How to handle the difficult airway, part 1

Airway management is one of the most crucial skill sets needed in the care of critically ill patients and of those requiring surgery. The timely and definitive approach to airway management leaves little margin for error. Indeed, the price of failure includes hypoxic brain injury and death.

One study estimated that 30% of deaths attributable to anesthesia are caused by the inability to successfully manage the difficult airway.¹ Fortunately, the incidence of this problem is relatively low. Various studies have demonstrated the incidence of significant complications to be 0.05% to 0.4% in the more controlled setting of the operating room and 6% in the more chaotic setting of the emergency department.^2-7

There are many underlying causes of difficult airway management, including facial deformity, cervical spine injury, airway hemorrhage, and uncooperativeness or combativeness on the part of the patient.⁸ To minimize the risks of poor outcomes, astute physicians must recognize the potential for difficulty and have an array of options for intervention.

In part 1 of this article, we will address factors that help clinicians predict difficult airway situations. We also will discuss management techniques, such as bag-valve mask ventilation, direct laryngoscopy, and use of light wands. In a coming issue of The Journal of Respiratory Diseases, we will continue our discussion of management techniques, including laryngeal mask airways and surgical techniques.

PREDICTING THE DIFFICULT AIRWAY

The American Society of Anesthesiologists (ASA) Task Force on Management of the Difficult Airway defines the difficult airway as a clinical situation in which an anesthesiologist has difficulty with face mask ventilation of the upper airway and/or difficulty with tracheal intubation.⁹ In difficult mask ventilation, the anesthesiologist cannot provide adequate face mask ventilation. Difficult tracheal intubation may result either from problematic direct laryngoscopy, in which it is not possible to visualize any portion of the vocal cords after multiple attempts, or from difficulty in inserting the endotracheal tube (ETT). Earlier ASA guidelines also considered a difficult intubation as a circumstance in which proper ETT insertion required more than 3 attempts or took longer than 10 minutes.¹⁰

Several scoring systems and anatomic landmarks can be helpful in the assessment of the airway and in determining whether intubation may be difficult. Mallampati¹¹ and others^4,12 devised a classification system to predict risk by correlating ease of laryngeal exposure with visibility of posterior pharyngeal structures. The Mallampati classification scheme has 4 classes based on which posterior pharyngeal structures can be visualized when a patient is seated with his or her head in a neutral position with the mouth open and tongue protruding. In class I, the soft palate, uvula, and pillars of fauces are observed; in class II, the soft palate and pillars of fauces are revealed; in class III, only the soft palate is seen; and, in class IV, only the hard palate is visible (Figure). Class III and IV airway views correlate with more difficult intubations.

While Mallampati scores are routinely ascertained in intubations for elective surgeries, they may be more difficult to assess in emergent situations. However, additional clinical features and measurements may be more useful to alert the practitioner to a potentially problematic airway.

The ASA describes several such findings, including relatively long length of upper incisors; prominent overbite; mandibular incisors anterior to maxillary incisors; less than 3 cm or 3 of the patient's finger breadths between upper and lower incisors on mouth opening; nar-row palate; edematous mandibular space; thyromental distance of less than 3 finger breadths; thick neck; and limited flexion of the neck, as in patients with cervical spine immobilization.⁹ Furthermore, trauma patients with facial distortion, secretions and blood in the airway, and mandibular injury, or those with possible cervical spine injuries can pose challenging airway management issues.^13,14

Walls and colleagues,¹⁵ developers of the National Emergency Management Airway Course, created the mnemonic "LEMON" to help practitioners remember the features that should be used to assess the potentially difficult airway:

• Look at the patient's external features, such as buckteeth, prominent mandible, obesity, and odd facial shape.

• Evaluate the 3-3-2 rule that describes facial and neck relationships and allows the practitioner to place 3 of the patient's finger breadths between the upper and lower teeth, 3 finger breadths between the tip of the mandible and the hyoid bone, and 2 finger breadths between the notch of the thyroid cartilage and the floor of the mandible or the mouth.

• Mallampati score: determine the class by checking the appearance of the oral pharynx.

• Obstruction: consider the presence of a foreign body, illness, or injury in the posterior larynx or upper larynx that could impede airflow and the ability to intubate the larynx.

• Neck mobility (or lack thereof in cervical spine immobilization): mobility is necessary to align the airway axes to facilitate intubation.

CONFIRMING ACCURATE ETT PLACEMENT

One of the last but most critical components of airway management is confirming that the ETT has been successfully placed in the trachea. It is no longer acceptable to use indirect methods to do this. Visualizing the tube passing between the vocal cords is the best way to confirm ETT placement.

Listening for breath sounds, auscultation over the stomach, or watching the tube "fog and clear" are unreliable techniques. Acceptable methods include detection of end-tidal carbon dioxide combined with monitoring of the oxygen saturation trend and/or use of an esophageal detector device. The most commonly used end-tidal carbon dioxide detector is a device that attaches to the end of the ETT and uses a qualitative sensor that changes color in the presence of carbon dioxide.

The esophageal detector device is a large syringe that attaches to the end of the ETT. To begin testing, suction is applied to the syringe. When the tube is correctly positioned, 30 to 40 mL of air is easily aspirated into the syringe. If the tube is in the esophagus, resistance will be felt during aspiration.

These devices are reasonably sensitive and specific, but false- positive results can occur if a large amount of air had been forced into the patient's stomach (as when there is a large quantity of carbonated beverage in the stomach). False-positive results also can occur in low-flow states. A false- negative result can occur if a large amount of mucus in the trachea obstructs airflow into the syringe or if the ETT was placed in the trachea or bronchus with the beveled side against the mucosa.

MANAGING THE PROBLEMATIC AIRWAY

Practitioners who expect airway difficulties can requisition equipment and appropriate personnel for definitive airway management and, thus, optimize patient outcome. A variety of methods are available, each with its own particular challenges.

Bag-valve mask ventilation

For decades, bag-valve mask ventilation has been a cornerstone in cardiopulmonary resuscitation. Although not classified as definitive airway management, it is an excellent temporizing measure for oxygenation and ventilation while the practitioner prepares for definitive intervention.

Difficulty with mask ventilation can result in dangerous situations, especially when there is difficulty with intubation. Anatomically, problematic bag-valve masking frequently results from difficulty in forming an adequate seal around the face or from posterior pharyngeal obstruction at the base of the tongue or the epiglottis.

One prospective study of 1502 patients found a 5% incidence of difficult mask ventilation.¹⁶ Associated risk factors included presence of a beard, lack of teeth, obesity, age of 55 years or older, macroglossia, and history of snoring. Problems in forming an adequate seal can be present in patients who do not have teeth and those who have facial hair, facial burns, or severe facial trauma.

In these cases, 2-person ventilation has been shown to be more effective than 1-person ventilation¹⁷; one person uses both hands to hold the mask flush with the face while the other compresses the bag. Obesity, macroglossia, or poor airway tone can exacerbate posterior airway obstruction.

Several methods may be used to alleviate posterior airway obstruction. Ventilation is improved when the head is in the optimum "sniffing position," in which the neck is slightly flexed and the atlanto- occipital joint is fully extended. Standard maneuvers, such as the chin lift and jaw thrust, lift the hyoid bone and epiglottis away from the airway and alleviate obstruction to bag-valve masking.¹⁸

Oral and nasal airways serve as useful adjuncts to overcome obstruction from the base of the tongue. Oral airways relieve soft tissue obstruction of the airway by anteriorly displacing the base of the tongue. Care should be taken not to use these devices in patients with intact airway reflexes because their insertion may precipitate vomiting, aspiration, and laryngospasm.

Nasopharyngeal airways, placed through the nostrils, project beyond the base of the tongue. Nasopharyngeal airways are much less likely to stimulate gag reflexes and, therefore, are better tolerated by patients, but they are contraindicated in patients with basilar skull fractures.⁸ Patients who cannot be ventilated by mask or intubated using traditional methods require surgical airways.

Direct laryngoscopy

The most effective and rapid way of securing definitive airway management remains direct laryngoscopy with subsequent placement of an ETT. While many situations may make intubation difficult, the likelihood of success can be maximized through proper positioning of the patient's head. The sniffing position facilitates a view of the vocal cords by the alignment of the oropharyngeal, pharyngeal, and laryngeal axes.

The oropharyngeal axis proceeds from the mouth to the posterior pharynx. The pharyngeal axis is parallel to the posterior pharynx, and the laryngeal axis is parallel to the larynx and extends into the trachea. Flexing the neck about 30 degrees aligns the pharyngeal and laryngeal axes. Extending the head about 20 degrees at the atlanto- occipital joint aligns the oropharyngeal and pharyngeal axes.¹⁹ Extension is limited (if not impossible) when the cervical spine is immobilized, and this probably accounts for the 20% incidence of class III and IV laryngoscopic views in this situation.¹⁴

Additional maneuvers may be performed to aid the laryngoscopic view. The first and easiest of these is external laryngeal manipulation, in which the laryngoscopist pushes the thyroid cartilage posteriorly and cephalad until the vocal cords come into view.²⁰ A similar technique, the "BURP" maneuver, consists of an assistant applying pressure on the larynx posteriorly (backward), 2 cm cephalad (upward), and 2 cm to the right (or to the patient's left).^21,22

Benumof and Cooper²³ reported that external laryngeal manipulation improved laryngoscopic visualization by an entire grade. Levitan and associates²⁴ found an additional maneuver improved the view. Their small pilot study of 7 cadavers suggested that lifting the head and flexing it forward with the hand not holding the laryngoscope significantly improved the vocal cord view.

The main value of any of these techniques lies in the speed and lack of adjunct equipment needed to perform definitive airway intervention. However, once it has been established that intubation is not possible using initial head and neck maneuvers, additional equipment should be considered.

Gum elastic bougie

This 60-cm latex-free semirigid rod is 5 mm in diameter with smooth round edges. The tip is angled about 36 degrees, 2 cm from its distal end. The gum elastic bougie is flexible and is straightened or curved as needed by the patient's anatomy.

The semirigid angled tip allows the user to feel a "washboard" sensation as the tip moves over the tracheal rings. The length of the bougie allows it to be advanced to the carina or into the main-stem bronchus, where resistance is met, thereby helping confirm tracheal (rather than esophageal) placement. The flexible-guide ETT introducer is an inexpensive plastic version of the gum elastic bougie.^25,26

The patient is placed in a supine position and a laryngoscope is used to elevate the pharyngeal soft tissues. The bougie is advanced with the angled tip pointed anteriorly toward the epiglottis. Gentle rotation of the bougie in a clockwise and counterclockwise maneuver may be required to traverse the vocal cords.

The assistant providing pressure on the cricoid may confirm proper placement of the bougie as it advances by feeling the tip move against the cricoid and tracheal cartilage. A black mark on the bougie, 37 cm from its distal tip, indicates that the tip is beyond the vocal cords and that the proximal portion is long enough to be used as an introducer along which to slide the ETT.

Occasionally, the tip of the ETT catches the arytenoids or aryepiglottic folds. Successful advancement can be achieved by withdrawing the ETT, rotating it 90 degrees counterclockwise, and then advancing it. After the ETT has been advanced into the trachea, the bougie can be removed and the ETT position can be confirmed using traditional maneuvers.

Although uncommonly used in the United States, the bougie is often the first alternative airway method of choice in Great Britain when anesthesiologists are faced with an unexpected difficult intubation.²⁷ One study compared the success rates of intubation with or without a bougie in patients who had simulated cervical spine injuries and Mallampati scores of III or IV.²⁸ The first-attempt success rate for intubation in the bougie group was 100%, which far surpassed that of the group with laryngoscopy alone.²⁸

In another study with simulated difficult intubations, there was a 96% success rate using the bougie compared with a 66% success rate using the traditional (laryngoscopic) stylet.²⁹ More important, all 17 patients who could not be intubated with a traditional stylet were intubated successfully with a bougie.

Light wand

This tool takes advantage of the anterior location of the trachea relative to the esophagus. A very bright light at the tip of a long stylet is the basic design of several types of light wands that have been produced. The stylet can be directed blindly or with the aid of a laryngoscope while the practitioner looks for the light in the patient's neck. If the tip of the stylet is in the trachea, a very distinct bright light shines through the skin. However, if the tip of the stylet is in the esophagus, the light is vague and diffused or not seen at all.

Most devices consist of 2 parts: a reusable handle and a disposable wand. Enclosed within the plastic tube of another type of wand is a rigid but malleable retractable stylet. The stylet is molded into the shape of a hockey stick, which is similar to the shape of a traditional stylet placed through an ETT.

A water-soluble lubricant must be used with the wand and ETT to facilitate the removal of the wand and stylet once the ETT has been placed. The provider should be positioned in a manner similar to that used for direct laryngoscopy. The patient's head should be held in a neutral position, and the nondominant hand of the practitioner should lift the jaw forward to help elevate the tongue and epiglottis. This step is essential to generate the space in the posterior pharynx through which the ETT/stylet assembly can be navigated.

The ETT/stylet assembly is held in the practitioner's dominant hand and is inserted into the patient's mouth and directed over the tongue toward the glottis. The assembly is advanced until the pre-tracheal glow of the stylet is identified. The assembly can be rocked gently right to left to facilitate navigation of the glottis opening into the trachea. The well-circumscribed bright glow of the tip of the assembly is the key to successful placement.

Withdrawing the stylet several centimeters from the assembly may make the tip of the device more pliable and facilitate its passage into the trachea. Once you are certain that the lighted stylet is in the trachea, you should move the nondominant hand from the mandible to advance the ETT along the stylet into the trachea. The lighted stylet is then removed, and ETT positioning is confirmed using traditional methods. The cuff is inflated to complete the process.

The ETT/stylet assembly is best used when the patient can be successfully ventilated with a bag-valve mask device between intubation attempts. Use of the light wand is contraindicated in situations in which illumination may be difficult to appreciate through the neck tissues, including morbid obesity, brightly lit rooms, and airway pathology (such as polyps, tumors, or abscesses). Extremely thin or frail patients may be poor candidates, because illumination in the esophagus may appear misleadingly bright as a result of the lack of interposing neck tissue.³⁰

Despite the drawbacks, excellent success rates for light-wand intubations have been reported in the literature. Of note, 2 studies showed successes in 203 of 206 patients and 250 of 253 patients, respectively.^31,32 In both studies, morbid obesity mainly contributed to the failures.

[Editor's note: In a coming issue of The Journal of Respiratory Diseases, the authors will continue their review of airway management techniques.]

References:

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