The airway comprises the nasopharynx, oropharynx, larynx, and trachea (Figure 1-1). The mucosa of the nasal passage is extremely vascular and fragile and therefore susceptible to trauma. 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. The mucous membrane of the nose is innervated anteriorly by the anterior ethmoid nerve (ophthalmic division of trigeminal nerve) and posteriorly by the sphenopalatine nerve (maxillary division of 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 "Airway Evaluation" and the anatomy of the larynx is covered in the next section.
The trachea measures 15 cm in an average adult. It bifurcates at the fifth thoracic vertebra into two primary bronchi. 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, foreign bodies) end up in 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 diameter of the trachea varies between normal adult males and females. It ranges from about 15 to 20 mm. Since the external diameter of a 7.5 mm internal diameter (ID) endotracheal tube is 11.0 mm, size must be taken into consideration in selecting an endotracheal tube. These considerations usually preclude using tubes much larger that 7.5 mm ID for normal adult females or larger that about 8.5 mm ID for normal adult males.
Anatomy of the Larynx
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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 1-2). The unpaired cartilages are the larger thyroid, cricoid, and epiglottic cartilages. Although not part of the larynx, the hyoid bone has many attachments to the larynx. The cricoid cartilage is signet ring–shaped, as opposed to the C-shaped cartilages of the trachea (Figure 1-2). Because it forms a complete circle, depression of the cricoid cartilage will put pressure on structures located posteriorly (e.g., the esophagus) without occluding the airway. The application of posteriorly directed pressure on the cricoid cartilage during intubation is known as the Sellick maneuver . 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 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 1-2). The cricothyroid membrane is usually located at the level of the sixth cervical vertebra. The cricothyroid membrane is the 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 1-2). This position renders them vulnerable to injury during intubation.2 By staying anterior and by not inserting a laryngoscope blade 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 being used.
One of the attachments of the hyoid bone to the larynx is the hyoepiglottic ligament located at the base of the vallecula (Figure 1-4). 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 1-2). 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 . At this point, the internal branch of the superior laryngeal nerve is superficial enough to be easily anesthetized with an injection of local anesthetic solution. |
Airway Evaluation
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The evaluation of the airway should always start with a thorough history. 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 1-1) and acquired conditions (Table 1-2) 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. Several brief evaluations are helpful in predicting a difficult intubation. External inspection should include mouth opening, atlantooccipital extension, and thyromental distance. Internally, inspect the teeth, palate, tongue, and other soft tissue for abnormal anatomy or masses. External inspection should identify obvious problems (cervical collars, face and/or neck trauma, severe micrognathia, massive obesity, etc.).
The next three 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
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 4 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 degrees and extend the cervical spine (atlantooccipital joint) 80 degrees from a neutral position.6 This range of neck movement allows for the alignment of the oral, pharyngeal, and laryngeal axes (Figure 1-5). This alignment of the axes provides the greatest chance for a successful intubation.
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. 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 faucial pillars, 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 three grades.7 Class 1 is when the faucial pillars, soft palate, and uvula can be fully visualized (Figure 1-6). In class 2, the faucial pillars and soft palate can be visualized but the uvula is partially masked by the base of the tongue. Class 3 is when only the soft palate can be visualized (Figure 1-7). The predictive value of this classification is that during direct laryngoscopy, the entire glottis can be exposed in 100 percent of class 1, 65 percent of class 2, and 0.1 percent of class 3 airways.7
Anatomic Differences between the Adult and the Infant
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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 1-3 and Figure 1-8.1
The most important difference is that the narrowest portion of the infant airway is below the level of the vocal cords at the cricoid cartilage. In an adult, the narrowest point is the vocal cords. 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. 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 1-8B).1 |
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 8 years of age. |
