Chapter 6. Essential Anatomy of the Airway

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.

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 on the left. The angle between the primary bronchus and the trachea on the left is more acute than on the right. This is due to the heart being located on the left side. This is clinically significant during aspiration and endobronchial intubations. Because of the more direct path on the right side due to the obtuse angle of the primary bronchi, objects (food, fluid, and foreign bodies) are more likely to enter the right lung. The tracheal mucosa removes waste products by producing and moving mucus toward the pharynx via ciliary action. The trachea has a rich innervation from the vagus nerve, which permits a vigorous cough reflex (accompanied by hypertension and tachycardia) if a foreign body is aspirated.

The inner diameter of the trachea varies between normal adult males and females. It ranges from about 15 to 20 mm.4 Since the external diameter of a 7.5 mm internal diameter (ID) endotracheal tube is 11.0 mm, the size must be taken into consideration in selecting an endotracheal tube. These considerations usually preclude using tubes much larger than 7.5 mm ID for normal adult females or larger than 8.0 or 8.5 mm ID for normal adult males.

The innervation of the larynx is relatively simple. The internal branch of the superior laryngeal nerve provides sensation above the vocal cords (vocal folds). The recurrent laryngeal nerve supplies sensation below the vocal cords. The recurrent laryngeal nerve provides the motor input to all of the intrinsic muscles of the larynx except to the cricothyroid muscle, which is supplied by the external branch of the superior laryngeal nerve. Bilateral injury to the recurrent laryngeal nerve will result in total airway closure due to unopposed stimulation of the vocal cord adductor, the cricothyroid muscle.1

There are three paired and three unpaired cartilages of the larynx.2 The paired cartilages are the smaller arytenoid, corniculate, and cuneiform cartilages (Figure 6-2). The unpaired cartilages are the larger thyroid, cricoid, and epiglottic cartilages. Although not part of the larynx, the hyoid bone has many ligamentous and muscular attachments to the larynx. The cricoid cartilage is signet ring-shaped, as opposed to the C-shaped cartilages of the trachea (Figure 6-2). Because it forms a complete circle, depression of the cricoid cartilage will put pressure on structures located posteriorly (i.e., the esophagus). The application of posteriorly directed pressure on the cricoid cartilage during intubation is known as the Sellick maneuver (Figure 6-3). The Sellick maneuver will not prevent regurgitation from active vomiting. It has been shown to be effective in the prevention of passive regurgitation and subsequent aspiration.3 However, the Sellick maneuver can impair insertion of the laryngoscope or an airway introducer, and can cause airway obstruction.

The cricoid cartilage is also an important landmark for locating the cricothyroid membrane, which lies inferior to the thyroid cartilage and superior to the cricoid cartilage (Figure 6-2). The cricothyroid membrane is usually located at the level of the sixth cervical vertebra. It is the anatomic location where emergency cricothyroidotomies and recurrent laryngeal nerve blocks are performed.

The three paired cartilages are located on the posterior aspect of the larynx (Figure 6-2). This position renders them vulnerable to injury during intubation.2 By maintaining an anterior insertion of the laryngoscope blade and by not inserting it too deeply during intubation attempts, it is less likely that these cartilages will become dislocated or otherwise injured. This is particularly true if a straight laryngoscope blade is used.

The hyoid bone is an important supporting structure of the upper airway. One of the attachments of the hyoid bone to the larynx is the hyoepiglottic ligament located at the base of the vallecula (Figure 6-4). This ligament is important because it is where the tip of the curved Macintosh laryngoscope blade is placed to move the epiglottis anteriorly and out of the path of vision during intubation. Another attachment of the hyoid bone to the larynx is the thyrohyoid membrane (Figure 6-2). As its name implies, it runs from the inferior border of the hyoid bone to the superior aspect of the thyroid cartilage. Just inferior to the lateral border of the hyoid bone, the internal branch of the superior laryngeal nerve passes through the thyrohyoid membrane (Figure 6-2). At this point, the internal branch of the superior laryngeal nerve is superficial and very easily anesthetized with an injection of local anesthetic solution.

The evaluation of the airway should always start with a thorough history. An airway history should be conducted, when feasible, prior to the initiation of airway management in all patients. It should include whether the patient has ever required intubation and if there was any difficulty. Additional history should focus on the patient's dentition and any surgery on or near the airway. There are many congenital syndromes (Table 6-1) and acquired conditions (Table 6-2) that can complicate airway management. These should be kept in mind when performing the airway history and physical examination.

External evaluation of the airway is a critical step to a successful intubation. These brief evaluations are helpful in predicting a difficult intubation. External inspection should identify some obvious problems that may interfere with airway management. These include as facial hair which prevents a good mask seal, cervical collars which restrict neck movement, face and/or neck trauma, severe micrognathia, or obesity.

The next steps in evaluating the airway may help to identify patients with potentially difficult airways. In adults, the distance between the thyroid cartilage (“Adam's apple”) and the inside of the anterior aspect of the mandible is known as the thyromental distance. It should be at least 5 cm or about three large finger breadths.1 A lesser distance suggests that the patient's vocal cords are positioned more anteriorly than normal. Distances less than 5 cm may indicate that visualization of the larynx during intubation may be difficult or impossible due to a lack of space in which to displace the tongue. The next evaluation requires the patient to open his or her mouth maximally. Ideally, the patient will be in a seated or semisitting position. The distance between the maxillary and mandibular incisors in an average adult is 3 to 5 cm or about two large finger breadths.5 Limited mouth opening may impair visualization of the airway as well as expose the teeth to damage during intubation. Adults should be able to flex their cervical spine 35° and extend the cervical spine (atlantooccipital joint) 80° from a neutral position.6 This range of neck movement allows for the alignment of the oral, pharyngeal, and laryngeal axes during orotracheal intubation (Figure 6-5). This alignment of the axes provides the greatest chance for a successful intubation. Recent evidence suggests that slight head extension in infants and young children by placing a rolled towel behind their shoulders better aligns the vision of the glottic and laryngeal axes.10 Observe the patient's neck for length and thickness. A short, excessively long, or thick neck may indicate difficulty in placing the patient in the “sniffing position” and align the airway axes.

The internal examination should evaluate the patient's dentition, palate, and tongue. Note any protuberant incisors, loose teeth, broken teeth, dental work, and dental devices. Prominent upper incisors may complicate the insertion of the laryngoscope blade, make laryngoscopy difficult, and predispose the patient to dental trauma. Assess the relation of the maxillary and mandibular incisors during normal jaw closure. Lack of an overbite forces the laryngoscope blade to enter the mouth in a more cephalad direction than normal. This can result in difficulty visualizing the airway. Observe the maxillary and mandibular incisors during voluntary protrusion of the mandible to determine the degree of temporomandibular joint mobility. Determine if the palate is normal, high and arched, or cleft. Determine if the tongue is elevated, larger, or wider than normal in comparison to the oral cavity. Any abnormality can make the procedure of orotracheal intubation more difficult.

A common classification used by Anesthesiologists to grade the difficulty of laryngoscopy and intubation involves the identification of the size of the tongue in relation to the tonsillar pillars, the fauces, the soft palate, and the uvula.7 It is important to perform this evaluation by first instructing patients to open their mouths and protrude their tongues maximally in the sitting position. The patient should not say “ahhh,” as this distorts the anatomy and may falsely improve the airway classification. The Mallampati classification, named after its author, has four grades or classes.7 The anterior and posterior tonsillar pillars, the fauces, the soft palate, and the uvula can be fully visualized in class I (Figure 6-6A). The fauces, the soft palate, and the uvula can be visualized in class II (Figure 6-6B). The anterior and posterior tonsillar pillars are covered by the base of the tongue and not visible. Only the soft palate and the base of the uvula are visible in class III (Figure 6-6C). None of the structures are visible in class IV (Figure 6-6D). The predictive value of this classification is that during direct laryngoscopy, the entire glottis can be exposed in 100% of class I airways, 65% of class II airways, 30% of class III airways, and 0.1% of class IV airways.7

The airway evaluation is imperfect in predicting potential problems and an airway strategy (combination of plans) should be drawn up for each patient. Additional evaluation may be indicated in some patients to characterize the likelihood or nature of the anticipated airway difficulty. The findings of the airway history and physical examination may be useful in guiding the selection of specific diagnostic tests and consultation.

There are numerous differences between the airway of an adult and that of a child. The head-to-body ratio is larger in the child. This causes the neck to be flexed when the child is supine. Placing a rolled towel under the child's shoulders will correct the flexion. A child has a small mouth with a relatively large tongue as compared to an adult. This can make orotracheal intubation difficult. The presence of adenoidal tissue in the child makes nasotracheal intubation difficult and orotracheal intubation the preferred method.

The anatomic differences between the larynx of an adult and that of a young child are summarized in Table 6-3 and Figure 6-7.1 The vocal cords are more obliquely inclined in the child. The cricothyroid membrane is very small and narrow. The most important difference is that the narrowest portion of the infant or young child's airway is below the level of the vocal cords at the level of the cricoid cartilage. In an adult, the narrowest point of the airway is at the level of the vocal cords. The cricoid cartilage is the only complete cartilaginous ring and the narrowest part of the trachea. An endotracheal tube may therefore pass through the vocal cords of a young child but might not advance past the cricoid cartilage due to normal anatomy. Forcing an endotracheal tube past the vocal cords in a young child may result in trauma to the airway and subsequent tracheal stenosis. Because the child's larynx is located more cephalad at the level of the third or fourth cervical vertebrae and angulated anterior to the glottis, blind nasal intubation in the case of airway emergencies is not possible. The child's laryngeal inlet is narrow and more susceptible to obstruction. The U-shaped epiglottis and a more acute angle between the epiglottis and glottis cause the aryepiglottic folds to be more in the midline (Figure 6-7B).

Differences also exist in the trachea. Children have a relatively shorter trachea. This makes both right main bronchial intubation and accidental extubation much easier. The narrower diameter of the trachea with smaller spaces between the cartilaginous rings makes a tracheostomy more difficult to perform. To avoid injury and subsequent subglottic stenosis, uncuffed endotracheal tubes should be used in children less than 28 days of age.11 A correctly sized endotracheal tube should have a leak at 15 to 25 cm water. Using the uncuffed endotracheal tube with high leak pressure more than 25 cm of H2O or the cuffed endotracheal tube can not only cause croup after extubation, but also create an erosion of the mucosa and subglottic stenosis. A cuffed endotracheal tube is recommended for anyone above the age of 28 days.10–13

The differences also continue into the chest. The infant thoracic cage is more compliant than the older child and adult. This can lead to more effective ventilation. The child's diaphragm is shorter and flatter than the adult, and thus has a decreased excursion during respiration. The child's relatively narrower airways result in an increased resistance to inspiratory and expiratory airflow. They are also more susceptible to airway edema and mucous plugs obstructing their narrow airway. Children have smaller and fewer alveoli. This results in less surface area for gas exchange compared to adults.

It is essential for Emergency Physicians to know the anatomy of the airway, especially the differences found in the young child. The airway assessment is an essential element in the preparation to intubate a patient. Unfortunately, there may be limited information and time available to perform the full assessment in an emergent setting. Performing the airway assessment in every patient before orotracheal intubation will allow one to perform it quickly in an emergency.