Miller's Airway Management PDF

40: Airway Management in the Adult Carlos A. Artime, Matteo Parotto, and Carin A. Hagberg K E Y PO IN TS ▪ One of the fundamental responsibilities of the anesthesiologist is to mitigate the adverse effects of anesthesia on the respiratory system by maintaining airway patency and ensuring ad...

40: Airway Management in the Adult Carlos A. Artime, Matteo Parotto, and Carin A. Hagberg K E Y PO IN TS ▪ One of the fundamental responsibilities of the anesthesiologist is to mitigate the adverse effects of anesthesia on the respiratory system by maintaining airway patency and ensuring adequate ventilation and oxygenation. The term airway management refers to this practice and is a cornerstone of anesthesia. ▪ Successful airway management requires a range of knowledge and skill sets – specifically, the ability to predict difficulty with airway management and to formulate an airway management strategy, as well as the skills to execute that strategy using the wide array of airway devices available. ▪ The American Society of Anesthesiologists’ (ASA) Practice Guidelines for Management of the Difficult Airway and the accompanying difficult airway algorithm provide guidelines for evaluating the airway and preparing for difficult airway management and can guide clinical decision-making when an anesthesiologist is faced with a known or potentially difficult airway. Cognitive aids, such as the Vortex Approach and the new  ASA Difficult Airway infographics that accompany the ASA adult and pediatric difficult airway algorithms, are useful to help implement airway algorithms in an emergency. ▪ A detailed understanding of airway anatomy is essential for successful airway management. ▪ A complete evaluation of the airway and knowledge of difficult airway predictors can alert the anesthesiologist to the potential for difficulty with airway management and allow for appropriate planning. ▪ Apneic oxygenation can be used to prolong the duration of apnea without desaturation and is increasingly being adopted during the management of both difficult and routine airways. ▪ Application of local anesthesia to the airway or induction of general anesthesia is usually required to facilitate airway management, to provide comfort for the patient, and to blunt airway reflexes and the hemodynamic response to airway instrumentation. ▪ Over the past 40 years, supraglottic airways (SGAs) have been one of the most important developments in airway devices. ▪ Tracheal intubation establishes a definitive airway, provides maximal protection against aspiration of gastric contents, and allows for positive-pressure ventilation with higher airway pressures than via a face mask or SGA. ▪ Flexible scope intubation of the trachea in an awake, spontaneously ventilating, and cooperative patient is the gold standard for the management of the anticipated difficult airway.  ▪ Invasive airway interventions are indicated when attempts at establishing a noninvasive airway fail. The anesthesiologist should become proficient with techniques for cricothyrotomy. ▪ Extubation is a critical component of airway management with the potential for significant complications. Extubation of the trachea must be planned in advance and includes a strategy for reintubation should the patient be unable to maintain an adequate airway after extubation. Introduction General anesthesia is associated with various effects on the respiratory system, including the loss of airway patency, loss of protective airway reflexes, and hypoventilation or apnea. Therefore, one of the fundamental responsibilities of the anesthesiologist is to establish airway patency and to ensure adequate ventilation and oxygenation. The term airway management refers to the practice of establishing and securing a patent airway and is a cornerstone of anesthetic practice. Traditionally, ventilation via a mask and tracheal intubation have been the foundations of airway management; however, in the past 40 years, supraglottic airways (SGAs) have emerged as one of the most important developments in airway devices. Because failure to secure a patent airway can result in hypoxic brain injury or death in only a few minutes, difficulty with airway management has potentially grave implications. Analysis of the American Society of Anesthesiologists’ (ASA) Closed Claims Project database demonstrated that the development of an airway emergency increases the odds of death or brain damage by 15-fold.1,2 Although the proportion of claims attributable to airway-related complications has decreased over the past three decades, airway complications remain associated with high mortality rates.2,3 In 2011, the Royal College of Anaesthetists and the Difficult Airway Society (DAS) of the United Kingdom reported the results of the 4th National Audit Project (NAP4), a one-year audit aimed at determining the incidence of major complications of airway management in anesthesia. NAP4 identified 133 major airway- related events in the perioperative period resulting in 16 deaths—a mortality incidence of 1 per 180,000 anesthetics—a number that could be as high as 1 per 50,000 anesthetics when considering underreporting.4 The most common airway problems in the NAP4 study were failure, delay, or difficulty in securing the airway; aspiration of gastric contents; and extubation-related complications. Inadequate assessment of the airway, poor planning, and a lack of personal and/or institutional preparedness for difficult airway management were the most common contributing factors.5 Studies such as these highlight the importance of successful airway management, which requires a range of knowledge and skill sets – specifically, the ability to predict difficulty with airway management, to formulate an airway management strategy, and to have the skills necessary to execute that strategy using the wide array of available airway devices.6 Development of these skills should be an ongoing endeavor for all anesthesiologists. As with any manual skill, continued practice improves performance and may reduce the likelihood of complications. New airway devices are continually being introduced, each with unique properties that may be advantageous in certain situations. Becoming familiar with new devices under controlled conditions is important – a patient’s difficult airway is not an appropriate setting during which to experiment with a new technique. Algorithms for Management of the Difficult Airway THE AMERICAN SOCIETY OF ANESTHESIOLOGISTS’ ALGORITHM In 1993, the ASA published the first Practice Guidelines for Management of the Difficult Airway, which was written with the intent to “facilitate the management of the difficult airway and to reduce the likelihood of adverse outcomes.”7 The most recent update to this report, published in 2022, defines the difficult airway as “the clinical situation in which anticipated or unanticipated difficulty or failure is experienced by a physician trained in anesthesia care, including, but not limited to one or more of the following: face mask ventilation, laryngoscopy, ventilation using a supraglottic airway, tracheal intubation, extubation, or invasive airway access,” and provides guidelines for the evaluation of the airway and preparation for difficult airway management, including a Difficult Airway Algorithm (DAA) intended to guide clinical decision-making when an anesthesiologist is faced with a known or potential difficult airway (Fig. 40.1).8 The ASA DAA begins with a decision-making tool guiding consideration of the relative clinical merits and feasibility of awake intubation versus intubation after induction of general anesthesia. The remainder of the algorithm is structured into three separate scenarios: (1) predicted difficult airway (awake intubation); (2) difficult intubation with adequate oxygenation/ventilation (the “non-emergency” pathway); and (3) difficult intubation without adequate oxygenation/ventilation (the “cannot intubate, cannot ventilate” [CICV] scenario or the “emergency” pathway). OTHER DIFFICULT AIRWAY ALGORITHMS Many other national anesthesia societies have published guidelines for management of the difficult airway, including the Difficult Airway Society (DAS) from the United Kingdom,9 the Canadian Airway Focus Group (CAFG),10,11 the French Society of Anesthesia and Intensive Care (SFAR),12 the German Society of Anesthesiology and Intensive Care Medicine (DGAI),13 the Italian Society for Anesthesia and Intensive Care (SIAARTI),14 the Japanese Society of Anesthesiologists,15 and the All India Airway Association (AIDAA).16 All these guidelines include recommendations for predicting the difficult airway, suggest awake intubation as a management strategy for the anticipated difficult airway, and incorporate algorithms for both unanticipated difficult intubation with adequate oxygenation and the CICV scenario. Common elements include a focus on awakening the patient in the setting of a difficult intubation with adequate ventilation, the use of SGAs as a rescue for difficult mask ventilation, and emergency invasive airway access in the CICV scenario. The primary differences in these algorithms are in specific details, such as the number of intubation attempts suggested, the specific initial and alternate devices recommended for difficult intubation, and the organization of the algorithm.17 HUMAN FACTORS AND COGNITIVE AIDS There has been growing attention to the influence of “human factors” on difficult airway management – namely, human behaviors, abilities, shortcomings, and biases as well as individual and team performance. Studies such as NAP4 have shown that these human factors contribute to an adverse airway outcome in over 40% of cases.4 The use of airway checklists, preprocedural team briefings, and cognitive aids are all strategies for addressing human factor challenges.18 The 2022 ASA Difficult Airway Guidelines now take human factors into consideration, including environmental factors, team behaviors, and individual performance. Practitioners may consider these factors before, during, and/or after the course of difficult airway management.8 The Vortex approach is one such cognitive aid designed to facilitate management of the unanticipated difficult airway.19 Rather than relying on complex algorithms that are based on decision trees, the Vortex model utilizes a visual aid in the shape of a funnel or vortex (Fig. 40.2) to guide the anesthesiologists through the three basic nonsurgical airway techniques (face mask ventilation, SGA, and tracheal intubation). If after an “optimal attempt” at each of these nonsurgical modalities adequate oxygenation has not been achieved, then one “travels down the vortex,” and an emergency surgical airway is indicated. Because this strategic approach is more conceptual, it is simple enough to be utilized and recalled during a stressful airway emergency. FIG. 40.1 The American Society of Anesthesiologists’ Difficult Airway Algorithm for Adult Patients.The airway manager’s choice of airway strategy and techniques should be based on their previous experience; available resources, including equipment, availability and competency of help; and the context in which airway management will occur. Low- or high-flow nasal cannula, head elevated position throughout procedure. Noninvasive ventilation during preoxygenation. Awake intubation techniques include flexible bronchoscope, videolaryngoscopy, direct laryngoscopy, combined techniques, and retrograde wire–aided intubation. Other options include, but are not limited to, alternative awake technique, awake elective invasive airway, alternative anesthetic techniques, induction of anesthesia (if unstable or cannot be postponed) with preparations for emergency invasive airway, and postponing the case without attempting the above options. Invasive airway techniques include surgical cricothyrotomy, needle cricothyrotomy with a pressure-regulated device, large-bore cannula cricothyrotomy, or surgical tracheostomy. Elective invasive airway techniques include the above and retrograde wire–guided intubation and percutaneous tracheostomy. Also consider rigid bronchoscopy and ECMO. Consideration of size, design, positioning, and first- versus second- generation supraglottic airways may improve the ability to ventilate. Alternative difficult intubation approaches include but are not limited to video- assisted laryngoscopy, alternative laryngoscope blades, combined techniques, intubating supraglottic airway (with or without flexible bronchoscopic guidance), flexible bronchoscopy, introducer, and lighted stylet or lightwand. Adjuncts that may be employed during intubation attempts include tracheal tube introducers, rigid stylets, intubating stylets, or tube changers and external laryngeal manipulation. Includes postponing the case or postponing the intubation and returning with appropriate resources (e.g., personnel, equipment, patient preparation, awake intubation). Other options include, but are not limited to, proceeding with procedure utilizing face mask or supraglottic airway ventilation. Pursuit of these options usually implies that ventilation will not be problematic. From Apfelbaum JL, Hagberg CA, Caplan RA, et al. Practice guidelines for management of the difficult airway: an updated report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Anesthesiology. 2013;118:251–270.  FIG. 40.2 (A) The Vortex implementation tool. (B) Lateral aspect of the Vortex in three dimensions, demonstrating the funnel concept. From Chrimes N. The Vortex: a universal ‘high-acuity implementation tool’ for emergency airway management. Br J Anaesth. 2016;117:i20–i27. Functional Airway Anatomy A detailed understanding of airway anatomy is essential for the anesthesiologist. Various aspects of airway management depend on a working knowledge of the anatomy involved, including airway assessment, preparation of the airway for awake intubation, and the proper use of airway devices. Knowledge of normal anatomy and anatomic variations that may render airway management more difficult helps with formulating an airway management plan. Because some critical anatomic structures may be obscured during airway management, the anesthesiologist must be familiar with the interrelationship between different airway structures.  The airway can be divided into the upper airway, which includes the nasal cavity, the oral cavity, the pharynx, and the larynx; and the lower airway, which consists of the tracheobronchial tree. NASAL CAVITY The airway begins functionally at the naris, the external opening of the nasal passages. The nasal cavity is divided into the right and left nasal passages (or fossae) by the nasal septum, which forms the medial wall of each passage. The septum is formed by the septal cartilage anteriorly and by two bones posteriorly: the ethmoid (superiorly) and the vomer (inferiorly). Nasal septal deviation is common in the adult population20; therefore, the more patent side should be determined before passing any device through the nasal passages. The lateral walls of the nasal passages are characterized by the presence of three turbinates (or conchae) that divide the nasal passage into three scroll-shaped meatuses (Fig. 40.3). The inferior meatus, between the inferior turbinate and the floor of the nasal cavity, is the preferred pathway for passage of nasal airway devices21; improper placement of devices anywhere in the nose can result in avulsion of a turbinate.22,23 The roof of the nasal cavity is formed by the cribriform plate, part of the ethmoid bone. This fragile structure, if fractured, can result in communication between the nasal and intracranial cavities and a resultant leakage of cerebrospinal fluid. Because the mucosal lining of the nasal cavity is highly vascular, vasoconstrictor should be applied, usually topically, before instrumentation of the nose to minimize epistaxis. The posterior openings of the nasal passages are the choanae, which lead into the nasopharynx.  FIG. 40.3 Lateral wall of the nasal cavity. From Redden RJ. Anatomic considerations in anesthesia. In: Hagberg CA, ed. Handbook of Difficult Airway Management. Philadelphia: Churchill Livingstone; 2000, p. 3, Fig. 1.2. ORAL CAVITY Because of the relatively small size of the nasal passages and the significant risk of trauma, the mouth is often used as a conduit for airway devices. Many airway procedures require adequate mouth opening, which is accomplished by rotation within the temporomandibular joint (TMJ) and subsequent opening by sliding (also known as protrusion or subluxation) of the condyles of the mandible within the TMJ.24 The oral cavity leads to the oropharynx and is inferiorly bounded by the tongue and superiorly by the hard and soft palates. The hard palate, formed by parts of the maxilla and the palatine bone, makes up the anterior two-thirds of the roof of the mouth; the soft palate (velum palatinum), a fibromuscular fold of tissue attached to the hard palate, forms the posterior one-third of the roof of the mouth. The tongue is anchored to various structures by its extrinsic musculature; of these, the most clinically relevant to the anesthesiologist is the genioglossus, which connects the tongue to the mandible. The jaw-thrust maneuver uses the sliding component of the TMJ to move the mandible and the attached tongue anteriorly, thereby relieving airway obstruction caused by the posterior displacement of the tongue into the oropharynx.24 Beneath the tongue, the mylohyoid muscles separate the floor of the mouth into the sublingual space superiorly and the submental space inferiorly. Infection or hematoma formation in these spaces can cause elevation and posterior displacement of the tongue and resultant airway obstruction.25 PHARYNX The pharynx is a muscular tube that extends from the base of the skull down to the level of the cricoid cartilage and connects the nasal and oral cavities with the larynx and esophagus. The posterior wall of the pharynx is made up of the buccopharyngeal fascia, which separates the pharynx from the retropharyngeal space. Improper placement of a gastric or tracheal tube can result in laceration of this fascia and retropharyngeal dissection.26,27 The pharyngeal musculature in the awake patient helps maintain airway patency; loss of pharyngeal muscle tone is one of the primary causes of upper airway obstruction during anesthesia.28,29 A chin lift with mouth closure increases longitudinal tension in the pharyngeal muscles, counteracting the tendency of the pharyngeal airway to collapse.30  The pharynx can be divided into the nasopharynx, the oropharynx, and the hypopharynx (Fig. 40.4). Along the superior and posterior walls of the nasopharynx are the adenoid tonsils, which can cause chronic nasal obstruction and, when enlarged, can cause difficulty when passing airway devices. The nasopharynx ends at the soft palate; this region is termed the velopharynx and is a common site of airway obstruction in both awake and anesthetized patients.28 The oropharynx begins at the soft palate and extends inferiorly to the level of the epiglottis. The lateral walls contain the palatoglossal folds and the palatopharyngeal folds, also termed the anterior and posterior faucial (tonsillar) pillars, respectively; these folds contain the palatine tonsils, which can hypertrophy and cause airway obstruction (Fig. 40.5). The base of the tongue lies in the anterior aspect of the oropharynx, connected to the epiglottis by the glossoepiglottic folds, which bound paired spaces known as the valleculae (although these are frequently referred to as a single space called the vallecula). The hypopharynx begins at the level of the epiglottis and terminates at the level of the cricoid cartilage, where it is continuous with the esophagus. The larynx protrudes into the hypopharynx, creating two piriform recesses on either side (Fig. 40.6). FIG. 40.4 Sagittal section through the head and neck showing the subdivisions of the pharynx. From Redden RJ. Anatomic considerations in anesthesia. In: Hagberg CA, ed. Handbook of Difficult Airway Management. Philadelphia: Churchill Livingstone; 2000, p. 7, Fig. 1.6.  FIG. 40.5 Oral cavity and oropharynx. From Redden RJ. Anatomic considerations in anesthesia. In: Hagberg CA, ed. Handbook of Difficult Airway Management. Philadelphia: Churchill Livingstone; 2000, p. 8, Fig. 1.7. LARYNX The larynx is a complex structure of cartilage, muscles, and ligaments that serves as the inlet to the trachea and performs various functions, including phonation and airway protection. The cartilaginous framework of the larynx is made up of nine separate cartilages: the thyroid and cricoid cartilages; the paired arytenoid, corniculate, and cuneiform cartilages; and the epiglottis. They are joined by ligaments, membranes, and synovial joints, and are suspended by the hyoid bone via the thyrohyoid ligaments and membrane (Fig. 40.7).  The thyroid cartilage is the largest of these cartilages and supports most of the soft tissues of the larynx. The superior thyroid notch and the associated laryngeal prominence (Adam’s apple) are appreciable from the anterior neck and serve as important landmarks for percutaneous airway techniques and laryngeal nerve blocks. The cricoid cartilage, at the level of the sixth cervical vertebra, is the inferior limit of the larynx and is anteriorly connected to the thyroid cartilage by the cricothyroid membrane (CTM). It is the only complete cartilaginous ring in the airway. The arytenoid cartilages articulate with the posterior cricoid and are the posterior attachments for the vocal cords. FIG. 40.6 Larynx as visualized from the hypopharynx. From Redden RJ. Anatomic considerations in anesthesia. In: Hagberg CA, ed. Handbook of Difficult Airway Management. Philadelphia: Churchill Livingstone; 2000, p. 8, Fig. 1.8.  FIG. 40.7 Cartilaginous and membranous components of the larynx. From Redden RJ. Anatomic considerations in anesthesia. In: Hagberg CA, ed. Handbook of Difficult Airway Management. Philadelphia: Churchill Livingstone; 2000, p. 10, Fig. 1.9. When viewed from the pharynx, as during direct laryngoscopy (DL), the larynx begins at the epiglottis, which is a cartilaginous flap that serves as the anterior border of the laryngeal inlet. It functions to divert food away from the larynx during the act of swallowing, although its role in this regard is not essential to prevent tracheal aspiration.31 The anterior surface of the epiglottis is attached to the upper border of the hyoid bone by the hyoepiglottic ligament. The laryngeal inlet is bound laterally by the aryepiglottic folds, and posteriorly by the corniculate cartilages and the interarytenoid notch (see Fig. 40.6). The space inferior to the laryngeal inlet down to the inferior border of the cricoid cartilage is the laryngeal cavity. The ventricular folds (also referred to as the vestibular folds or false vocal cords) are the most superior structure within the laryngeal cavity. Beneath these are the true vocal cords, which attach to the arytenoids posteriorly and the thyroid cartilage anteriorly, where they join to form the anterior commissure. The space between the vocal cords is termed the glottis, the portion of the laryngeal cavity above the glottis is known as the vestibule, and the portion inferior to the vocal cords is known as the subglottis. TRACHEA AND BRONCHI The trachea begins at the level of the cricoid cartilage and extends to the carina at the level of the fifth thoracic vertebra; this length is 10 to 15 cm in the adult. It consists of 16 to 20 C-shaped cartilaginous rings that open posteriorly and are joined by fibroelastic tissue; the trachealis muscle forms the posterior wall of the trachea. At the carina, the trachea bifurcates into the right and left mainstem bronchi. In the adult, the right mainstem bronchus branches off at a more vertical angle than the left mainstem bronchus, resulting in a greater likelihood of foreign bodies and endotracheal tubes (ETTs) entering the right bronchial lumen.32 Airway Assessment Although the anesthesiologist should always be prepared for potential difficulty with airway management, the ability to predict the difficult airway in advance is obviously desirable. Certain details from the patient’s history or physical findings can be prognostic of difficulty with mask ventilation, SGA placement, laryngoscopy, tracheal intubation, or the need for a surgical airway. No single test has been devised to predict a difficult airway accurately 100% of the time; however, a complete evaluation of the airway and knowledge of the difficult airway predictors can alert the anesthesiologist to the potential for difficulty and allow for appropriate planning. TRADITIONAL METRICS Airway assessment should begin with a directed patient history whenever possible.8 One of the most predictive factors for difficult intubation is a history of previous difficulty with intubation.33 On the other hand, a history of a previously easy airway does not rule out the possibility of difficulty with ventilation or intubation. In either case, the patient interview should specifically address changes in weight, symptoms, and pathologic conditions since the last general anesthetic (if there was one), and attempts should be made to obtain prior anesthetic records as they may yield useful information concerning airway management. The patient should be questioned about the presence of pathologic states that increase the risk of a difficult airway. A focused systems review can alert the anesthesiologist to other potential factors that may predict difficult airway management; for example, a history of snoring has been shown to be predictive of difficult mask ventilation.34,35 B ox 4 0. 1 C omponent s of t he P hy sic al E xaminat ion of t he A ir w ay ▪ Visual inspection of the face and neck ▪ Assessment of mouth opening ▪ Evaluation of oropharyngeal anatomy and dentition ▪ Assessment of neck range of motion (ability of the patient to assume the sniffing position) ▪ Assessment of the submandibular space ▪ Assessment of the patient’s ability to slide the mandible anteriorly (test of mandibular prognathism) A physical examination of the airway should be performed to detect any physical characteristics that may suggest a difficult airway.8 The specific characteristics that should be evaluated in this examination are listed in Box 40.1. The visual inspection of the face and neck should focus on physical characteristics that may indicate the potential for difficulty with airway management. These include obvious facial deformities, neoplasms involving the face or neck, facial burns, a large goiter, a short or thick neck, or a receding mandible. The presence of a beard has been shown to be associated with difficult ventilation attributable to the difficulty in obtaining a mask seal.34,35 Cervical collars or cervical traction devices can interfere with both mask ventilation and DL. A neck circumference greater than 43 cm (17 inches) is associated with difficulty with tracheal intubation36; a large neck circumference is, in fact, more predictive of difficulty with tracheal intubation than a high body mass index (BMI).37 Assessment of mouth opening and inspection of the oropharyngeal anatomy is achieved by instructing the patient to open their mouth as wide as possible. An interincisor distance of less than 3 cm (or 2 fingerbreadths), as measured from the upper to the lower incisors with maximal mouth opening, can suggest the possibility of difficult intubation7; some studies have used 4 or 4.5 cm as the cutoff.38 A thorough inspection of the oropharynx can help identify pathologic characteristics that may result in difficulty with intubation, such as neoplasm, a high arched palate, or macroglossia. In 1983, Mallampati and associates described a clinical sign to predict difficult tracheal intubation based on the size of the base of the tongue.39 A Mallampati classification of I to III is assigned, based on the visibility of the faucial pillars, uvula, and soft palate when the patient is seated upright with the head neutral, the mouth open, the tongue protruded, and no phonation.40 Higher scores on the Mallampati classification indicate poor visibility of the oropharyngeal structures attributable to a large tongue relative to the size of the oropharyngeal space, and, subsequently, a more difficult DL/VL. The modified Mallampati classification described by Samsoon and Young,41 which adds a fourth classification, is defined as follows (Fig. 40.8): ▪ Class I: Faucial pillars, uvula, and soft palate are visualized. ▪ Class II: Base of the uvula and soft palate are visualized.  FIG. 40.8 Modified Mallampati classification as described by Samsoon and Young.Classes are differentiated on the basis of the structures visualized: class I—soft palate, fauces, uvula, tonsillar pillars; class II —soft palate, fauces, uvula; class III—soft palate, base of the uvula; class IV—soft palate not visible. From Mallampati SR. Recognition of the difficult airway. In: Benumof JL, ed. Airway Management Principles and Practice. St Louis: Mosby; 1996, p. 132. ▪ Class III: Soft palate only is visualized. ▪ Class IV: Hard palate only is visualized. As a stand-alone test, the modified Mallampati classification is insufficient for accurate prediction of difficult intubation; however, it may have clinical utility in combination with other difficult airway predictors.42 Some studies support obtaining a Mallampati score with the head in full extension to improve the predictive value of the test.40,43 A Mallampati zero classification has been proposed when the epiglottis can be visualized during examination of the oropharynx; this finding is usually associated with easy laryngoscopy,44,45 although difficulty with airway management attributable to a large, floppy epiglottis in patients with a Mallampati zero classification can occur.46,47 An examination of dentition should be performed when the oropharyngeal anatomy is being evaluated.8 Relatively long upper incisors can impair DL/VL. Poor dentition and loose teeth increase the risk of dental trauma and present a risk of tooth dislodgment with subsequent aspiration; very loose teeth should be removed before laryngoscopy. Dental work, such as veneers, caps, crowns, and bridges, are particularly susceptible to damage during airway management. Edentulousness is predictive of easy tracheal intubation but potentially difficult mask ventilation.48 The ideal positioning for DL is achieved by cervical flexion and atlantooccipital extension and is most commonly referred to as the sniffing position49 (see Direct Laryngoscopy: Preparation and Positioning, below). Assessment of a patient’s ability to assume this position should be included in the airway examination; an inability to extend the neck at the atlantooccipital joint is associated with difficult intubation.50 Head and neck mobility can also be quantitatively assessed by measuring the sternomental distance between the sternal notch and the point of the chin with the head in full extension and the mouth closed. Distances less than 12.5 cm are associated with difficult intubation.51 An assessment of overall neck range of motion can be performed by measuring the angle created by the forehead when the neck is fully flexed and then fully extended; a measurement of less than 80 degrees is predictive of difficult intubation.52 During DL, the tongue is displaced into the submandibular space; glottic visualization may be inadequate if this space is diminished because of a small mandible. This scenario is frequently referred to as an anterior larynx. A thyromental distance of less than 6.5 cm (3 fingerbreadths), as measured from the thyroid notch to the lower border of the chin, is indicative of reduced mandibular space and may predict difficulty with intubation.38,51 Compliance of this space should also be assessed; a lack of compliance or the presence of a mass is a nonreassuring finding.8 Tests of the ability for mandibular protrusion (prognathism) have predictive value and should be included in the airway assessment. The inability to extend the lower incisors beyond the upper incisors may be indicative of difficult laryngoscopy.53 A similar evaluation, the upper lip bite test (ULBT), has been shown to predict difficult laryngoscopy with higher specificity and less interobserver variability than the Mallampati classification; an inability of the lower incisors to bite the upper lip is associated with more difficult laryngoscopy.54,55 Although individual airway tests are limited by low sensitivity and positive predictive value, some multivariable assessments have been shown to have higher predictive power. The Mallampati score has been shown to have improved predictive value when combined with thyromental, sternomental, and/or interincisor distances.51 Models that use several risk factors, such as the Wilson risk sum score (weight, head and neck movement, jaw movement, receding mandible, and buck teeth) and the El-Ganzouri risk index (mouth opening, thyromental distance, Mallampati class, neck movement, prognathism, weight, and history of difficult intubation) have been developed in an attempt to improve the predictive value of airway assessment.52,56 On the other hand, a large database study of an airway risk index that utilizes seven independent risk factors found that it does not improve prediction of difficult intubation.57 A computer-assisted model that uses complex interactions among several risk factors (BMI, mouth opening, thyromental distance, Mallampati class, and receding mandible) predicted difficult intubation more accurately than other models based on simpler statistical analyses.58 NEW MODALITIES Owing to the poor sensitivity and specificity of traditional metrics for airway assessment, several new modalities are being studied. The use of point-of-care ultrasonography for the prediction of difficult laryngoscopy and intubation has shown some promise in small studies, but its overall value has yet to be established.59 Computed tomographic images of the head and neck can be used to create three-dimensional virtual endoscopic images that can be used for planning difficult airway management, particularly for patients with complex airway pathology.60 Early studies of facial image analysis have also shown promise for predicting difficult intubation.61 Physiologic Concepts for Airway Management PREOXYGENATION With the induction of anesthesia, hypoxemia can quickly develop as a result of hypoventilation or apnea in combination with decreases in functional residual capacity (FRC) attributable to the supine position, muscle paralysis, and the direct effects of the anesthetic agents. Preoxygenation, the process of replacing nitrogen in the lungs with oxygen, provides an increased length of time before oxyhemoglobin desaturation occurs in an apneic patient. This lengthened safe apnea time provides an improved margin of safety while the anesthesiologist secures the airway and resumes ventilation. Adequate preoxygenation is essential when mask ventilation after the induction of anesthesia is contraindicated or anticipated to be difficult, when intubation is anticipated to be difficult, and in patients with a smaller FRC (i.e., patients who are obese or pregnant).62 Because difficulty with airway management can unexpectedly occur, routine preoxygenation before induction of general anesthesia is recommended.63 Preoxygenation is typically performed via a face mask attached to an anesthesia circuit. To ensure adequate preoxygenation, 100% oxygen must be provided at a flow rate high enough to prevent rebreathing (10 to 12 L/min), and no leaks around the face mask must be present. An end-tidal concentration of oxygen greater than 90% is considered to maximize safe apnea time. With maximal preoxygenation, the time to oxyhemoglobin desaturation below 80% can vary from 9 minutes in a healthy, nonobese adult to 3 minutes or less in children or obese adults.64 Two methods have been traditionally described to accomplish preoxygenation. The first method uses tidal volume ventilation through the face mask for 3 minutes, which allows the exchange of 95% of the gas in the lungs.62 The second method uses vital capacity breaths to achieve adequate preoxygenation more rapidly. Four breaths over 30 seconds is not as effective as the tidal volume method but may be acceptable in certain clinical situations; eight breaths over 60 seconds has been shown to be more effective.62 Another approach to preoxygenation is to continue tidal volume ventilation through the facemask until an end-tidal oxygen concentration of 90% is reached.65 Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) at 60 L/min for 3 minutes is as effective as tidal volume preoxygenation by face mask (see Apneic Oxygenation, below).66 Head-up positioning improves the quality of preoxygenation in both obese67 and nonobese patients.68 Noninvasive positive-pressure ventilation (PPV) for preoxygenation also prolongs apnea time.69,70 APNEIC OXYGENATION Apneic oxygenation is a physiologic phenomenon by which oxygen from the oropharynx or nasopharynx diffuses down into the alveoli as a result of the net negative alveolar gas exchange rate resulting from oxygen removal and carbon dioxide excretion during apnea. Assuming the airway is patent, and oxygen is insufflated through the nose and/or mouth, oxygenation occurs, prolonging safe apnea time beyond that of standard face mask preoxygenation.71 Oxygen can be insufflated at up to 15 L/min with nasal cannulae (nasal oxygen during efforts securing a tube [NO DESAT])72 or with a catheter placed through the nose or mouth with the tip in the pharynx (pharyngeal oxygen insufflation).73 These techniques are effective in delaying oxyhemoglobin desaturation in morbidly obese patients74,75 and during emergency tracheal intubation.76,77  THRIVE involves the administration of warmed, humidified oxygen, allowing higher oxygen flow rates than the previously described techniques – up to 70 L/min. These higher flows extend the safe apnea time even further and improve the clearance of carbon dioxide, preventing the potential development of severe respiratory acidosis. In 25 patients with a difficult airway at risk for rapid desaturation, THRIVE was used to achieve a median apnea time without arterial desaturation to