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24 Airway Management...

24 Airway Management Jeremy S. Heiner Effective airway management is a cornerstone of safe anesthesia practice, bone is preformed in cartilage. #e bones form initially from the optic, and an essential skill for all anesthesia providers. In addition to the man- olfactory, and otic capsules. #ese capsules merge with the midline car- aging the operating room (OR), nurse anesthetists are also responsible for tilaginous structures to form the embryologic vestiges of the ethmoid, managing the airway in many other health care settings. Maintenance of the sphenoid, the petrous portion of the temporal bone, and the base oxygenation and ventilation are primary goals during airway management of the occipital bone. Direct ossi%cation of membranous tissue known for both difficult and routine (nondifficult) airways. Proper management as the mesenchyme occurs during early embryologic development to of a difficult airway occurs as a result of experience and regular manage- form membranous bone. #e membranous bones include the temporal ment of normal airways, thoughtful planning, and effective preparation. bone, the parietal bone, the frontal bone, and portions of the occipital #erefore it is important that anesthesia providers become familiar with bones and the pharyngeal arches. #e pharyngeal arches are complex appropriate decision-making strategies and methods for providing ade- structures also known as the branchial arches that extend anterior to quate ventilation during airway management. #ese strategies and meth- posterior. Development of these structures begins at day 22 (week 4 ods pertain not only to routine airways but also to anticipated difficult after fertilization).1 airways, unanticipated difficulty with intubation and/or ventilation, failed Embryologically, there are six arches that develop from %ve struc- airways, patients at risk for aspiration of gastric contents, and patients tures. #e %rst through fourth arches and the sixth arch go on to who present with any other type of airway compromise. develop the airway structures, and the %fth arch disappears with fetal An understanding of airway anatomy and appropriate airway assess- development. #e arches all contain a covering of tissue that will even- ment techniques facilitates the development of a comprehensive airway tually become the nerves, muscles, and cartilage of the airway. #ese management plan. Complete and thorough assessment of the airway will become the tissues of the oropharynx, the middle ear, the hyoid guides management plans, which may involve placing an airway while bone, and the laryngeal cartilages. #e %rst arch becomes the jaws, the patient is awake or after the induction of anesthesia, using any one the second arch becomes the facial structures and the ears, the third of a variety of airway adjuncts. Familiarization with the different airway arch becomes the hyoid bone and structures of the upper pharynx, the adjuncts is important so that anesthesia providers may become com- fourth and sixth arches become the structures of the larynx and the fortable with their use in a variety of airway management situations and lower pharynx, and the %fth (as mentioned) disappears. #e tongue is changing airway dynamics, in order to facilitate safe practices for the formed from the mesoderm of multiple arches. #e anterior two-thirds establishment of a protected airway. of the tongue is developed from the %rst arch. #e mesoderm of the Removal of an airway management device should be part of the over- third and fourth arches comprises the posterior third of the tongue. all airway management plan. Consideration should be given to patient, Spaces found between the arches are known externally as clefts and surgical, and anesthetic risk factors before removing any airway adjunct. internally as pouches. #e cleft between the %rst two arches becomes Ultimately, the removal of an airway is determined by the patient’s ability the external auditory meatus. #e internal pouch between the %rst to meet extubation criteria and maintain adequate spontaneous venti- and second arches forms the majority of the tympanic cavity and the lation and oxygenation. Finally, it is important to understand the risk eustachian tubes. #e other clefts disappear as the fetus develops. #e factors and complications of airway management in order to develop the pouches contribute to the development of the glandular structures of most appropriate and safe airway management plan for the patient. the head and neck. #e palatine tonsils arise from the second pouch, the inferior parathyroid glands and the thymus arise from the third pouch, the superior parathyroid glands arise from the fourth pouch, ANATOMY AND PHYSIOLOGY OF THE AIRWAY and the ultimobranchial structures arise from the inferior portion of #e airway is divided into upper and lower sections. #e cricoid carti- the fourth pouch as well.1 lage separates the upper and lower airways. #us anatomic structures Nose. #e nose and mouth are the external openings to the included in the upper airway are the nose, mouth, pharynx, hypophar- respiratory tree. #e large surface area of the nasal mucosa warms and ynx, larynx, and cricoid cartilage. Anatomic structures that constitute humidi%es inspired air while causing signi%cant resistance to breathing. the lower airway include trachea, bronchi, bronchioles, terminal bron- #e nose is the primary passage by which air enters the lungs. Because chioles, respiratory bronchioles, and alveoli. #is section reviews pri- of the surface area over the turbinates and the sinuses, the nasal passages mary structures, innervation, blood supply, and normal and abnormal are well suited for the humidi%cation of air and primary %ltration. functions of the upper airway structures. As air passes through the nose, it meets the turbinates, which cause directional changes in the air&ow. Branches of three arteries (e.g., the Developmental Anatomy maxillary [sphenopalatine], ophthalmic, and facial [septal]) provide a Upper Respiratory Tract rich supply of blood to the nasal mucosa. #e innervation of the nose Unlike the structures of the lower respiratory tract, the upper respi- is from the maxillary and opthalmic branches of the Trigeminal nerve. ratory tract arises from bony structures of the head. Endochondral #ese nerves also supply the nasopharynx, nasal septum, and palate. 429 430 UNIT V Intraoperative Management Parasympathetic innervation arises from the seventh cranial nerve and the pterygopalatine ganglion. Sympathetic innervation is derived Sup. laryngeal n. from the superior cervical ganglion. Sympathetic stimulation results in vasoconstriction and shrinkage of the nasal tissue. Depression of Int. laryngeal n. the sympathetic nervous system, as occurs with general anesthesia, Ext. laryngeal n. may cause engorgement of the nasal tissues, resulting in a potential increase in bleeding during manipulation of nasal airways. Carotid artery Mouth. #e oral cavity is separated from the nasal passages by Vagus n. the hard and soft palates. #e hard palate is stationary and remains in the same position. #e soft palate covers the posterior third to half of the oral cavity. #e soft palate rises during eating to prevent Recurrent Recurrent food and liquids from passing from the mouth into the nose, and laryngeal n. laryngeal n. thereby decreases the chance of aspiration. With age, obesity, and other conditions, this structure may stretch and become more movable. When an individual is asleep or paralyzed, as with general anesthesia, this structure can fall back against the nasal passages, Recurrent blocking air movement and causing symptoms similar to sleep laryngeal n. apnea. #e tongue is a large muscular organ that %lls most of the oral cavity and is involved in the tasting and ingestion of food. It relaxes when the individual is either asleep or paralyzed, which Fig. 24.1 Anatomy of the right and left, superior and recurrent laryngeal increases the potential for airway obstruction. #e uvula protects the nerves. ext, External; int, internal; n, nerve; sup, superior. passageway from the oral cavity into the oropharynx. #is pendulous piece of tissue extends from the posterior edge of the middle of the soft palate into the oral cavity. If swollen, enlarged, or injured, it can cavity. Afferent (sensory) stimuli elicited when the posterior wall of be a cause of airway obstruction. #e palatine tonsils are walnut- the pharynx is touched are carried by the glossopharyngeal nerve to shaped structures that sit on both sides of the posterior opening the medulla, where they synapse with nuclei of the vagus nerve and the of the oral cavity. #ey are partially buried in the soft tissue at the cranial portion of the spinal accessory nerve. #e efferent response base of the tongue and are protected by the anterior and posterior returns primarily through the vagus nerve, resulting in the gag re&ex as tonsillar pillars. Underdevelopment of the tongue, maxilla, and/ the muscles of the pharynx elevate and constrict. or mandible can result in upper airway obstructive disorders such Two branches of the vagus nerve innervate the hypopharynx: the as Pierre Robin sequence, Apert syndrome, and Treacher Collins superior laryngeal nerve (SLN) and the recurrent laryngeal nerve syndrome. Other disorders, such as Beckwith-Wiedemann syndrome (RLN) (Fig. 24.1). #e superior laryngeal nerve divides into the inter- and Down syndrome, can result in airway obstruction as a result of nal and external branches. #e internal branch of the SLN provides sen- macroglossia.2 sory input to the hypopharynx above the vocal cords, which includes Pharynx. #e pharynx is divided into three compartments: the base of tongue, epiglottis, aryepiglottic folds, and arytenoids. #e nasopharynx, oropharynx, and hypopharynx (laryngopharynx). #e external branch provides motor function to the cricothyroid muscle of pharynx extends from the base of the skull to the level of the cricoid the larynx. cartilage. #e nasopharynx lies anterior to C1 and is bound superiorly #e RLN provides sensory innervation to the subglottic area and by the base of the skull and inferiorly by the soft palate. #e openings the trachea. #e RLN is so named because it recurs (loops) around to the auditory (eustachian) tubes and the adenoids are found in the other structures. #e right RLN branches from the vagus nerve and nasopharynx. Sensory innervation of the mucosa is derived from recurs (loops) around the right subclavian artery, and the left RLN the maxillary division of the trigeminal nerve. #e oropharynx lies at branches from the vagus nerve and recurs (loops) around the aortic the C2 to C3 vertebral level, and it is bound superiorly by the soft palate arch. Traction on either of these structures during thoracic surgery and inferiorly by the epiglottis. It opens, from the oral cavity, through can cause injury to the RLN, causing hoarseness, stridor, or possi- the anterior tonsillar pillar (i.e., the palatoglossus muscle) and posterior bly respiratory distress. #e motor component of the RLN provides tonsillar pillar (i.e., palatopharyngeus muscle). #is opening into the motor function to all the muscles of the larynx except the cricothy- oropharynx is also known as the fauces. #e fauces is located directly roid muscle. posterior to the oral cavity, with its borders including the soft palate #e SLN and RLN may be damaged by surgery, neoplasms, and superiorly, the palatoglossal and palatopharyngeal arches (i.e., anterior neck trauma. Dissecting aortic arch aneurysms and mitral stenosis and posterior tonsillar pillars) laterally, and the tongue inferiorly. can place pressure on the left RLN, causing hoarseness. Unilateral Identi%cation of the fauces is a part of the Mallampati classi%cation RLN injury usually results in hoarseness but does not compromise during airway assessment. #e hypopharynx lies posterior to the respiratory status. #e vocal cords compensate by shifting the mid- larynx, and it is bound by the superior border of the epiglottis and the line toward the uninjured side. In the acute phase of bilateral injury inferior border of the cricoid cartilage at the C5 to C6 level. #e upper to the RLN, unopposed tension and adduction of the vocal cords esophageal sphincter lies at the lower edge of the hypopharynx and result in stridor, which may deteriorate into severe respiratory dis- arises from the cricopharyngeus muscle. #is muscle acts as a barrier tress and possibly death. Patients with chronic injury develop com- (within the upper esophagus) to prevent regurgitation in a conscious pensatory mechanisms that allow for normal respiration and gruff or patient. husky speech. Injury to the superior laryngeal nerve is not associated Numerous nerves supply motor and sensory %bers to the airway. with respiratory distress. #e glossopharyngeal, vagus, and spinal accessory nerves share nuclei Larynx. #e larynx begins with the epiglottis and extends to the in the medulla and innervate all the muscles of the pharynx, larynx, cricoid cartilage. #e larynx is composed of (1) three single cartilages and soft palate. Branches of the trigeminal nerve innervate the nasal (e.g., thyroid, cricoid, and epiglottis), (2) three paired cartilages (e.g.,  CHAPTER 24 Airway Management 431 Epiglottis tongue to the posterior cornu. #e thyroid cartilage is connected to the Hyoid hyoid bone by the thyrohyoid fascia and muscles of the larynx. C3 Thyrohyoid #e posterior portion of the thyroid and cricoid cartilages forms the membrane posterior border of the larynx. Internal to the larynx are the epiglottis C4 and three paired cartilages. #e epiglottis exists as a single lea&ike car- Superior horn Thyroid notch tilage. #e epiglottis rests above the glottic opening, where it closes the Body of thyroid glottic aperture during swallowing. #e superior vallecula is formed by cartilage the space between the epiglottis and the base of the tongue. #e infe- Cricothyroid rior vallecula is formed by the space between the inferior edge of the membrane Cricoid epiglottis and the true vocal cords. C6 #e intrinsic muscles of the larynx control the tension of the vocal Cricotracheal cords, as well as the opening and closing of the glottis (Table 24.1). In membrane contrast, the extrinsic muscles of the larynx connect the larynx, hyoid A bone, and neighboring anatomic structures (Box 24.1). #e primary Epiglottis function of the extrinsic muscles is to adjust the position of the larynx during phonation, breathing, and swallowing. Hyoid Blood supply to the larynx originates from the external carotid, which branches into the superior thyroid artery. #e superior thyroid Aryepiglottic Thyrohyoid artery eventually gives rise to the superior laryngeal artery, which sup- fold membrane plies blood to the supraglottic region of the larynx. #e inferior laryn- geal artery, a terminal branch of the inferior thyroid artery, supplies the infraglottic region of the larynx. Corniculate cartilage Lower Respiratory Tract Muscular process of Arytenoid As the fetus develops, the respiratory system evolves into complex arytenoid cartilage developmental interactions between the endodermal-derived epithe- lium and the mesoderm. Both contribute to lung development. #e Ligaments of lungs and airways develop through a %ve-stage process. #ese include cricothyroid the embryonic, pseudoglandular, canalicular, and terminal sac phases, articulation and maturation.3 During the embryonic phase, the endodermal respiratory divertic- Cricoid ulum (laryngotracheal groove) develops. #is occurs during weeks 4 lamina through 7. #e laryngotracheal groove develops from the ventral sur- face of the foregut. During this period, %broblast growth factor (FGF- 10) causes stimulation and proliferation of cells that will eventually B express %broblast homologous factor (FHF). As the laryngotracheal Fig. 24.2 (A) Anterior view of the laryngeal cartilages. External frontal groove grows and develops, it becomes the primitive lung bud. By (left) and anterolateral (right) views of the larynx. Notice the location of day 28, it has grown caudally to the splanchnic mesoderm. It divides the cricothyroid membrane and thyroid gland in relation to the thyroid into the right and left bronchial buds. #is then progresses through and cricoid cartilages in the frontal view. The horn of the thyroid carti- the development and expression of the epithelial lining of the lower lage is also known as the cornu. In the anterolateral view, the shape of the cricoid cartilage and its relation to the thyroid cartilage are shown. respiratory system. Cartilage, muscle, and connective tissue arise from (B) Posterior view of laryngeal cartilages. Cartilages and ligaments of the same tissues that form the smooth muscle of the blood vessels. #e the larynx are seen posteriorly. Notice the location of the corniculate bronchopulmonary segments appear by day 42 of fetal development. cartilage within the aryepiglottic fold. (Modified from Ellis H, Feldman During the pseudoglandular stage, there is rapid growth and prolif- S. Anatomy for Anaesthetists. 6th ed. Oxford, UK: Blackwell Scientific; eration of the peripheral airways. #is occurs during weeks 6 through 1993; Hagberg CA. Hagberg and Benumof’s Airway Management. 4th 16. Repeated branching of the distal ends of the epithelial tubes results ed. Philadelphia: Elsevier; 2018.) in 16 or more generations of the bronchial tubes and the develop- ment of the terminal bronchioles. #e airways are %lled with liquid at this time. #e cellular structure is characterized by tall columnar arytenoid, corniculate, and cuneiform), and (3) intrinsic and extrinsic epithelium. muscles (Figs. 24.2 and 24.3). #ese structures function in an intricate #e next phase of development is known as the canalicular stage. manner to provide (1) protection to the lower airway against aspiration, #is occurs most often during weeks 16 through 26, when the airways (2) patency between the hypopharynx and trachea, (3) protective gag widen and lengthen. #is space will eventually become the large vol- and cough re&exes, and (4) phonation. In the adult, the larynx begins ume of air space in the expanded lung after birth. Terminal and respi- between the third and fourth cervical vertebrae and ends at the level ratory bronchioles along with terminal saccules develop. Cuboidal cells of the sixth cervical vertebra (e.g., cricothyroid muscle). #e anterior of the terminal sacs differentiate into alveolar type II cells. Secretion and lateral aspects of the larynx are formed by the thyroid and cricoid of surfactant begins at this time. Type II alveolar cells that are adja- cartilages. Anteriorly, the thyroid cartilage fuses and forms the thyroid cent to a vessel &atten and differentiate into type I cells. As the type notch, which connects to the hyoid bone superiorly by the thyrohyoid II and type I cells develop, vascularization appears. #e vasculariza- membrane. #e thyroid cartilage connects to the cricoid cartilage tion is associated with the development of the respiratory bronchioles anteriorly and inferiorly by the cricothyroid membrane. Posteriorly, and the alveoli necessary for air exchange after birth. Along with other the thyroid cartilage rises toward the hyoid bone at the base of the growth factors, vascular endothelial growth factor participates in the 432 UNIT V Intraoperative Management Internal laryngeal nerve (pulled forward) Aryepiglottic muscle Thyroepiglottic muscle Oblique and Thyroid transverse cartilage interarytenoid Posterior Thyroarytenoid cricoarytenoid muscle muscle Lateral cricoarytenoid Cricoid muscle Cricothyroid cartilage muscle Tracheal ring Recurrent laryngeal nerve A B Fig. 24.3 Posterior view of the larynx showing the laryngeal cartilages and muscles. (A) Intrinsic muscles of the larynx and their nerve supply. (B) The cricothyroid muscle and its attachments. (Modified from Ellis H, Feldman S. Anatomy for Anaesthetists. 6th ed. Oxford, UK: Blackwell Scientific; 1993; Hagberg CA. Hagberg and Benumof’s Airway Management. 4th ed. Philadelphia: Elsevier; 2018.) TABLE 24.1 Intrinsic Muscles of the Larynx BOX 24.1 Extrinsic Muscles of the Larynx Muscles Involved Nervous Innervation Main Function Muscles That Elevate the Larynx Posterior Recurrent laryngeal nerve Abduction of vocal Suprahyoid Muscles cricoarytenoids cords Stylohyoid Digastric Lateral cricoarytenoids Recurrent laryngeal nerve Adduction and Mylohyoid lengthening of Geniohyoid arytenoids causing glottis closure Pharyngeal Muscle Interarytenoid Recurrent laryngeal nerve Closing of posterior Stylopharyngeus (transverse and commissure of the oblique arytenoids) glottis causing vocal Infrahyoid Muscle cord narrowing Thyrohyoid: depresses hyoid, causing laryngeal elevation Thyroarytenoids and Recurrent laryngeal nerve Shortening (reduced vocalis tension) and Muscles That Depress the Larynx adduction of vocal Infrahyoid Muscles cords Omohyoid Sternohyoid Aryepiglottic Recurrent laryngeal nerve Constriction of Sternothyroid laryngeal vestibule Cricothyroids Superior laryngeal nerve Adduction and From Tarrazona V, Deslauriers J. Glottis and subglottis: a thoracic (external branch) increasing tension surgeon’s perspective. Thorac Surg Clin. 2007;17:561−570. of vocal cords From Tarrazona V, Deslauriers J. Glottis and subglottis: a thoracic surgeon’s perspective. Thorac Surg Clin. 2007;17:561−570. By week 36, mature alveoli can be seen. #is requires FGF and platelet-derived growth factor. Development of alveoli will continue for approximately 3 years after birth. A change in the relative relationship formation of blood vessels that will surround the alveoli. At the end of of parenchyma to total lung volume contributes to lung growth until this phase, air exchange is possible, though inefficient. the second year of life. From the third year of life until adulthood, lung #e terminal sac phase occurs during weeks 24 through 36. Branch- growth continues.3 ing of the respiratory bud continues, and further development of the Trachea. #e trachea originates at the inferior border of the cricoid terminal buds is expressed as primitive alveoli. Capillaries begin to cartilage and extends to the carina (Fig. 24.4). It is approximately 10 to develop around the terminal buds and proliferate at the same time as 20 cm long in adults. #e cricoid cartilage is the only cartilage of the the primitive alveoli develop. Cells further differentiate throughout this trachea that is a complete cartilaginous ring. #e remainder of the period, and by week 26 a primitive blood-gas barrier has developed. trachea is composed of 16 to 20 C-shaped cartilaginous rings. #e  CHAPTER 24 Airway Management 433 Hyoid bone cervical nerves, and travels with the descending diaphragmatic struc- Thyrohyoid ture as the phrenic nerve. #e phrenic nerves lie within the pericar- membrane dium 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 potential Thyroid Superior congenital deformities, including diaphragmatic hernia.3 cartilage thyroid notch Cricothyroid AIRWAY EVALUATION membrane Evaluation of the airway is central to any airway management plan. A Cricoid cartilage proper airway evaluation should be conducted in a thorough and sys- tematic fashion to determine potential problems. A substantial amount of research over the past 20 years has identi%ed how individual airway examinations can be poor and unreliable predictors of difficulty. Cur- rently, no single examination consistently demonstrates high sensitivity Trachea and speci%city with minimal false-positive or false-negative reports.4,5 Instead, many researchers have advocated for a more comprehensive airway management plan that involves the use of multiple airway assess- ments to identify a difficult airway.6-8 One area of airway management that may now render some airway examinations obsolete is the advent of Carina videolaryngoscopy (VL). Ultimately, an appropriate airway management Right main plan considers multiple airway examinations, time constraints, the avail- Left main bronchus bronchus ability of equipment, and presence of experienced personnel. Current recommendations concerning evaluation of the airway are to employ several preoperative assessments (Table 24.2).8-11 To recog- nize possible difficult airway conditions, and to make “sensible airway management decisions,” airway assessments should be tailored to the Fig. 24.4 Principal features of the larynx and trachea (anterior view). patient, the operative procedure, and the situation to formulate a com- Tracheobronchial angles vary widely. The angle of the right mainstem prehensive airway management plan.11,12 bronchus is approximately 25 to 30 degrees, whereas the angle of the left mainstem bronchus is 45 degrees. (From Minnich DJ, Mathisen #ere is no system that reliably and consistently predicts airway dif- DJ. Anatomy of the trachea, carina, and bronchi. Thorac Surg Clin. %culty with 100% certainty.13 Instead of focusing on conditions that 2007;17:571–585.) affect the ability to intubate only, a more all-encompassing assessment of the airway has been described.14 #e following four areas of airway management focus the airway assessment on conditions that could posterior side of the trachea is formed by the trachealis muscle. It lacks lead to difficulty with: cartilage and instead it connects the free ends of the tracheal cartilages 1. Bag-mask ventilation (BMV) with smooth muscle, which helps to accommodate the esophagus during 2. Direct laryngoscopy (DL) and VL, including endotracheal tube the act of swallowing. #e primary function of the trachealis muscle is (ETT) delivery and tracheal intubation (TI) to constrict the trachea causing air to more forcibly exit the trachea (i.e., 3. Supraglottic airway ventilation during coughing). #e cartilaginous rings and plates continue deeper 4. Cricothyrotomy airway placement (e.g., needle, percutaneous or into the tracheal bronchial tree until the bronchi reach 0.6 to 0.8 mm in surgical) size. At this point, the cartilage disappears, and the bronchi are termed A series of acronyms was developed to facilitate a thorough and bronchioles. #e function of the bronchi is to provide humidi%cation systematic assessment of airway features (Box 24.2) that may lead to and warming of inspired air as it passes to the alveoli. difficulty with hand mask, supraglottic device, ETT, and cricothyrot- #e angle of bifurcation of the right mainstem bronchus is approx- omy airway ventilation and placement.14 imately 25 to 30 degrees. #e bifurcation to the right upper lobe is In addition to an airway examination, a history of a difficult air- approximately 2.5 cm from the carina. #e angle of the left mainstem way is a strong indication for current airway difficulties. #erefore an bronchus is 45 degrees. #e left mainstem bronchus is approximately evaluation of the patient’s anesthetic history should be included in the 5 cm long, before it bifurcates into the left superior and inferior lobe airway assessment.15 A careful review of prior anesthetic records and bronchi. information obtained directly from the patient or family members can #e tracheobronchial tree receives sympathetic innervation from reveal past difficulties and offer insight concerning speci%c techniques the %rst through %fth thoracic ganglia. Parasympathetic innervation is used to previously manage the patient’s airway. Clues that may indicate derived from branches of the vagus nerve. #e carina is richly inner- a history of difficult airway management may include chipped or bro- vated, making it extremely sensitive to sensory stimulation. ken teeth, bruised lips, previous sore throat after general surgery, past postoperative dysphonia, a memory of TI, an unexpected admission to Diaphragm an intensive care unit (ICU), or a pharyngeal, esophageal, or tracheal #e diaphragm arises from four structures: (1) the septum transver- perforation.15,16 Weight gain or loss can in&uence airway anatomy, and sum, (2) the dorsal esophageal mesentery, (3) the pleuroperitoneal the patient may not necessarily present with the same airway condi- folds, and (4) the body wall mesoderm. #e diaphragm develops in tions as in the past. Furthermore, pathologies or conditions such as an the cephalic region and descends into position between the abdominal airway tumor or hematoma, which may have previously caused diffi- and pleural cavity contents as the embryo develops. #e nerve supply culty but have since been treated or removed, may not in&uence the for the diaphragm arises from the cords of the third, fourth, and %fth current airway to the same degree as in the past. 434 UNIT V Intraoperative Management TABLE 24.2 Components of the BOX 24.2 Four Areas of Airway Preoperative Airway Physical Examination* Management With Factors Associated With Airway Examination Indication of Airway Difficulty Component Difficulty Indication of Difficulty With Bag Mask Ventilation Length of upper incisors Relatively long Mask seal impeded by beards, altered anatomy, or nasogastric tubes Relation of maxillary and Prominent “overbite” (maxillary incisors Obstruction of the upper or lower airway mandibular incisors during anterior to mandibular incisors) Obesity with redundant upper airway soft tissue and greater chest and normal jaw closure abdominal mass compressing the lungs Age >55 related to loss of upper airway tissue elasticity Relation of maxillary and Inability to protrude mandibular incisors No teeth, leading to improper facial structure for the bag mask mandibular incisors during anterior to maxillary incisors Stiff lungs (e.g., increases in airway resistance and decreases in pulmonary voluntary protrusion (ULBT) compliance) Interincisor distance 43 cm in circumference) Neck mobility that is impaired by disease of immobilization (direct Range of motion of head and neck Patient cannot touch tip of chin to chest laryngoscopy) or patient cannot extend neck Operator experience *This table displays some findings of the airway physical examination Indication of Difficulty With Supraglottic Airway Device that may suggest difficulty with laryngoscopy and intubation. The deci- Placement and Ventilation sion to examine some or all of the airway components shown in this Restricted mouth opening table depends on the clinical context and judgment of the practitioner. The table is not intended as a mandatory or exhaustive list of the com- Obstruction of the upper airway ponents of an airway examination. The order of presentation in this Distortion of airway anatomy preventing an adequate seal table follows the “line of sight” that occurs during conventional oral Stiff lungs (e.g., increases in airway resistance or decreases in pulmonary laryngoscopy. compliance) ULBT, Upper lip bite test. From Berkow LC. Strategies for airway management. Best Pract Res Indication of Difficulty With Cricothyrotomy Airway Clin Anaesthesiol. 2004;18(4):531–548. Placement Distortion of neck anatomy (e.g., hematoma, infection, abscess, tumor, scarring from radiation) Multiple airway assessments exist that help the anesthetist predict Obesity or a short neck limiting cricothyroid identification difficulty with BMV, DL and VL with TI, supraglottic airway ventila- Trauma in or around the cricothyroid area tion, and cricothyrotomy airway placement (see Box 24.2). #e follow- Impediments causing limited access to the neck (e.g., halo device, fixed ing sections consider airway evaluations speci%c to each of the four flexion abnormality) areas of airway management. Surgery causing limited access to anatomic landmarks * 3-3-2 rule = three fingerbreadths between incisors; three Bag-Mask Ventilation Assessment fingerbreadths between tip of the chin (mentum) and chin-neck Adequate BMV skills are important for effective airway management. junction (hyoid bone); two fingerbreadths between chin-neck junction A sufficient seal between the facemask and the patient’s face is impera- (hyoid bone) and thyroid notch. tive. Proper BMV can be achieved by placing the left thumb and index Adapted from Walls RM, Murphy MF, eds. Manual of Emergency %nger around the collar of the facemask at both the mask bridge and Airway Management. 4th ed. Philadelphia: Lippincott Williams & Wilkins; 2012:8–21. chin curve, while compressing the left side of the mask onto the face with the palm of the left hand (Fig. 24.5). #e middle and ring %ngers can then be placed on the bony part of the mandible to help compress then the placement of an oropharyngeal airway (OPA) may help bypass the mask to the patient’s face and to raise the chin. #e %fth %nger the obstruction. However, the placement of an OPA can be considered can be placed at the angle of the mandible to provide an anterior jaw- prior to any attempt at BMV. #ird, if BMV continues to remain inad- thrusting maneuver. Mask-retaining straps can be placed behind the equate, then place both thumbs on either side of the facemask bridge patient’s head and then be connected to the collar of the mask to apply with both index %ngers on either side of the mask chin curve on the pressure at various angles and promote a better mask seal. body of the mask. #e middle %ngers can then be placed on either side If BMV is believed to be inadequate, then a series of steps can be of the mandible at the chin, the ring %ngers on the bony part of the performed (Box 24.3) to facilitate ventilation. First, the patient’s head mandible, and the %fth %ngers on each angle of the mandible to pro- and neck can be repositioned into a sniffing position. Second, if the vide a jaw thrust while a second provider in&ates and depresses the tongue or airway soft tissue is thought to be the cause of obstruction, anesthesia bag. Alternatively, a second provider may stand at the side  CHAPTER 24 Airway Management 435 A B C Fig. 24.5 Suggested hand technique for facemask ventilation. (A) The thumb, index finger, and thenar emi- nence of the palm encircle the collar, while the hypothenar eminence extends below the mask. (B) One- handed mask technique. The thumb and index finger encircle the collar of the mask, while the middle finger grasps the mental protuberance (chin), and the ring and little fingers grip the body of the mandible to pull the mask securely onto the face while gently extending the head. (C) Jaw thrust with one-handed mask technique. Middle and ring fingers grasp the mandible and pull upward into the mask, while the little finger is moved to the angle of the jaw and pulls backward and upward to maintain subluxation (jaw thrust). (From Hagberg CA. Benumof and Hagberg’s Airway Management. 3rd ed. Philadelphia: Elsevier; 2013:335.) awaken the patient is considered. Consider laryngoscopy, using either BOX 24.3 Proper Sequence for Bag-Mask direct or video technique, and tracheal intubation if it has not previ- Ventilation When Difficulty Is Encountered ously been attempted. In the event that all these maneuvers fail, then a 1. Assure proper positioning of the patient that avoids compression of the cricothyrotomy should be considered. airway. #e incidence of difficult BMV has been described as being between 2. Attempt ventilation with one (left) hand (can consider use of mask strap on 0.9% and 7.8%, and the incidence of impossible BMV as 0.15%.17-21 opposite side of mask [right] to improve mask seal). Difficulty with BMV can occur because of an inappropriate mask seal 3. Reposition the head into a sniffing position. (such as with the presence of a beard). Inadequate ventilation during 4. Place oropharyngeal airway (may consider earlier placement). BMV is evidenced by (1) minimal or no chest movement, (2) inade- 5. Proceed with two-handed mask ventilation while assistant compresses quate or de%cient exhaled carbon dioxide (e.g., lack of condensation ventilation bag. and spirometric reading), (3) reduced or absent breath sounds, and 6. Consider placement of a supraglottic device and consider awakening the (4) a decreasing oxygen saturation (e.g., 30 kg/m2) in both obese and lean patients. However, the authors observed that Tumors or lesions Pregnancy apnea-induced desaturation occurred more quickly, and hypoxemia Trauma was more profound, in the obese population.23 Time to desaturation should be considered in the overall airway management plan because From Berkow LC. Strategies for airway management. Best Pract Res any time delay after the induction of anesthesia in the obese population Clin Anaesthesiol. 2004;18(4):531–548. may result in signi%cant hypoxemia. Redundant tissue of the upper airway may be another factor that the cause of difficulty and obstruction. Patients with a history of OSA leads to difficulty with BMV because excessive soft tissue in the oro- should be encouraged to bring their positive pressure device (e.g., con- pharyngeal and pharyngeal cavities can cause resistance to air&ow tinuous positive airway pressure [CPAP] or biphasic positive airway during positive pressure ventilation.14,24 #erefore ideal positioning pressure [BiPAP] machine) from home to the hospital for use in the (e.g., ramping the patient), adequate preoxygenation, a secondary air- postanesthesia care unit.22 way management plan with alternative airway adjuncts readily avail- Signi%cant obesity (body mass index >30 kg/m2) has been identi%ed able, and assistance from other anesthesia professionals should be as a potential risk factor for difficult BMV.17,18,21 Difficulty with BMV considered prior to the induction of anesthesia in the obese patient. 438 UNIT V Intraoperative Management Obesity is a risk factor for both effective ventilation and laryngos- A primary goal during airway evaluation is to determine factors copy because excessive upper airway soft tissue may inhibit direct visu- that predispose a patient to difficulty with DL, VL, and TI. #e dis- alization of the glottis. Morbid obesity, now known as class 3 obesity or tinction among these procedures has not always been clear in the “severe obesity”, is generally associated with a body mass index greater literature, and the de%nition of difficulty can vary. However, the inci- than 40 kg/m2.25,26 dence of difficult TI has been described to be between 1.5% and 8.5%, and the incidence of failed TI has been estimated to be between 0.3% Apneic Oxygenation and 0.5%.6,28-30 #e actual incidence of failure will vary depending Apneic oxygenation is a strategy used to provide a patient with oxygen on patient characteristics and the current situation (i.e., obstetric during times of apnea, and more speci%cally during intubation of the emergency, airway trauma, airway pathology). As described earlier, trachea. Even without lung expansion and diaphragmatic movements, individual airway assessments have not demonstrated an ability to alveoli will continue to receive oxygen if a higher concentration gradient reliably and consistently predict difficulty. #erefore, multiple airway exists in the upper respiratory areas. Approximately 250 mL/min of oxy- assessments should be used when evaluating for potential difficulty gen diffuses from the alveoli into the bloodstream during apnea, whereas with DL, VL, and TI. only 8 to 20 mL/min of carbon dioxide is transferred into the alveoli. #is Combinations of difficult airway assessments have been used difference is the result of blood gas solubility and hemoglobin’s affinity with varying degrees of predictive success.8,28,31-33 A significant for oxygen which causes the alveoli’s net pressure to become slightly concern when performing the different airway assessments is the subatmospheric, facilitating gas to &ow from the pharynx into the alve- variation in measurements.6 The variability among practitioners oli. Apneic oxygenation can sustain a patient’s partial pressure of arte- when performing airway evaluation techniques alters the validity rial oxygen (Pa(2) for signi%cant amounts of time without ventilation. of findings between studies. This highlights the importance of con- However, hypercapnia and acidosis are probable if either spontaneous ducting airway evaluations the way they were intended every time. or assisted breaths are not administered within a reasonable amount of The most commonly cited airway assessments used to evaluate for time. #e process of providing apneic oxygenation during an intubation DL, VL, and TI include (1) the modified Mallampati classification, procedure requires a patent upper airway and a nasal cannula. Initially (2) thyromental distance (TMD), (3) interincisor gap distance, (4) the nasal cannula can be placed under an anesthesia facemask during atlantooccipital joint mobility and cervical range of motion, (5) preoxygenation and is then set to a high &ow rate (15 L/min) during mandibular protrusion test, (6) evaluation for obstruction of the apnea to drive oxygen into the hypopharynx and become entrained upper airway, and (7) measurement of neck circumference (specifi- within the trachea. #e effects of such high nasal cannula oxygen &ows cally for obese patients). on airway tissues can be minimized as long as this technique is utilized for short-term apneic episodes. Apneic oxygenation may be considered Mallampati Classification for use in obese patients who require intubation or in other patients who #e Mallampati classi%cation is a commonly used technique of assess- are at risk for rapid desaturation during apnea.27 ing the mouth opening, size of the tongue, size of the oral pharynx, and Advanced age (>55 years) is associated with difficult BMV.17,18 Older posterior oropharyngeal structures. First developed by Rao Mallam- patients may have decreased upper airway muscle tone as a result of inelas- pati, this classi%cation originally described three classes.34 Later, Sam- tic or aged tissue. In addition, elderly patients commonly have loose or soon and Young35 added a fourth class, which is now often referred to missing teeth, or are edentulous, which can cause the airway soft tissue to as the modi%ed Mallampati test (MMT) (Fig. 24.9; Table 24.3). When sink inward. #ese factors contribute to a poor mask seal and an inability performing the assessment, the patient is instructed to sit upright, to perform adequate BMV. Anesthesia practitioners may consider leaving extend the neck, open the mouth as much as possible, protrude the dentures in place (as long as they are easily removable) in the edentulous tongue, and avoid phonation. #e MMT uses a classi%cation system to patient prior to instrumentation of the airway. #is may be done to con- evaluate tongue size relative to the oropharyngeal space and is catego- serve the facial structure and facilitate a better mask seal. rized as follows: Decreases in lung compliance can contribute to ineffective BMV Class I: Full visualization of the entire oropharynx, including soft because of the inability to effectively move ventilated gases into the palate, uvula, fauces (created by two tonsillar pillars known as the alveoli. Conditions such as bronchospasm, pulmonary edema, acute palatoglossal and palatopharyngeal arches), and possibly the pala- respiratory distress syndrome, or pneumonia (see Box 24.4) can cause tine tonsils located between the tonsillar pillars (not always identi- peak airway pressures to rise and compliance to decrease, resulting in %ed) ineffective BMV and poor oxygenation of the patient. #e ability of the Class II: Visualization of the soft palate, fauces, and uvula anesthetist to recognize potential and actual upper and lower airway Class III: Visualization of the soft palate and base of the uvula problems early is essential when managing the airway. Class IV: Visualization of the hard palate only Ezri et)al.36 suggested adding a class “zero” to the MMT, which is the Direct Laryngoscopy and Videolaryngoscopy With ability to see any part of the epiglottis on mouth opening and tongue Tracheal Intubation Assessment protrusion, and found a 1.2% incidence. However, most researchers DL, VL, and TI are different procedures. DL is the process of airway and clinical practitioners use the Samsoon and Young I to IV modi%ca- instrumentation with a laryngoscope to acquire a direct line of sight with tion of the Mallampati classi%cation. the laryngeal opening and supporting structures. VL is an indirect proce- Evaluations of the MMT have shown some correlations with dure for viewing the laryngeal opening using a camera that is embedded difficulty when either class III or IV is measured.35,36 However, on the tip of the laryngoscope which is then linked to a video monitor. TI by itself, the MMT has been unreliable at consistently predicting is the process of placing an ETT into the trachea proximal to the carina the presence or absence of a difficult airway.37,38 Variability in and can be done utilizing either direct or indirect laryngoscopy. Usually the administration of the MMT and practitioner subjectivity of when anesthesia professionals perform a DL or VL it is with the intent of assessment can result in the assessment of different classifications completing TI, and as a result laryngoscopy and intubation are generally between similar patients. Nevertheless, when used in combination performed together in sequence. Airway assessments speci%c to DL, VL, with other airway assessments, the MMT can help increase the pre- and TI are therefore discussed together in this section. dictive ability of both laryngoscopic and intubation difficulties. For  CHAPTER 24 Airway Management 439 Mallampati Classification Laryngoscope Hard palate Soft palate Epiglottis Uvula Pillar Pillar Grade I Grade II Class I Class II Grade III Grade IV Fig. 24.10 Cormack and Lehane grading system. Grade I is visualization of the entire glottic opening. Grade II is visualization of only the poste- rior portion of the glottic opening. Grade III is visualization of the epiglot- tis only. Grade IV is visualization of the soft palate only. (From Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia. 1984;39:1105–1111.) Cormack and Lehane Grading System An objective scoring system frequently used by researchers and clinicians to describe laryngoscopic difficulty is the Cormack and Lehane grading scale (Fig. 24.10). This scale was originally Class III Class IV described in 1984 and updated in 1998 to better define the grade II Fig. 24.9 Modified Mallampati classification. Performed with the patient views.39,40 It offers an objective assessment of the pharyngeal struc- sitting up and maximally protruding the tongue; visibility of the soft pal- tures, glottic structures, and glottic opening during laryngoscopy ate, fauces, tonsillar pillars, and uvula are noted. Class I describes full visualization of all structures. Class II allows visualization of the soft and is rated as follows: palate, fauces, and uvula. Class III allows visualization of the soft palate Grade I: Full view of the glottic opening, including the anterior and the base of the uvula. With class IV, only the hard palate is visible. commissure and posterior laryngeal cartilages (Courtesy Cleveland Clinic.) Grade IIa: Partial view of the vocal cords (anterior commissure not seen) and full view of posterior laryngeal cartilages Grade IIb: Only the posterior portion of the glottic opening can be visualized (posterior laryngeal cartilages) TABLE 24.3 Mallampati Airway Grade III: Only the epiglottis can be visualized; no portion of the Classification glottic opening can be seen Classification Description of Visualized Structures Grade IV: Epiglottis cannot be seen; only view is of the soft palate Cormack and Lehane grades I and IIa and IIb are generally asso- I Soft palate, fauces, uvula, pillars visible ciated with easier intubations, although a grade IIb is likely more dif- II Soft palate, fauces, uvula visible %cult than the IIa view. Grades III and IV correspond with higher III Soft palate, base of uvula visible degrees of intubation difficulty.39 Other authors have focused on an IV Hard palate visible only assessment to quantify the percentage of glottic visualization in an effort to limit the ambiguity that exists between Cormack and Lehane From Berkow LC. Strategies for airway management. Best Pract Res grades I and II. #e percentage of glottic opening (POGO) is the per- Clin Anaesthesiol. 2004;18(4):531–548. centage of the vocal cords visualized. It is de%ned as the linear span from the anterior commissure to the interarytenoid notch. A 100% example, the combination of high Mallampati classification levels POGO score is a full view of the glottis and includes the anterior of III or IV and a large neck circumference (>43 cm, as measured commissure to the interarytenoid notch. A POGO score of 0% signals at the thyroid cartilage) were predictors of intubation problems.12,25 that no portion of the glottic opening is seen (including the interary- Other researchers identified the combination of high Mallampati tenoid notch). Objective scoring between 0% and 100% is dependent classifications, decreased TMDs, and limited interincisor open- on the judgment of the individual performing the laryngoscopy, and ings as strong predictors of both laryngoscopic and intubation POGO scoring has been shown to provide good intra- and interob- difficulties.6,28,33 server reliability.41 440 UNIT V Intraoperative Management Thyromental Distance #e TMD is a measurement of the thyromental space. #is space is an available pliable compartment, directly anterior to the larynx, where the tongue can be displaced during laryngoscopy to improve direct line of site with the glottic opening. #e tongue is a malleable structure that can be displaced within the oropharyngeal and pharyngeal com- partments. #e thyromental space is bordered laterally by the neck, superiorly by the mentum, and inferiorly by the semi%xed hyoid bone. #e TMD is measured from the thyroid notch to the lower border of the mentum (at the chin) when the patient’s head is extended and the A mouth is closed (Fig. 24.11B). A TMD less than 6 cm, or three ordinary %ngerbreadths, is associated with a higher incidence of difficult intuba- tion. Research has indicated that the thyromental space should accom- modate most tongue sizes but that a small space would only facilitate a relatively small tongue.32,42 Some conditions, such as radiation or pathologic factors (e.g., tumors), may render the thyromental space noncompliant, hindering tongue placement and causing it to protrude into the pharyngeal space. A malplaced tongue that does not %t into the thyromental space can obstruct the line of site to the glottic opening during DL. Another condition affecting tongue placement into the thyromental B space is known as mandibular hypoplasia. #is condition results in a TMD of less than three %ngerbreadths and may cause difficulty with DL and TI because the tongue cannot be displaced into the small sub- mandibular space. Furthermore, the larynx may be tucked underneath the tongue or be positioned relatively anterior to the base of the tongue in the pharyngeal space, causing difficulty with laryngoscopic views. #is condition is commonly called an anterior larynx, and unless the tongue can be detracted anteriorly away from the pharyngeal space, little can be done to improve the direct line of site view. In contrast, VL has shown to provide improved laryngeal visualization in patients with difficult direct line of site views.43,44 C #e assessment of a long TMD (>9 cm) may also indicate a poten- Fig. 24.11 The 3-3-2 assessment. (A) Mouth opening (distance tially difficult laryngoscopy and intubation. #e indication for dif- between upper and lower incisors) at least three fingerbreadths. (B) %culty is due to a large hypopharyngeal tongue, caudal larynx, and Mentum (protruding part of chin) to thyroid notch, as known as thy- longer mandibulohyoid distance (MHD). #e MHD measures the ver- romental distance, at least three fingerbreadths. (C) Thyroid notch to tical distance from the angle of the mandible to the hyoid bone and is hyoid bone at least two fingerbreadths. (From Elisha S, Nagelhout JJ, usually assessed by radiograph. A long TMD and MHD may position Heiner JS. Current Anesthesia Practice. St. Louis: Elsevier; 2021:48.) the glottic opening caudally in the neck and beyond the visual horizon. If the larynx is more caudally situated in the pharyngeal compartment, then the tongue is also likely to be positioned lower because the tongue laryngoscopy. #e “2” is a modi%cation of the MHD, which is a non- muscle is hinged to the hyoid bone.45,46 #is can lead to both a greater radiographic measurement from the mandible-neck junction to the distance to the glottic opening during laryngoscopy and an obstructed tip of the thyroid notch and assesses the position of the larynx (glottic view caused by a greater amount of tongue mass in the hypopharyn- opening) in relation to the base of the tongue. More than two %nger- geal space. In addition, a history of sleep apnea and snoring can pro- breadths indicates that the larynx may be positioned too far down the vide further evidence of a possible hypopharyngeal tongue, which has neck and could be difficult to visualize. Less than two %ngerbreadths been shown to cause difficulty with both mask ventilation and intu- indicates that the larynx may be tucked under the base of the tongue, bation.46,47 Similar to other airway assessment tests, researchers have which would be indicative of an anterior larynx. #is combination of found limited predictability when the TMD test was used by itself and airway assessments allows the anesthetist to perform multiple airway therefore recommend that it be used as part of a multivariable airway measurements in a rapid and sequential manner. assessment.25,30 #e sternomental distance (SMD) is measured from the sternal #e 3-3-2 rule has been described by some authors as a nonscien- notch to the lower border of the mentum at the chin with the mouth ti%cally based test to ensure that upper airway geometry is adequately closed. #ere is some debate regarding a normal SMD length with assessed.48 #e 3-3-2 rule is an assessment that evaluates various the number ranging from greater than 12.5 cm to 15 cm. However, airway proportions using %ngerbreadths as a measurement (see Fig. most researchers will agree that a SMD of less than 12.5 cm is a pre- 24.11). It is a combination of different geometric dimensions that relate dictor for a difficult laryngoscopy and intubation.49 #e SMD is helpful mouth opening and the size of the mandibular space to the position of when determining the length of head and neck extension. #is can be the larynx in the neck.14 #e %rst “3” estimates oral access (the inter- assessed by %rst measuring the SMD with the head and neck in the incisor gap distance), which should accommodate at least three %nger- neutral position and then again with the head and neck extended. #e breadths. #e second “3” assesses the mandibular length (TMD) from measurements are then used to calculate the difference. Extensions of the tip of the mentum to the mandible-neck junction and gauges the less than 5 cm have been reported as an indicator for difficult laryn- ability of the tongue to displace within the submandibular space during goscopy.49 Similar to other preoperative airway exams, the SMD in  CHAPTER 24 Airway Management 441 OA OA PA LA PA LA A B OA OA PA LA LA PA C D Fig. 24.12 Oral axis (OA), pharyngeal axis (PA), and laryngeal axis (LA) for intubation. (A) Nonaligned position. (B) Head resting on a pad causes flexion of the neck and aligns the PA and LA. (C) Head resting on pad causes flexion of the neck, and neck extension into the sniffing position aligns the OA, PA, and LA. (D) Extension of the neck without head elevation aligns PA and LA, but not the OA. (From Miller RD, Pardo MC, eds. Basics of Anesthesia. 6th ed. Philadelphia: Elsevier; 2011:226.) isolation is not an adequate predictor of difficult laryngoscopy and/ the oral, pharyngeal, and laryngeal axes (Fig. 24.12). Evaluation of or intubation, and instead should be used in combination with several atlantooccipital joint extension is conducted with the patient seated other preoperative airway exams in an effort to effectively predict diffi- upright in a neutral face-forward position; the patient is then asked to culty with airway management. lift the head back with the chin up as far as possible. When extension is reduced to 23 degrees, visualization may become difficult.50 If the Interincisor Gap patient demonstrates substantial or complete immobility of the atlan- As discussed previously, the interincisor gap is important to assess tooccipital joint, then signi%cant compromise with DL should be antic- when performing a DL or VL because the size of the mouth opening ipated. Furthermore, cervical spine diseases such as cervical pathology can affect the ability to introduce a laryngoscope device and to create (e.g., degenerative disease, rheumatic disease, neurologic pathology, a direct line of sight with the laryngeal opening.15 #e challenge with a trauma, or previous surgical intervention) or spinal abnormalities narrow mouth opening is placing both the laryngoscope blade’s 2-cm (e.g., Down syndrome) may lead to difficult laryngoscopy and diffi- &ange and the ETT into the mouth while maintaining good visualiza- cult airway management in general. VL has been shown to be an effec- tion of the vocal cords. #us a predictor of difficult intubation is an tive method for placing an ETT in patients with limited cervical spine interincisor opening of less than 3 cm. An increase in maxillary inci- mobility or in patients where the cervical spine requires minimal or sor length can reduce the interincisor gap and create a sharper angle no movement.43 Either VL or &exible endoscopy (i.e., %beroptic scope) between the oral and glottic openings. Furthermore, prominent inci- has shown to be effective when managing the airway with patients who sors or “buck teeth” increase the risk of dental damage from any hard have limited cervical spine mobility or those who require cervical spine device used to manipulate the airway. Finally, loose or awkwardly sized immobilization.51 teeth can either impede the placement of a laryngoscope blade, VL device, and ETT or create an obstruction if a tooth is dislodged and Mandibular Protrusion Test falls posteriorly into the laryngeal opening or trachea. #e mandibular protrusion test, also known as the upper lip bite test (ULBT), is an evaluation technique that demonstrates the patient’s abil- Atlantooccipital Joint Mobility ity to extend the mandibular incisors anterior past the maxillary incisors. #e full range of neck &exion and extension varies from 90 to 165 #e purpose of this airway test is to %rst assess the patient’s maxillary inci- degrees and decreases approximately 20% between ages 16 and 75. sor length (e.g., presence of buck teeth), and then to assess the mobility of #e atlantooccipital joint provides the highest degree of mobility in the patient’s temporomandibular joint function and forward subluxation the neck, with a normal head extension of up to 35 degrees. Proper of the jaw. #e presence of these indicators can lead to problematic oral atlantooccipital joint mobility is required for an adequate sniffing posi- airway placement, noneffective relief of soft tissue obstruction from poor tion. #e sniffing position is important because it helps to improve mandibular movement, difficulty introducing the laryngoscope blade DL views by promoting displacement of the tongue by better aligning into the mouth, and an obstructive view of the glottic opening caused by 442 UNIT V Intraoperative Management MANDIBULAR difficult laryngoscopies when used in combination with the modi%ed PROTRUSION TEST Mallampati classi%cation.52,54 However, the ULBT has been shown to be an unreliable assessment technique when used in the edentulous popu- lation and has demonstrated poor predictability when used as a single screening test for the assessment of laryngoscopic difficulty.55 Supraglottic Airway Assessment Supraglottic airway ventilation may be the primary means of managing Class A: Lower incisors an airway or it can be used for rescue ventilation in the event that face- can be protruded anterior mask ventilation is difficult or fails. Similar predictors related to diffi- to the upper incisors culty with mask ventilation, laryngoscopy, and intubation can also be used to indicate difficulty in placing and ventilating with a supraglot- tic airway device (SAD). #ese predictors include a restrictive mouth opening, a distortion in the upper airway anatomy, and both upper and lower airway obstruction (see Box 24.2). #e interincisor gap is an important assessment to consider when introducing any airway adjuncts into the mouth. As identi%ed earlier, an assessment of less than 3 cm indicates a restricted oral access caus- ing difficult SAD placement.15 Furthermore, some researchers have indicated that reductions in atlantooccipital joint movement due to Class B: The lower incisors conditions such as ankylosing spondylitis and rheumatoid arthritis can be brought edge to edge may cause difficulty with some SAD placements, such as the laryngeal with the upper incisors mask airway (LMA).56,57 Obstruction at the level of the larynx, trachea, or below can reduce or completely block ventilation from a SAD (see Box 24.4). In addition, %xed upper airway lesions, such as oropharyngeal tumors or a disrup- tion or distortion in the upper airway anatomy, may make SAD place- ment difficult and lead to ineffective ventilation from a compromised “seat and seal.”48,58 Finally, conditions affecting the lower airways resulting in decreases in pulmonary compliance or increases in airway resistance can cause peak airway pressures to rise, which may also make supraglottic airway ventilation difficult. For example, bronchospasm or acute respiratory Class C: The lower incisors distress syndrome require higher ventilatory pressures to facilitate ade- cannot be brought edge to quate gas exchange within the lungs. Severe obstruction or restriction edge with the upper incisors as a result of lung pathophysiology may limit the amount of ventilation that can be accomplished with a SAD and even allow for the possible MC risk of vomiting and pulmonary aspiration.58,59 Fig. 24.13 Mandibular protrusion test classes A, B, and C (also known as the upper lip bite test). (Redrawn from Munnur U, et al. Airway prob- Cricothyrotomy Airway Assessment lems in pregnancy. Crit Care Med. 2005;33:S259–2S68.) An assessment of the neck is part of any comprehensive airway exam- ination and should be conducted with the intent of identifying factors that may indicate difficulty with cricothyrotomy placement. #e need malplacement of the tongue. Difficulty with laryngoscopy arises when for an invasive airway usually occurs on an emergency basis. Invasive the patient is unable to protrude the mandibular incisors anterior past airway management consists of a needle cricothyrotomy, percutaneous the maxillary incisors (Fig. 24.13). #e test has three classi%cations: (wire guided) cricothyrotomy, surgical cricothyrotomy, or a tracheot- Class A: Patient can protrude the lower incisors anteriorly past the omy. Since surgeons are more likely to perform a tracheotomy, the pri- upper incisors and can bite the upper lip above the vermilion bor- mary focus will be on assessments aimed at identifying factors that lead der (line where the lip meets the facial skin). to difficulty with cricothyrotomy procedures. Class B: Patient can move the lower incisors in line with the upper Even though there is no absolute contraindication for the place- incisors and bite the upper lip below the vermilion border but can- ment of an emergency cricothyrotomy, there are conditions that not protrude lower incisors beyond. make performing this procedure difficult or even impossible (see Box Class C: Lower incisors cannot be moved in line with the upper 24.2). Murphy and Walls14 described using the mnemonic SHORT as incisors and cannot bite the upper lip. a means for remembering surgery, hematoma, obesity, radiation, and Assessment of an ULBT class C indicates a potential difficult laryn- tumors, which are conditions associated with difficult cricothyrotomy goscopic view, whereas class A indicates a good view using conven- placement. Any condition that causes a distortion of the airway, such tional laryngoscopy.52 as trauma, surgery, infection, tumors, or physical impediments in or #e ULBT was found to be a valuable assessment tool for the predic- around the cricothyroid area, may cause difficulty with invasive airway tion of difficult ventilation when used in combination with other assess- access. A hematoma from surgery (e.g., carotid endarterectomy), an ment techniques such as increased neck circumference and a history of infection of the soft tissue around the airway (e.g., Ludwig angina), an snoring.53 Furthermore, the ULBT has been shown to be useful in the oral or pharyngeal abscess, a tumor in or around the neck, or radia- assessment of temporomandibular joint function and the prediction of tion causing scarring of the neck tissue can distort the neck anatomy,  CHAPTER 24 Airway Management 443 making a cricothyrotomy neck incision and tube placement into the #e TI procedure is preceded by careful attention to optimal trachea challenging. However, none of these factors should be consid- head and neck positioning for laryngoscopy. Flexion of the neck and ered a contraindication when emergency airway access is required. In extension of the atlantooccipital joint, or sniffing position, allows for addition, recent or past neck trauma or surgery can alter neck anatomy proper alignment of the oral, pharyngeal, and tracheal axis (see Fig. and cause scarring or acute bleeding, again making cricothyrotomy 24.12). #is can be accomplished using pillows, towels, blankets, or procedures difficult. Finally, neck impediments such as a halo device the OR table’s positioning functions (Fig. 24.16). Furthermore, ele- or a rigid cervical collar may impede emergency access or make the vation of the shoulders, head, and neck (ramping; see Fig. 24.8) in procedure technically difficult and should be removed when possible. an effort to align the tragus of the ear with the sternum may help A short neck, obesity, scarring, or %xed &exion abnormality are all indi- in patients identi%ed as potentially difficult to intubate. Manipula- cations for a difficult cricothyrotomy placement. Both a short neck and tion of the head and neck must be used with caution in patients with excessive soft tissue around the neck can make %nding the cricothyroid decreased cervical range of motion caused by degenerative disease, membrane and surrounding anatomic landmarks challenging. When per- rheumatic disease, neurologic pathology, trauma, or previous surgi- forming the preoperative airway assessment on patients who exhibit any cal intervention (Fig. 24.17). of these characteristics, mark the cricothyroid membrane prior to airway Sufficient preoxygenation may help to delay arterial desaturation manipulation so it can be located rapidly in the event of an emergency. prior to the induction of anesthesia and during subsequent apneic sit- Furthermore, make readily available the appropriate equipment and per- uations. Effective preoxygenation increases pulmonary oxygen content sonnel to save time when and if emergency airway access is indicated. and eliminates much of the nitrogen (∼79% of room air) from the FRC. A functional FRC can theoretically oxygenate a healthy individual for Radiologic and Ultrasonographic Imaging up to 8 minutes. However, without preoxygenation, the oxygen reserve Imaging technologies such as magnetic resonance imaging (MRI) and in the FRC is limited, leading to a decrease in the time an anesthe- computed tomography (CT) can identify pathologies and alterations tist has to secure the airway. Preoxygenation should include 100% in airway anatomy and increase the predictive value of anticipated dif- inspired oxygen, a tight mask seal, instructing the patient to breathe %culty. #ese technologies can help anesthetists formulate appropri- at normal tidal volumes for 3 to 5 minutes, and providing a minimum ate airway management plans. Advantages of the CT scan include the fresh gas &ow of at least 5 L/min. Signs of adequate preoxygenation are ability to accurately depict pathology involving bones and soft tissues a respiratory bag that moves with each inspiration/expiration, a well- and the ability to view images in the sagittal, coronal, axial, and three- de%ned end-tidal CO2 waveform, and a fraction of expired oxygen that dimensional views. Furthermore, CT scans have become the gold stan- increases to 90% or greater. If time is limited, there is some evidence dard for ruling out fractures of the cervical spine.60 that supports a patient taking eight vital capacity breaths within 60 sec- Whereas CT scans can image both bone and soft tissue, an MRI onds, before the induction of anesthesia, with equivalent results to 3 offers detailed information on soft tissues only. An MRI can help assess minutes of tidal volume breathing.64 Finally, as discussed earlier, the the impact that pathologic processes have on soft tissue, such as the concept of apneic oxygenation using a nasal cannula with &ows reach- altered patency of an airway caused by a tumor in the neck. However, ing 15 L/min to insufflate oxygen into the pharyngeal airway may help CT and MRI tests are not currently used on a routine basis because of to delay desaturation during apnea in a patient with a patent airway. the economic costs and time-delaying factors associated with their use, #e technique for DL is dependent on the type of laryngoscope blade especially if there is no indication to perform them. used for the procedure, but the premise of control/displacement of the Ultrasonography has been shown to be useful as a predictor for diffi- tongue from right to left and elevation of the epiglottis remains the same cult laryngoscopy. Excessive pretracheal soft tissue, as assessed by ultra- for both techniques. #e use of the curved (e.g., Macintosh) laryngo- sound, in combination with a large neck circumference and a history of scope blade requires the anesthetist to (1) place the tip of the laryngo- sleep apnea, were all positive predictors for difficult laryngoscopy in an scope in the vallecula and (2) apply tension to the hyoepiglottic ligament obese population.61 Furthermore, point of care ultrasound (POCUS) using a gentle lifting force, which (3) promotes indirect elevation of the provides bedside imaging of airway anatomy, including the cricoid epiglottis. In contrast, the use of the straight (e.g., Miller) laryngoscope cartilage and vocal cords.62 As such, ultrasonography may be a useful blade requires the anesthetist to (1) place the tip of the laryngoscope pos- technique for the identi%cation of the cricoid cartilage and cricothyroid terior to the epiglottis and (2) apply gentle force to directly lift the epi- membrane in an obese individual (Figs. 24.14 and 24.15).63 #e effec- glottis. Levering action should never be applied to the patient’s dentition. tiveness of POCUS as an airway assessment tool has yet to be deter- Such action risks dental damage and diminishes the quality of the laryn- mined. However, as it is being used with more frequency, it is likely goscopic view. #e technique backwards-upwards-rightwards-pressure to emerge as a useful adjunct in the assessment of the difficult airway. (BURP) has been demonstrated to improve visualization of the vocal When assessing the airway and preparing for manipulation, the cords and can be achieved by the clinician manipulating the larynx at the anesthetist should perform a multivariable assessment. #is assess- neck with the right hand during laryngoscopy.65 ment is based on patient, surgical, and situational requirements and is If an adequate laryngoscopic view cannot be achieved after DL, crucial for the identi%cation of factors that are associated with difficult then strategies and algorithms for the difficult or failed airway should mask ventilation, laryngoscopy and intubation, supraglottic device, be followed. Maintenance of adequate oxygenation and ventilation is and cricothyrotomy placement. of the utmost priority during TI and can be facilitated by the use of various adjunctive airway equipment and techniques. TRACHEAL INTUBATION MANAGEMENT OF THE DIFFICULT AND Tracheal intubation remains a cornerstone of traditional airway man- FAILED AIRWAY agement and may be performed by various methods (e.g., DL, &exible intubating scope, intubating LMA, VL). #e technique chosen for TI Management of the difficult airway is a multifaceted process that is dependent on the patient’s history, physical examination (including begins with the airway assessment, considers decision-making strat- airway assessment), and previous anesthetic history (e.g., difficult intu- egies suggested by the various difficult airway algorithms, continues bation). Indications for TI can be found in Table 24.4. with a primary airway management plan, and is then &exible with an 444 UNIT V Intraoperative Management Thyroid Cartilage Cricothyroid Membrane Cricoid Cartilage