The most effective aids work well in all clinical situations, as everyday practice adjuncts. The following mnemonics fall into this category8:
1.6.1 Difficult Bag-Mask-Ventilation: MOANS
The importance of BMV in airway management is not taken lightly by airway practitioners, particularly as a rescue maneuver when orotracheal intubation has failed. If the airway practitioner is uncertain that neuromuscular blockade facilitated tracheal intubation will be successful, they must be confident that BMV will be adequate, the use of an EGD will be successful, or at the very least, a successful cricothyrotomy can rapidly be performed.
The bag-mask devices most commonly used in resuscitation settings are capable of generating 50 to 100 cm of water pressure in the upper airway, provided that they do not have positive pressure relief valves, and an adequate mask seal can be obtained (Figure 1-7).
Optimum mask hold to achieve a mask seal using a two-hand technique.
Pediatric and neonatal devices often incorporate positive pressure relief valves that can be easily defeated if needed. This degree of positive pressure is often sufficient to overcome the moderate degree of upper airway obstruction offered by redundant tissue (eg, the obese) or edematous tissue (eg, angioedema, croup, or epiglottitis). Research51-59 has validated many of those anatomical features that over the years have been implicated in heralding difficult BMV (difficult mask ventilation or DMV). Those features can be grouped into five indicators that can be easily recalled by using the mnemonic MOANS8:
- Mask seal, high Mallampati grades, Minimal jaw protrusion, or Male gender: Bushy beards, crusted blood on the face, or a disruption of lower facial continuity are the commonest examples of conditions that may make an adequate mask seal difficult. Some recommend smearing a substance such as Vaseline or KY Jelly on a beard as a remedy to this problem. However, in the experience of the authors, it simply makes a bad situation worse in that the entire face becomes too slippery to hold the mask in place. Several studies have identified additional risk factors of difficult mask ventilation, including male sex, Mallampatti III or IV airways, and limited jaw protrusion55,56,59.
- Obese or Obstructing lesions: Patients who are obese (defined by Langeron et al57 as BMI >26 kg·m−2 as opposed to the conventional definition of obese as 30-35 kg·m−2) are often difficult to ventilate adequately by bag and mask. BMV can also be difficult in parturients at term and in patients with upper-airway obstruction, angioedema, Ludwig angina, upper airway abscesses (eg, peritonsillar), and epiglottitis. There is a sense among experienced practitioners that edematous lesions (eg, angioedema, croup, epiglottitis, etc) are more amenable to bag-mask rescue should sudden obstruction occur or be induced, although the authors would not rely on this opinion. On the other hand, firm, immobile lesions such as hematomas, cancers, and foreign bodies usually cannot be circumvented by BMV. Total airway obstruction must be avoided in these patients, and care must be taken with airway manipulation (positioning, avoidance of bleeding, sedative hypnotic medications, etc).
- Aged: Age more than 55 is associated with a higher risk of difficult BMV, perhaps because of a loss of muscle and tissue tone in the upper airway.52,57
- No teeth or Neck radiation: An adequate mask seal may be difficult in the edentulous patient as the face tends to cave in. An option is to leave dentures in situ (if available) for BMV and remove them for intubation. Alternatively, gauze may be inserted in the cheeks to puff them out in an attempt to improve the seal (vigilance to prevent dislodgement into the airway is required). Radiation therapy in the past to the head and neck may hinder mask ventilation.55
- Snores or Stiff: For the former, this mnemonic affords one a reminder to check for sleep apnea, an increasingly important consideration in anesthetic practice today. BMV may be difficult or impossible in the face of substantial increases in airways resistance (eg, deadly asthma) or decreases in pulmonary compliance (eg, pulmonary edema).
As discussed in Chapter 8, several studies involving large patient populations have validated the above findings.55,56,59 In a large study by Kheterpal et al involving over 53,000 adult patients receiving a general anesthetic at a tertiary care hospital, the reported incidence of impossible BMV (IMV) defined as "the inability to establish face-mask ventilation despite multiple airway adjuncts and two-hand mask ventilation" was 0.15%. Despite there being a diverse clinician group (trainees; nurse and physician anesthetists), having a junior anesthesia provider was not found to be an independent predictor for IMV. The presence of three or more predictors (neck radiation, male, OSA, Mallampati III or IV, beard) significantly increased the risk of IMV with an odds ratio of 8.9 compared to patients without these risk factors. Another important finding from this study is that of the IMV group, 25% were also difficult to intubate. It should be remembered, however, that these studies did not examine the incidence of DMV in patients requiring emergency airway management.
1.6.2 Difficult Laryngoscopy and Intubation: LEMON
Difficult laryngoscopy and intubation ordinarily implies that the operator had a poor view of the glottis. Cormack and Lehane36 provided some clarity to the way we think of the difficult airway by parsing the act of intubation into its two subcomponents: laryngoscopy and intubation. They also introduced the most widely utilized system of categorizing the degree to which the glottis can be visualized during laryngoscopy (Figure 1-5). Cormack/Lehane view Grades 3 (epiglottis only visible) and 4 (no glottic structures at all visible) are often used as surrogates to define a difficult laryngoscopy and predict difficult intubation. View Grades 1 (visualization of the entire laryngeal aperture) and 2 (visualization of the posterior cords and arytenoids) are not typically associated with difficult intubation, though some Grade 2s may be difficult or impossible to intubate. Tough Grade 2s and 3s are tailor-made for intubating introducers such as the Eschmann Tracheal Introducer and Frova devices (see Sections 11.2.1 and 11.2.2).
As can be gleaned from the descriptions, the Cormack/Lehane grading system is insensitive to the degree to which the laryngeal aperture is visible during laryngoscopy: a little bit of it (Grade 2) or all of it (Grade 1). The question often asked is: How much of the cords must be viewed to assure intubation success? How much is enough? In attempting to provide a framework or an approach to answering this question, Levitan et al37-39 devised a scoring system to quantify the POGO visible. While attractive in many ways, this scale has yet to gain wide acceptance (Figure 1-6).
The Cormack/Lehane grading system is predicated upon grading during the best attempt at conventional laryngoscopy, and best attempt in turn requires definition. Benumof5 defines best attempt as being composed of six components:
Performance by a reasonably experienced practitioner
No significant muscle tone
The use of the optimal sniffing position
The use of external laryngeal manipulation (backward upward rightward pressure [BURP] or optimum external laryngeal manipulation [OELM])60
Length of the blade
Type of blade
Most times, an intubation demands that the first attempt be the best attempt, particularly in an emergency, although some compromises may be necessary (eg, residency training). Should an orotracheal intubation attempt fail and an additional attempt be contemplated, it seems reasonable to change something on the subsequent attempt to enhance the chances of success. That something may be one, some, or all of these factors. Reminding oneself of the components of the optimum or best attempt provides a framework to address "what to change?"
Optimization of all six components may not be in the patient's best interest in an emergency. For example, if difficulty is anticipated, it may not be advisable to paralyze the patient. Additionally, in the event the cervical spine is immobilized, it may not be possible to place them in the sniffing position. Most experts in airway management agree that positioning the head and neck is an important step in optimizing conventional laryngoscopy as a prelude to orotracheal intubation.61
If it is possible to consistently and precisely predict intubation failure, the initial selection of laryngoscopic oral intubation could be eliminated as a strategy and alternative techniques employed (eg, flexible bronchoscopic intubation, cricothyrotomy). However, they may be technically more challenging, risky, and time consuming. During the last several decades, this has not proven to be possible. Lists of anatomical features, radiologic findings, and complex scoring systems have all been explored without consistent success.
Therefore, we are left to assemble the known risks, match them to the skill, experience, and judgment of the practitioner, and make a decision: Does this airway meet the threshold of being sufficiently difficult to warrant using a Difficult Airway Algorithm, or am I safe to proceed directly to induction, paralysis, and intubation (eg, rapid sequence intubation or rapid sequence induction, commonly known as RSI)8
So, how do we quickly identify as many of the risks as possible? The mnemonic LEMON is a useful guide:
- Look externally: If the airway looks difficult, it probably is (Figure 1-8). A litany of physical features have been associated with difficult laryngoscopy and intubation—a small mandible may indicate that the tongue is retro-fitted over the larynx; a large mandible elongates the pharyngeal axis serving to extend the distance to the larynx and perhaps move it beyond the horizon of view. Buck teeth block access to the oral cavity and elongate the length of the oral axis. A high, arched palate is often associated with a long, narrow oral cavity making access a problem. A short neck may mean the larynx is positioned higher in the neck relative to the base of the tongue making it more difficult to bring the glottis into view. Lower facial disruption is inconsistent with adequate mask seal and may make the glottis impossible to find. It is often said that when it comes to orotracheal intubation, the "tongue is your enemy" because it gets in your way and the "epiglottis is your friend" because once you find it, you ought to be able to find the glottic opening. In upper airway disruption, the tongue may actually be a friend as it leads to the epiglottis and the glottic opening.
- Evaluate 3-3-2: Although there is no scientific basis to support the 3-3-2 rule, it serves to ensure that the relevant geometry of the upper airway is assessed adequately. The first 3 assesses the adequacy of oral access (Figure 1-9). One ought to be able to open one's mouth three of one's own finger breadths (approximately 5 cm). The second 3 and the 2 recognize the interplay of the geometric relationships among the various components of the upper airway as first articulated by Patil in 1983.31 A thyromental distance of less than 6 cm was associated with difficult intubation (Figure 1-3). As described earlier, the thyromental distance is the hypotenuse of Patil's triangle (Figure 1-3), the base being the length of the mandible (Figure 1-10) and the third leg being the distance between the base of the tongue (neck–mandible junction at the level of the hyoid bone) and the top of the larynx (Figure 1-11). One ought to be able to accommodate three of one's own fingers (approximately 5 cm) between the tip of mentum and the mandible–neck junction (Figure 1-10) and fit two fingers between the mandible–neck junction and the thyroid notch (Figure 1-11). The second 3 steers one in assessing the capacity or volume of the mandibular space to accommodate the tongue on laryngoscopy. More than, or less than, three fingers (approximately 5 cm) are associated with greater degrees of difficulty in visualizing the larynx. The length of the oral axis is elongated if it is longer than three fingers, and the mandibular space may be too small to accommodate the tongue during laryngoscopy if it is shorter than three fingers, leaving it to obscure the view of the glottis. This volume is determined by three dimensions: its length, its width, and its depth. The 2 identifies the location of the larynx in relation to the base of the tongue. If more than two fingers are accommodated, meaning the larynx is further below the base of the tongue, it may be difficult to visualize the glottis on laryngoscopy because it is too far down the neck and beyond the visual horizon. Fewer than two fingers may mean that the larynx is tucked up under the base of the tongue and may be difficult to expose. This condition is often called "anterior larynx."
- Mallampati class:33,34 Mallampati studied the relationship between the visibility of the posterior oropharyngeal structures and success rate of laryngoscopic intubation. He had patients sit on the side of the bed, open their mouth as widely as possible, and protrude their tongue as far as possible, without phonating. Figure 1-4 depicts how the scale is constructed. Although Class I and II patients are associated with low intubation failure rates, circumspection with respect to the wisdom of utilizing neuromuscular blockade to facilitate intubation rests with those in Classes III and IV, particularly Class IV in which intubation failure rates may exceed 10%. This scale, by itself, is neither sensitive nor specific.62 However, it is commonly used because it is easily performed, particularly in an emergency, and it may reveal important information about access to the oral cavity and potentially difficult glottic visualization.
- Obstruction: There are three cardinal signs of upper airway obstruction: muffled voice (hot potato voice); difficulty in swallowing secretions, either because of pain or obstruction; and stridor. The first two signs do not ordinarily herald imminent total upper airway obstruction. The presence of stridor generally indicates that the diameter of the airway has been reduced to 4.0 mm or less.63 Upper airway obstruction should always be considered a difficult airway and managed with extreme care. The administration of small doses of opioids and benzodiazepines to manage anxiety may induce total obstruction as the stenting tone of the upper airway musculature relaxes.
- Neck mobility: The ability to position the head and neck is one of the six components of achieving an optimal view of the larynx on oral laryngoscopy. Although there is some dissention,61 it has long been taught that the "sniffing the morning air," or "sipping English tea" positioning (neck flexion, head extension) of the head and neck, when possible, is at least the best place to start. While cervical spine immobilization alone may not constitute a difficult laryngoscopy, airway practitioners should be cautious in managing patients with limited cervical spine movement.
This patient provides an image recognizable instantly as a difficult airway.
Airway evaluation: The first 3 of 3-3-2 evaluation indicates the extent of the mouth opening.
Airway evaluation: The second 3 of 3-3-2 evaluation indicates the length dimension of the mandibular space.
Airway evaluation: The 2 of 3-3-2 evaluation indicates the position of the larynx relative to the base of the tongue.
1.6.3 Difficult Use of an Extraglottic Device: RODS
The insertion of an EGD may be a planned backup maneuver (Plan B) when faced with a failed conventional orotracheal intubation. It may also serve as a bridging technique to reestablish gas exchange in a CICV setting while one prepares to perform a cricothyrotomy (see Chapter 2). To minimize the wasting of valuable time, airway practitioners should place the EGD concurrently while setting up to perform a surgical airway.
In the former case, when Plan B is an EGD, one ought to have performed an evaluation for difficult EGD placement before it is relied on as a primary or backup plan. While there are no prospective studies to evaluate predictors of difficult use of EGDs, there are many clinical reports of difficult use of EGDs, such as the LMA. RODS is a mnemonic that is intended to identify problem patients when an EGD is contemplated:
- Restricted mouth opening: Depending on the EGD to be employed, more or less oral access may be needed. For instance, at least 2 cm of mouth opening is required to accommodate an LMA Fastrach™.64,65
- Obstruction: Upper airway obstruction at the level of the larynx or below. An EGD will not bypass this obstruction. The use of an LMA can be potentially difficult in patients with lingual tonsillar hypertrophy.66,67
- Disrupted or distorted airway: At least in as much as the seat and seal of the EGD may be compromised.68
- Stiff lungs or Stiff cervical spine: Ventilation with an EGD may be difficult or impossible in the face of substantial increases in airway resistance (eg, deadly asthma) or decreases in pulmonary compliance (eg, pulmonary edema). Seal may be exceedingly difficult or impossible to achieve in the face of a fixed flexion deformity of the neck.56 In addition, there are reports of difficult LMA insertion in patients with limited neck movement (eg, ankylosing spondylitis).69,70
1.6.4 Difficult Cricothyrotomy: SHORT
There are no absolute contraindications to performing an emergency cricothyrotomy. However, some conditions may make it difficult or impossible to perform the procedure, making it imperative to identify those conditions upfront, particularly if one is relying on a rapidly performed cricothyrotomy as a rescue technique. Similarly, while there are no prospective trials to determine predictors of difficult cricothyrotomy, a number of clinical reports have identified situations associated with difficulties in performing a surgical airway. The mnemonic SHORT is used to quickly identify features that may indicate a difficult cricothyrotomy:
- Surgery/disrupted airway: The anatomy of the neck may be subtly or obviously distorted due to previous surgery, making the airway difficult to access. Patel reported a case of difficult surgical airway following a recent thyroidectomy.71
- Hematoma or infection: An infective process or hematoma in the pathway of the cricothyrotomy incision may make the procedure technically difficult but should never be considered a contraindication in a life-threatening situation.
- Obese/access problem: Obesity should be considered a surrogate for any problem that makes percutaneous access to the anterior neck problematic. A fixed flexion deformity of the cervical spine, halo traction, and other situations may also make access to the neck difficult. Patel reported a case of surgical airway failure in a patient with an obese and short neck.71
- Radiation: The tissue changes associated with past radiation therapy may alter tissues, making the procedure difficult.
- Tumor: Tumor either in or around the airway may present difficulty, both from an access perspective as well as bleeding.