Summary
Airway management is a vital component of administering anesthesia, allowing for the exchange of gases between the patient and the surrounding atmosphere. Difficult or unsuccessful management of the airway is a significant source of anesthesia-related morbidity and mortality [1]. As such, it is important for anesthesia providers to be adept at all aspects of managing the airway. A thorough understanding of the pertinent anatomy and physiology, the ability to use clinical evaluation to identify potential difficulties, and a mastery of interventional techniques and procedures are crucial to safe and effective airway management. This chapter presents a comprehensive overview of the elements related to effective airway management.
Introduction
Airway management is a vital component of administering anesthesia, allowing for the exchange of gases between the patient and the surrounding atmosphere. Difficult or unsuccessful management of the airway is a significant source of anesthesia-related morbidity and mortality [Reference Klinger, Infosino, Pardo and Miller1]. As such, it is important for anesthesia providers to be adept at all aspects of managing the airway. A thorough understanding of the pertinent anatomy and physiology, the ability to use clinical evaluation to identify potential difficulties, and a mastery of interventional techniques and procedures are crucial to safe and effective airway management. This chapter presents a comprehensive overview of the elements related to effective airway management.
Airway Anatomy
Respiration is a complex process that involves the exchange of gases and the breakdown of glucose to yield energy [Reference Klinger, Infosino, Pardo and Miller1]. Understanding the anatomy of the airway is important when performing intubation. The airway can be divided into multiple subsections: the nasal cavity, oral cavity, and pharynx. The pharynx is further divided into the nasopharynx, oral pharynx, and hypopharynx, running from superior to inferior. The nasal cavity consists of the nares, septum, and turbinates (superior, middle, and cheap). It is bound superiorly by the ethmoid bone. It is continuous posteriorly with the nasopharynx (the most prominent portion of the pharynx). The oral cavity consists of the upper/lower rows of teeth, the tongue, hard palate, and soft palate, and is continuous posteriorly with the oropharynx. The oropharynx stretches down to the epiglottis (the cartilaginous structure that serves as a flap to cover the trachea or esophagus). The hypopharynx runs from the epiglottis down to the superior edge of the trachea. This is the region where the vocal cords will be visualized (along with the larynx region). These lie at around the level of the thyroid cartilage. Breathing, or ventilation, is the process of conducting air to and from the lungs. Simultaneously, gaseous exchange is the diffusion of oxygen into the blood vessels and the removal of carbon dioxide and other gases into the air [Reference Ward, Ward and Leach2]. The respiratory tract organs form a continuous passage for air, and they are divided into upper and lower airways.
The lower airways include the trachea, bronchi, bronchioles, and alveoli. Their primary function is to facilitate the movement of air between the lungs and the atmosphere. The trachea is a hollow tube supported by cartilage. It begins from the larynx and branches into the bronchi. The cartilage helps to ensure that it does not collapse or overexpand. The bronchi branch from the trachea and subdivide into bronchioles. They serve as passages for bringing air in and out of the lungs. Unlike the trachea and bronchi, the bronchioles do not have cartilage and their diameter is much smaller [Reference Davies and Moores3]. They are ciliated and have a simple epithelium with mucus-secreting cells. The final portion of the lower airway is made of the alveoli, single-cell layered and near the capillaries. They facilitate the actual exchange of gases in the lungs. The general function of the airway is to allow for airflow to facilitate gaseous exchange, which is essential for respiration. However, they perform other functions to maintain adequate protection and homeostasis. They serve as moisture barriers to prevent loss of excessive moisture through humidification of air. They work as temperature barriers by warming the air from the environment as it gets into the airways. Finally, they work as barriers to infection, primarily through the mucosa-associated lymphoid tissue (MALT).
From the base of the trachea, the airways branch into the right and left sides. Two bronchi further divide into lobar (secondary) bronchi, which in turn divide into segmental (tertiary) bronchi that eventually form the bronchioles [Reference Ball, Hossain and Padalia4]. The right- and left-sided airways connect to the respective lungs. The branching of the airways into the left and right sides forms an extensive pulmonary tree. The right lung is broader and shorter, whereas the left lung is thinner and longer. The reason why the right lung is shorter is because the liver rests beneath it. On the other hand, the left lung has to make room for the heart, hence is narrower.
The right lung refers to the right side of the pair of lungs at the front of the thoracic cavity, whereas the left side is known as the left lung. One of the key differences is the number of lobes, with the right lung having three (superior, middle, and inferior). By contrast, the left lung has only two lobes (upper and lower). There is a thick cardiac notch at the left lung, making it distinct, although it does not serve any role in the right lung. Finally, the left lung has a horizontal and an oblique fissure, whereas the left has only the oblique fissure.
These different upper airway areas are innervated differently by branches and terminal ends of some cranial nerves. The primary nerves that give sensation to the airway are the trigeminal nerve (CN V), the glossopharyngeal nerve (CN IX), and the vagus nerve (CN X). The trigeminal nerve is almost exclusively a cranial sensory nerve and gives off three main branches: ophthalmic (V1), maxillary (V2), and mandibular (V3). The ophthalmic nerve and its smaller branches provide sensory innervation to the superior region of the internal nasal cavity. Many internal components supply the medial and lateral parts of the ethmoidal area and the superior nares. The maxillary nerve provides sensory innervation to the inferior nasal region, nasal septum, and soft palate in the oral cavity. It also provides some innervation to the external nasal area via the infraorbital nerve (one of the terminal branches of the maxillary nerve). The pterygopalatine ganglion lies in between the palatine and maxilla bones, receives fibers from the maxillary nerve, and then sends smaller components out (nasopalatine, greater/lesser palatine, etc.). Remember, the nasopalatine nerve comes from the pterygopalatine ganglion, runs along inside the nasal septum mucosa, dives through the incisive canal, and terminates in the anterior hard palate. Finally, the mandibular nerve gives sensation to the anterior two-thirds via the lingual nerve [Reference Bates5].
The posterior one-third of the tongue and posterior pharyngeal mucosa (down to the aryepiglottic fold level) receive sensory innervation from the glossopharyngeal nerve. This nerve also provides fibers to the dense pharyngeal nerve plexus, which innervates the palatopharyngeal arch. The pharyngeal nerve plexus receives some glossopharyngeal nerve fibers. However, the plexus is mainly made up of motor fibers from the vagus nerve. The vagus nerve mediates sensory innervation to the larynx and laryngopharynx, and gives rise to the superior laryngeal nerve. The superior laryngeal nerve branches into the internal and external. The inner laryngeal nerves provide sensory innervation to the epiglottic region’s mucosa, extending to the level of the vocal folds. Below the vocal folds, sensory and motor innervation is supplied by the left and right recurrent laryngeal nerves (also branches of the vagus). Therefore, they innervate all the larynx’s intrinsic muscles (sparing the cricothyroid muscle, which is innervated by the external laryngeal nerve). All of these innervations are important for airway management when performing intubation because the endotracheal (ET) tube will pass through most, if not all, of these regions to end up in the trachea to provide oxygenation to the lungs [Reference Bates5].
During routine intubation, the blade should move the patient’s tongue out of the visual field to directly see the vocal cords. The vocal cords are the most medial. They connect to the cricoid cartilage anteriorly, and posteriorly to the arytenoid cartilage on the larynx’s posterior edge. Visualization of the ET tube moving midline through the cords should give successful intubation. The ET tube was kept in place just superior to the carina level (e.g., bifurcation of the trachea into the two primary bronchi). Although adult and pediatric patients have all of the same airway structures, there can be differences in length, size, and width. In pediatric patients, note the prominent occiput will cause neck flexion in the supine position, so a towel should be placed under the shoulders to keep a direct airway ready for intubation. The hypopharynx will also be shorter and narrower than in an adult – the cricoid cartilage resting higher at the C4 vertebra (the adult cricoid cartilage is at C6).
Additionally, the pediatric vocal cords are not at a 90-degree angle to the larynx wall as the adult vocal cords are. The pediatric cords slope downwards anteriorly, providing difficulty with the tube rubbing against the cords and possibly causing trauma. Lastly, the epiglottis in a pediatric patient is not as flat as in an adult, presenting problems with using a Macintosh blade instead of a Miller blade. Some physicians prefer the Miller blade during pediatric intubation because it has a better shape to move the pediatric epiglottis out of the visual field, compared to a curved Macintosh blade [Reference Bates5].
Airway Assessment
Airway Assessment
A thorough assessment of the patient’s airway should be conducted in the preoperative setting. This consists of obtaining a history of any previous airway instrumentation, reviewing the patient’s medical record, with particular attention to previous anesthesia and/or intubations, and noting any disease states that may have implications on airway management. Typically, patients who have presented difficulties with airway management have been informed of this and/or documentation of such can be found in the patient’s medical record. The medical record may also contain information regarding which techniques were used in order to successfully manage the airway [Reference Harless, Ramaiah and Bhananker6].
A comprehensive history should be accompanied by a physical examination, with the aim of identifying features that may portend difficulty with airway management. Examination of the mouth opening, dentition, oropharyngeal space, submandibular compliance, cervical spine mobility, and body habitus can all help stratify the risk of difficult airway management, and several tools exist to assist in this assessment. The sensitivity and specificity of any single one of these tools are low. However, when used in combination, they can be helpful in predicting which patients may present difficulty in airway management. The Mallampati test is used to evaluate the oropharyngeal space. It consists of visual examination of the oropharyngeal space. A “Mallampati score” is derived based upon which structures are visible in the patient’s mouth. To properly administer the test, the observer should be at eye level, with the patient holding the head in a neutral position, opening the mouth maximally, and protruding the tongue without phonating. The Mallampati classification is as follows:
I: The soft palate, fauces, uvula, and tonsillar pillars are visible.
II: The soft palate, fauces, and uvula are visible.
III: The soft palate and base of the uvula are visible.
IV: The soft palate is not visible [Reference Harless, Ramaiah and Bhananker6].
The mnemonic PUSH (tonsillar Pillars, Uvula, Soft palate, Hard palate) is often used to remember the Mallampati score. A patient with a Mallampati score of I has all four elements of PUSH visible on examination, whereas a patient with a Mallampati score of IV has only the hard palate visible. A Mallampati score of III or IV correlates with difficult laryngoscopy. Mouth opening is assessed by measurement of the interincisor distance – that is, the distance between the upper and lower incisors. An interincisor distance of <3–4.5 cm correlates with difficult laryngoscopy. Patients with overbites have a reduced effective interincisor distance and therefore may present difficulties with laryngoscopy. The upper lip bite test (ULBT) is also used to predict ease of laryngoscopy and intubation. The ULBT assesses an individual’s mandibular prognathic ability. The ULBT is broken down into three classes. Patients who fall in Class III of the ULBT may present difficulty with laryngoscopy and intubation:
Class I: The lower incisors can bite above the vermilion border of the upper lip.
Class II: The lower incisors cannot reach the vermilion border.
Class III: The lower incisors cannot bite the upper lip.
As the soft tissues of the pharynx are displaced into the submandibular space during laryngoscopy, anything that limits the size or compliance of the submandibular space can make laryngoscopy and intubation more challenging. The thyromental distance, which is the distance between the tip of the jaw and the thyroid cartilage, can also be informative. A distance less than three fingerbreadths or of 6–7 cm correlates with difficult laryngoscopy. Difficulty with neck extension and certain physical features, such as obesity and increased neck circumference, also indicate a potential difficult airway [Reference Harless, Ramaiah and Bhananker6].
Predictors of Difficult Intubation/Ventilation
The purpose of conducting a thorough preoperative history and physical examination is to identify patients with potential difficult airways and formulate a plan for successful management. The unanticipated difficult airway must be avoided at all costs. A number of congenital and acquired conditions are associated with difficult airway management. Table 2.1 outlines factors that are associated with difficult ventilation and/or intubation. It should be noted that factors that are associated with difficult intubation are not always also associated with difficult ventilation. The reverse is also true.
Table 2.1 Factors associated with difficult ventilation and intubation
| Predictors of difficult ventilation | Predictors of difficult intubation |
|---|---|
| Edentulous | History of difficult intubation |
| History of snoring | Prognathic inability |
| Macroglossia | Limited neck range of motion |
| Micrognathia | Long incisors |
| Uvula not visible | Mallampati class III or IV |
| Obesity | Mandibular space stiff, indurated, or occupied |
| Obstructive sleep apnea | Mouth opening <3–4 cm |
| Presence of beard | Uvula not visible |
| Short neck | Obesity |
| Thick neck | Prominent overbite |
| Thyromental distance <6 cm | |
| Short neck | |
| Thick neck |
Physiology of Airway Management
Preoxygenation
Prior to any intubation, it is important to practice clinical fundamentals of management of the airway with preoxygenation. If possible, preoxygenation with oxygen via face mask should be initiated prior to all airway management interventions or anesthetic induction. Nitrogen makes up about 80% of the concentration in the lung. During preoxygenation, or denitrogenation, the functional residual capacity (FRC) is purged of nitrogen and filled with oxygen, increasing the duration of apnea without desaturation. The four maximal breath technique of preoxygenation was found to be associated with a statistically significant shorter time to onset of oxygen desaturation of blood, when compared to normal breathing of 100% oxygen for 3 minutes [Reference Nimmagadda, Salem and Crystal7]. A healthy patient breathing room air will experience desaturation of <90%, with apnea within 1–2 minutes versus up to 8 minutes following preoxygenation. Goals for preoxygenation include: bringing the patient’s saturation as close to 100% as possible; denitrogenating the residual capacity of the lungs (maximizing oxygen storage in the lungs); and denitrogenating and maximally oxygenating the bloodstream [Reference Weingart and Levitan8]. Utilizing this technique to achieve the goals mentioned will increase the duration of safe apnea, as oxygen is poorly soluble, and can provide preferable conditions for intubation.
Apneic Preoxygenation
The process by which gases are entrained into the alveolar space during apnea is referred to as apneic preoxygenation. Since oxygen enters blood from the alveoli at a faster rate than carbon dioxide leaving blood, a negative pressure is generated in the alveolus, driving oxygen into the lungs. This method extends the duration of safe apnea after the use of sedatives and muscle relaxants.
Airway Reflexes
Laryngoscopy and tracheal intubation directly stimulate airway reflexes that may elicit protective responses to this stimulation, leading to hypertension and tachycardia. This is commonly seen in a pediatric setting or “light” planes of anesthesia. In the larynx, glottic stimulation, innervated by the superior laryngeal nerve, causes closure of the distal airways, leading to glottic closure to prevent aspiration. When exaggerated, this response can lead to complete glottic closure, and consequently impending respiratory collapse. If laryngospasm occurs and persists, positive pressure ventilation or small doses of succinylcholine may be required to abate this response. However, in certain cases, the spasm is sustained as long as the stimulus continues, and morbidity, such as cardiac arrest, arrhythmia, pulmonary edema, bronchospasm, or gastric aspiration, may occur [Reference Alalami, Ayoub and Baraka9]. Bronchospasm, a reflex more commonly seen in asthmatics, tends to also occur in pediatric patients and those under “light” planes of anesthesia. This response may also be an indicator of bronchial intubation. Aside from respiratory reflexes, intubation may cause an increase in intracranial or intraocular pressure, primarily due to a sympathetic surge.
Aspiration Risk and Ways to Minimize the Risk
Intraoperative pulmonary aspiration, a rare anesthesia-related complication, is associated with potentially fatal complications with significant associated morbidity. Awareness of the risk factors, predisposing conditions, maneuvers to decrease risk, and immediate management options by both the surgeon and the anesthesia team is imperative to reducing risk and optimizing patient outcomes associated with acute intraoperative pulmonary aspiration [Reference Nason10]. Risk factors include, but are not limited to: obesity; medications that reduce lower esophageal tone; gastrointestinal (GI) obstruction; need for emergency surgery; nasogastric tube placement; meal within 8 hours prior to surgery; previous esophageal surgery; lack of coordination of swallowing or respiration; esophageal cancer; hiatal hernia; patient positioning; and provider factors. Assessing preoperative risk factors may assist in planning preventative measures and minimize the risk of intraoperative pulmonary aspiration. The first step in successful management of an intraoperative aspiration is immediate recognition of gastric content in the oropharynx or airways [Reference Nason10]. Signs of aspiration include persistent hypoxia, high airway pressures, bronchospasm, and abnormal breath sounds following intubation. It is important to suction the airway prior to positive pressure ventilation and to position the patient with the head down and rotated laterally if possible. If the patient is not intubated, it is recommended that the airway be secured as rapidly as possible to prevent further aspiration and facilitate airway clearance.
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