MÉTODOS DE MEDICIÓN DE LA CALIDAD DEL AIRE

The trachea lies in the midline of the neck and upper mediastinum. It begins at the lower border of the cricoid cartilage at the level of the sixth cervical vertebra and extends to its bifurcation in to right and left main bronchi at the carina. It is composed

of C shaped rings of cartilage that form the anterior and lateral walls and a musculo- membranous posterior wall. The smooth muscle in the musculo-membranous portion of the tracheal wall contains both transverse and longitudinal fibres. The transverse fibres make up the trachealis muscle connecting the ends of the tracheal cartilages. Tracheal size is related to the size of the individual but in the adult male there are approximately 18-22 tracheal rings extending inferiorly from the cricoid to carina for 11-12 cm. The lateral diameter is approximately 2.3 cm coronally and 1.8 cm

sagitally60,61.

The shape of the tracheal lumen varies with age and in the presence or absence of disease states. The lumen in the child is almost circular reaching its usual adult D shape at adolescence (figure 2.1). Luminal shape also alters dynamically in response to changes in intra-luminal pressure during respiration, coughing and mechanical ventilation. During forced expiration the trachealis muscle approximates the ends of the tracheal cartilages creating an elongated antero-posterior tracheal diameter. In the presence of ageing or significant chronic obstructive pulmonary disease, this

reduction in lateral diameter may result in a sabre sheath or scabbard trachea. Chronic obstructive airways disease may also result in a softening of the tracheal cartilage and a widening of the lateral diameter and an antero-posterior narrowing (figure 2.1). This conformational change may ultimately lead to luminal obstruction during coughing and active expiration.

Figure 2.1. Cross sectional tracheal morphology – normal and common variants.

A; Juvenile circular trachea. B; Adult D-shaped trachea. C; Sabre sheath or scabbard trachea. D; Tracheal changes seen in chronic obstructive pulmonary disease.

Image reprinted with permission from Medscape Drugs & Diseases (http://emedicine.medscape.com), 2015, available at

http://emedicine.medscape.com/article/1949391-overview#a3

The tracheal wall is lined by a ciliated pseudo-stratified columnar epithelium that also contains Goblet cells and mucous glands. Also present within the epithelium are “brush  cells”  with  surface  microvilli and neuroendocrine cells, the function of both cell types is not entirely clear. Mucosal irritation may result in cilia damage and squamous metaplasia resulting in an increased dependence upon coughing to clear secretions. As a result of such injury the tracheal mucosa must be able to regenerate. This occurs from a sub-population of basal stem cells in the columnar epithelium which are able to develop in to ciliated surface cell and Goblet cells62.

The trachea receives its blood supply segmentally via the lateral walls. The cervical trachea is supplied predominantly from the inferior thyroid artery most frequently via three tracheo-oesophageal vessels. The mid to lower trachea receives its arterial supply from the superior, middle and inferior bronchial arteries. At the segmental level each artery to the trachea will branch superiorly and inferiorly over several tracheal rings forming a series of longitudinal anastomoses. Additionally anterior and posterior braches will run in the inter-cartilaginous space to eventually anastomose with contralateral vessels. The posterior vessels also anastomose with oesophageal arteries60,61.

2.1.1.1 Anatomical relations

The anatomical relations of the trachea explain many of the early and late complications seen following tracheostomy (figure 2.2). The oesophagus lies

posteriorly and slightly to the left of the trachea from the level of the cricoid cartilage with the pre-vertebral fascia and vertebral bodies lying posteriorly to the right. The isthmus of the thyroid gland is anterior to the second to fourth tracheal rings with the inferior thyroid artery supplying the proximal trachea as well as the thyroid gland itself. Lateral to the cervical part of the trachea lie the lobes of the thyroid gland, carotid artery, internal jugular vein, vagus nerve and cervical lymph nodes within the carotid sheath.

Figure 2.2. Anatomical relations of the trachea including major vessels, thyroid and nerves. Modified from: Deslauriers.61

Reprinted from Thoracic Surgery Clinics 2007;17(4):529-547. Deslauriers J. Anatomy of the neck and cervicothoracic junction. Copyright (2007), with permission from Elsevier.

As the trachea descends in to the superior mediastinum anteriorly lies the thymus, the brachio-cephalic (innominate) artery and vein and the aortic arch. The tracheo-

innominate artery arises as the first branch of the aortic arch. It ascends obliquely and posteriorly to cross the trachea at the level of the ninth tracheal cartilage (range from sixth to thirteenth). There is however, a degree of variability in the course of the tracheo-innominate artery with passage across the trachea being possible as high as the second – third tracheal ring63. Posteriorly lie the oesophagus, pre-vertebral fascia and recurrent laryngeal nerves in the tracheo-oesophageal grooves. Laterally lie the vagus and phrenic nerves, the superior vena cava on the right, left common carotid and left subclavian vessels with the thoracic duct on the left and azygous vein on the right.