Surgical Anatomy Of Tracheobronchial Tree

The important aspects of the surgical anatomy of the tracheobronchial tree are given below.

Embryology

The respiratory system is a derivative of the second part of the foregut.

Timing: It begins to develop at approximately 4 weeks, or day 22, of embryonic development.

Appearance: It begins as a laryngo-tracheal groove on the ventral aspect of the foregut, which deepens and forms a respiratory diverticulum

Groove Formation: The respiratory diverticulum expands, leading to the formation of the laryngeal tracheal groove. This groove deepens and separates from the foregut.

Two longitudinal grooves develop, separating the lung bud from the foregut.

Foregut Division: The foregut is divided into dorsal and ventral portions.

• Dorsal Portion: Forms the esophagus.
• Ventral Portion: Forms the trachea and lung buds.

Lung Bud Development: During the separation, the lung bud gives rise to the trachea. Two lateral pockets on the outside of the lung bud form the bronchial buds.

(The above diagram shows the stages in the development of the respiratory diverticulum, indicating the tracheoesophageal ridges and the septum formation. This septum splits the foregut into the esophagus and trachea with lung buds.)

Also read: Bronchi and Their Divisions

Bronchial Tree Development (Fifth Week Onward)

From the fifth week, buds grow into the right and left main bronchi. Asymmetric branching occurs during the following two weeks to form secondary bronchi: three on the right and two

on the left, forming the main divisions of the bronchial tree. The right bronchus forms 3 secondary bronchi (upper, middle, and lower lobes). The left bronchus forms 2 secondary bronchi (upper and lower lobes).

On the right bronchus, there are ten subdivisions.

The upper lobe is divided into apical, anterior, and posterior. The middle lobe is divided into medial and lateral. The lower lobe is divided into apical, lateral, anterior, and posterior basals.

On the left bronchus, there are eight subdivisions.

The upper lobe is divided into apical, anterior, posterior, and lingular. The lower lobe is divided into apical, lateral, anterior, and posterior basals.

Also read: Reinke's Edema

Tracheobronchial Tree

The tracheobronchial tree constitutes the entire ventilator pathway to the lungs.

1. Initiation of the System: The respiratory system begins with the trachea.
2. Primary Divisions: Two primary divisions emerge: one on the right side and one on the left.
3. Branching Pattern: Primary bronchi extend from these initial divisions. Secondary bronchi originate from the primary bronchi. 

Further branching occurs with tertiary bronchi emerging from the secondary bronchi. Bronchioles, in turn, branch off from the tertiary bronchi. The bronchioles progress into respiratory bronchioles. Respiratory bronchioles evolve into alveolar ducts. Alveolar ducts eventually open into the alveoli.

Trachea

Initiation of the Tracheobronchial System: The trachea marks the beginning of the tracheobronchial system.

Position: Corresponds to C6 to T4-T5 vertebrae.

Composition: Composed of cartilage and fibromuscular tissue.

Length and structure: Length: 10 to 11 cm. Midline structure.

Start and end points: Superiorly begins at the lower border of the cricoid. Inferiorly end at the carina.

Diameter: Transverse diameter: 20 mm. Anterior-posterior diameter: 15mm. Diameter is 3 mm at 1 year of age and corresponds to the age in years during childhood, maximum of 12 mm at puberty.

Tracheal Stenosis: Reduction in diameter, due to injury or illness, leads to tracheal stenosis. Trachea has cartilage anteriorly and trachealis muscle posteriorly.

Also read: Otoacoustic Emissions (OAE)/ KEMP Echoes

Tracheal Cartilages

Number and Variability: There are 16 to 20 cartilage rings in the trachea, with individual variation.

Shape and Arrangement: Cartilages are D-shaped in cross-section with incomplete cartilaginous rings. They are placed horizontally, one above the other

Cartilage Type: The type of cartilage is hyaline.

Dimensions: Tracheal cartilage measures 4 mm deep (vertically) and 1 mm thick.

Separation: Cartilages are separated by an annular ligament.

(In a cross-section of the tracheal cartilage, you can see the esophagus behind the trachea in the image below. The D-shaped trachea is posteriorly deficient and is covered by the trachealis muscle. The cartilage is lined with respiratory epithelium and has mucus glands.)

Pediatric Airway

1. Size and Characteristics: The pediatric airway is smaller in diameter, narrow, fragile,

and easily collapsible compared to an adult airway.

2. Mobility and Tracheal Rings: It is more deeply placed and more mobile than in the adults. It has immature tracheal rings and the bifurcation is at a higher level until the age of 10-12 years

3. Airway: In children, the airway is located higher and more anterior

compared to adults. As children grow, their airways will enlarge and move more caudally as the cervical spine elongates.

4. Challenges in Tracheostomy: Due to the easy collapsibility of pediatric airways, performing a tracheostomy in young children becomes challenging. Different procedures must be followed to address this challenge.

5. Cartilage Composition: Hyaline cartilage is more abundant in the airways of adults compared to the pediatric population.

6. Submucosal Glands: Pediatric airways have a higher presence of submucosal glands compared to adults.

Also read: Inner Ear Anatomy : Embryology and Parts

Relations of Cervical Trachea

Extent of Cervical Trachea: From the first tracheal cartilage below the cricoid cartilage at

C6 vertebra to the thoracic inlet. Cross-Sectional Relations. Anterior: Thyroid isthmus is in front of the trachea. Posterior: Esophagus and C6 vertebra. Lateral: Left and right lobes of thyroid glands are on either side. Posterior and Lateral: Carotid sheath with contents

Muscular Relations: In front of the thyroid isthmus, strap muscles sternohyoid and

sternothyroidism is present. In front of these strap muscles, sternomastoid muscles cover

Them.

Fascial covering: covering sternomastoid muscle is a superficial layer of deep cervical fascia. Pre-tracheal fascia covers the trachea

Relations of Thoracic Trachea

Thoracic Trachea Extent: Thoracic part extends from the thoracic inlet to the

tracheal bifurcation, where trachea divides into right and left main bronchi at the level of T4 vertebra

Anterior Relations: Thymus gland, Second costal cartilage is located in front of the thymus gland.

Posterior Relations: Esophagus, Thoracic duct, T4 vertebra

Left Side Relations: Aortic arch; left vagus nerve is lateral to the aortic arch. Recurrent laryngeal nerve is located between the trachea and esophagus and medial to the aortic arch.

Right Side Relations: Superior vena cava, Azygos vein, Right vagus nerve

Also read: Upper Airway Obstruction: Causes, Management

Different Types of Tracheoesophageal Fistula

These congenital fistulas are classified into five types

1. First Type: Proximal and distal atresia of the esophagus. No communication between the trachea and esophagus.

2. Second Type: Distal atresia of the esophagus. Proximal communication between the trachea and esophagus (proximal TE fistula).

3. Third Type: Proximal atresia of the esophagus. Distal atresia of the trachea.

4. Fourth Type: Both proximal and distal tracheoesophageal fistula.

5. Fifth Type (H-type): Trachea and esophagus communicate via a common opening.

Carina

Location: The carina is situated at the lowermost point of the trachea, where the main bronchi divides into left and right.

Appearance: The main bronchi are separated at their origin by a narrow ridge that resembles the keel of an upturned boat and is called Carina.

Main Bronchi Divisions: From the carina, two main divisions arise: the right main bronchus and the left main bronchus.

Lymph nodes: The carina may exhibit a bulge because of the projection of subcarinal lymph nodes. Virtual Bronchoscopy. The carina and main bronchi can also be visualized using

virtual bronchoscopy. Virtual bronchoscopy is a non-invasive, non-painful, and less risk-associated entirely virtual method for observing these structures.

Endoscopy with bronchoscope: It is invasive and is associated with risk such as airway edema, laryngospasm and bronchospasm because of irritation to mucosa.

Also read: Staging of Supraglottic, Glottic & Subglottic Tumour

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