1

What is the trachea?

The trachea is a cartilaginous and membranous tube that allows the passage of air between the upper respiratory tract and the lungs.

2

Describe the embryology of the trachea

The trachea develops at approximately week 4 of gestation from an endodermal bud growing into the splanchnic mesoderm.

The epithelial covering (including cilia) and the glandular component of the trachea are derived from the endoderm whereas the cartilage, muscles and connective tissue are derived from the mesoderm.

3

Describe the anatomical characteristics of the trachea (Figure 1)

The boundaries of the trachea are:

a)

superior – lower border of the cricoid cartilage in the larynx (C6 level);

b)

inferior – origin of the main stem bronchi (T4 level at end expiration to T6 at peak inspiration).

The total length of the trachea is 11-13cm and can be divided into:

a)

cervical part (approximately 5cm) – cricoid cartilage to suprasternal notch;

b)

thoracic part (approximately 6-8cm) – suprasternal notch to manubriosternal junction.

The trachea is elliptical and flattened posteriorly.

The anterolateral two-thirds (cartilaginous trachea) is reinforced with cartilage whereas the posterior one-third (membranous trachea) constitutes outer longitudinal and inner transverse non-striated muscle fibres.

The transverse diameter of an adult trachea remains constant, whereas in children up to 4 years of age it is funnel-shaped, with the proximal part being wider than the distal part.

The transverse diameter of an adult male and female trachea measures about 2.3cm and 2cm, respectively, whereas the anteroposterior diameter measures approximately 1.8cm.

There are 18-22 tracheal rings of hyaline cartilage, which constitute the relatively fixed part of the trachea. The dynamic parts are the posterior wall and the muscles between the cartilage rings, which are responsible for change in the tracheal lumen size.

Each tracheal ring is approximately 4mm wide and 1mm thick. The rings are thicker in the middle than the margins.

The tracheal rings are highly elastic in childhood but with age they calcify and become less flexible.

4

What are the arterial supply, venous and lymphatic drainage, and innervation of the trachea (Figure 2)?

Arterial supply:

a)

cervical trachea – inferior thyroid arteries (shared with the oesophagus);

b)

thoracic trachea – bronchial arteries (shared with the oesophagus).

Venous drainage:

a)

entire length – inferior thyroid venous plexus, which drains into the brachiocephalic veins and then onto the superior vena cava.

Lymphatic drainage – via perivascular lymphatics and adjacent 1st lymph node groups:

a)

cervical trachea – inferior group of deep cervical lymph nodes;

b)

thoracic trachea – pretracheal, paratracheal and subcarinal lymph nodes.

Innervation:

a)

parasympathic nerve supply – vagus nerve;

b)

sympathetic nerve supply – sympathetic trunk;

c)

rich sensory innervation – which is linked to coughing.

5

Describe the anatomical characteristics of the main carina

The main carina lies slightly to the right of the midline, where it divides into the right and left main bronchi.

The angle between the two main bronchi (subcarinal angle) is wider in children than in adults.

The right main bronchus is wider, shorter and continues more vertically than the left. This makes inhalation of foreign bodies into the right side more common.

The last cartilage ring of the trachea is thicker and broader in the middle, as compared to the rest of the tracheal rings (except the 1st), and curves inferiorly and posteriorly between the two bronchi.

The carina has a very rich blood supply from the bronchial arteries.

The subcarinal group of lymph nodes drain both sides of the tracheobronchial tree and lung, and can be easily punctured via the airway (transbronchial needle aspiration or biopsy) to obtain tissue samples.

When describing lesions of the trachea, it is useful to do so relative to the position of the cricoid and carina.

6

What are important anatomical relations of the trachea (Figure 3)?

Anteriorly:

a)

cervical part:

i)

thyroid isthmus;

ii)

anastomosis of the two superior thyroid arteries;

iii)

inferior thyroid veins;

iv)

anterior jugular vein;

v)

jugular arch in the suprasternal space of Burns;

vi)

in children – thymus, left brachiocephalic vein, brachiocephalic artery and even aortic arch;

b)

thoracic part:

i)

manubrium;

ii)

remnant of thymus;

iii)

left innominate vein;

iv)

aortic arch;

v)

innominate artery;

vi)

left common carotid artery;

vii)

deep cardiac plexus.

Posteriorly:

a)

cervical part:

i)

oesophagus;

ii)

recurrent laryngeal nerves in the tracheo-oesophageal groove;

b)

thoracic part:

i)

oesophagus;

ii)

left recurrent laryngeal nerve.

Laterally:

a)

cervical part:

i)

medial surface of the lateral lobe of the thyroid;

ii)

inferior thyroid arteries;

iii)

internal jugular veins;

iv)

common carotid arteries;

b)

thoracic part:

i)

pleura (bilaterally);

ii)

superior vena cava (right side);

iii)

right vagus nerve (right side);

iv)

arch of azygos vein (right side);

v)

left common carotid artery (left side);

vi)

left subclavian artery (left side);

vii)

phrenic nerve (left side);

viii)

vagus nerve (left side);

ix)

aortic arch (left side).

7

Describe the physiological characteristics of the trachea

The trachea is a dynamic conduit for ventilation, where the length and diameter increase during inspiration and decrease in expiration.

The tracheal mucosa is formed by pseudostratified ciliated columnar epithelium with interspersed mucous glands and goblet cells.

Parasympathetic activity of the vagus nerve increases secretions from tracheal mucous glands, while local irritants increase secretions from goblet cells.

These secretions form a thin layer (5μm) on the surface of the airway to protect (with the help of dissolved immunoglobulins and lysozymes) and reduce evaporation.

In normal conditions, the amount of secretions varies between 10-100mL/day.

Rhythmic contractions of the cilia transport debris dissolved in the mucous layer with an outward movement at a rate of approximately 166mm/min. Debris is finally either expectorated or swallowed.

8

What are the factors that influence airways resistance?

Airway resistance is governed by:

a)

Ohm’s law:

b)

Poiseuille law:

R = resistance; ∆P = pressure difference between the ends of the airway; μ = dynamic viscosity; L = length of the airway; V = volumetric flow rate; r = radius of the airway.

As described in these laws, tracheal resistance depends on dynamic viscosity, length and radius of the trachea, volumetric flow rate and pressure difference between the ends of the trachea.

There is an inverse relation of the 4th power between tracheal diameter and airway resistance. Therefore, a small narrowing of the trachea can result in an important rise in airway resistance producing symptoms.

9

What are the common causes of tracheal lesions (Table 1)?

Tracheal lesions can broadly be classified as intrinsic or extrinsic.

Most tracheal lesions cause stenosis that can be static (structural) or dynamic (functional).

10

What are the common symptoms of tracheal lesions?

Symptoms and their severity can vary with the respiratory cycle, especially in dynamic lesions.

Mild to moderate stenosis leads to dyspnoea (exertional and/or at rest).

Severe stenosis can cause stridor.

Recurrent respiratory tract infections, chronic cough, haemoptysis and dysphagia are other common symptoms.

11

Describe the characteristic features of congenital tracheomalacia

Tracheomalacia is characterised by significant flaccidity of the tracheal supporting cartilage causing an exacerbation of the physiological dilation during inspiration and narrowing during expiration.

As a result, the airway collapses in expiration, creating dynamic airway obstruction and symptoms.

It is more commonly seen in premature infants.

Tracheomalacia is classified as diffuse or segmental and may be associated with laryngomalacia.

Diffuse tracheomalacia without extrinsic compression is the most common type.

12

Describe the characteristic features of congenital tracheal stenosis

Congenital tracheal stenosis is characterised by the absence of the membranous trachea, resulting in almost complete cartilaginous tracheal rings, and can be subclassified as:

a)

segmental (50%);

b)

generalised (30%);

c)

funnel-shaped (20%).

It is often associated with other abnormalities, especially in a low position, with webs and diaphragms representing the simplest forms of congenital tracheal stenosis.

13

Describe the characteristic features of a tracheo-oesophageal fistula

Tracheo-oesophageal fistula (TOF) can be congenital or acquired.

Congenital TOF is part of VACTERL syndrome, which has an incidence of 1:4000 births and a male preponderance:

a)

vertebral anomalies;

b)

anal atresia;

c)

cardiovascular defects;

d)

tracheo-oesophageal fistula;

e)

renal or radial anomalies;

f)

limb defects.

Acquired TOF can be caused by:

a)

tumour (50-70%);

b)

trauma;

c)

aortic surgery;

d)

tracheostomy;

e)

infection;

f)

inflammatory processes.

There are five distinct types of TOF seen:

a)

Type A (5%) – isolated oesophageal atresia without TOF;

b)

Type B (rare) – oesophageal atresia with proximal TOF;

c)

Type C (85%) – oesophageal atresia with distal TOF;

d)

Type D (rare) – oesophageal atresia with proximal and distal TOF;

e)

Type E (5%) – isolated TOF without oesophageal atresia.

14

Describe the characteristic features of vascular anomalies and rings (Figure 4)

Vascular anomalies and rings can cause extrinsic compression of a structurally normal trachea and result from abnormal development of the aortic arch complex.

During fetal development, there are six pairs of aortic arches which connect the two primitive dorsal and ventral aortae:

a)

most of the 1st, 2nd and 5th arches regress;

b)

the 3rd arch becomes the carotid arteries;

c)

a distinct branch from the ventral bud of the 6th arch meets the lung bud to form the pulmonary arteries. On the right side the dorsal contribution to the 6th arch disappears while on the left it persists as the ductus arteriosus.

Double aortic arch is the commonest complete vascular ring abnormality leading to tracheal and oesophageal compression.