Tracheobronchial tree and diaphragm. Anatomy and anaesthetic, clinical importance. Viva points
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Tracheobronchial Tree and Diaphragm: Anatomy and Anaesthetic/Clinical Importance
PART 1: TRACHEOBRONCHIAL TREE
Basic Anatomy
Trachea
- Fibromuscular tube, 10-12 cm long in adults, outer diameter ~20 mm
- Begins at vertebral level C6 (continuous with the larynx/cricoid cartilage above)
- Supported by U-shaped hyaline cartilage rings - the opening of the U faces posteriorly (where the posterior wall is membranous/muscular - the trachealis muscle)
- Lies directly anterior to the oesophagus
- Bifurcates at the sternal angle (angle of Louis) / T4-T5 disc level - this point is the carina
- Half the trachea is intrathoracic, half extrathoracic
Carina
- The ridge of cartilage at the bifurcation (T4-T5 level / sternal angle)
- Highly sensitive - stimulation causes severe cough reflex and bronchospasm
- Can move superiorly up to 5 cm from its resting position
Right Main Bronchus
- Wider, shorter (~2.5 cm before first branch), more vertical (25° from vertical axis)
- Departs at ~25 degrees from tracheal axis
- Right upper lobe bronchus branches almost directly posterior (~90° from right main bronchus)
- In 10% of adults, the right upper lobe bronchus departs less than 2.5 cm from the carina
- In 2-3% of adults, the right upper lobe bronchus opens into the trachea superior to the carina (anomaly important for double-lumen tubes)
Left Main Bronchus
- Narrower, longer (~5 cm before first branch)
- More horizontal - departs at ~45 degrees from tracheal axis
Bronchioles
- ~1 mm diameter, no cartilaginous support - highest proportion of smooth muscle
- Terminal bronchiole = last generation incapable of gas exchange
Anaesthetic/Clinical Importance: Tracheobronchial Tree
| Point | Detail |
|---|---|
| ETT depth and neck movement | With neck flexion, the ETT tip moves 3.8 cm toward the carina; with extension, it moves away - "tip away, base toward" |
| Endobronchial intubation | More likely to enter the right bronchus because it is wider and more vertical |
| Foreign body aspiration | Goes to the right (erect) or either side (supine); right lower lobe most common site |
| Double-lumen tube | Right-sided DLT technically harder due to short right main bronchus and variable right upper lobe take-off |
| Paediatric angles | In children <3 years, right and left bronchial angles are approximately equal (~55°) - aspiration can go either way |
| Carina sensitivity | Lightest anaesthetic plane - touching the carina causes bucking, coughing, laryngospasm, bronchospasm |
| Airway resistance | As cross-sectional area of tracheobronchial tree increases peripherally, resistance drops from central to peripheral airways |
| Turbulent vs laminar flow | Turbulent flow at branch points (carina, lobar bronchi) - turbulence increased by high-flow rates; Reynolds number >4000 = turbulent |
| Helium-oxygen | Lower gas density → promotes laminar flow → reduces resistance in upper airway oedema/narrowing |
| Suction catheter tip | When inserted through ETT without bias, preferentially enters right bronchus - deliberately turning head left helps access left bronchus |
Viva point: "Which side does unintentional endobronchial intubation occur, and why?"
Right side - the right main bronchus is wider and departs at only 25° from vertical (vs 45° for the left). - Barash Clinical Anesthesia, 9e, p.1116
PART 2: DIAPHRAGM
Basic Anatomy
The diaphragm is a musculotendinous sheet that seals the inferior thoracic aperture. It is the principal muscle of inspiration.
Shape and Position
- Dome-shaped - balloons superiorly on both sides
- Right dome is higher than left, reaching as far as rib V (due to liver below)
- Posterior attachment is more inferior than anterior (oblique inferior thoracic aperture)
- On contraction, dome height decreases → thoracic volume increases
Origins (peripheral muscular attachments)
- Sternal part: posterior surface of xiphoid process
- Costal part: inner surfaces of ribs 7-12 and costal cartilages
- Lumbar part: crura (medial - from L1-L3 vertebral bodies) + arcuate ligaments (medial, lateral)
Central tendon: fibres converge centrally; the IVC passes through it
The Three Major Openings (High-Yield Viva Table)
| Opening | Level | Structures passing through |
|---|---|---|
| Caval hiatus | T8 | IVC, right phrenic nerve, lymphatic vessels |
| Oesophageal hiatus | T10 | Oesophagus, anterior & posterior vagal trunks, left gastric artery & vein |
| Aortic hiatus | T12 | Aorta, thoracic duct (+ occasionally azygos vein) |
Memory aid: 8, 10, 12 → I Eat Aorta / Vena cava (8) - Oesophagus (10) - Aorta (12)
Additional openings:
- Sternocostal hiatus (foramen of Morgagni): between sternal and costal fibres - lymphatics from liver pass through (site of Morgagni hernia)
- Lumbocostal triangle (Bochdalek): potential weak area posterolaterally - site of Bochdalek hernia (most common congenital diaphragmatic hernia, usually left-sided)
Why does aorta not transmit pulse pressure to diaphragm? The aorta passes posterior to the median arcuate ligament, not through the diaphragm muscle - so diaphragmatic contraction does not compress it.
Inferior view of the diaphragm - Fischer's Mastery of Surgery, 8e
Innervation
Phrenic nerve origin from C3, C4, C5 - Gray's Anatomy for Students
| Component | Detail |
|---|---|
| Motor | Entire diaphragm via phrenic nerve (C3, C4, C5 - major from C4) |
| Sensory (central) | Phrenic nerve (C3-C5) - referred pain to shoulder tip |
| Sensory (peripheral) | Lower intercostal nerves (T9-T11) |
| Course | Down anterior surface of scalenus anterior → thoracic inlet → mediastinum (anterior to lung roots) → diaphragm |
| Left phrenic | Enters muscular part of left hemidiaphragm, lateral to left cardiac surface |
| Right phrenic | Courses through the caval opening or central tendon |
Mnemonic for level: "C3, 4, 5 keeps the diaphragm alive"
Blood Supply
| Surface | Arteries | Veins |
|---|---|---|
| Superior | Superior phrenic arteries; pericardiophrenic and musculophrenic branches of internal thoracic artery | Pericardiophrenic and musculophrenic veins → IVC |
| Inferior | Inferior phrenic arteries (from abdominal aorta / celiac trunk) | Inferior phrenic veins → IVC (right) / left suprarenal vein (left) |
Lymphatics:
- Superior surface → anterior, middle, posterior phrenic nodes
- Inferior surface → phrenic nodes + aortic nodes (follow inferior phrenic vessels)
Anaesthetic/Clinical Importance: Diaphragm
| Scenario | Relevance |
|---|---|
| Spinal cord injury | Injuries at C2 and above require immediate ventilatory support. Between C3-C5, variable; below C5 diaphragm function preserved |
| Referred shoulder tip pain | Diaphragmatic irritation (blood, pus, inflammation) → pain referred to shoulder tip via C3-C4 (phrenic nerve) - e.g., ruptured ectopic, subphrenic abscess, ruptured spleen |
| Phrenic nerve palsy | Unilateral: raised hemidiaphragm, 20-25% reduced VC. Bilateral: respiratory failure requiring ventilation |
| Hiatal hernia | Sliding (commonest, 95%) vs paraesophageal - sliding type causes GORD; paraesophageal risks strangulation |
| Congenital diaphragmatic hernia (CDH) | Usually left Bochdalek hernia (gut herniates into chest) → pulmonary hypoplasia, neonatal respiratory distress - anaesthetic challenge: avoid high airway pressures, treat pulmonary hypertension |
| Diaphragm position in anaesthesia | Supine position and GA → diaphragm shifts cephalad (~4 cm), reducing FRC - especially important in obese patients, pregnant patients |
| Pneumoperitoneum (laparoscopy) | CO₂ insufflation pushes diaphragm cephalad → increased airway pressure, reduced compliance, potential basal atelectasis |
| Incisions near diaphragm | Below the 8th rib (midclavicular line) / 10th rib (midaxillary line) may enter abdominal cavity without entering pleural cavity |
| Phrenic nerve block | Occurs as a complication in ~100% of interscalene brachial plexus blocks - bilateral blocks contraindicated in severe respiratory compromise |
| Morgagni hernia | Anterior/retrosternal; Bochdalek hernia = posterior/lateral. Bochdalek more common (left > right) |
| Tracheal intubation effect | ETT and IPPV bypasses normal diaphragmatic breathing mechanics; in COPD hyperinflation flattens diaphragm → poor mechanical advantage |
QUICK VIVA SUMMARY POINTS
- Trachea begins at C6, bifurcates at T4-T5 (sternal angle / carina)
- Trachea is 10-12 cm, half intrathoracic, half extrathoracic
- Right bronchus: wider, shorter, 25° - preferred side for foreign bodies, endobronchial intubation
- ETT moves 3.8 cm toward carina on neck flexion
- Children <3 years: equal bronchial angles (~55°) - aspiration equal both sides
- Bronchioles have no cartilage - highest smooth muscle content - prone to spasm
- Carina at T4-T5; exquisitely sensitive - light anaesthesia + carina contact = bronchospasm
- Diaphragm = principal muscle of inspiration; dome-shaped; right dome higher (rib V)
- Three openings: T8 (IVC), T10 (oesophagus + vagus), T12 (aorta + thoracic duct)
- Aorta passes behind median arcuate ligament - not through muscle - no compression
- Phrenic nerve: C3, C4, C5 - motor to whole diaphragm; sensory to central part
- Peripheral diaphragm sensation: T9-T11 intercostals
- Referred shoulder tip pain from diaphragmatic irritation = via C3-C4 phrenic nerve
- Spinal injury at C2 or above = immediate ventilatory support needed
- GA + supine → diaphragm shifts cephalad ~4 cm → FRC falls
- Interscalene block → 100% ipsilateral phrenic nerve palsy - avoid bilateral blocks
Sources: Barash Clinical Anesthesia 9e, pp. 1115-1117 | Gray's Anatomy for Students | Fischer's Mastery of Surgery 8e, pp. 2589-2591 | Gray's Anatomy for Students (Innervation of Diaphragm, p. 174)
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