and causing lung collapse.
• Can be spontaneous (ruptured subpleural blebs), traumatic, or iatrogenic (central line, thoracentesis); the cervical pleural dome's proximity to the neck root makes it vulnerable during central venous access attempts there.

Tension Pneumothorax

• A one-way valve mechanism allows air into the pleural space on inspiration but prevents its escape on expiration, progressively increasing intrapleural pressure.
• Causes mediastinal shift away from the affected side, compressing the contralateral lung and, critically, kinking the great veins (SVC/IVC) at the mediastinum, impairing venous return and causing obstructive shock, a clinical emergency requiring immediate needle decompression, classically at the 2nd intercostal space, midclavicular line (just above the 3rd rib, to avoid the neurovascular bundle).

Pleural Effusion

• Fluid accumulation collecting first in the dependent costodiaphragmatic recess.
• Transudative (heart failure, hypoalbuminaemia) or exudative (infection, malignancy) causes, reflecting different underlying fluid-formation mechanisms.

Chest Drain Anatomy

• Standard insertion site is the "triangle of safety" (lateral pectoralis major border, lateral latissimus dorsi border, 5th intercostal space/nipple line).
• Inserted just above the rib below the chosen space, to avoid the neurovascular bundle running in the costal groove of the rib above.

Thoracentesis Anatomy

• Performed below the identified fluid level (percussion/ultrasound-guided), typically in the mid-scapular or posterior axillary line.
• Needle placed just above the superior border of the rib below, targeting the dependent costodiaphragmatic recess while avoiding the neurovascular bundle and underlying lung.

Pericardial Effusion

• Fluid accumulation within the pericardial cavity, between the visceral and parietal serous layers.
• Because the fibrous pericardium is non-distensible, a rapidly accumulating effusion (even a relatively small volume) raises intrapericardial pressure quickly, unlike a slowly accumulating effusion, which allows the pericardium to stretch and accommodate a much larger volume before becoming symptomatic.

Cardiac Tamponade

• Critical elevation of intrapericardial pressure (from effusion, blood, or pus) compresses the cardiac chambers, impairing diastolic filling and reducing cardiac output.
• The thin-walled right atrium and right ventricle are the most susceptible chambers to external compression, given their comparatively low intracavitary pressure relative to the left heart.

Pericardiocentesis Anatomy

• Classic approach is the subxiphoid (subcostal) route, needle inserted just left and inferior to the xiphoid process, angled superiorly and posteriorly toward the left shoulder, aiming to enter the pericardial space while avoiding the pleura, lung, and internal thoracic vessels.
• Alternative approaches exist (parasternal, apical), each requiring awareness of the adjacent internal thoracic vessels (parasternal) or the pleural reflections/lung margins (apical), to avoid iatrogenic injury.

Coronary Artery Occlusion

• Produces ischaemia/infarction of the myocardial territory supplied by the occluded vessel, given coronary arteries' functional end-artery behaviour in the acute setting.
• Anatomical territory correlates with ECG lead changes (anterior/septal = LAD, lateral = circumflex, inferior = RCA).

Aortic Dissection Relationships

• A tear in the aortic intima allows blood to dissect within the aortic wall (between intima and media), potentially propagating along the aorta's length and compromising branch vessel perfusion wherever the dissection extends.
• Type A (involving the ascending aorta) can extend retrograde to involve the coronary ostia (causing myocardial infarction), the aortic valve (causing acute regurgitation), or rupture into the pericardial sac (causing tamponade), reflecting the ascending aorta's close relations to these structures.
• Type B (confined to the descending aorta, distal to the left subclavian artery origin) can compromise intercostal, spinal, and abdominal visceral branch perfusion depending on the extent of dissection.

SVC Obstruction

• Most often caused by malignant compression (bronchogenic carcinoma, lymphoma), given the SVC's thin walls, low pressure, and central mediastinal position surrounded by lymph nodes.
• The azygos vein (entering the SVC at the sternal angle/T4 level) provides a key collateral pathway if the obstruction lies above its entry point.

Oesophageal Compression

• Can result from left atrial enlargement (directly anterior relation, e.g., severe mitral stenosis), thoracic aortic aneurysm (posterior/left relation), or mediastinal masses/lymphadenopathy at any level along the oesophagus's course.
• Produces dysphagia, with the anatomical level of compression corresponding to the causative structure's position relative to the three oesophageal constrictions.

Recurrent Laryngeal Nerve Injury

• The left nerve's long intrathoracic course (hooking under the aortic arch) makes it more vulnerable to thoracic pathology (aortic aneurysm, mediastinal masses, cardiac/thoracic surgery) than the right nerve, which hooks at the root of the neck around the subclavian artery.
• Unilateral injury causes hoarseness (ipsilateral vocal fold paralysis); bilateral injury is a much more serious airway emergency (bilateral vocal fold paralysis, often in the paramedian position, causing stridor and potential airway obstruction).

Phrenic Nerve Injury

• Vulnerable given its course anterior to the lung root and closely applied to the pericardium, at risk during cardiothoracic surgery, mediastinal tumour invasion, or neck trauma affecting its cervical (C3-C5) origin.
• Causes unilateral diaphragmatic paralysis, since each hemidiaphragm depends solely on its own ipsilateral nerve.

Diaphragmatic Paralysis

• Unilateral paralysis is often reasonably well tolerated at rest but causes dyspnoea on exertion or when supine; shows characteristic paradoxical upward movement on inspiration (positive sniff test) due to loss of active contraction.
• Bilateral paralysis is far more serious, causing significant respiratory compromise, given the diaphragm's contribution to the majority of quiet tidal volume.