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’ Practice Guidelines for Management of the Difficult Airway and the accompanying Difficult Airway Algorithm provide guidelines for the evaluation of the airway and preparation 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, are useful to help implement airway algorithms in an emergency situation.
A detailed understanding of airway anatomy is essential for the anesthesia provider.
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 30 years, the laryngeal mask airway (LMA) has emerged as 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 supraglottic airway.
Flexible scope intubation of the trachea in an awake, spontaneously ventilating, and cooperative patient is the gold standard for the management of the difficult airway.
Invasive airways are indicated as a rescue technique when attempts at establishing a noninvasive airway fail. The anesthesia practitioner should become proficient with techniques for transtracheal jet ventilation and cricothyrotomy.
Extubation is a critical component of airway management with the potential for significant complications. The plan for extubation of the trachea must be preemptively formulated and includes a strategy for reintubation should the patient be unable to maintain an adequate airway after extubation.
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 foundation of airway management; however, in the past 30 years, the laryngeal mask airway (LMA) has 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 has demonstrated that the development of an airway emergency increases the odds of death or brain damage by 15-fold. Although the proportion of claims attributable to airway-related complications has decreased over the past 3 decades, airway complications are still the second-most common cause of claims. 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 1-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 underreporting is considered. 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. Poor assessment of the airway, poor planning, and a lack of personal and/or institutional preparedness for managing difficulty with airway management were the most common contributing factors.
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. 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 into the clinical arena, each with unique properties that may be advantageous in certain situations. Becoming familiar with new devices under controlled conditions is important for the anesthesia practitioner—the 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.” The most recent update to this report, published in 2013, defines the difficult airway as “the clinical situation in which a conventionally trained anesthesiologist experiences difficulty with ventilation of the upper airway via a mask, difficulty with tracheal intubation, or both” 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. 44.1 ). The ASA DAA begins with a consideration of the relative clinical merits and feasibility of four basic management choices: (1) awake intubation versus intubation after induction of general anesthesia, (2) noninvasive techniques versus invasive techniques (i.e., surgical or percutaneous airway) for the initial approach to intubation, (3) video-assisted laryngoscopy (VAL) as an initial approach to intubation, and (4) preservation versus ablation of spontaneous ventilation.
The ASA DAA does not follow a linear decision-making tree, as the advanced cardiac life support (ACLS) algorithms do. It can be better understood and remembered by considering it as 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 oxygenate” [CICO] scenario or the “ emergency ” pathway).
Other Difficult Airway Algorithms
In addition to the ASA, several different national anesthesia societies have published their own guidelines for management of the difficult airway, including the Difficult Airway Society (DAS) from the United Kingdom, the Canadian Airway Focus Group (CAFG), the French Society of Anesthesia and Intensive Care (SFAR), the German Society of Anesthesiology and Intensive Care Medicine (DGAI), the Italian Society for Anesthesia and Intensive Care (SIAARTI), and the Japanese Society of Anesthesiologists. All of these include recommendations for the prediction of the difficult airway and suggest awake intubation as a management strategy (with the exception of the DAS guidelines) and all incorporate algorithms for both unanticipated difficult intubation with adequate oxygenation and the CICO scenario. Common elements include a focus on awakening the patient in the setting of a difficult intubation with adequate ventilation, the use of the LMA as a rescue for difficult mask ventilation, and emergency front of neck access (FONA) in the CICO scenario. The primary differences in these algorithms are in specific details, such as the number of intubation attempts suggested, the specific alternate devices recommended for difficult intubation, and the organization of the algorithm.
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. The use of airway checklists, preprocedural team briefings, and cognitive aids are all strategies for addressing human factor challenges.
The Vortex approach, conceived by Dr. Nicholas Chrimes, a specialist anaesthetist in Melbourne, Australia, is one such cognitive aid designed to facilitate management of the unanticipated difficult airway. 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. 44.2 ) to guide the airway practitioner through the three basic nonsurgical airway techniques (face-mask ventilation, supraglottic airway [SGA], and tracheal intubation). If after an “optimal attempt” at each of these nonsurgical modalities alveolar oxygen delivery 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.
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 the formulation of 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.
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 population ; therefore the more patent side should be determined before passing instrumentation through the nasal passages. The lateral wall of the nasal passages is characterized by the presence of three turbinates (or conchae) that divide the nasal passage into three scroll-shaped meatuses ( Fig. 44.3 ). The inferior meatus, between the inferior turbinate and the floor of the nasal cavity, is the preferred pathway for passage of nasal airway devices ; improper placement of objects in the nose can result in avulsion of a turbinate. 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.
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.
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.
Beneath the tongue, the mylohyoid muscles separate the floor of the mouth into the sublingual space superiorly and the submental space inferiorly. Cellulitis (Ludwig’s angina) or hematoma formation in these spaces can cause elevation and posterior displacement of the tongue and resultant airway obstruction.
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 the formation of a retropharyngeal dissection. 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. A chin lift with mouth closure increases longitudinal tension in the pharyngeal muscles, counteracting the tendency of the pharyngeal airway to collapse.
The pharynx can be divided into the nasopharynx, the oropharynx, and the hypopharynx ( Fig. 44.4 ). 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 passaging 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. 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. 44.5 ). 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. 44.6 ).
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. 44.7 ).
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.
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. 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. 44.6 ).
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 together 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.
Although the anesthesia provider should always be prepared for potential difficulty with airway management, the ability to predict the difficult airway in advance is obviously desirable. Certain physical findings or details from the patient’s history can be prognostic of difficulty with mask ventilation, supraglottic airway placement, laryngoscopy, tracheal intubation, or the performance of 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.
Airway assessment should begin with a directed patient history whenever possible. One of the most predictive factors for difficult intubation is a history of previous difficulty with intubation. 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, symptomatology, and pathologic conditions since the last induction of an anesthetic (if there was one), and attempts should be made at obtaining prior anesthetic records—they may yield useful information concerning airway management. The presence of pathologic states that increase the risk of a difficult airway should be elicited by performing a medical history. A focused review of systems 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.
A physical examination of the airway should be preoperatively performed, when possible, to detect any physical characteristics that may suggest a difficult airway. The specific characteristics that should be evaluated in this examination are listed in Box 44.1 .
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)
The visual inspection of the face and neck should focus on any 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. 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 intubation ; Brodsky demonstrated that a large neck circumference is, in fact, more predictive of difficulty with tracheal intubation than a high body mass index (BMI).
Assessment of mouth opening and inspection of the oropharyngeal anatomy is achieved by instructing the patient to open his or her 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 intubation ; some studies have used 4 or 4.5 cm as the cutoff. 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. 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. 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 laryngoscopy. The modified Mallampati classification described by Samsoon and Young, which adds a fourth classification, is the most commonly used airway assessment test in current anesthesia practice and is defined as follows ( Fig. 44.8 ):
Class I: Faucial pillars, uvula, and soft palate are visualized.
Class II: Base of the uvula and soft palate are visualized.
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. Some studies support obtaining a Mallampati score with the head in full extension to improve the predictive value of the test. 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, although difficulty with airway management attributable to a large, floppy epiglottis in patients with a Mallampati zero classification can occur.
An examination of dentition should be performed when the oropharyngeal anatomy is being evaluated. Relatively long upper incisors can impair DL. 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. Cosmetic 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.
The ideal positioning for DL is achieved by cervical flexion and atlantooccipital extension and is most commonly referred to as the sniffing position (see Direct Laryngoscopy: Preparation and Positioning). 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. 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. 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.
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 mentum, is indicative of reduced mandibular space and may predict difficulty with intubation. Compliance of this space should also be assessed; a lack of compliance or the presence of a mass is a nonreassuring finding.
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. A similar evaluation, the upper lip bite test (ULBT) described by Khan and colleagues, 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.
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. 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. On the other hand, a recent large database study of an airway risk index that utilizes seven independent risk factors found that it does not improve prediction of difficult intubation. Langeron and associates developed a computer-assisted model that uses complex interactions among several risk factors (BMI, mouth opening, thyromental distance, Mallampati class, and receding mandible) to predict difficult intubation more accurately than other models based on simpler statistical analyses.
Owing to the poor sensitivity and specificity of traditional metrics for airway assessment, a number of 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. 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. Early studies of facial image analysis have also shown promise for the use of this technology in predicting difficult intubation.
Physiologic Concepts for Airway Management
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 themselves. Preoxygenation, the process of replacing nitrogen in the lungs with oxygen, provides an increased length of time before hemoglobin desaturation occurs in an apneic patient. This lengthened 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). Because difficulty with airway management can unexpectedly occur, routine preoxygenation before induction of general anesthesia is recommended.
Preoxygenation is typically performed via a face mask attached to either the anesthesia machine or a Mapleson 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 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.
Two primary methods are used 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. 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.
Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) at 60 L/min for 3 minutes has been demonstrated to be as effective as tidal volume preoxygenation by face mask (see Apneic Oxygenation). Head-up positioning has been shown to improve the quality of preoxygenation in both obese and nonobese patients. The use of noninvasive positive-pressure ventilation (PPV) for preoxygenation also prolongs apnea time.
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 apnea time beyond that of standard face-mask preoxygenation.
Oxygen can be insufflated at up to 15 L/min with nasal cannulae (nasal oxygen during efforts securing a tube [NO DESAT]) or with a catheter placed through the nose or mouth with the tip in the pharynx (pharyngeal oxygen insufflation). Studies have demonstrated that these techniques are effective in delaying oxyhemoglobin desaturation in morbidly obese patients and during emergency tracheal intubation.
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 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 of 14 minutes, with a range of 5 to 65 minutes, and an average rate of carbon dioxide rise of only 1.1 mm Hg per minute.
Pulmonary Aspiration of Gastric Contents
In 1946, Mendelson was the first to describe aspiration pneumonitis attributable to the pulmonary aspiration of acidic gastric secretions in pregnant women undergoing anesthesia. This potentially fatal complication, occasionally referred to as Mendelson syndrome, has since been the intense focus of preventive efforts among the anesthesia community. Prevention of aspiration of gastric contents is primarily accomplished by adherence to established preoperative fasting guidelines, premedication with drugs that may decrease the risk of aspiration pneumonitis, and specialized induction techniques, which are discussed later in this chapter.
Traditionally, patients who were scheduled for elective procedures requiring sedation, regional anesthesia, or general anesthesia were instructed to remain NPO (Latin for nulla per os or nothing by mouth ) after midnight to ensure an empty stomach to decrease the risk of regurgitation. Based on evidence that allowing ingestion of clear liquids 2 to 4 hours before surgery resulted in lower gastric volumes and higher gastric pH, the ASA published Practice Guidelines for Preoperative Fasting and the Use of Pharmacologic Agents to Reduce the Risk of Pulmonary Aspiration in 1999 that liberalized the traditional NPO policy and allowed clear liquids up to 2 hours before beginning elective procedures requiring anesthesia. The guidelines, most recently updated in 2017, recommend 4 hours of fasting from breast milk and 6 hours of fasting from solid foods, infant formula, and nonhuman milk. Fried or fatty foods may require longer fasting times (e.g., 8 hours or more). Although the ASA guidelines do not specifically address chewing gum, hard candies, or smoking, guidelines published by the European Society of Anaesthesiology on the topic do not recommend delaying the start of anesthesia if a patient has consumed any of these immediately before the induction of anesthesia.
The routine use of drugs as prophylaxis against aspiration pneumonitis is not recommended by the ASA guidelines but may be beneficial in patients with specific risk factors for aspiration, such as a full stomach, symptomatic gastroesophageal reflux disease (GERD), hiatal hernia, presence of a nasogastric tube, morbid obesity, diabetic gastroparesis, or pregnancy. The goal of aspiration prophylaxis is twofold: to decrease gastric volume and to increase gastric fluid pH. Commonly used agents include nonparticulate antacids (e.g., Bicitra), promotility drugs (e.g., metoclopramide), and H 2 -receptor antagonists. These drugs may be used alone or in combination.
Airway Reflexes and the Physiologic Response to Intubation of the Trachea
One of the most important teleologic functions of the larynx is that of airway protection, which is primarily provided by the glottic closure reflex. This reflex is triggered by sensory receptors in the glottic and subglottic mucosa and results in strong adduction of the vocal cords. An exaggerated, maladaptive manifestation of this reflex, referred to as laryngospasm , is a potential complication of airway management. Laryngospasm is usually provoked by glossopharyngeal or vagal stimulation attributable to airway instrumentation or vocal cord irritation (e.g., from blood or vomitus) in the setting of a light plane of anesthesia (stage II of the Guedel classification), but it can also be precipitated by other noxious stimuli and can persist well after the removal of the stimulus. Treatment of laryngospasm includes removal of airway irritants, deepening of the anesthetic, and the administration of a rapid-onset neuromuscular blocking drug (NMBD), such as succinylcholine. Continuous positive airway pressure with 100% oxygen is commonly cited as a therapeutic maneuver, although the pressure may push the aryepiglottic folds closer together and may actually promote laryngospasm by acting as a mechanical stimulus. Bilateral pressure at the laryngospasm notch between the condyle of the mandible and the mastoid process can be effective at treating laryngospasm by causing an intense, painful stimulus, which may function to terminate laryngospasm by arousing a semiconscious patient or by activating autonomic pathways.
The tracheobronchial tree also possesses reflexes to protect the lungs from noxious substances. Irritation of the lower airway by a foreign substance activates a vagal reflex–mediated constriction of bronchial smooth muscle, resulting in bronchospasm. Untreated bronchospasm can result in an inability to ventilate because of an extremely elevated airway resistance. Treatment includes a deepening of anesthetic with propofol or a volatile agent and the administration of inhaled β 2 -agonist or anticholinergic medications. Administration of intravenous (IV) lidocaine has been studied, but the evidence does not support its use for treatment of bronchospasm.
Tracheal intubation, as well as laryngoscopy and other airway instrumentation, provides an intense noxious stimulus via vagal and glossopharyngeal afferents that results in a reflex autonomic activation, which is usually manifested as hypertension and tachycardia in adults and adolescents; in infants and small children, autonomic activation may result in bradycardia. Hypertension and tachycardia are usually of short duration; however, they may have consequences in patients with significant cardiac disease. Central nervous system activation as a result of airway management results in increases in electroencephalographic (EEG) activity, cerebral metabolic rate, and cerebral blood flow, which may result in an increase in intracranial pressure in patients with decreased intracranial compliance.
Anesthesia for Airway Management
To facilitate airway management, some form of anesthesia is usually required to provide comfort for the patient, to blunt airway reflexes, and to blunt the hemodynamic response to airway instrumentation. Most commonly, airway management is performed after induction of general anesthesia. Alternatively, an awake technique, which entails establishing an airway (including tracheal intubation) by using local anesthesia of the airway and/or sedation, can be used to meet these goals when clinically indicated. In emergency scenarios where the patient is obtunded or comatose, such as in the event of acute respiratory or cardiac arrest, anesthetic drugs may not be required.
Airway Management after the Induction of General Anesthesia
Airway management is usually performed after the induction of general anesthesia if the anesthesiologist determines that it is safe to do so. Several pharmacologic techniques are used for the induction of anesthesia, each with its own implications for airway management. The decision of which induction technique to use should be made with careful consideration of the specific clinical circumstances at hand.
Standard Intravenous Induction with Neuromuscular Blockade
The most common technique for induction of general anesthesia is the standard IV induction, which entails the administration of a rapid-acting IV anesthetic, followed by an NMBD. Muscle relaxation achieved by the administration of NMBDs improves intubating conditions by facilitating laryngoscopy, preventing both reflex laryngeal closure and coughing after intubation.
Propofol is the most frequently used IV anesthetic drug; other options include etomidate, ketamine, thiopental, and midazolam. The choice of drug depends on a variety of factors including the patient’s hemodynamic status, comorbidities, and allergies, as well as drug pharmacokinetics, side effects, physician preference, and availability. Whether the choice of an anesthetic drug has any effect on the quality of intubating conditions when NMBDs are also administered is not well established. Studies comparing propofol, etomidate, and thiopental in combination with NMBDs showed no difference in intubating conditions between the different anesthetics. On the other hand, one study, during which patients received cisatracurium, showed that larger doses of propofol were associated with improved intubating conditions, as compared with smaller doses.
For many years, succinylcholine was the most frequently used NMBD for routine IV induction ; however, nondepolarizing NMBDs have gained greater popularity attributable to the risk of adverse effects from succinylcholine administration, including bradycardia, myalgia, hyperkalemia, increased intracranial pressure, and increased intragastric pressure. Succinylcholine, the only depolarizing NMBD in clinical use, has the benefit of a rapid onset combined with a short duration of action, and it is currently used most often when those properties are desired. Most notably, succinylcholine is still commonly used in the setting of a suspected difficult airway; its short duration of action theoretically allows for the resumption of spontaneous ventilation before severe hypoxia develops in a preoxygenated patient, although evidence suggests that this may not predictably occur .
Nondepolarizing NMBDs are the more frequently used relaxants for routine IV induction of anesthesia. The most commonly used nondepolarizing NMBDs in current practice—rocuronium, vecuronium, and cisatracurium—are notable for having a favorable safety profile with relatively few side effects. The primary limitation of these drugs is a significantly longer duration of action; once administered, a functional airway must be established within minutes to avoid life-threatening hypoxia. Sugammadex is a selective relaxant-binding agent for rocuronium that has the ability to reverse profound neuromuscular blockade rapidly in a time comparable with spontaneous recovery from succinylcholine (also see Chapter 28 ).
Traditional teaching in the United States has advocated withholding NMBDs until the ability to mask ventilate has been established. If ventilation via a mask cannot be achieved, a preoxygenated patient can then theoretically resume spontaneous ventilation or be awakened before the onset of hypoxia. This practice has been increasingly questioned in the literature in part because of a number of studies demonstrating that ventilation via a mask is not rendered more difficult by muscle relaxation ; rather, mask ventilation is, in fact, facilitated by muscle relaxation. One issue with the traditional paradigm is that the theoretical advantage of the practice—the ability to awaken the patient if mask ventilation fails—is rarely used. The desire to preserve that ability may, in fact, result in giving an inadequate dose of anesthetic during induction, resulting in a difficult mask ventilation situation when one would not have otherwise occurred. Delaying the administration of NMBDs can result in the onset of hypoxia before spontaneous recovery (with succinylcholine) or reversal (with sugammadex) is possible.
The authors do not recommend withholding NMBDs in patients who are predicted to be easy to mask ventilate and/or intubate. For patients in whom difficulty with both mask ventilation and intubation are predicted, awake intubation or inhalation induction of anesthesia should be considered, and the administration of NMBDs is best withheld until the ability to ventilate is proven.
Rapid-Sequence Induction and Intubation
Rapid-sequence induction and intubation (often simply referred to as rapid sequence induction [RSI] in the anesthesia literature) is a specialized method of IV induction commonly used when an increased risk of gastric regurgitation and pulmonary aspiration of gastric contents exists. After adequate preoxygenation and while cricoid pressure is applied, an induction dose of IV anesthetic is rapidly followed by 1 to 1.5 mg/kg of IV succinylcholine, and the trachea is intubated without attempts at PPV. The goal is to achieve optimal intubating conditions rapidly to minimize the length of time between the loss of consciousness (LOC) and securing of the airway with a cuffed endotracheal tube (ETT). Cricoid pressure, eponymously referred to as the Sellick maneuver after the physician who first described it, involves the application of pressure at the cricoid ring to occlude the upper esophagus, thereby preventing the regurgitation of gastric contents into the pharynx. The recommended force to be applied is 10 Newtons (N) while the patient is awake, increased to 30 N after LOC. These values are based on esophageal manometry on patients undergoing induction of anesthesia and cadaver studies of safe amounts of pressure. RSI is widely practiced and approaches a standard of care in patients with a full stomach (i.e., when NPO guidelines have not been observed) and in the setting of bowel obstruction. RSI has historically been recommended for patients who are pregnant, starting in the second trimester, but this dogma has been called into question. Other clinical situations for which RSI may be considered due to a higher than normal risk for aspiration of gastric contents, include poorly controlled GERD, presence of a nasogastric tube, morbid obesity, and diabetic gastroparesis. RSI is also a useful induction technique when mask ventilation is predicted to be difficult, but intubation is not, such as with an edentulous, bearded patient with an otherwise reassuring airway examination.
Some common variations to RSI have developed from the technique first described in 1970. When succinylcholine is contraindicated or its side effects are undesired, RSI can be accomplished using nondepolarizing NMBDs (rocuronium 1.0 to 1.2 mg/kg or vecuronium 0.3 mg/kg); these doses provide adequate intubating conditions in less than 90 seconds. The primary disadvantage with the nondepolarizing NMBDs used to be the prolonged duration of neuromuscular blockade; however, since the introduction of sugammadex these agents are increasingly employed in place of succinylcholine for RSI (also see Chapter 27, Chapter 28 ). Although traditional RSI calls for induction with a fixed dose of thiopental, the use of other anesthetics such as propofol, etomidate, or ketamine is common. Some advocate for the titration of the chosen anesthetic agent to LOC rather than the delivery of a fixed, predetermined dose.
The application of cricoid pressure is the most controversial aspect of RSI. Opponents point to studies demonstrating that cricoid pressure results in a decrease in lower esophageal sphincter tone, potentially increasing the risk for regurgitation, and to magnetic resonance imaging (MRI) studies showing that cricoid pressure does not, in fact, result in compression of the esophagus, but rather a lateral displacement. Cricoid pressure also worsens laryngeal visualization during DL, potentially lengthening the time to intubation and increasing the risk of pulmonary aspiration, and can result in occlusion of the subglottic airway, resulting in difficulty with tracheal intubation or mask ventilation. On the other hand, advocates argue that properly applied cricoid pressure is effective in reducing the risk of aspiration and that reports of problems are due to incorrect application. The authors of an MRI study of cricoid pressure argue that the position of the esophagus is irrelevant because the effectiveness of cricoid pressure is due to occlusion of the hypopharynx. In general, because of the relatively low risk of application of cricoid pressure, its use is encouraged for RSI unless glottic visualization proves difficult, in which case it can be easily released.
The term modified RSI is frequently used, but no standardized definition exists. A survey of anesthesia residents and attending anesthesiologists in the United States showed that the term was most commonly used to refer to the use of mask ventilation in conjunction with cricoid pressure. Indications for this technique include patients at risk for rapid development of hypoxemia (e.g., patients who are obese, pregnant, or critically ill; pediatric patients) in emergent situations during which preoxygenation cannot be satisfactorily completed, or when a longer time to acceptable intubating conditions is required because of the use of standard doses of nondepolarizing NMBDs. Although the effect of PPV with cricoid pressure applied in terms of gastric insufflation of air is not definitively known, gentle PPV (inspiratory pressure <20 cm water [H 2 O]) in conjunction with cricoid pressure may be acceptable in these clinical scenarios.
Inhalational Induction of Anesthesia
Another option for the induction of general anesthesia is inhalational induction with volatile anesthetic. This technique is commonly used in pediatric anesthesia to provide a painless, needle-free experience for the child. In adults, an inhalational induction of anesthesia is used when IV access is not available or when the specific advantages of the technique are desirable. Advantages of an inhalational induction of anesthesia are the maintenance of spontaneous ventilation and the potential for gradual changes in the depth of anesthesia and associated respiratory and cardiovascular effects. Inhalational induction of anesthesia has also been used for RSI, with a rapid-onset NMBD administered at LOC (also see Chapter 27 ).
Sevoflurane is currently the most commonly used volatile anesthetic for inhalational induction because of its lack of pungency and low blood:gas solubility, allowing for a smooth induction of anesthesia that can provide suitable conditions for airway management with or without adjuvant drugs such as NMBDs or opioids. The two principal techniques for sevoflurane induction of anesthesia are a tidal volume induction , in which patients are instructed to breathe normally through the face mask, and a vital capacity induction , in which patients are instructed to exhale to residual volume and then take a vital capacity breath from the face mask. High delivered concentrations of sevoflurane (8%) are used for vital capacity induction, whereas tidal volume inductions may start with lower sevoflurane concentrations before the concentration is increased. Nitrous oxide (N 2 O) can be used with either method to speed induction via the second-gas effect. Both methods are effective and can be used for either LMA placement or tracheal intubation. Deep levels of anesthesia are required to achieve satisfactory intubating conditions when using sevoflurane as a sole induction agent, increasing the risk of adverse effects, such as hypotension. The administration of propofol, rapid-onset opioids, NMBDs, and ketamine have all been shown to improve intubating conditions and allow for lower end-tidal concentrations of sevoflurane.
Halothane, which is still used in developing countries, can also be used for inhalational induction of anesthesia. One main disadvantage of halothane is its high blood:gas partition coefficient, which leads to relatively long induction times. It also can produce cardiac dysrhythmias, myocardial depression, and halothane-induced hepatitis. Because of the inability to achieve deep levels of anesthesia with halothane as a result of its side effects, the use of NMBDs, opioids, or both, is often required. The use of desflurane for inhalational induction of anesthesia is limited by its tendency to cause airway irritation, although reports of its use for induction in combination with opioids has been reported.
Intravenous Induction Without Neuromuscular Blocking Drugs
IV induction of general anesthesia without the use of NMBDs is commonly used for LMA placement but can be used to achieve satisfactory intubating conditions as well. This technique is useful when the use of succinylcholine is contraindicated and the prolonged recovery time from nondepolarizing NMBDs is undesirable and their reversal not possible (e.g., when sugammadex is not available). Of the commonly available IV anesthetics, propofol is the best suited for induction without muscle relaxation because of its unique ability to suppress airway reflexes and to produce apnea. Larger doses are required, however, when propofol is used as a sole anesthetic, increasing the risk of significant hypotension. Improvement of intubating conditions and smaller doses of propofol are possible when rapid-onset opioids (e.g., alfentanil, remifentanil) or IV magnesium are administered. Remifentanil is more effective than comparable doses of alfentanil ; in combination with propofol 2 mg/kg, remifentanil 4 to 5 μg/kg can reliably provide good-to-excellent intubating conditions. When combined with cricoid pressure and an avoidance of mask ventilation, this induction technique can be used for RSI.
Disadvantages of this technique include a potentially more frequent incidence of difficult intubation, pronounced hemodynamic side effects such as bradycardia and hypotension, and an increased risk for laryngeal morbidity. This technique also introduces the risk of opioid-induced muscle rigidity resulting in difficulty with mask ventilation. Although this risk is commonly attributed to chest wall rigidity, studies in intubated patients and patients with tracheostomies have shown that decreases in pulmonary compliance due to chest wall rigidity are not sufficient to explain an inability to mask ventilate after a large dose of an opioid. Examination of the vocal cords during induction with opioids has shown that vocal cord closure is the primary cause of difficult ventilation after opioid-induced anesthesia. Treatment with small doses of NMBD or topical lidocaine (laryngotracheal anesthesia [LTA]) can be effective in relaxing the vocal cords to allow for mask ventilation and/or intubation.
Airway Management in an Awake (Non-Anesthetized) Patient
As noted in the ASA DAA, a consideration of whether the airway should be secured before or after induction of general anesthesia is one of the basic management choices that should be considered when an airway management plan is being devised. The benefits of awake airway management include the preservation of pharyngeal muscle tone and patency of the upper airway, the maintenance of spontaneous ventilation, an ability to obtain a quick neurologic examination, and a safeguard against aspiration attributable to the preservation of protective airway reflexes. In general, when difficult mask ventilation and difficult intubation are expected, the safest approach to airway management is to secure the airway while the patient remains awake. Other indications for awake airway management include the risk of severe aspiration of gastric contents, facial or airway trauma, severe hemodynamic instability, and unstable cervical spine pathology.
Because of the nature of these indications, tracheal intubation is most often chosen as the goal of awake airway management; however, awake placement of an LMA for diagnostic bronchoscopy has been described. The most useful technique for awake intubation is the flexible scope intubation (FSI), although other techniques have been successfully used, including VAL, optical stylets, lighted stylets, intubating LMAs, and retrograde intubation (RI).
Topical application of local anesthetic to the airway should, in most cases, be the primary anesthetic for awake airway management. Lidocaine is the most commonly used local anesthetic for awake airway management because of its rapid onset, high therapeutic index, and availability in a wide variety of preparations and concentrations. Benzocaine and Cetacaine (a topical application spray containing benzocaine, tetracaine, and butamben; Cetylite Industries, Pennsauken, NJ) provide excellent topical anesthesia of the airway, but their use is limited by the risk of methemoglobinemia, which can occur with as little as 1 to 2 seconds of spraying. Topical cocaine is primarily used for anesthesia and vasoconstriction of the nasal mucosa during awake nasotracheal intubation. A mixture of lidocaine 3% and phenylephrine 0.25%, which can be made by combining lidocaine 4% and phenylephrine 1% in a 3:1 ratio, has similar anesthetic and vasoconstrictive properties as topical cocaine and can be used as a substitute.
Topical application of local anesthetic should primarily be focused on the base of the tongue (pressure receptors here act as the afferent component of the gag reflex), the oropharynx, the hypopharynx, and the laryngeal structures; anesthesia of the oral cavity is unnecessary. If a nasotracheal intubation is planned, then the nasal cavity should also be topicalized. Before topical application of local anesthetic to the airway, administration of an anticholinergic agent should be considered to aid in the drying of secretions, which helps improve both the effectiveness of the topical local anesthetic and visualization during laryngoscopy. Glycopyrrolate is usually preferred because it has less vagolytic effects than atropine at doses that inhibit secretions and does not cross the blood-brain barrier. It should be administered as early as possible to maximize its effectiveness.
Direct application of topical cocaine, lidocaine 4% with epinephrine, or lidocaine 3%/phenylephrine 0.25% solution via cotton swabs or cotton pledgets is effective for anesthesia of the nasal mucosa. Oropharyngeal anesthesia can be achieved by the direct application of local anesthetic or by the use of an atomizer or nebulizer. Topical application of local anesthetic to the larynx can be achieved by directed atomization of a local anesthetic or by the spray-as-you-go (SAYGO) method, which involves intermittently injecting local anesthetic through the suction port or working channel of a flexible intubation scope (FIS) or optical stylet, as it is advanced toward the trachea.
Topical application of local anesthetic to the airway mucosa using one or more of these methods is often sufficient. If supplemental anesthesia is required, then a variety of nerve blocks may be used. Three of the most useful are the glossopharyngeal nerve block, superior laryngeal nerve block, and translaryngeal block.
The glossopharyngeal nerve supplies sensory innervation to the posterior third of the tongue, vallecula, the anterior surface of the epiglottis, and the posterior and lateral walls of the pharynx, and is the afferent pathway of the gag reflex. To block this nerve, the tongue is displaced medially, forming a gutter (glossogingival groove). A 25-gauge spinal needle is inserted at the base of the anterior tonsillar pillar, just lateral to the base of the tongue, to a depth of 0.5 cm ( Fig. 44.9 ). After negative aspiration for blood or air, 2 mL of 2% lidocaine is injected. The process is then repeated on the contralateral side. The same procedure can be performed noninvasively with cotton-tipped swabs soaked in 4% lidocaine; the swabs are held in place for 5 minutes ( Video 44.1 ).
The superior laryngeal nerve, a branch of the vagus nerve, provides sensory input from the lower pharynx and the upper part of the larynx, including the glottic surface of the epiglottis and the aryepiglottic folds. Blockade of this nerve may be achieved using one of three landmarks ( Fig. 44.10 ). Using either the superior cornu of the hyoid or the superior cornu of the thyroid cartilage, a 25-gauge spinal needle is walked off the cornu anteriorly toward the thyrohyoid ligament. Resistance is felt as the needle is advanced through the ligament, usually at a depth of 1 to 2 cm. After negative aspiration for blood and air, 1.5 to 2 mL of 2% lidocaine is injected and then repeated on the opposite side. The third landmark for the superior laryngeal nerve block is particularly useful in patients who are obese, in whom palpation of the hyoid or the superior cornu of the thyroid cartilage may be difficult or uncomfortable for the patient. In this approach, the needle is inserted 2 cm lateral to the superior notch of the thyroid cartilage and directed in a posterior and cephalad direction to 1 to 1.5 cm depth, where 2 mL of 2% lidocaine is infiltrated and, again, repeated on the contralateral side.
Translaryngeal (or transtracheal) block provides anesthesia of the trachea and vocal cords. This block may be particularly useful in situations where a neurologic examination is needed after intubation; it makes the presence of the ETT in the trachea more comfortable. The CTM is identified, and a 20- to 22-gauge needle attached to 5-mL syringe is directly advanced posteriorly and slightly caudally until air is aspirated, at which point 4 mL of either 2% or 4% lidocaine is quickly injected. This causes the patient to cough, anesthetizing the vocal cords and the trachea. To minimize the risk of trauma, a catheter may first be placed over the needle and the local anesthetic then injected through the catheter ( Fig. 44.11 and Video 44.2 ).
These techniques may be used in various different combinations as long as the maximum dose of local anesthetic is not exceeded. The maximum dose of lidocaine for application to the airway is not well established; different sources suggest total doses in the range of 4 to 9 mg/kg. Monitoring for signs and symptoms of lidocaine toxicity, including tinnitus, perioral tingling, metallic taste, lightheadedness, dizziness, and sedation is important. Severe lidocaine overdose can cause hypertension, tachycardia, seizures, and cardiovascular collapse.
Depending on the clinical circumstance, IV sedation may facilitate airway management in an awake patient by providing anxiolysis, amnesia, and analgesia. Benzodiazepines, opioids, IV hypnotics, α 2 agonists, and neuroleptics can be used alone or in combination. A summary of common medications used for sedation can be found in Table 44.1 . These drugs should be carefully titrated to effect; oversedation can render a patient uncooperative and make awake intubation more difficult. Spontaneous ventilation should always be maintained. Care should be taken in situations with critical airway obstruction since awake muscle tone is sometimes necessary in these patients to maintain airway patency. Avoiding oversedation is also important in the patient at increased risk for aspiration of gastric contents, because an awake patient can protect his or her own airway if regurgitation should occur.
|Midazolam||Benzodiazepine||1-2 mg IV, repeated prn (0.025-0.1 mg/kg)||Frequently used in combination with fentanyl.|
|Fentanyl||Opioid||25-200 μg IV (0.5-2 μg/kg)||Usually used in combination with other agents (e.g., midazolam, propofol).|
|Alfentanil||Opioid||500-1500 μg IV (10-30 μg/kg)||Has a faster onset, shorter duration than fentanyl.|
|Remifentanil||Opioid||Bolus 0.5 μg/kg IV, followed by an infusion of 0.1 μg/kg/min||Infusion can be subsequently titrated by 0.025-0.05 μg/kg/min in 5-minute intervals to achieve adequate sedation.|
|Propofol||Hypnotic||0.25 mg/kg IV in intermittent bolusesorContinuous IV infusion of 25-75 μg/kg/min, titrated to effect||Can also be used in combination with remifentanil (decrease dose of both drugs).|
|Ketamine||Hypnotic||0.2-0.8 mg/kg IV||Pretreat with an antisialagogue. |
Consider administration of midazolam to attenuate undesirable psychologic effects.
|Dexmedetomidine||α 2 Agonist||Bolus 1 μg/kg IV over 10 minutes, followed by an infusion of 0.3-0.7 μg/kg/hr||Reduce dose in older adults and in patients with depressed cardiac function.|
Mask ventilation is a straightforward, noninvasive technique for airway management that can be used as a primary mode of ventilation for an anesthetic of short duration or as a bridge to establish a more definitive airway. The use of a face mask is common for preoxygenation, inhalational induction of anesthesia, and as a means to provide oxygen and anesthetic gases to both a spontaneous ventilating patient and an anesthetized, apneic patient via PPV. Mask ventilation is not only used to ventilate and oxygenate before conditions for tracheal intubation have been achieved, but it is also a valuable rescue technique when tracheal intubation proves difficult. For this reason, mask ventilation is an important part of the ASA DAA and an essential skill for the anesthesia practitioner.
Mask ventilation is relatively contraindicated when the risk for regurgitation is increased; no protection from pulmonary aspiration of gastric contents exists. Mask ventilation should also be performed with caution in patients with severe facial trauma and in patients in whom head and neck manipulation must be avoided (e.g., those with an unstable cervical spine fracture).
Anesthesia face masks are designed to form a seal around the patient’s nose and mouth, allowing for PPV and the administration of anesthetic gases; they should not be confused with oxygen face masks, which are designed only to administer supplemental oxygen. Early anesthesia face masks were reusable and composed of black rubber. These have been almost entirely replaced in clinical use by disposable, clear plastic masks, which are less frightening for patients and have the added benefit of allowing for better visualization of cyanosis or the need for oral suctioning. Face masks are available in various styles and sizes but share a basic design: a main body, seal, and connector. The seal is the portion of the mask that comes in contact with the face, and in clear plastic masks is comprised of a plastic, air-filled, high-volume, low-pressure cushion that conforms to the facial anatomy while minimizing the chance for pressure ischemia; some models have a valve on the cushion to allow changing the volume of the air within. The connector is a standard 22-mm female adapter that allows a connection to a standard anesthesia circuit or a bag-valve device; pediatric masks usually have a 15-mm male adapter that allows the same connections.
The technique for mask ventilation is dependent on two key elements: (1) maintenance of a seal between the face mask and the patient’s face, and (2) an unobstructed upper airway. The mask is usually held with the left hand, with the thumb and index finger forming a “C” around the collar of the connector, the third and fourth digits on the ramus of the mandible, and the fifth digit on the angle of the mandible ( Fig. 44.12 ). The thumb and index finger are used to produce downward pressure to ensure a tight mask seal, while the remaining digits provide upward displacement of the mandible (jaw thrust) to aid with airway patency. The right hand is free to provide manual ventilation. Ensuring that pressure from the digits is placed on the bony ridge of the mandible and not the soft tissue is important—compression of the submandibular space can cause obstruction of the airway and difficulty with mask ventilation. Many face masks have hooks around the collar for use with mask straps that can facilitate formation of a seal.
The one-handed technique is occasionally ineffective, especially in patients who are obese or edentulous, attributable to the failure to maintain a seal and/or a patent upper airway. In these situations, a two-handed technique can be more successful. Two-handed techniques depend on either an assistant or the use of pressure-control ventilation (PCV) with the anesthesia machine to provide PPV. The use of PCV for mask ventilation results in lower peak airway pressures and reduced inspiratory flow rates when compared with manual ventilation, providing an additional measure of safety against gastric insufflation. In one approach to the two-handed technique, the left hand is positioned as in the one-handed technique and the right hand is placed on the other side of the mask in an identical conformation. A more effective approach involves using the second and third digits to perform a jaw thrust while the mask is held in place with the thumbs ( Video 44.3 ). A study in anesthetized patients showed that this technique improved upper airway patency, compared with the traditional one-handed technique, as measured by greater tidal volumes during PCV. Additional techniques to improve the mask seal in difficult scenarios include leaving dentures in place in edentulous patients and placing an adhesive plastic dressing over facial hair.
Once a seal is established between the face mask and the patient’s face, ventilation is achieved by either spontaneous ventilation or PPV. The effectiveness of mask ventilation should be ascertained by observing for chest rise, exhaled tidal volumes, pulse oximetry, and capnography. During controlled ventilation in patients with normal lungs and a patent airway, adequate tidal volumes should be achieved with peak inspiratory pressures less than 20 cm H 2 O; higher pressures should be avoided to prevent gastric insufflation. If PPV is inadequate at acceptable inspiratory pressures, then airway patency and pulmonary compliance should be assessed.
Because of a reduction in muscle tone as a result of general anesthesia, tissues fall backward under the influence of gravity in a supine patient and can obstruct the upper airway. Upper airway obstruction most commonly takes place at the level of the soft palate (velopharynx), epiglottis, and tongue. To maximize airway patency, mask ventilation can be performed with maximal atlantooccipital extension in combination with the forward displacement of the mandible (jaw thrust) involved in the mask-holding techniques. The addition of cervical flexion to head extension (i.e., placing the patient in the sniffing position) improves pharyngeal patency. If the sniffing position and jaw thrust fail to relieve airway obstruction, then oropharyngeal or nasopharyngeal airways may be used to facilitate airway patency.
Oropharyngeal airways are the most commonly used. They follow the curvature of the tongue, pulling it away from the posterior pharynx ( Fig. 44.13 ). Because they place pressure on the base of the tongue and may come in contact with the epiglottis, oropharyngeal airways can precipitate coughing, retching, or laryngospasm if laryngeal and pharyngeal reflexes are not sufficiently blunted; therefore they are not appropriate for use in conscious patients who have not had local anesthetic applied to the airway. The oropharyngeal airway is sized by measuring from the corner of a patient’s mouth to the angle of the jaw or the earlobe. Inappropriately sized oropharyngeal airways can actually worsen airway obstruction; therefore correct size selection is important. Proper placement is accomplished by inserting the oropharyngeal airway with the curvature facing posteriorly and then rotating 180 degrees; alternatively, a tongue depressor can be used to displace the tongue anteriorly as the oropharyngeal airway is inserted with the curvature facing anteriorly. Complications from oropharyngeal airways include lingual nerve palsy and damage to the teeth. Nasopharyngeal airways are less stimulating than oropharyngeal airways once in place and thus are more appropriate for conscious patients ( Fig. 44.14 ). They should be well lubricated before insertion and inserted perpendicularly to the longitudinal axis of the body with the bevel facing the nasal septum. To avoid epistaxis, force should never be used during insertion of a nasopharyngeal airway.
Difficult mask ventilation occurs when ventilating via the face mask is not possible because of an inadequate mask seal, excessive gas leak, and/or excessive resistance to the ingress or egress of gas. Predictors for difficult mask ventilation that can be identified during the preoperative airway assessment are listed in Box 44.2 .
Obstructive sleep apnea or history of snoring
Age older than 55 years
Body mass index of 30 kg/m 2 or greater
Mallampati classification III or IV
Presence of a beard
The term supraglottic airway or extraglottic airway refers to a diverse family of medical devices that are blindly inserted into the pharynx to provide a patent conduit for ventilation, oxygenation, and delivery of anesthetic gases without the need for tracheal intubation. SGAs have the advantage of being less invasive than tracheal intubation while providing a more definitive airway than a face mask and can be used for either spontaneous ventilation or PPV. One of the first SGAs, the LMA, was described in 1983 by Dr. Archie Brain and introduced into clinical practice in 1988. Since that time, the LMA has proved to be one of the single most important developments in both routine and difficult airway management and is a pivotal component of the ASA DAA. Various different designs of SGAs are now available and are widely used in current anesthesia practice as a primary airway management device, a rescue airway device, and a conduit for tracheal intubation.
The specific advantages of SGAs include the ease and speed of placement, improved hemodynamic stability, reduced anesthetic requirements, lack of a need for muscle relaxation, and an avoidance of some of the risks of tracheal intubation (e.g., trauma to the teeth and airway structures, sore throat, coughing on emergence, or bronchospasm). The primary disadvantages are that SGAs have comparatively smaller seal pressures than ETTs, which can lead to ineffective ventilation when higher airway pressures are required, and they provide no protection from laryngospasm. First-generation SGAs also provide little protection from gastric regurgitation and aspiration, although newer devices have incorporated design elements to minimize this risk.
SGAs have many applications. They are considered the first choice for airway management for diagnostic and minor surgical procedures. No standardized classification system exists for the different designs of SGAs, although several have been proposed. This chapter uses the terminology described by Donald Miller: perilaryngeal sealers; cuffless, anatomically preshaped sealers; and cuffed pharyngeal sealers. Second-generation SGAs are differentiated from first-generation SGAs in that they incorporate features designed to reduce the incidence of aspiration.
Laryngeal Mask Airway
The LMA (LMA North America, San Diego, CA) is the most widely used, well-studied SGA and is the archetype of the perilaryngeal sealer. The original version, the LMA Classic (cLMA), consists of an oval-shaped, silicone mask with an inflatable cuff that sits in the hypopharynx and forms a seal around the periglottic tissues ( Fig. 44.15 ). An airway tube attached to the mask exits the mouth and has a standard 15-mm connector for attachment to an anesthesia circuit or to a bag-valve device. The seal around the laryngeal inlet allows for the delivery of oxygen and inhaled anesthetics during spontaneous ventilation and permits PPV at pressures up to 20 cm H 2 O. The cLMA is reusable up to 40 times and is available in a variety of sizes from size 1 (neonate) to size 6 (large adult, >100 kg).
The LMA Classic Excel is an updated version that incorporates design features to facilitate tracheal intubation through the device, including an epiglottic-elevating bar, a wider-bore airway tube, and a removable connector. A disposable, single-use version of the cLMA, the LMA Unique, is available with either a polyvinyl chloride (PVC) or silicone cuff and has gained popularity because of its lower cost and maintenance, as well as concerns over the perceived risk of cross-contamination and the transmission of infection with reusable medical devices. The LMA Flexible, available in reusable and single-use models, has a flexible, kink-resistant airway tube that can be positioned away from the surgical field for head and neck procedures.
To achieve a proper fit, the manufacturer of the LMA suggests placing the largest size LMA possible; an airtight seal is achieved more frequently with a size 5 LMA in the average adult man and a size 4 LMA in the average adult woman. Using an undersized LMA can result in overinflation of the cuff to achieve a seal, which can predispose the patient to oropharyngolaryngeal morbidity and nerve damage. Smaller LMA sizes have also been shown to be associated with placement failure. Larger sizes, however, may be associated with a more frequent incidence of sore throat; therefore a smaller size may be appropriate when spontaneous ventilation through the LMA is planned.
The manufacturer’s instructions for the placement of the cLMA are summarized in Fig. 44.16 . Adequate depth of anesthesia for LMA insertion can be achieved with propofol or sevoflurane ; short-acting opioids such as fentanyl, alfentanil, and remifentanil may be coadministered to facilitate placement and to decrease the incidence of coughing, gagging, and laryngospasm. Before insertion, the LMA cuff should be deflated and the posterior aspect of the mask should be lubricated with a water-based lubricant. Once positioned (see Fig. 44.16 ), the cuff should be inflated with the minimum effective volume of air, with a target cuff pressure of 40 to 60 cm H 2 O. To allow the LMA to position itself correctly, the device should not be secured or attached to the anesthesia circuit until the cuff has been inflated. Confirmation of proper placement is performed by attempting gentle PPV while checking capnography and auscultation and by quantifying the inspiratory pressure at which a leak is audible, which should be 18 to 20 cm H 2 O. Once proper positioning is confirmed, a roll of gauze is inserted as a bite block and the LMA is secured in place with tape. Several modifications to the recommended insertion technique have been described, including a thumb insertion method by the manufacturer ( Video 44.4 ). Cuff pressure should be periodically monitored if N 2 O is being used; cuff pressures may increase above the recommended threshold of 60 cm H 2 O as a result of diffusion of N 2 O into the cuff.
Initial difficulty with ventilation after the placement of an LMA may be due to a down-folded epiglottis. The up-down maneuver described by Dr. Brain may help correct this problem; the LMA is withdrawn 2 to 4 cm and reinserted without deflating the cuff. Head extension and LMA repositioning may also improve ineffective ventilation. If these actions do not correct the problem, then a different size may be needed. Insufficient depth of anesthesia, resulting in laryngospasm or bronchospasm, may make ventilation through an LMA impossible; the administration of topical, inhaled, or IV anesthesia can help to correct this. Although not necessary, DL can also facilitate proper LMA placement.
Serious complications from LMA use are relatively rare. More commonly, minor oral, pharyngeal, or laryngeal injury occurs, expressed as complaints of a dry or sore throat. The incidence of sore throat is approximately 10% to 20%, and has been linked to higher cuff pressures and larger LMA sizes. More serious cases of oropharyngolaryngeal injury have been described, such as trauma to the uvula and pharyngeal necrosis. Injury to the lingual, hypoglossal, and recurrent laryngeal nerves has also been reported; these usually spontaneously resolve over a period of weeks to months. Predisposing factors include high cuff pressures (often attributable to the use of N 2 O), using too small of an LMA, and nonsupine positions.
The LMA ProSeal (PLMA, LMA North America, San Diego, CA) is a reusable second-generation SGA that incorporates a posterior cuff, improving the perilaryngeal seal and allowing for PPV at pressures up to 30 cm H 2 O. It also incorporates a gastric drainage tube that allows for gastric access with an orogastric tube and channels any regurgitated gastric contents away from the airway, effectively isolating the respiratory and gastrointestinal tracts. Additional features include an incorporated bite block and a softer cuff.
The insertion technique is similar to the cLMA but requires deeper anesthetic levels. An optional introducer can be used to facilitate insertion. As with the cLMA, cuff pressure should not exceed 60 cm H 2 O. Once inserted, assessment of proper placement is accomplished by providing PPV; adequate tidal volumes should be accomplished with reasonable peak inspiratory pressures, leak pressure should be above 20 cm H 2 O, and the capnography waveform should appear normal. An additional test to confirm proper placement and separation of the airway and gastrointestinal tract is performed by placing a small layer (<5 mm) of water-based lubricant over the drainage tube orifice; PPV and suprasternal notch palpation should result in a small up-down movement of the gel meniscus. Easy passage of an orogastric tube through the gastric drainage tube confirms proper positioning.
The LMA Supreme (SLMA) is a single-use, second-generation SGA based on the PLMA design. Similar to the PLMA, the SLMA has an improved cuff design that produces higher airway leak pressures, a drainage tube that allows for gastric access, and an integrated bite block ( Fig. 44.17 ). A fixation tab allows for determination of proper sizing (the tab should rest 1 to 2.5 cm above the upper lip) and provides an improved perilaryngeal seal when inward pressure is maintained by securing the mask into position by taping cheek to cheek across the fixation tab.
Although not clinically proven, evidence suggests that second-generation SGAs, such as the PLMA and the SLMA, reduce the risk of aspiration of gastric contents. This property, along with the improved airway seal and higher leak pressures, have enabled SGA devices to be used in various applications where the cLMA is potentially unsuitable, such as in nonsupine positions (e.g., lateral, prone), in laparoscopic surgery (e.g., cholecystectomy, gynecologic surgery), and in patients who are obese. The successful, routine use of the SLMA in fasted, nonobese patients for cesarean section has also been reported.
Newer LMA Models
The LMA Protector is an all-silicone second-generation SGA with integrated Cuff Pilot Technology, which allows constant cuff pressure monitoring. Color-coded indicator bands alert the clinician to changes in cuff pressure attributable to temperature, N 2 O, and movement within the airway, allowing the clinician to maintain the recommended cuff pressure of 40 to 60 cm H 2 O. The LMA Protector is designed to channel fluids away from the airway in the unlikely event of regurgitation and allows for gastric suctioning. The airway channel is wide enough to allow intubation with a standard-sized ETT (see Tracheal Intubation Through a Supraglottic Airway Device). The LMA Gastro is a single-use silicone LMA designed for upper gastrointestinal endoscopy procedures, simultaneously protecting the airway and facilitating passage of an endoscope.
Other Perilaryngeal Sealers
Over the past 15 years, a multitude of manufacturers have produced SGAs that incorporate the basic perilaryngeal sealing design of the cLMA. Because the term LMA is a protected trademark, these devices are referred to as laryngeal masks (LMs). Each has its own unique characteristics that may afford it specific advantages over other designs. Although an exhaustive description of every available LM is outside the scope of this chapter, some unique features merit mentioning.
Some design features address the issue of high cuff pressures, which can lead to oropharyngolaryngeal morbidity, nerve palsies, and improper device positioning. The line of LMs manufactured by AES, Inc. (Black Diamond, WA) incorporates a cuff pilot valve (CPV) that allows constant cuff pressure monitoring. The air-Q SP (Cookgas LLC, St. Louis, MO; distributed by Mercury Medical, Clearwater, FL) has a self-pressurizing cuff that uses the positive pressure that ventilates the patient to also pressurize the cuff, obviating the need for an inflation line and eliminating the possibility of cuff overinflation. On exhalation, the mask cuff deflates to the level of positive end-expiratory pressure (PEEP), decreasing the total mucosal pressure over the course of an anesthetic, thereby potentially reducing the incidence of cuff pressure–related complications.
Cuffless Anatomically Preshaped Sealers
Cuffless anatomically preshaped sealers do not have a cuff; rather, they provide an airway seal by their anatomically preshaped design. Advantages include simplicity of insertion and positioning and the lack of a need to inflate a cuff. The first of these devices, the SLIPA (Curveair, London, UK), contains a hollow chamber that can trap regurgitated liquid and prevent aspiration. Other cuffless devices such as the i-gel (Intersurgical Inc., Wokingham, Berkshire, UK) and the Baska Mask (Strathfield, NSW, Australia) can also be included in this classification.
Cuffed Pharyngeal Sealers
Cuffed pharyngeal sealers have an airway with a pharyngeal cuff that seals at the level of the base of the tongue and can be subclassified as to whether they also possess an esophageal sealing cuff. SGAs with only a pharyngeal cuff include the Cobra Perilaryngeal Airway (CobraPLA; Engineered Medical Systems, Indianapolis, IN) and the Tulip Airway (Marshall Medical, Bath, UK); they are not detailed in this chapter. The following devices all have an esophageal sealing cuff.
The esophageal-tracheal combitube (ETC) (Covidien, Mansfield, MA) is a uniquely designed SGA with both a pharyngeal and esophageal sealing cuff and two lumina. The ETC is primarily designed for emergency intubation and is mostly used in the prehospital setting, although it has occasionally been used during general anesthesia as both a primary airway and as a rescue airway device. It is inserted blindly through the mouth in a curved, downward motion until the printed ring marks lie between the teeth. Both the proximal, oropharyngeal cuff and the distal esophageal-tracheal cuff are inflated. Greater than 90% of the time, esophageal placement of the device occurs, in which ventilation should be performed via the longer, blue, #1 (esophageal) lumen. This lumen has a closed distal end with eight small perforations located between the two cuffs, which allow oxygenation and ventilation. When the device is placed into the trachea, ventilation should occur via the shorter, clear, #2 (tracheal) lumen, which is open at its distal end. When the ETC is placed in the esophagus, an orogastric tube may be passed through the tracheal lumen to empty the stomach. Use of the ETC as a primary airway is limited by a higher risk of complications, compared with the LMA or tracheal intubation, including hoarseness, dysphagia, and bleeding. Because the oropharyngeal cuff of the ETC contains latex, this device should not be used in latex-sensitive individuals.
The Rüsch EasyTube (Teleflex Medical, Research Triangle Park, NC) is a double-lumen SGA that is similar to the ETC. The primary differences are its nonlatex construction and a proximal lumen that ends just below the oropharyngeal balloon, allowing for the passage of a tube exchanger or FIS. The insertion technique and risks are similar to the ETC; a comparative study showed shorter insertion times with the EasyTube.
The King LT series of SGAs (King Systems Corporation, Noblesville, IN) are similar in design to the ETC and EasyTube, with a ventilation port between the pharyngeal and esophageal cuffs. The King LT and the King LT-D (reusable and disposable, respectively) are single-lumen devices with a tapered distal tip that allows easy passage into the esophagus. The distal (esophageal) portion of the tube is occluded. The King LTS and the disposable King LTS-D, on the other hand, have an open distal tip with a secondary channel to allow suctioning of gastric contents. Although tracheal placement of a King LT device has not be reported, if it should occur, then the device should be removed and reinserted.
Tracheal intubation is the gold standard for airway management. It establishes a definitive airway, provides maximal protection against the aspiration of gastric contents, and allows for PPV with higher airway pressures than with a face mask or an SGA. Tracheal intubation is usually facilitated by DL; however, a wide variety of alternative intubation devices and techniques have been developed to circumvent the problems encountered when conventional DL is difficult.
In the fasted patient undergoing elective surgery with general anesthesia, an SGA is often suitable. Certain conditions or clinical situations, however, favor tracheal intubation, although the advent of second-generation SGAs has somewhat narrowed this list. Absolute indications for tracheal intubation include patients with a full stomach or who are otherwise at increased risk for aspiration of gastric secretions or blood, patients who are critically ill, patients with significant lung abnormalities (e.g., low lung compliance, high airway resistance, impaired oxygenation), patients requiring lung isolation, patients undergoing otorhinolaryngologic surgery during which an SGA would interfere with surgical access, patients who will likely need postoperative ventilatory support, and patients in whom SGA placement has failed. Other indications for intubation include a surgical requirement for NMBDs, patient positioning that would preclude rapid tracheal intubation (e.g., prone or turned away from the anesthesia provider), a predicted difficult airway, and prolonged procedures.
The modern, standard ETT is a disposable, single-use, cuffed, plastic tube that is designed to be inserted through the nose or mouth and sit with its distal end in the mid-trachea, providing a patent airway to allow for ventilation of the lungs. A variety of different types of ETTs are available for use in specialized situations. Several features are commonplace among the different styles, however, including a universal 15-mm adapter that allows the attachment of the proximal end to different ventilating circuits and devices; a high-volume, low-pressure cuff; a beveled tip to facilitate passage through the vocal cords; and an additional distal opening in the side wall of the ETT known as a Murphy eye , which serves to provide an additional portal for ventilation should the distal end of the lumen become obstructed by either soft tissue or secretions.
Cuffed ETTs are routinely used for tracheal intubation in most patients; cuffless ETTs are used in neonates and infants. The high-volume, low-pressure cuff is inflated with air to provide a seal against the tracheal wall to protect the lungs from pulmonary aspiration and to ensure that the tidal volume delivered ventilates the lungs rather than escapes into the upper airway. A pilot balloon with a one-way valve allows for the inflation of the cuff and an assessment of the cuff pressure. The cuff should be inflated to the minimum volume at which no air leak is present with positive pressure inspiration; the cuff pressure should be less than 25 cm H 2 O. Excessive cuff pressure may result in tracheal mucosal injury, vocal cord dysfunction from recurrent laryngeal nerve palsy, and sore throat. Monitoring the cuff pressure with a pressure gauge is recommended. When N 2 O is used as part of the anesthetic, cuff pressure should be periodically measured throughout the surgery; N 2 O diffusion into the cuff can result in increases in cuff pressure to potentially dangerous levels.
ETT size is normally described in terms of its internal diameter (ID); the relationship of the ID to the external diameter varies between different designs and manufacturers. Selection of the ETT size depends on the reason for placement and patient-specific factors such as gender and airway pathologic conditions. Smaller ETTs result in increased airway resistance and work of breathing, and ETTs with a smaller ID may preclude therapeutic fiberoptic bronchoscopy. Larger ETTs are more likely to be associated with laryngeal or tracheal mucosal trauma and have a higher incidence of sore throat after general anesthesia. Generally, in patients intubated only for the purposes of a general anesthetic, a smaller ETT may be used than on the patient who will remain intubated in the medium to long term as a result of respiratory failure; typically a 7-mm ETT is used for women and a 7.5- or 8-mm ETT is used for men.
A variety of specialized tracheal tubes are available for use in specific clinical situations. Preformed tubes, such as the nasal and oral Ring-Adair-Elwin (RAE) tubes, have a specific contour to maintain a low profile and to avoid surgical interference. Armored (reinforced) tubes have an embedded coil that minimizes kinking of the tube when it is subjected to angulation. Microlaryngeal tubes, which have small IDs with a longer length tube, are useful in laryngeal surgery or for specific applications, such as intubation through a cLMA. The VivaSight ETT (Ambu, Inc., Ballerup, Denmark) has an integrated video camera at the tip, useful during intubation and for confirming ETT position throughout the procedure. Other specialized tubes include laser-resistant tubes and both single- and double-lumen tubes that allow for one-lung ventilation.
Endotracheal Tube Introducers
ETT introducers are long, slender devices used to assist in guiding an ETT through the glottis. They are particularly useful for performing a blind intubation when the glottic opening cannot be visualized during laryngoscopy.
The original ETT guide was the Eschmann introducer, developed by Venn in 1973. This device, also known as the gum elastic bougie , is long enough to allow advancement of an ETT over its distal end after being placed through the vocal cords. It also possesses an anterior angulation at the distal end (coudé tip) to facilitate maneuvering underneath the epiglottis toward the glottic opening, even when the glottic structures are not visualized. A variety of similar introducers with different sizes and features are available; some are hollow to allow for ventilation if the need arises.
Coudé-tip introducers are particularly useful when only a portion of the laryngeal structures, such as only the tip of the epiglottis, can be visualized. Proper placement of the stylet is indicated by the perception of tracheal clicks as the coudé tip passes along the tracheal rings and by a distal hold-up as it reaches the small bronchi. An ETT is subsequently advanced over the introducer into the correct position ( Video 44.5 ).
Orotracheal Versus Nasotracheal Intubation
Tracheal intubation can proceed via the orotracheal or nasotracheal route—this decision should be made before deciding which airway management technique will be used. Nasotracheal intubation is generally indicated when the orotracheal route is not possible (e.g., when the mouth opening is severely limited) or when the need for surgical access precludes an orotracheal route. In addition, certain intubation techniques, such as blind intubation, awake intubation, and FSI, are significantly easier when performed through the nose.
When the nasotracheal route is not specifically indicated, however, the orotracheal route is usually preferred for several reasons. The orotracheal route is potentially less traumatic and presents a lower risk of bleeding, it usually allows for the placement of a larger ETT, and it provides for more options in terms of airway management techniques. The major disadvantages include the potential for damage to the teeth and stimulation of the gag reflex during awake intubation, requiring denser airway anesthesia and potentially being less comfortable for the patient. Nasotracheal intubation, on the other hand, bypasses the gag reflex and is usually more easily tolerated by the awake patient. However, the risks of epistaxis, trauma to the nasal turbinates, and submucosal tunneling in the nasopharynx must be taken into account. Nasotracheal intubation is relatively contraindicated in the setting of maxillary or skull base fractures.
The most commonly used technique for tracheal intubation is DL, which involves direct visualization of the glottis with the assistance of a laryngoscope. The ETT is inserted through the glottic opening into the trachea under continuous observation.
Preparation and Positioning
Preparation for DL includes proper patient positioning, adequate preoxygenation, and ensuring the availability and proper functioning of all necessary equipment—laryngoscopes, tracheal tubes, tube stylets, an empty syringe for inflating the tracheal tube cuff, a suction apparatus, and the essential equipment for mask ventilation, including an oxygen source. A skilled assistant should be present to help with external laryngeal manipulation and stylet removal, among other tasks. Adequate preparation is of the utmost importance; as with any airway procedure, the first attempt should be the best attempt.
For DL to be successful, a line of sight from the mouth to the larynx must be achieved. The classical model used to describe the anatomic relationships necessary to achieve this was proposed in 1944 by Bannister and Macbeth and involves the alignment of three anatomic axes—oral, pharyngeal, and laryngeal. Positioning the patient in the sniffing position approximates this alignment. Cervical flexion aligns the pharyngeal and laryngeal axes, and maximal head extension at the atlantooccipital joint brings the oral axis closer into alignment ( Fig. 44.18 ). The accuracy of this model has been questioned, and various alternative models to explain the anatomic advantage of the sniffing position have been proposed. Regardless of the explanatory model, the evidence in the literature supports the assertion that the sniffing position is the optimal position for DL.
Proper positioning in the sniffing position involves approximately 35 degrees of cervical flexion, which is accomplished by a 7- to 9-cm elevation of the head on a firm cushion; patients with shorter necks may require less head elevation. Patients who are obese often require elevation of the shoulders and upper back to achieve adequate cervical flexion, which can be accomplished by placing the patient in the ramped position using either a specialized device, such as the Troop Elevation Pillow (Mercury Medical, Clearwater, FL), or folded blankets. Confirming horizontal alignment of the external auditory meatus with the sternal notch is useful for ensuring optimal head elevation in both obese and nonobese patients. Adequate cervical flexion also facilitates maximal atlantooccipital extension, which provides optimal alignment of the oral and pharyngeal axes (the primary determinant for quality of laryngeal view) and enhanced mouth opening.
The laryngoscope is a handheld instrument consisting of a blade attached to a handle containing a light source. Most are reusable and made of steel, although disposable, plastic versions are available. The curved blade and the straight blade are the two basic types of laryngoscope blades available for DL; multiple variations of both styles exist. The Macintosh is the most commonly used curved blade, whereas the Miller is the most commonly used straight blade. Both are designed to be held in the left hand, and both have a flange on the left side that is used to retract the tongue laterally. Each type of blade has its benefits and drawbacks and is associated with its own technique for use.
The technique for laryngoscopy consists of the opening of the mouth, inserting the laryngoscope blade, positioning of the laryngoscope blade tip, applying a lifting force exposing the glottis, and inserting a tracheal tube through the vocal cords into the trachea. Mouth opening is best achieved using the scissors technique; the right thumb pushes caudally on the right lower molars while the index or third finger of the right hand pushes on the right upper molars in the opposite direction ( Fig. 44.19 ).
The decision of whether to use a Macintosh or a Miller blade is multifactorial; however, the personal preferences and experience of the laryngoscopist is a significant consideration. In general, the Macintosh is most commonly used for adults, whereas the straight blades are typically used in pediatric patients. Curved blades provide greater room for passage of an ETT through the oropharynx, attributable to their larger flange, and are generally considered less likely to cause dental damage. Straight blades are preferred in patients with a short thyromental distance or prominent incisors, and usually provide a better view of the glottis in patients with a long, floppy epiglottis. Often, when one style of laryngoscope does not provide an adequate view of the glottis, the other may be more effective. For most adults, a Macintosh size 3 or a Miller size 2 blade is usually the proper size; in larger patients or patients with a very long thyromental distance, a larger blade may be more appropriate.
The Macintosh blade is inserted in the right side of the mouth, and the flange is used to sweep the tongue to the left. Once the laryngoscope has been inserted in the mouth, the right hand can be used to ensure that the upper lip is not impinged between the laryngoscope and the upper incisors. The blade is advanced along the base of the tongue until the epiglottis is visualized; the tip of the blade is then advanced further and positioned in the vallecula. A force oriented at a 45-degree angle up and away from the laryngoscopist indirectly lifts the epiglottis by placing tension on the hyoepiglottic ligament, exposing the glottic structures ( Fig. 44.20 ). The tip of the blade should not be lifted by using the laryngoscope as a lever, rocking back on the upper incisors, which can damage the teeth and provides an inferior view of the glottis. A properly oriented vector of force is achieved by using the anterior deltoid and triceps, not by radial flexion of the wrist. Once a complete view of the glottis is achieved, the ETT is grasped similar to a pencil with the right hand and guided through the vocal cords into the trachea. Passage of the ETT may be facilitated by an anterior angulation of the tip, which can be accomplished by shaping the ETT with a malleable stylet into a hockey stick shape, with approximately a 60-degree angle formed 4 to 5 cm from the distal end, or by accentuating the natural anterior curvature of the ETT by inserting the tip into the 15-mm connector, forming a circle, for several minutes before performing DL.