Chapter 22
Airway Management
Anatomy and Physiology of the Airway
Developmental Anatomy
Unlike the structures of the lower respiratory tract, the upper respiratory tract arises from bony structures of the head. Endochondral bone is preformed in cartilage. The bones form initially from the optic, olfactory, and otic capsules. These merge with the midline cartilaginous structures to form the embryologic vestiges of the ethmoid, sphenoid, the petrous portion of the temporal bone, and the base of the occipital bone. Direct ossification of membranous tissue known as the mesenchyme occurs during early embryologic development to form membranous bone. The membranous bones include the temporal, parietal, frontal, and portions of the occipital bones and the pharyngeal arches. The pharyngeal arches are complex structures also known as the branchial arches that extend anterior to posterior. Development of these structures begins at day 22 (week 4 after fertilization).1
Embryologically, there are six arches that develop from five structures. Arches one through four and six go on to develop the airway structures, and the fifth arch disappears with fetal development. The arches all contain a covering of tissue that will eventually become the nerves, muscles, and cartilage of the airway. These will become the tissues of the oropharynx, middle ear, the hyoid bone, and the laryngeal cartilages. Arch one becomes the jaws; arch two becomes the facial structures and the ears; arch three becomes the hyoid bone and structures of the upper pharynx; arches four and six become the structures of the larynx and the lower pharynx; and arch five disappears. The tongue is formed from the mesoderm of multiple arches. The anterior two thirds of the tongue is developed from the first arch. The mesoderm of the third and fourth arches comprises the posterior third of the tongue. Spaces found between the arches are known externally as clefts and internally as pouches. The cleft between the first two arches becomes the external auditory meatus. The internal pouch between the first and second arches forms the majority of the tympanic cavity and the eustachian tubes. The other clefts disappear as the fetus develops. The pouches contribute to the development of the glandular structures of the head and neck. The palatine tonsils arise from pouch two; the inferior parathyroid glands and the thymus come from pouch three; the superior parathyroid glands arise from pouch four, and the ultimobranchial structures arise from the inferior portion of pouch four.1
Nose
The nose and mouth are the external openings to the respiratory tree. The large surface area of the nasal mucosa warms and humidifies inspired air but also provides almost two thirds of the resistance to breathing. The nose is the primary passage by which air enters the lungs. Because of the surface area over the turbinates and the sinuses, the nasal passages are well suited for the task of humidification of air and primary filtration. As air passes through the nose, it meets the turbinates, which cause directional changes in the airflow. Branches of three arteries (e.g., the maxillary [sphenopalatine], ophthalmic, and facial [septal]) provide a rich supply of blood to the nasal mucosa. The innervation of the nose is from the nasopalatine and ethmoidal branches of the facial nerve. These nerves also supply the nasopharynx, nasal septum, and palate. Sensory-nerve supply to the nasal mucosa is from the ophthalmic and maxillary divisions of the trigeminal nerve. Parasympathetic innervation arises from the seventh cranial nerve and pterygopalatine ganglion. Sympathetic innervation is derived from the superior cervical ganglion. Sympathetic stimulation results in vasoconstriction and shrinkage of the nasal tissue. Depression of the sympathetic nervous system, as occurs with general anesthesia, may cause engorgement of the nasal tissues, increasing the likelihood of bleeding with manipulation from nasal airways or endotracheal tubes.
Pharynx
Two branches of the vagus nerve innervate the hypopharynx: the superior laryngeal nerve and the recurrent laryngeal nerve (RLN) (Figure 22-1). The superior laryngeal nerve divides into the internal and external branches. The internal branch of the superior laryngeal nerve provides sensory input to the hypopharynx above the vocal folds (cords). The external branch provides motor function to the cricothyroid muscle of the larynx.
Larynx
The larynx begins with the epiglottis and extends to the cricoid cartilage. The larynx is composed of (1) three single cartilages (e.g., thyroid, cricoid, and epiglottis), (2) three paired cartilages (e.g., arytenoid, corniculate, and cuneiform), and (3) intrinsic and extrinsic muscles (Figures 22-2 and 22-3). These structures function in an intricate manner to provide (1) protection to the lower airway from aspiration, (2) patency between the hypopharynx and trachea, (3) protective gag and cough reflexes, and (4) phonation. In the adult, the larynx begins between the third and fourth cervical vertebrae and ends at the level of the sixth cervical vertebra (e.g., cricothyroid muscle). The anterior and lateral larynx is formed by the thyroid cartilage. Anteriorly the thyroid cartilage fuses and forms the thyroid notch. Posteriorly the thyroid cartilage rises toward the hyoid bone at the base of the tongue as the posterior cornu. The thyroid cartilage is connected to the hyoid bone by the thyrohyoid fascia and muscles of the larynx.
The intrinsic muscles of the larynx control the tension of the vocal cords as well as the opening and closing of the glottis (Table 22-1). In contrast, the extrinsic muscles of the larynx connect the larynx, hyoid bone, and neighboring anatomic structures (Box 22-1). The primary function of the extrinsic muscles is to adjust the position of the trachea during phonation, breathing, and swallowing.
TABLE 22-1
Intrinsic Muscles of the Larynx
From Tarrazona V, Deslauriers J. Glottis and subglottis: a thoracic surgeon’s perspective. Thorac Surg Clin. 2007;17(4):561-570.
Lower Respiratory Tract
As the fetus develops, the respiratory system evolves into complex developmental interactions between the endodermal-derived epithelium and the mesoderm. Both contribute to lung development. The lungs and airways develop through a process of five stages. These include the embryonic, pseudoglandular, canalicular, terminal sac phases, and maturation.2
During the embryonic phase, the endodermal respiratory diverticulum (laryngotracheal groove) develops. This occurs during week 4 through week 7. The laryngotracheal groove develops from the ventral surface of the foregut. During this period, fibroblast growth factor (FGF-10) causes stimulation and proliferation of cells that will eventually express fibroblast homologous factor (FHF). As the laryngotracheal groove grows and develops, it becomes the primitive lung bud. By day 28, it has grown caudally to the splanchnic mesoderm. It divides into the right and left bronchial buds. This then progresses through the development and expression of the epithelial lining of the lower respiratory system. Cartilage, muscle, and connective tissue arise from the same tissues that form the smooth muscle of the blood vessels. The bronchopulmonary segments appear by day 42 of fetal development.
By the week 36, mature alveoli are seen. This requires FGF and platelet-derived growth factor (PDGF). Development of alveoli will continue for approximately 3 years after birth. A change in the relative relationship of parenchyma to total lung volume contributes to lung growth until the second year of life. From the third year of life until adulthood, lung growth continues.2
Trachea
The trachea originates at the inferior border of the cricoid cartilage and extends to the carina (Figure 22-4). It is approximately 10 to 20 cm long in adults. The cricoid cartilage is the only cartilage of the trachea that is a complete ring. The remainder of the trachea is composed of 16 to 20 C-shaped cartilaginous rings. The posterior side of the trachea lacks cartilage, thereby accommodating the esophagus during the act of swallowing. The cartilaginous rings and plates continue until the bronchi reach 0.6 to 0.8 mm in size. At this point the cartilage disappears, and the bronchi are termed bronchioles. The function of the bronchi is to provide humidification and warming of inspired air as it passes to the alveoli.
Diaphragm
The diaphragm arises from four structures: (1) septum transversum, (2) dorsal esophageal mesentery, (3) the pleuroperitoneal folds, and (4) the body-wall mesoderm. The diaphragm develops in the cephalic region and descends into the position between the abdominal and pleural cavity contents as the embryo develops. The nerve supply for the diaphragm arises from the cords of the third, fourth, and fifth cervical nerves and travels with the descending diaphragmatic structure as the phrenic nerve. Owing to this process of descent, the phrenic nerves lie within the pericardium as the fetus matures and after birth. Because of the development of the diaphragm in the cephalic position and the merging of four structures, drugs that impair fetal development can result in many potential congenital deformities, including diaphragmatic hernia.2
Airway Evaluation
Evaluation of the airway is central to any airway management plan. A proper airway evaluation should be conducted in a thorough and systematic fashion on every patient to determine potential problems. A substantial amount of research has been conducted over the past 20 years and has identified individual airway assessment examinations as poor and unreliable predictors of difficulty. No single examination has emerged that has a consistently high sensitivity and specificity with minimal false-positive or false-negative reports.3–6 Instead, many researchers have advocated for a more comprehensive airway management plan that involves the use of multiple airway tests.6,7 Furthermore, an appropriate airway management plan considers the availability of equipment and personnel needed in the event that difficult airway management is encountered. Additional airway anatomy and physiology can be found in Chapters 26 and 38.
Multiple rating systems have been developed that assist in the assessment and recognition of a difficult airway.8–11 However, no one rating system has proven to be superior over another. Instead, current recommendations concerning evaluation of the airway are to employ several assessments (Table 22-2). In order to recognize possible difficult airway conditions and to make “sensible airway management decisions,” these assessments should be tailored to the patient, operative procedure, and situation (e.g., outside of the operating room [OR]).6,12,13 It is generally agreed upon that an airway physical evaluation can improve the detection of a difficult airway. Then, after a thorough assessment, the anesthetist can formulate a comprehensive airway management plan that addresses the pertinent identified conditions.
TABLE 22-2
Components of the Preoperative Airway Physical Examination∗
| Airway Examination Component | Non-Reassuring Findings |
| Length of upper incisors | Relatively long |
| Relation of maxillary and mandibular incisors during normal jaw closure | Prominent “overbite” (maxillary incisors anterior to mandibular incisors) |
| Relation of maxillary and mandibular incisors during voluntary protrusion (ULBT) | Inability to protrude mandibular incisors anterior to maxillary incisors |
| Interincisor distance | Less than 3 cm |
| Visibility of uvula | Not visible when tongue is protruded with patient in sitting position (e.g., Mallampati class III or greater) |
| Shape of palate | Highly arched or very narrow |
| Compliance of mandibular space | Stiff, indurated, occupied by mass, or nonresilient |
| Thyromental distance | Less than three ordinary finger-breadths |
| Length of neck | Short |
| Thickness of neck | Thick |
| Range of motion of head and neck | Patient cannot touch tip of chin to chest or patient cannot extend neck |
∗This table displays some findings of the airway physical examination that may suggest the presence of a difficult intubation. The decision to examine some or all of the airway components shown in this table depends on the clinical context and judgment of the practitioner. The table is not intended as a mandatory or exhaustive list of the components of an airway examination. The order of presentation in this table follows the “line of sight” that occurs during conventional oral laryngoscopy.
From Berkow LC. Strategies for airway management. Best Pract Res Clin Anaesthesiol. 2004;18(4):531-548.
Because of the lack of a system that reliably and consistently predicts airway difficulty with 100% certainty, some authors encourage a more focused approach on “ventilatability” rather than “intubatability.”14 Instead of focusing on conditions that affect the ability to intubate only, Murphy and Walls15 advocated for a more all-encompassing assessment of the airway and described “four dimensions of difficulty” with airway management. The following four areas of airway management represent a modification of their “dimensions” and focus the airway assessment on conditions that could lead to difficulty with:
2. Direct laryngoscopy with direct tracheal intubation
3. Supraglottic airway ventilation
4. Invasive airway placement (e.g., needle cricothyrotomy, surgical cricothyrotomy)
A series of acronyms was developed to facilitate a thorough and systematic assessment of airway features (Box 22-2) that may lead to difficulty with hand mask, supraglottic device, endotracheal tube, and invasive airway ventilation and placement.15
Because a history of a difficult airway is a strong indication for current airway difficulties, an evaluation of the patient’s anesthetic history should be included in the airway assessment.16 A careful review of prior anesthetic records and information obtained directly from the patient or family members can reveal past difficulties and offer insight concerning specific techniques used to previously manage the patient’s airway. Clues that may indicate a history of difficult airway management may include chipped or broken teeth, bruised lips, previous sore throat after general surgery, past postoperative dysphonia, a memory of tracheal intubation, an unexpected admission to an intensive care unit, or a pharyngeal, esophageal, or tracheal perforation.16,17 Weight gain or loss can influence an airway and may not necessarily portray the same airway conditions as in the past. Furthermore, pathologies or conditions such as a tumor or hematoma, which may have previously caused difficulty, but have since been treated or removed, may not influence the current airway to the same degree they once did.
Multiple airway assessments exist that help the anesthetist predict difficulty with bag mask ventilation, direct laryngoscopy and tracheal intubation, supraglottic airway ventilation, and invasive airway placement (see Box 22-2). The following sections consider airway evaluations specific to each of the four areas of airway management.
Bag Mask Ventilation Assessment
Every anesthetist needs to possess adequate bag mask ventilation (BMV) skills. An adequate seal between the facemask and patient’s face is imperative. Proper BMV can be achieved by placing the left thumb and index finger around the body of the facemask at both the mask bridge and chin curve, while compressing the left side of the mask onto the face with the palm of the left hand (Figure 22-5). The middle and ring fingers can then be placed on the bony part of the mandible to help compress the mask to the patient’s face and to raise the chin. The fifth finger can be placed at the angle of the mandible to provide an anterior jaw-thrusting maneuver. Mask retaining straps can be placed behind the patient’s head and then be connected to the collar of the mask to apply pressure at various angles and promote a better mask seal.
If bag mask ventilation is believed to be inadequate, then a series of steps can be performed (Box 22-3) to facilitate ventilation. First, the patient’s head and neck can be repositioned into a sniffing position. Second, if the tongue or airway soft tissue is thought to be the cause of obstruction, then the placement of an oropharyngeal airway (OPA) may help bypass the obstruction. However, the placement of an OPA can be considered prior to any attempt at BMV. Third, if BMV continues to remain inadequate, then the anesthetist should perform two-handed mask ventilation by placing both thumbs on either side of the facemask bridge with both index fingers on either side of the mask chin curve on the body of the mask. The middle fingers can then be placed on either side of the mandible at the chin, the ring fingers on the bony part of the mandible, and the fifth fingers on each angle of the mandible to provide a jaw-thrust while maintaining a secure mask seal. A second anesthetist or assistant can then compress the anesthesia bag for ventilation (Figure 22-6). If ventilation continues to remain inadequate, then a supraglottic airway device may be placed while the decision to awaken the patient is considered. In the event that all of these maneuvers fail, then invasive airway ventilation should be considered.
The incidence of difficult BMV has been described as being between 0.9% and 7.8%, and the incidence of impossible BMV as 0.15%.18–22 Difficulty with BMV can result from problems in establishing an appropriate mask seal (such as with the presence of a beard). Inadequate ventilation during BMV is evidenced by (1) minimal or no chest movement; (2) inadequate or deficient exhaled carbon dioxide (e.g., lack of condensation and spirometic reading); (3) reduced or absent breath sounds; and (4) a decreasing oxygen saturation (e.g., less than 92%).12,19 If difficulty is believed to be a possibility with BMV, positioning can be improved by elevating the shoulders, neck, and head, which is known as “ramping” the patient (Figure 22-7). Multiple factors have been identified as predictors for difficulty with BMV (see Box 22-2); these include mask seal impediments, upper airway obstructions, obesity, elderly patients, Mallampati scores of III or IV, a short thyromental distance, snoring, and poor lung compliance.18,19,21,22
Obstruction may occur in the upper or lower airway and can severely limit the effectiveness of BMV. Upper airway obstructions should be considered an emergency and managed with extreme care because of the potential to become total airway obstructions. The hallmark signs of an upper airway obstruction in the unanesthetized patient include a hoarse or muffled voice, difficulty swallowing secretions, stridor, and dyspnea. Stridor and dyspnea are ominous signs of severe respiratory obstruction and indicate a 50% decrease in circumference from normal or a diameter reduced to 4.5 mm or less.15 There are several potential causes of an upper airway obstruction (Boxes 22-4 and 22-5). The management of these conditions is discussed later in this chapter under “Management of the Difficult and Failed Airway.” It is important to treat the airway with care because these conditions have the potential to severely impair ventilation and oxygenation, as well as cause difficulty with laryngoscopy and tracheal intubation.
