---

Anatomy of the Airway and Tracheobronchial Tree - With Emphasis on Airway Management and Endotracheal Intubation

---

PART 1: OVERVIEW OF THE AIRWAY

1. Upper airway - nose, mouth, pharynx, larynx
2. Lower airway - trachea, bronchi, bronchioles

---

PART 2: UPPER AIRWAY ANATOMY

A. Nose and Nasal Cavity

• Divided by the nasal septum
• Lined by highly vascular mucosa (Kiesselbach's plexus anteriorly) - vulnerable to epistaxis during nasal intubation
• The turbinates (inferior, middle, superior) project from the lateral wall; the inferior turbinate is the most common obstruction during nasotracheal passage
• The nasopharynx connects posteriorly via the choanae
• Anesthesia relevance: Vasoconstriction (e.g., xylometazoline or cocaine) reduces bleeding risk before nasotracheal intubation. The tube passes below the inferior turbinate along the floor of the nasal cavity.

B. Oral Cavity and Oropharynx

• The oral cavity is the primary route for orotracheal intubation
• The tongue is the most common cause of upper airway obstruction in the unconscious patient
• The tonsillar pillars, uvula, and posterior pharyngeal wall form the oropharynx
• Mallampati classification assesses the visibility of these structures and predicts intubation difficulty
• The vallecula - the space between the base of the tongue and the epiglottis - is the target for the Macintosh laryngoscope blade during intubation

C. Pharynx

• Nasopharynx - behind nose, above soft palate
• Oropharynx - behind oral cavity
• Hypopharynx (laryngopharynx) - behind larynx, extending to C6

---

PART 3: THE LARYNX

A. Overview and Position

• Continuous below with the trachea
• Opens above into the hypopharynx
• Suspended from the hyoid bone above
• A valve (sphincter) protecting the lower airway AND an instrument for phonation

B. Cartilaginous Framework

C. Membranes and Ligaments

• Thyrohyoid membrane - connects hyoid to thyroid cartilage; contains the internal laryngeal nerve and superior laryngeal vessels (a route for superior laryngeal nerve block)
• Cricothyroid membrane (CTM) - connects thyroid to cricoid anteriorly; approximately 9 mm high and 30 mm wide; avascular in its upper two-thirds; site for emergency cricothyrotomy and transtracheal jet ventilation
• Hyoepiglottic ligament - attaches epiglottis to hyoid; renders the vallecula useful as a lever point for Macintosh blade

D. Interior of the Larynx

1. Vestibule (supraglottis) - from laryngeal inlet to false vocal cords
   • Contains the aryepiglottic folds (lateral borders of laryngeal inlet)
   • Contains ventricular folds (false cords) - redundant mucosa superior to true cords
2. Glottis - the space between the true vocal cords
   • The true vocal cords attach anteriorly to the thyroid cartilage (anterior commissure) and posteriorly to the arytenoid cartilages
   • In adults: anterior-posterior diameter ~23 mm (male), ~17 mm (female)
   • The narrowest part of the adult airway is the glottis
3. Subglottis - from below the vocal cords to the inferior border of the cricoid
   • In children, the subglottis (at the level of the cricoid ring) is the narrowest part of the airway - this is why uncuffed ETTs are used in young children; using an oversized tube causes subglottic edema and post-extubation stridor

E. Laryngeal Muscles

---

PART 4: THE TRACHEA AND TRACHEOBRONCHIAL TREE

A. Trachea

• Begins at the inferior border of the cricoid cartilage (C6) and extends to the carina at T4-T5 (sternal angle of Louis)
• Length: 10-15 cm in adults; 4-5 cm in neonates
• Diameter: ~2 cm in adults
• Composed of 16-20 C-shaped cartilaginous rings open posteriorly
• The trachealis muscle forms the posterior (membranous) wall
• The trachea is mobile: it can be palpated and displaced laterally by masses
• In neck flexion the trachea moves cephalad (ETT may enter bronchus); in neck extension it moves caudal

• The optimal ETT tip position is mid-trachea, 2-4 cm above the carina
• An ETT inserted too far will enter the right mainstem bronchus (right-sided endobronchial intubation) due to the less acute angle of the right bronchus
• Tracheal rings are palpable during emergency cricothyrotomy/tracheostomy

B. Carina

• Located at T4-T5, the level of the sternal angle (Louis)
• The trachea bifurcates here into right and left mainstem bronchi
• Rich in cough receptors - most reactive part of the airway
• Anesthesia relevance: ETT cuff should be above the carina; confirm position with bilateral breath sounds and end-tidal CO2

C. Right and Left Mainstem Bronchi

• ETTs inserted too far enter the right bronchus preferentially
• Foreign bodies preferentially enter the right bronchus
• One-lung ventilation: Double-lumen tubes (Robertshaw) are placed with the bronchial limb in the left mainstem bronchus for most thoracic procedures, given the longer length of the left bronchus

D. Bronchial Divisions

• Mainstem bronchi → lobar bronchi (3 right, 2 left lobes)
• Lobar → segmental (tertiary) bronchi (10 right, 8-10 left)
• Segmental → subsegmental → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli

---

PART 5: NERVE SUPPLY OF THE LARYNX

A. Complete Sensory Map of the Airway

B. Branches of the Vagus Relevant to the Larynx

1. Superior Laryngeal Nerve (SLN)

2. Recurrent Laryngeal Nerve (RLN)

• Right RLN: Loops around the right subclavian artery, ascends in the right tracheoesophageal groove
• Left RLN: Loops around the arch of the aorta (ligamentum arteriosum), ascends in the left tracheoesophageal groove - has a longer intrathoracic course, making it more vulnerable to pathology (lung tumors, aortic aneurysm, mediastinal masses)

• Motor: All intrinsic laryngeal muscles except cricothyroid
• Sensory: Mucosa below the vocal cords and upper trachea

---

PART 6: VOCAL CORD PALSIES AND SIGNIFICANCE IN ANESTHESIA

A. Pathophysiology

B. Classification of Vocal Cord Palsies

1. Superior Laryngeal Nerve Palsy

2. Recurrent Laryngeal Nerve Palsy

• Thyroid surgery (most common - especially total thyroidectomy)
• Lung carcinoma (left hilar lesion - left RLN)
• Aortic aneurysm
• Mediastinal masses, lymph nodes
• Post-intubation injury (cuff pressure on RLN at anterior cricoarytenoid joint)
• Neck dissection, carotid endarterectomy
• Idiopathic (20-25%)

• Paralyzed cord lies paramedian (near midline)
• Hoarseness, breathiness, weakened cough, difficulty swallowing liquids
• Diplophonia (two pitch levels) due to unequal cord vibration
• Mild inspiratory stridor possible
• Compensation occurs over weeks as the normal cord crosses the midline to meet the paralyzed one - voice may normalize
• Anesthesia: Generally does not compromise airway; intubation is safe; be aware of aspiration risk (weakened cough reflex, incomplete glottic closure)

C. Anesthesia-Specific Significance of Vocal Cord Palsies

1. Pre-op assessment: Always enquire about voice changes and stridor before thyroid, esophageal, aortic, or mediastinal surgery. A pre-existing unilateral palsy must be documented because inadvertent injury to the contralateral nerve during surgery causes bilateral palsy.
2. Intubation-related RLN injury: Post-intubation vocal cord paralysis can occur due to:
   • Cuff pressure compressing the anterior branch of the RLN at the cricoarytenoid joint
   • Arytenoid dislocation during laryngoscopy
   • Prevention: Use the lowest effective cuff pressure; use appropriate-sized ETT
3. Extubation after thyroid surgery: Always inspect vocal cord mobility (direct laryngoscopy or fiberoptic) before extubation if bilateral RLN injury is suspected. Have a tracheostomy set and reintubation equipment immediately available.
4. Awake intubation planning: Presence of RLN palsy does not contraindicate awake fiberoptic intubation but requires careful topicalization; the superior laryngeal nerve block alone is sufficient above the cords if the RLN territory is already anesthetized by the palsy.
5. Seimon's Law clinical implication: Early partial RLN injury shows adductor function preserved (cord moves to midline on phonation) but abductor function lost. Do not mistake this for a normal cord.
6. Treatment of bilateral palsy:
   • Acute: Reintubation/tracheostomy
   • Chronic: Endoscopic cordotomy or arytenoidectomy (CO2 laser) to widen posterior glottic airway; thyroplasty or injection medialization for unilateral palsy

---

PART 7: CLINICALLY IMPORTANT DIMENSIONS FOR INTUBATION

---

Summary: Key Points for Exam

1. The cricoid cartilage is the only complete cartilaginous ring - landmark for Sellick's maneuver and emergency airway access.
2. The CTM is the site for emergency surgical airway (cricothyrotomy) - avascular, accessible.
3. Macintosh blade tip sits in the vallecula; Miller blade tip goes posterior to the epiglottis.
4. The right bronchus is more vertical - ETTs and foreign bodies preferentially enter it.
5. All intrinsic laryngeal muscles are supplied by the RLN except cricothyroid (external laryngeal nerve).
6. Only abductor of the cords = posterior cricoarytenoid (PCA).
7. Bilateral RLN palsy = adducted cords → stridor → airway emergency.
8. Left RLN has a longer intrathoracic course → more susceptible to mediastinal/thoracic pathology.
9. Children's narrowest airway is at the cricoid (subglottis), not the glottis.
10. Semon's Law - abductor fibers injured first in partial RLN palsy.

---

• Miller's Anesthesia, 10e - Chapter 40 (Airway Management), pp. 5841-5845
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Chapter 19 (Airway Management), pp. 567-568
• Gray's Anatomy for Students - Chapter 8 (Larynx), p. 1192
• Bailey and Love's Short Practice of Surgery, 28e - Chapter 52 (Vocal Fold Palsy), pp. 702-724
• Localization in Clinical Neurology, 8e - Lesions of the Recurrent Laryngeal Nerve, p. 871