AIRWAY ANATOMY

Successful execution of any procedure demands a thorough understanding of the procedure: indications, technical requirements, and necessary equipment to complete the task. In addition, the operator must be cognizant of potential complications and their management. Obtaining an airway, whether by conventional or surgical means, is no different than any other procedure.^3,4 The anatomy and normal functions of the structures that comprise the upper airway are complex. The surgeon must recognize the essential anatomic structures to effectively manage the airway.

The nose and mouth are the two natural external openings to the airway. The nasal cavity is separated by the nasal septum into two pyramids containing bone, cartilage, and sinus orifices. The roof of the nose is formed by the nasal and frontal bones, the ethmoid cribiform plate, and the body of the sphenoid bone. The floor is formed by the maxilla and palatine bones, and the medial wall by septal cartilage, the vomer, and the perpendicular plane of the ethmoid. Laterally, the wall contains a portion of the ethmoid bone along with the superior, middle, and inferior turbinate bones. In addition, it contains the openings to the paranasal sinuses and the nasolacrimal duct. The nerve supply to the nasal cavities is from the olfactory and the trigeminal nerves (cranial nerves I and V, respectively). The blood supply is from the anterior and posterior branches of the ophthalmic artery, as well as branches of the maxillary and facial arteries.

The mouth is defined by the lips and cheeks externally; the gingiva, teeth, and palate superiorly; and mucosa and tongue inferiorly. The palate forms the roof of the mouth and the floor of the nasal cavity. The hard palate forms the anterior four-fifths of the palate and is a bony framework covered with a mucus membrane. The soft palate, comprising the posterior one-fifth of the palate, is a fibromuscular fold that moves posteriorly against the pharyngeal wall to close the oropharyngeal cavity when swallowing or speaking.

Although separated anteriorly by the palate, the mouth and nasal cavities join posteriorly at the end of the soft palate to form the pharynx. The pharynx is separated into the nasopharynx and the oropharynx. The pharynx extends to the inferior border of the cricoid cartilage anteriorly and the inferior border of the body of C6 posteriorly. The wall of the pharynx is composed of two layers of pharyngeal muscles: the external circular layer consists of the constrictor muscles and the internal longitudinal layer consists of muscles that elevate the larynx and pharynx during swallowing and speaking. The nasopharynx follows directly from the nasal cavity at the level of the soft palate, and communicates with the nasal cavities through the nasal choanae and with the tympanic cavity through the Eustachian tube. It contains the pharyngeal tonsils in its posterior wall. The oropharynx begins at the soft palate and continues to the tip of the epiglottis. It contains the palatine tonsils, important landmarks in the Mallampati airway classification.⁵

The epiglottis is a spoon-shaped plate of elastic cartilage that lies behind the tongue. It prevents aspiration by covering the glottis—the opening of the larynx—during swallowing. The laryngopharynx/hypopharynx extends from the upper border of the epiglottis to the lower border of the cricoid cartilage. It is separated laterally from the larynx by the arytenoepiglottic folds, which contain the piriform recesses. The piriform sinuses are found at each side of the opening of the larynx, in which the inferior laryngeal nerve lies and swallowed foreign materials may be lodged.

The laryngeal skeleton consists of several cartilages connected by ligaments and muscles: the thyroid, cricoid, epiglottic, and (in pairs) the arytenoid, corniculate, and cuneiform. The larynx serves as a sphincter to prevent the passage of food and drink into the trachea and lungs during swallowing. It also contains the vocal cords and regulates the flow of air to and from the lungs for phonation. It is through the abducted vocal cords of the larynx that an endotracheal tube is advanced during endotracheal intubation.

The thyroid cartilage is a large anterior structure that consists of right and left laminae that meet in the midline, forming the thyroid notch and thyroid prominence. The right and left superior projections (horns) of the thyroid cartilage connect to the hyoid bone, and the cricothyroid membrane connects the inferior edge to the cricoid cartilage. This latter anatomic relationship is of particular importance when an emergency surgical airway must be obtained.

Unlike the thyroid cartilage, which does not project posteriorly, the cricoid cartilage is a complete signet-shaped cartilaginous ring connected to both the thyroid cartilage and the first tracheal ring. This anatomic relationship was used by Sellick,⁶ who described the prevention of passive regurgitation of gastric contents by posterior pressure on the cricoid cartilage during the induction of general anesthesia, a technique that has since become the accepted standard of care.

The epiglottis is an elastic cartilaginous structure covered by a mucous membrane. The reflection of this mucous membrane forms a slight depression known as the vallecula. It is at this point that the tip of the laryngoscope blade is placed so as to elevate the epiglottis and visualize the vocal cords. The epiglottis is anteriorly attached, superiorly to the hyoid bone by the hypoepiglottic ligament, and inferiorly to the thyroid cartilage by the thyroepiglottic ligament. The quadrangular ligament extends between the lateral aspects of the arytenoid and epiglottic cartilages. Its free inferior edge is the vestibular ligament, and, covered with mucosa, it forms the vestibular fold, which lies above the vocal cord (fold) and extends from the thyroid to the arytenoid cartilage. The arytenoids articulate with the superolateral aspect of the cricoid, and each has an anterior vocal cord process that attaches to the vocal cords via the cord ligament. The free superior margin forms the aryepiglottic ligament, and its mucosal covering forms the aryepiglottic fold. It is within the posterior aspects of aryepiglottic folds that the cuneiform and corniculate cartilages can be seen. These cartilages rest on the apex of the arytenoids. Their elastic properties facilitate the return of the arytenoid cartilages to their anatomic position of rest after abduction, and can usually be seen during direct laryngoscopy. They can, therefore, be used as a landmark for the tracheal opening when it is difficult to visualize the vocal cords (Figure 1).

Figure 1 Landmarks of the tracheal opening.

(From Putz R, Pabst R, editors: Sobotta Atlas of Human Anatomy, 13 ed Baltimore, Williams & Wilkins, 2001, with permission.)

The glottis is contained within the larynx, and is the narrowest part of the adult airway. It is composed of the vocal cords (or folds) and the space between them is called the rima glottides. Because the true vocal cords are covered by stratified epithelium, they have the characteristic pearly white color when illuminated. The cords are innervated by the recurrent laryngeal nerves, and the musculature of the larynx receives its innervation from branches of the vagus nerve (cranial nerve X). The larynx is inclusive of the structures noted previously.

The trachea extends from the inferior border of the cricoid cartilage for a variable distance, dividing into the right and left mainstem bronchi at the carina, which anatomically corresponds to the T4-T5 junction posteriorly and the sternal notch anteriorly. Like the thyroid cartilage, the sixteen to twenty tracheal cartilages form incomplete C-shaped rings that open posteriorly, allowing for variability in the tracheal diameter needed for normal airway functional changes. The trachea is in direct apposition posteriorly to the esophagus. This may be of significance in patients who have esophageal tumors, mediastinal hematomas (either traumatic or postoperative in origin), or have ingested large foreign bodies. All of these can cause upper (tracheal) airway obstruction.

ASSESSING THE AIRWAY

A thorough but rapid initial assessment of the trauma patient begins with the airway and breathing as the first priorities. This must be done with the understanding that, unless the cervical spine has already been “cleared” clinically, every patient has a cervical spine injury until proven otherwise. Breathing and speech are impossible without an intact airway. As such, one of the easiest determinants of an intact airway is the patient’s ability to respond to the simple question, “My name is Dr. Jones, who are you?” If the patient is unable to answer in a normal voice because of a compromised airway or depressed mental status, then the clinician knows that an airway must be secured immediately.

Any patients with a Glasgow Coma Scale (GCS) of 8 or less, regardless of etiology, will need to be intubated to protect their airway. Intoxication and use of illicit substances are frequent causes of an altered mental status, as are closed head injuries and hypoxia. One must, therefore, err on the side of caution, and always assume that the depressed GCS is injury related, and that intubation will be needed to protect the airway. Hoarseness, shortness of breath, or, in the case of burn victims, carbonaceous sputum, should alert the clinician that an airway may be in jeopardy even if the patient can respond appropriately at that time. These patients must have careful and frequent interval reevaluations, with a low threshold for intubation. In addition, these patients may pose particularly difficult challenges to intubation, and underscore to the need to understand the need for a rapid and careful assessment of each individual airway.

Patient anatomy, both inherent and as altered by injury, is an important factor that requires careful assessment by the clinician prior to attempting intubation, whether nasal, orotracheal, or surgical. In the trauma patient, the clinician must always be cognizant of the potential for cervical spine injury. This is particularly important in the patient whose physical exam is compromised by an altered mental status, regardless of the cause. The presence of maxillofacial trauma or injuries of the soft tissues of the neck must also be considered when deciding the best way to secure and protect the airway. In the absence of injury or pathologic conditions, the ability to ventilate and control the supraglottic airway is a function of the size of the airway and the compliance of the supralaryngeal tissues. It should be remembered that under normal conditions air flow through the nasal cavity is not usually a concern in that this portion of the airway is usually supported by rigid elements in its walls. Conversely, the pharynx does not have a rigid support system, and therefore, it is predisposed to collapse. It can also be obstructed by the tongue in obtunded or comatose patients; in most cases this collapse and/or obstruction can initially be dealt with effectively with the use of the oral airway.⁷

A more ominous situation arises when the airway is compromised in the immediate vicinity of the laryngeal inlet because of the size of the base of the tongue or the epiglottis. Air flow in this region will be inversely proportional to the size of these structures, as well as the condition of the perilaryngeal tissues. Trauma to the neck causing anatomic disruption and/or tissue edema, prior irradiation to the neck, head and neck tumors, or redundant fatty tissue in the perilaryngeal area secondary to obesity will all restrict normal air flow. These conditions may lead to the “cannot-ventilate-and-cannot-intubate scenario,” a potentially lethal situation in even the most competent intubators.⁷

As with any other medical procedure, it is always prudent to perform a methodical evaluation of the situation, and determine the preferable approach and equipment needed to perform the task efficiently. In addition to factors previously discussed, one must also assess for scarring and fibrosis to the airway and surrounding tissues, injury to the trachea; congenital anomalies; acute inflammatory or neoplastic diseases of the airway; and finally gingival, mandibular, and dental anomalies/disease.⁸ Short, muscular individuals with very short necks and obese patients tend to be difficult to intubate. Morbidly obese patients can compromise the airway with excessive redundant tissue in the paratonsillar and paraglottic areas, masking visualization and accessibility of the glottis during laryngoscopy. Anatomic factors that should be assessed include mandibular mobility at the temporomandibular joints; mobility of the head at the atlanto-occipital joint (when cervical spine injury has been ruled out); the length, size, and muscularity of the neck; size and configuration of the palate; the proportionate size of the mandible in relation to the face; and the presence of an overbite of the maxillary teeth.

Difficulty in laryngoscopic visualization varies in accordance with the extent of anatomic variation or pathologic conditions. The Mallampati classification is routinely used by anesthesiologists to evaluate the airway in elective cases (see Figure 1 on page 59).

In order to classify a patient, the patient is asked to open the mouth as widely as possible and protrude the tongue out as far as possible while seated, with the head in the neutral position,⁸ something that is almost impossible to accomplish in the trauma setting. Evaluation of the atlanto-occipital extension is another tool that is used by anesthesiologists in the elective setting. Because this requires an awake patient who can sit upright and flex and extend the neck, it is not useful in the trauma setting. However, a clinician familiar with the Mallampati classification can perform a cursory evaluation of the oropharynx using a tongue blade or laryngoscope, which provides the clinician some idea as to the potential for difficulty in intubation. It will also demonstrate the existence of any evidence of injury to the oropharynx, the presence of foreign bodies, blood, or vomitus.

Because one cannot always rely on the anesthesiologist to secure the airway, all clinicians caring for the trauma patient must have the ability to provide immediate and effective airway protection and ventilation. Clinicians must, therefore, be able to recognize limitations to airway access, and must possess techniques to overcome these limitations. There are many tools and techniques available to the anesthesiologist in the elective setting, and while many of these may not be practical in the emergent trauma setting, several can be used, and these will be the focus of the rest of this discussion.

CONTROLLING THE AIRWAY

The best airway is the airway that the patient can safely maintain without any external intervention. The Eastern Association for the Surgery of Trauma has sited the need for emergency tracheal intubation in trauma patients with airway obstruction, hypoventilation, severe hypoxia in spite of supplemental oxygen administration, severe cognitive impairment (GCS score <8), cardiac arrest, and severe hemorrhagic shock.⁹ Once it has been decided that the airway is not adequate, or that the patient is unable to protect the airway, then intubation must be accomplished in an efficient and safe manner. It is important to remember that all trauma patients must be intubated with one person maintaining in-line cervical immobilization throughout the procedure to protect the possibly injured cervical spine.

As with all other procedures, a successful outcome depends on the operator’s experience and on having immediate access to all of the correct equipment. Most emergency departments have airway carts that contain all of the equipment that will routinely be needed for emergency intubations, as well as specialty carts designed for the “difficult airway.” While the contents of these carts will vary from institution to institution, they will usually be standardized within each hospital. Examples of stocking lists for the standard airway cart (Figure 2) and the difficult airway cart appear in Tables 1 and 2, respectively.

Figure 2 Standard respiratory/airway cart.

Table 1 Respiratory/Airway Cart

Table 2 Difficult Intubation Cart

Once the decision to intubate has been made, all patients should be preoxygenated with 100% oxygen. The jaw thrust and chin lift can be done without endangering the cervical spine, and should be performed in an attempt to open an airway that may be obstructed by the tongue, which tends to fall posteriorly in the obtunded patient. The chin lift is accomplished by placing one’s fingers under the mandible and gently lifting the mandible upward. The thumb of the same hand is used to depress the lower lip and open the mouth. Care must be taken to prevent hyperextension of the neck while performing the chin lift. The jaw thrust can be performed alone or used in conjunction with bag-valve-mask (BVM) ventilation of the obtunded, but breathing patient, and the respiratory efforts are assisted with compression of the bag. By grasping both mandibular angles and displacing the mandible forward, one is able to simultaneously open the airway and obtain and maintain an effective seal with the mask during assisted bag mask ventilation.

Oropharyngeal and nasopharyngeal airways (see Figure 2) are useful, but must be inserted with care. While neither one is well tolerated by the awake patient, the nasopharyngeal airway is better tolerated in the semiresponsive patient, and is less likely to stimulate gagging and vomiting.¹⁰ They can both be used to help maintain a patent upper airway in the obtunded patient. In the adult, the oral airway is inserted with the convexity facing down initially, and then rotated 180 degrees once in the oropharynx. It must be placed carefully, so that it does not push the tongue posteriorly, but rather, is seated posterior to the tongue displacing the tongue anteriorly. In children, the oral airway must always be placed with the convex surface facing cephalad initially to avoid injury to the soft tissues of the mouth and pharynx. It is important to remember that in patients with a persistent (intact) upper airway gag reflex, the insertion of the oral airway can precipitate laryngospasm and bronchospasm, as well as coughing, gagging, vomiting, and ultimately, aspiration.¹¹