Airway management is a crucial skill for clinical anesthesiologists. It is an integral part of general anesthesia, allowing ventilation and oxygenation as well as a mode for anesthetic gas delivery. Major complications of airway management in the operating room are very rare but maybe life-threatening.
Anatomy of the airway
The upper airway
The upper airway refers to the nasal passages, oral cavity (teeth, tongue), pharynx (tonsils, uvula, epiglottis) and larynx. Although the larynx is the narrowest structure in the adult airway and a common site of obstruction, the upper airway can also become obstructed by the tongue, tonsils, and epiglottis.
The lower airway
The lower airway begins below the level of the larynx. The lower airway is supported by numerous cartilaginous structures. The most prominent of these is the thyroid cartilage (Adam’s apple) which acts as a shield for the delicate laryngeal structures behind it. Below the larynx, at the level of the sixth cervical vertebra (C6), the cricoid cartilage forms the only complete circumferential ring in the airway. Below the cricoid, many horseshoe-shaped cartilaginous rings help maintain the rigid, pipe-like structure of the trachea. The trachea bifurcates at the level of the fourth thoracic vertebra (T4) where the right mainstem bronchus takes off at a much less acute angle than the left.
The airway is innervated by both sensory and motor fibers. The purpose of the sensory fibers is to allow detection of foreign matter in the airway and to trigger the numerous protective responses designed to prevent aspiration. The swallowing mechanism is an example of such a response whereby the larynx moves up and under the epiglottis to ensure that the bolus of food does not enter the laryngeal inlet. The cough reflex is an attempt to clear the upper or lower airway of foreign matter and is also triggered by sensory input.
There are many different laryngeal muscles. Some adduct, while others abduct the cords. Some tense, while others relax the cords. With the exception of one, they are all supplied by the recurrent laryngeal nerve. The cricothyroid muscle, an adductor muscle, is supplied by the external branch of the superior laryngeal nerve.
Sensory innervation of the airway
- Lingual nerve – anterior 2/3 of tongue
- Glossopharyngeal nerve – posterior 1/3 of tongue
- Superior laryngeal nerve (internal branch) – epiglottis and larynx
- Recurrent laryngeal nerve – trachea, lower airways
Assessment of the airway
The anesthesiologist must always perform a thorough preoperative airway assessment, regardless of the planned anesthetic technique. The purpose of the assessment is to identify potential difficulties with airway management and to determine the most appropriate approach. The airway is assessed by history, physical examination and occasionally, laboratory exams.
On history, one attempts to determine the presence of pathology that may affect the airway. Examples include arthritis, infection, tumors, trauma, morbid obesity, burns, congenital anomalies, and previous head and neck surgery. As well, the anesthesiologist asks about symptoms suggestive of an airway disorder: dyspnea, hoarseness, stridor, sleep apnea. Finally, it is important to elicit a history of previous difficult intubation by reviewing previous anesthetic history and records.
The physical exam is focused on the identification of anatomical features which may predict airway management difficulties. It is crucial to assess the ease of intubation. Traditional teaching maintains that exposure of the vocal cords and glottic opening by direct laryngoscopy requires the alignment of the oral, pharyngeal and laryngeal axes. The “sniffing position” optimizes the alignment of these axes and optimizes the anesthesiologist’s chance of achieving a laryngeal view.
Easy intubation can be anticipated if the patient is able to open his mouth widely, flex the lower cervical spine, extend the head at the atlanto-occipital joint and if the patient has enough anatomical space to allow a clear view. Each of these components should be assessed in every patient undergoing anesthesia:
- Mouth opening: Three fingerbreadths is considered adequate mouth opening. At this point in the exam, the anesthesiologist also observes the teeth for an overbite, poor condition and the presence of dental prosthetics.
- Neck motion: The patient touches his chin to his chest and then looks up as far as possible. The normal range of motion is between 90 and 165 degrees.
- Adequate space: Ability to visualize the glottis is related to the size of the tongue relative to the size of the oral cavity as a large tongue can overshadow the larynx. The Mallampati classification assigns a score based on the structures visualized when the patient is sitting upright, with the head in a neutral position and the tongue protruding maximally. Class 1 corresponds well with easy intubation. Class 4 corresponds well with difficult intubation. Classes 2 and 3 less reliably predict ease of intubation. The thyromental distance is also an important indicator. The distance from the lower border of the mandible to the thyroid notch with the neck fully extended should be at least three to four fingerbreadths. A shorter distance may indicate that the oral-pharyngeal-laryngeal axis will be too acute to achieve good visualization of the larynx. As well, a short thyromental distance may indicate inadequate “space” into which to displace the tongue during laryngoscopy.
Combining Mallampati classification with thyromental distance and other risk factors (morbid obesity, short, thick neck, protuberant teeth, retrognathic chin), will increase the likelihood of identifying a difficult airway. No assessment can completely rule out the possibility and so the clinician must always be prepared to manage a difficult airway. Laboratory investigations of the airway are rarely indicated. In some specific settings, cervical spine x-rays, chest rays, flow-volume loops, computed tomography or magnetic resonance imaging may be required.
Management of the airway
Airway patency and protection must be maintained at all times during anesthesia. This may be accomplished without any special maneuvers such as during regional anesthesia or conscious sedation. If the patient is deeply sedated, simple maneuvers may be required: jaw thrust, chin lift, oral airway (poorly tolerated if gag reflex is intact) or nasal airway (well tolerated but can cause epistaxis).
During general anesthesia (GA), more formal airway management is required. The three common airway techniques are:
- Mask airway (airway supported manually or with oral airway)
- Laryngeal mask airway (LMA)
- Endotracheal intubation (nasal or oral)
The choice of airway technique depends on many factors:
- Airway assessment
- Risk of regurgitation and aspiration
- Need for positive pressure ventilation
- Surgical factors (location, duration, patient position, degree of muscle relaxation required)
A patient who is deemed to be at risk of aspiration requires that the airway be “protected” with a cuffed endotracheal tube regardless of the nature of the surgery. If the surgery requires a paralyzed patient, then in most cases the patient is intubated to allow mechanical ventilation.
Bag mask ventilation may be used to assist or control ventilation during the initial stages of resuscitation or to pre-oxygenate a patient as a prelude to anesthetic induction and intubation. A mask airway may be used as the sole airway technique during inhalational anesthesia (with the patient breathing spontaneously) but it is only advisable for relatively short procedures as it “ties up” the anesthesiologist’s hands. It does not protect against aspiration or laryngospasm (closure of the cords in response to noxious stimuli at light planes of anesthesia). Upper airway obstruction may occur, particularly in obese patients or patients with very large tongues. In current practice, the use of a mask as a sole airway technique for anesthesia is rarely-seen although it may be used for very brief procedures in the pediatric patient.
Laryngeal mask airway (LMA)
The LMA is an airway device that is a hybrid of the mask and the endotracheal tube. It is inserted blindly into the hypopharynx. When properly positioned with its cuff inflated, it sits above the larynx and seals the glottic opening. It is usually used for spontaneously breathing patients but positive pressure ventilation can be delivered through an LMA. The LMA does not protect against aspiration. Like an endotracheal tube, it frees up the anesthesiologist’s hands and allows surgical access to the head and neck area without interference. While airway obstruction due to laryngospasm is still a risk, the LMA prevents upper airway obstruction from the tongue or other soft tissues. The LMA also has a role to play in the failed intubation setting particularly when mask ventilation is difficult. The #3, #4 and #5 LMA are used in adults. Many modifications have followed the original “classic” LMA including a design that facilitates blind endotracheal intubation through the LMA (Fastrach LMA™) and one that is specially designed for use with positive pressure ventilation with or without muscle relaxation (Proseal LMA™).
There are 3 basic indications for intubation:
- To provide a patent airway. An endotracheal tube (ETT) may be necessary to provide a patent airway as a result of either patient or surgical factors (or both). For example, an ETT is required to provide a patent airway when surgery involves the oral cavity (e.g. tonsillectomy, dental surgery). An ETT provides a patent airway when the patient must be in the prone position for spinal surgery. Airway pathology such as tumor or trauma may compromise patency, necessitating an ETT.
- To protect the airway. Many factors predispose a patient to aspiration. A cuffed endotracheal tube, although not 100% reliable, is the best way to protect the airway of an anesthetized patient.
- To facilitate positive pressure ventilation. Some surgical procedures, by their very nature, require that the patient be mechanically ventilated which is most effectively and safely achieved via an ETT.
Mechanical ventilation is required when:
- The surgery requires muscle relaxation (abdominal surgery, neurosurgery).
- The surgery is of long duration such that respiratory muscles would become fatigued under anesthesia.
- The surgery involves the thoracic cavity.
In rare cases, an ETT may be required to improve oxygenation in patients with critical pulmonary disease such as Acute Respiratory Distress Syndrome (ARDS), where 100% oxygen and positive end-expiratory pressure (PEEP) may be needed.
While intubation is most commonly performed orally, in some settings nasotracheal intubation is preferable such as during intra-oral surgery or when long-term intubation is required. Nasotracheal intubation may be accomplished in a blind fashion (i.e. without performing laryngoscopy) in the emergency setting if the patient is breathing spontaneously.
Nasotracheal intubation is contraindicated in patients with coagulopathy, intranasal abnormalities, sinusitis, extensive facial fractures or basal skull fractures.
While there are myriad devices and techniques used to achieve intubation (oral or nasal), most often it is performed under direct vision using a laryngoscope to expose the glottis. This technique is called direct laryngoscopy. The patient should first be placed in the “sniffing position” in order to align the oral, pharyngeal and laryngeal axes. The curved Macintosh blade is most commonly used in adults. It is introduced into the right side of the mouth and used to sweep the tongue to the left.
The blade is advanced into the vallecula which is the space between the base of the tongue and the epiglottis. Keeping the wrist stiff to avoid levering the blade, the laryngoscope is lifted to expose the vocal cords and glottic opening. The ETT is inserted under direct vision though the cords. A size 7.0 or 7.5 ETT is appropriate for oral intubation in the adult female and a size 8.0 or 8.5 is appropriate in the male. A full size smaller tube is used for nasal intubation.
The Cormack Lehane views
The view of the larynx on laryngoscopy varies greatly. A scale represented by the “Cormack Lehane views” allows anesthesiologists to grade and document the view that was obtained on direct laryngoscopy. Grade 1 indicates that the entire vocal aperture was visualized; grade 4 indicates that not even the epiglottis was viewed. This is a realistic depiction of the range of what one might see when performing laryngoscopy.
After intubation, correct placement of the ETT must be confirmed and esophageal intubation ruled out. The “gold standard” is direct visualization of the ETT situated between the vocal cords. The presence of a normal, stable end-tidal carbon dioxide (CO2) waveform on the capnograph confirms proper placement except in the cardiac arrest setting. Both sides of the chest and the epigastrium are auscultated for air entry. Vapor observed moving in and out of the ETT is supportive but not confirmative of correct tracheal placement.
If the ETT is advanced too far into the trachea, a right mainstem intubation will occur. This is detected by noting the absence of air entry on the left as well as by observing that the ETT has been advanced too far. The appropriate distance of ETT insertion, measured at the lips, is approximately 20 cm for an adult female and 22 cm for the adult male.
Complications may occur during laryngoscopy and intubation. Any of the upper airway structures may be traumatized from the laryngoscope blade or from the endotracheal tube itself. The most common complication is damage to teeth or dental prosthetics. It is imperative to perform laryngoscopy gently and not to persist with multiple attempts when difficulty is encountered. Hypertension, tachycardia, laryngospasm, raised intracranial pressure and bronchospasm may occur if airway manipulation is performed at an inadequate depth of anesthesia. Sore throat is the most common complication that presents post-extubation and is self-limited. Airway edema, sub-glottic stenosis, vocal cord paralysis, vocal cord granulomata, and tracheomalacia are some of the more serious consequences that can occur and are more common after a prolonged period of intubation.
Airway devices and adjuncts
After performing a history and physical examination and understanding the nature of the planned procedure, the anesthesiologist decides on the anesthetic technique. If a general anesthetic is chosen, the anesthesiologist also decides whether endotracheal intubation is indicated or whether another airway device such as an LMA could be used instead.
When endotracheal intubation is planned, the technique used to achieve it depends in large part on the assessment of the patient’s airway. When intubation is expected to be routine, direct laryngoscopy is the most frequent approach. In settings where the airway management is not routine, then other techniques and adjuncts are used. Airway devices that can be used to achieve an airway (either as a primary approach or as a “rescue” method to use when direct laryngoscopy has failed) are categorized below.
- Methods for securing the upper airway only. These methods achieve what is sometimes termed a “noninvasive airway” and include the oral airway with the mask; the LMA; and the King Laryngeal Tube™.
- Adjuncts for increasing the likelihood of achieving endotracheal intubation through direct laryngoscopy: alternate laryngoscope blades, endotracheal introducers (commonly referred to as gum elastic bougies), stylet.
- Methods of achieving endotracheal intubation using “indirect” visualization of the larynx: video laryngoscope, (the Glidescope™, McGrath™); Bullard™ laryngoscope, fibreoptic bronchoscope.
- Methods of achieving endotracheal intubation in a “blind” fashion (without visualization of the larynx): blind nasal intubation, lighted stylet, retrograde intubation, Fastrach LMA™.
What is a difficult airway?
Airway mismanagement is a leading cause of anesthetic morbidity and mortality and accounts for close to half of all serious complications. The best way to prevent complications of airway management is to be prepared. The anticipation of the difficult airway (or difficult intubation) and formulation of a plan to manage it when it occurs, save lives.
Anticipated difficult intubation
The use of an alternate anesthetic technique (regional or local) may be the most practical approach. If a general anesthetic is chosen, then airway topicalization and awake intubation (with fiberoptic bronchoscope) is the preferred technique. In pediatric patients, neither a regional technique nor awake intubation is feasible. In this case, induction of anesthesia with an inhaled agent such that the patient retains spontaneous respiration is the safest approach. Efforts are undertaken to secure the airway once the child is anesthetized.
Unanticipated difficult intubation
Unanticipated difficult intubation, able to ventilate by mask: In this situation, one calls for help, repositions the patient and reattempts laryngoscopy. The guiding principle is to avoid multiple repeated attempts which can lead to airway trauma and edema resulting in the loss of the ability to ventilate the patient. During the subsequent attempts at intubation, the anesthesiologist considers using alternate airway techniques or awakening the patient to proceed with awake intubation.
Unanticipated difficult intubation, unable to ventilate by mask: This is an emergency situation. One calls for help and attempts to insert an LMA which is likely to facilitate ventilation even when mask ventilation has failed. If an airway is not achievable by non-surgical means, then a surgical airway (either needle cricothyrotomy or tracheostomy) must not be delayed.
When a difficult airway is encountered, the anesthesiologist must respond quickly and decisively. As in many clinical situations that occur infrequently but are associated with high rates of morbidity and mortality, the management of the difficult airway is improved by following well-developed algorithms. The American Society of Anesthesiologists has published a “Difficult Airway Algorithm” which is widely accepted as a standard of care.
Source: Understanding Anesthesia. Karen Raymer, MD, MSc, FRCP(C) McMaster University
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