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Bronchopulmonary Segments: Anatomy, Anaesthetic Significance, and Postural Drainage

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1. Definition and Structure

• The segmental bronchus and pulmonary artery branch run intrasegmentally (centrally within the segment)
• Pulmonary vein tributaries run intersegmentally — passing between segments through connective tissue septa, draining adjacent segments
• Segments are separated by connective tissue septa (subpleural in origin), making each segment a functionally independent unit
• Each segment is the smallest unit of lung that can be surgically resected without compromising adjacent regions

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2. Number and Nomenclature of Segments

Right Lung — 10 segments:

Left Lung — 8–9 segments (some fused):

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3. Bronchopulmonary Segments — Detailed View

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4. Anaesthetic Significance

A. Lung Isolation and One-Lung Ventilation (OLV)

• Double-lumen endotracheal tube (DLT) placement — the left DLT is positioned so the bronchial lumen sits in the left main bronchus proximal to the left upper lobe bronchus, exploiting the longer left main bronchus (~5 cm) versus the right (~2 cm). The anatomy of the right upper lobe bronchus (arising ~1–2.5 cm from the carina) necessitates a specially designed right-sided DLT with a Murphy eye.
• Bronchial blocker placement — fibreoptic-guided blockers are advanced into specific lobar or segmental bronchi; knowledge of the segmental map is essential for accurate positioning.
• OLV allows operative access for thoracic surgery (lobectomy, pneumonectomy, oesophagectomy, thoracoscopic procedures) by collapsing the operative lung.

B. Aspiration and Dependent Segment Contamination

• The right main bronchus is wider (12–16 mm) and makes a more acute angle with the trachea (~25°), compared to the left (~45°) — making the right lower lobe (especially posterior basal and superior segments) the most common site of aspiration pneumonia in both supine and upright patients.
• During anaesthesia with vomiting or regurgitation, contents tend to enter the posterior basal segments of the lower lobes and the superior segments (S VI) in the supine patient.
• This has implications for patient positioning during induction (head-up/lateral) and for postoperative physiotherapy targeting these segments.

C. Segmental Resection (Segmentectomy)

• Because each bronchopulmonary segment is a functionally independent bronchoarterial unit, surgeons can perform anatomic segmentectomy — a lung-sparing procedure — preserving adjacent segments.
• Pre-operative lung function prediction uses the formula:

  Predicted post-operative FEV₁ = Pre-operative FEV₁ × (segments remaining ÷ total segments)

  • Total = 19 (10 right + 9 left) or 18, depending on nomenclature used
  • This guides anaesthetic and surgical risk stratification

D. Fibreoptic Bronchoscopy and Airway Management

• Segmental bronchial anatomy guides bronchoscopic inspection, selective BAL, endobronchial biopsy, and foreign body removal.
• Rigid bronchoscopy under general anaesthesia allows retrieval from the right lower lobe bronchi and basal segments, which are the commonest sites of foreign body lodgement.

E. Endobronchial Spread of Disease and Isolation

• In patients with lung abscess, bronchiectasis, tuberculosis, or massive haemoptysis, the anaesthetist must protect the healthy lung from spillover using OLV. The diseased segment must be identified preoperatively and the double-lumen tube or blocker positioned accordingly.

F. Selective Bronchography / CT Bronchography

• Segmental anatomy underpins CT-guided localisation for video-assisted thoracoscopic surgery (VATS), where small peripheral nodules are identified by their segment for preoperative wire/dye marking.

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5. Postural Drainage

Definition

Positions for Each Segment

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6. Significance in Pre- and Postoperative Care

Preoperative Significance

Postoperative Significance

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7. Summary Table: Segments → Drainage Positions → Clinical Relevance

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References

• Gray's Anatomy for Students, pp. 208–211 (Bronchopulmonary segments)
• Color Atlas of Human Anatomy, Vol. 2: Internal Organs, pp. 214–216 (Divisions of bronchi, segments)
• Murray & Nadel's Textbook of Respiratory Medicine, 2-Volume (Spinal cord disease; postural drainage in ventilated patients)
• Fishman's Pulmonary Diseases and Disorders, 2-Volume (Postoperative pulmonary morbidity)
• StatPearls — Postural Drainage and Vibration (NCBI Bookshelf)
• AARC Clinical Practice Guideline — Postural Drainage Therapy
• Kenhub — Bronchopulmonary Segments

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Anatomy of the Diaphragm, Anaesthetic Significance, and Functions in Various Phases of Anaesthesia

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1. Anatomy of the Diaphragm

Overview

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Structure

1. A central tendinous portion — the central tendon, a trefoil-shaped fibrous aponeurosis into which all peripheral muscle fibers converge. The pericardium is directly attached to its middle part.
2. A peripheral muscular portion — arranged circumferentially, arising from the margins of the inferior thoracic aperture.

• Sternal part: two small slips from the posterior surface of the xiphoid process
• Costal part: inner surfaces of the lower six costal cartilages and ribs (ribs 7–12) — the largest part
• Lumbar (vertebral) part: the crura and arcuate ligaments

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Attachments

• Right crus — bodies of L1, L2, L3 and intervening discs (longest and broadest)
• Left crus — bodies of L1, L2 and intervening disc
• Crura blend with the anterior longitudinal ligament of the vertebral column

• Median arcuate ligament — connects the two crura over the front of the aorta at T12
• Medial arcuate ligament (psoas arch) — from lateral body of L1/L2 to transverse process of L1, arching over psoas major
• Lateral arcuate ligament (quadratus arch) — from transverse process of L1 to rib XII, arching over quadratus lumborum

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Openings in the Diaphragm

• Greater, lesser, least splanchnic nerves — through the crura
• Hemiazygos vein — through left crus
• Left phrenic nerve — through muscular part of left dome
• Sympathetic trunks — behind medial arcuate ligaments
• Superior epigastric vessels — between sternal and costal slips (foramen of Morgagni)

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Blood Supply

• Pericardiacophrenic arteries (branches of internal thoracic artery) — with phrenic nerves
• Musculophrenic arteries (terminal branches of internal thoracic artery)
• Superior phrenic arteries (from lower thoracic aorta)
• Small branches from lower intercostal arteries

• Inferior phrenic arteries — the largest supply; direct branches of the abdominal aorta (or occasionally from the coeliac axis)

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Innervation

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Domes (Cupulae)

• Right dome: reaches rib V in the midclavicular line (elevated by the liver)
• Left dome: fifth intercostal space (pushed by stomach fundus and spleen)

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2. Anaesthetic Significance

A. Phrenic Nerve — at Risk During Regional Procedures

B. Hiatus and Reflux — Aspiration Risk

• The oesophageal hiatus at T10, formed by the right crus, is the site of hiatus hernia (the lower oesophageal sphincter is partly maintained by diaphragmatic crural contraction)
• During general anaesthesia, the diaphragm relaxes — crural tone is lost — lower oesophageal sphincter competence is reduced, increasing risk of regurgitation and aspiration
• This is a key reason for the rapid sequence induction (RSI) technique with Sellick's manoeuvre (cricoid pressure) in patients with hiatus hernia, reflux, or full stomach

C. Diaphragmatic Splinting — Postoperative Pain

• Upper abdominal and thoracic surgery lead to reflex inhibition and pain-mediated splinting of the diaphragm
• This causes: reduced tidal volume, inability to cough, retained secretions, atelectasis, and hypoxia
• Managed by: adequate analgesia (regional/neuraxial), incentive spirometry, early mobilisation, physiotherapy

D. Effect on FRC (Functional Residual Capacity)

• The supine position alone shifts the diaphragm ~4 cm cephalad (due to abdominal contents)
• Induction of general anaesthesia causes a further cephalad shift of the dependent (posterior) diaphragm → FRC falls by ~0.5 L (about 20%) immediately on induction
• This fall in FRC reduces the oxygen reserve at intubation, contributing to faster desaturation

E. Neuromuscular Blocking Agents

• The diaphragm is relatively resistant to non-depolarising neuromuscular blockers (NMBAs) compared to the adductor pollicis (peripheral neuromuscular junction monitoring site)
• It recovers faster from blockade than laryngeal muscles; however, complete diaphragmatic paralysis requires higher doses
• This means: a patient showing thumb adductor twitches may still have partial diaphragmatic impairment, relevant during reversal of neuromuscular block
• Residual neuromuscular block (Train-of-Four ratio <0.9) impairs diaphragmatic function, increases airway obstruction risk, and compromises hypoxic ventilatory response

F. Thoracic Epidural and Neuraxial Anaesthesia

• A high thoracic epidural (T1–T4) blocks intercostal muscles but spares the diaphragm (phrenic nerve, C3–C5, is above the block level) — ventilation is maintained, though expiratory reserve volume falls
• A high spinal or total spinal that reaches cervical levels abolishes diaphragmatic function — requires immediate ventilatory support

G. Ultrasound Assessment

• Point-of-care ultrasound (POCUS) of the diaphragm at the 10th rib interspace in the midaxillary line assesses:
  • Diaphragm thickness (normal 1.6–2.9 mm at FRC; ~4.5 mm at TLC)
  • Thickening fraction on inspiration (correlates with effort and respiratory muscle strength)
  • Excursion (>10 mm = adequate function; <10 mm at 24 h post-surgery = higher atelectasis risk)
  • Paradoxical cephalad movement = phrenic palsy
• Used for weaning prediction from mechanical ventilation in the ICU

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3. Functions of the Diaphragm in Various Phases of Anaesthesia

Awake Baseline (Reference)

• In the upright position: diaphragm provides ~70% of tidal volume; accessory muscles (intercostals, scalenes) contribute the rest
• In the supine position (pre-anaesthesia): the diaphragm shifts ~4 cm cephalad; posterior (dependent) diaphragm becomes more convex and has longer fibres → by Starling's law, contracts more forcefully → greater ventilation of dependent (posterior/basal) zones where perfusion is also greatest → optimal V/Q matching

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Phase 1: Induction of Anaesthesia

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Phase 2: Maintenance of Anaesthesia

A. Spontaneous Ventilation (Light Anaesthesia, LMA)

• Volatile anaesthetics at 1 MAC reduce diaphragmatic activity in a dose-dependent manner
• Tidal volume is maintained (diaphragm still contracts) but respiratory rate and minute volume fall
• Expiratory activity of abdominal muscles increases under volatile anaesthesia (parasternal activity abolished) — contributes to dependent atelectasis
• CO₂ response curve shifts right (higher PaCO₂ threshold for breathing); hypoxic drive is obtunded

B. Controlled Mechanical Ventilation (CMV) with Muscle Relaxants

• The diaphragm is completely paralysed
• Positive pressure ventilation (PPV) inflates lungs from above — the non-dependent (anterior) lung is preferentially ventilated (opposite of spontaneous breathing)
• The dependent (posterior) lung remains poorly ventilated (compressed by cephalad diaphragm and abdominal contents) but continues to receive the most perfusion → V/Q mismatch, shunt, hypoxaemia
• Atelectasis in dependent zones can form/persist throughout maintenance
• PEEP (5–10 cmH₂O) recruits dependent alveoli and restores FRC toward normal
• In the prone position: the diaphragm is displaced caudally by gravity, dependent (now anterior/ventral) lung is decompressed → dramatically improved V/Q matching (basis for prone ventilation in ARDS)

C. Trendelenburg / Steep Head-Down Position

• Abdominal contents push diaphragm further cephalad — FRC falls a further 12%
• Particularly problematic in robotic pelvic surgery (steep Trendelenburg + pneumoperitoneum)
• Increased peak airway pressures; higher risk of barotrauma and atelectasis

D. Pneumoperitoneum (Laparoscopic Surgery)

• Insufflation of CO₂ into the peritoneal cavity markedly elevates intra-abdominal pressure
• Pushes diaphragm cephalad → FRC falls, atelectasis increases, peak airway pressures rise
• CO₂ absorbed from peritoneum adds to arterial CO₂ load — requires increase in minute ventilation

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Phase 3: Emergence and Recovery

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Summary: Diaphragm Position Across Anaesthetic Phases

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References

• Gray's Anatomy for Students, pp. 173–174, 198–199, 320, 432–433 (Anatomy, openings, innervation, blood supply)
• Morgan & Mikhail's Clinical Anesthesiology, 7e, pp. 788–789, 917–919 (Innervation, spontaneous ventilation, anaesthetic effects)
• Murray & Nadel's Textbook of Respiratory Medicine (Diaphragm ultrasound, p. 588)
• Perioperative Pulmonary Atelectasis — Biology and Mechanisms. PMC9869183
• Respiratory Function Under Anaesthesia — Indian Journal of Respiratory Care