A thorough understanding of anatomy is essential for the performance of any medical procedure.1–10 Untoward events due to a procedure are usually the result of inexperience and/or an inadequate understanding of the regional anatomy. The anatomy of the airway and airway procedures are no exception. From the evaluation of external anatomic landmarks to the performance of nerve blocks for fiberoptic intubation, an understanding of the anatomy of the airway will result in fewer attempts at intubation and improved success with fewer iatrogenic misadventures.
The upper airway comprises the nasal and oral cavities, the pharynx, and the larynx. The lower airway consists of the subglottic larynx, the trachea, and the bronchi.8 Airway management typically involves the upper airway, the focus of this chapter. The anatomy of the pharynx, larynx, and trachea are depicted in Figure 6-1.
Anatomy of the airway as visualized in a midsagittal section through the head and neck.
The nares serves as the functional beginning of the airway, namely warming and humidification of air.4 The mucosa of the nasal passage is extremely vascular and fragile and therefore susceptible to trauma. The nasal blood supply originates from branches of the internal and external carotid arteries. It is wise to consider the use of a vasoconstricting agent, when appropriate, to help avoid epistaxis which may obscure further attempts at securing the airway. Although patients tolerate nasal intubation better than oral intubation for a longer period of time, it is more important in an emergency to definitively secure the airway using a straightforward oral intubation if possible.
The sensory innervation of the upper airway is provided by branches of several cranial nerves. The mucous membrane of the nose is innervated anteriorly by the anterior ethmoid nerve (ophthalmic division of the trigeminal nerve) and posteriorly by the sphenopalatine nerve (maxillary division of the trigeminal nerve). The tongue is innervated by the lingual nerve on its anterior two-thirds (a branch of the facial nerve) and by the glossopharyngeal nerve posteriorly. The glossopharyngeal nerve also innervates the adjacent areas, including the palatine tonsils, the undersurface of the soft palate, and the roof of the pharynx.1 The anatomy of the oropharynx is discussed further under the “Airway Evaluation” section and the anatomy of the larynx is covered in the next section.
The trachea measures 10 to 16.5 cm in an average adult.4 The trachea is a tubular structure that begins at the level of the fifth or sixth cervical vertebrae and bifurcates at the level of the fifth thoracic vertebra into two primary bronchi. The posterior aspect of the trachea is flat and membranous, while its anterior and lateral aspect is lined by 16 to 20 horseshoe-shaped cartilaginous rings. The primary bronchi subsequently branch into three secondary bronchi on the right and two secondary bronchi ...