Definitions

A difficult airway can be defined in several ways. A practitioner may be said to encounter a difficult airway when he or she experiences difficulty providing adequate maintenance or protection of the airway that leads to hypoxemia or soiling of the tracheobronchial tree. This definition includes difficulty in providing ventilation via a face mask or supraglottic airway (e.g., laryngeal mask airway [LMA]) or tracheal intubation. The American Society of Anesthesiologists (ASA) Task Force on Management of a Difficult Airway defines a difficult airway as the clinical situation in which a conventionally trained anesthesiologist experiences difficulty with face mask ventilation of the upper airway, difficulty with tracheal intubation, or both.

The prevalence of difficult face mask ventilation is dependent on the definition. In one study, 5% of 1502 nonpregnant patients experienced difficulty in face mask ventilation, which was defined as an oxyhemoglobin saturation value less than 92%. A multivariate analysis identified five independent risk factors for difficult face mask ventilation: (1) age older than 55 years, (2) body mass index (BMI) greater than 26 kg/m ² , (3) presence of a beard, (4) lack of teeth, and (5) a history of snoring. Impossible face mask ventilation, defined as an inability to exchange air during bag-and-mask ventilation despite multiple providers, airway adjuncts, and neuromuscular blockade, was reported in 77 of 50,000 (0.15%) nonobstetric anesthetic procedures. Independent predictors of impossible face mask ventilation were previous neck irradiation, male sex, diagnosis of sleep apnea, and a Mallampati class III or IV (see later discussion). Difficult laryngeal mask ventilation may be defined as the inability within three attempts of device insertion to produce expired tidal volumes more than 7 mL/kg (leak pressure greater than 15 to 20 cm H ₂ O). In a study of 11,910 nonobstetric patients, the incidence of difficult laryngeal mask ventilation was 0.19%.

Although failed tracheal intubation may appear to be a tangible endpoint, its definition is not standardized. Various definitions have been used, ranging from tracheal intubation not accomplished with a single dose of succinylcholine to simply an inability to intubate during general anesthesia. Such variation inevitably leads to a difference in the reported rate of failed intubation. Defining difficult tracheal intubation is even more complex. Difficulty may be encountered because of failure to visualize the glottis (difficult laryngoscopy) or due to an anatomic laryngeal or tracheal abnormality. Difficulty has been variously defined by (1) the time taken to intubate, (2) the number of attempts, (3) the view at laryngoscopy, and (4) the requirement for special equipment.

Although a dramatic decrease in the number of anesthesia-related deaths has been reported in the UK Confidential Enquiries into Maternal Deaths over the past 40 years, complications from general anesthesia, primarily complications of airway management, continue to be a leading cause of anesthesia-related maternal mortality. Similarly, data from the United States have demonstrated a higher case-fatality rate with general anesthesia compared with neuraxial anesthesia. Although the development of national guidelines has resulted in a more systematic approach to the management of the difficult airway, deaths directly resulting from anesthesia still occur owing to failures in ventilation, tracheal intubation, or airway management following extubation. Despite widespread use of neuraxial anesthesia for operative delivery, general anesthesia may still be required in emergency situations, if neuraxial anesthesia is contraindicated or patients refuse it, or, most commonly, if neuraxial anesthesia fails to provide adequate anesthesia.

Incidence and Epidemiology

The incidence of failed intubation in obstetrics has long been considered to be approximately 1 in 250 to 300 ( Table 29.1 ). A 2015 systematic review including data from published studies, abstracts, and databases reported a failed intubation rate at cesarean delivery of 1 in 433. The incidence of failed intubation for all obstetric procedures was slightly higher at 1 in 390, which likely reflects intubation difficulties resulting from airway swelling following crystalloid resuscitation in postpartum hemorrhage. The authors noted no increase in incidence from 1985 to 2014. However, there was significant heterogeneity among reports. Importantly, the authors highlighted the perceived emphasis on the need to achieve tracheal intubation. This emphasis (see later discussion) may encourage repeated efforts with their accompanying risk for morbidity.

Regardless, the failed intubation rate in obstetric patients is approximately 8 times higher than estimates of the rate in nonobstetric surgical populations. A number of reasons have been proposed to explain the increased difficulty with obstetric airway management. Significant physiologic and anatomic changes of pregnancy (see later discussion) affect the airway, oxygenation, and metabolism. The majority of obstetric general anesthetics are administered for emergency deliveries, often during off-hours ; these anesthetic procedures may be conducted by inexperienced anesthesia providers with less proficiency in difficult airway management. Excessive cricoid pressure applied by a poorly trained assistant can worsen the glottic view at laryngoscopy, as can positioning the parturient with left lateral tilt. With a decrease in the number of cesarean deliveries performed under general anesthesia, trainees have fewer opportunities to become familiar with challenges of the obstetric difficult airway. Changes in anesthesia training, notably the reduction in trainee working hours and the advent of supraglottic airway (SGA) devices, mean that laryngoscopy and intubation are now less commonly performed than previously. Therefore, the skills required to manage a challenging tracheal intubation are less likely to have been gained before working on the labor and delivery unit without direct supervision.

Changes in maternal demographics, most notably an increase in the prevalence of maternal obesity, may increase the risk for complications from general anesthesia, especially when performed for emergency procedures. Obese women are at increased risk for obstetric interventions requiring anesthesia and are at increased risk for failed neuraxial anesthesia, necessitating the use of general anesthesia for emergency delivery (see Chapter 49 ). Difficulty with intubation has been reported to occur in 15.5% of the nonobstetric obese population. A large Danish cohort study of more than 90,000 nonobstetric patients found that a BMI greater than 35 kg/m ² was a significant risk factor for difficult intubation (odds ratio [OR], 1.34) ; BMI was a more accurate predictor of difficult intubation than weight alone. Data collected from one UK region from 1993 to 1998 identified 26 parturients with failed intubation at cesarean delivery; the mean BMI was 33.1 kg/m ² . Poor head and neck positioning at induction of anesthesia, inappropriately applied cricoid pressure, macromastia, shorter interval from start of apnea until significant oxygen desaturation, and operator anxiety may be responsible for a higher incidence of difficult airway management in obese patients.

In contrast to some experts, others have questioned whether the rate of difficult and failed intubation is increasing in obstetric anesthesia practice. A more liberal attitude toward the use of general anesthesia has been suggested to lead to greater familiarity with maternal airway management and subsequent reduced rates of difficulty. Other explanations include increased use of neuraxial blocks in those with predicted airway difficulty, focused training with a bougie as a tracheal tube guide, senior specialist–level involvement, and increasing use of video laryngoscopy and fiberoptic intubation.

Certainly the presence of experienced anesthesia staff during induction of general anesthesia is recommended and should reduce the morbidity and mortality, and perhaps the frequency, of difficulty with airway management. It is hoped that the introduction and widespread acceptance of simulation training in obstetrics will lead to improvement in staff performance during critical events such as difficult airway management (see Chapter 11 ). A need for situational awareness and recognition of the risk for fixation error, especially when dealing with emergency airway issues, was highlighted in a recent edition of the UK Confidential Enquiries into Maternal Deaths.

Maternal Morbidity and Mortality

For many years the UK Confidential Enquiries into Maternal Deaths have reported thromboembolism, hypertensive disease, hemorrhage, and infection as the leading direct causes of maternal mortality. In the report covering the 2012 to 2014 triennium, pregnancy-related mortality from anesthetic causes was the 11th most common cause, accounting for 1% of maternal deaths. In the United States between 1991 and 2002, 1.6% of maternal deaths were related to complications of anesthesia care, representing a 59% reduction in anesthesia-related mortality compared with data from 1979 to 1990. Experience from both countries demonstrates dramatic improvements in anesthesia-related maternal mortality in the past three decades. This improvement likely reflects the tremendous efforts by national anesthesia organizations in defining standards of care that lead to improved maternal safety.

Compared with neuraxial anesthesia, general anesthesia is associated with a greater risk for maternal mortality ( Table 29.2 ; see also Chapter 39 ). Using data from the Centers for Disease Control and Prevention (CDC), the estimated case-fatality risk ratio for general anesthesia compared with neuraxial anesthesia was 16.7 between 1985 and 1990. However, the estimated risk ratio for the period between 1997 and 2002 was only 1.7 (95% CI, 0.6 to 4.6, P = 0.2). Improvements in monitoring and rescue airway equipment, and the publication of algorithms for difficult airway management may account for the reduction in mortality from general anesthesia. The case-fatality risk for general anesthesia from the earlier period may have overstated the relative risk because the accuracy of data was questionable and it is likely that general anesthesia was used for more complex cases for which mortality was expected to be greater. Unfortunately, maternal deaths directly attributable to general anesthesia are still reported. Although protocols for the management of a difficult airway are now ubiquitous, they are not always followed.

Hypoventilation and airway obstruction after extubation are now increasingly recognized as causes of maternal mortality. In Michigan between 1985 and 2003, eight maternal deaths were believed to be related to anesthesia care; all deaths occurred during emergence from general anesthesia or the recovery period, and six of the eight patients were obese. System errors in which the care of the patient did not meet recognized standards were identified in five of the eight cases. These errors included inadequate supervision by an anesthesiologist and lapses in postoperative monitoring.

Physiologic and Anatomic Changes in Pregnancy

Of the multitude of anatomic and physiologic changes that occur in pregnancy (see Chapter 2 ), some have a significant effect on the degree of difficulty of laryngoscopy and tracheal intubation ( Box 29.1 ).

Respiratory and Metabolic Changes

As pregnancy progresses, the gravid uterus increasingly encroaches on the diaphragm and lung volumes are reduced. By term, expiratory reserve volume decreases by 25% and residual volume decreases by 15%, resulting in a 20% reduction in functional residual capacity (FRC). This decrease is more marked in the supine than in the upright position, and in the obese than in the lean patient. Closing volume is unchanged in pregnancy, but the decrease in FRC results in airway closure in 50% of women in the supine position. Metabolic requirements for oxygen increase by nearly 60% during pregnancy, predominantly because of fetal demands. Oxygen requirement is further increased during labor (see Chapter 2 ). These changes make pregnant women more likely to become hypoxemic during periods of apnea such as during the induction of general anesthesia. Therefore, adequate denitrogenation (so-called preoxygenation—replacing nitrogen in the FRC with oxygen) is vital to delay the onset of hypoxemia during periods of apnea (see later discussion).

Preoxygenation and the rate of hemoglobin desaturation have been investigated by computer modeling. In these models, labor, morbid obesity, and sepsis all hasten preoxygenation; however, desaturation also occurs more rapidly in the moderately ill and the obese ( Fig. 29.1 ). Importantly, the time to life-threatening hypoxemia is significantly shorter than that for recovery from paralysis from succinylcholine. Therefore, should ventilation be impossible, it cannot be assumed that the patient will recommence breathing before dangerously low levels of oxygen saturation have been reached. Alternatively, if rocuronium is used instead of succinylcholine and ventilation and oxygenation are impossible, emergency reversal of neuromuscular blockade with sugammadex 16 mg/kg is recommended. Using physiologic simulations in 40-year-old male patients in three BMI categories, Naguib et al. demonstrated that sugammadex reversal of neuromuscular function with rocuronium 1.2 mg/kg was faster than spontaneous recovery of ventilation after succinylcholine 1 mg/kg. However, ventilatory depression leading to a significant fall in oxygen saturation was more likely in obese subjects. Studies of emergency sugammadex reversal in obstetric patients are currently lacking.

Time course of Sa o ₂ during apnea in pregnant (horizontal hatching) and nonpregnant (vertical hatching) virtual subjects predicted to tolerate short, average, and long periods of apnea. Heavy lines represent average subjects. (a) Pregnant subjects, no preoxygenation; (b) nonpregnant subjects, no preoxygenation; (c) pregnant subjects, 99% complete denitrogenation; (d) nonpregnant subjects, 99% complete denitrigenation.

(From McClelland SH, Bogod DG, Hardman JG. Apnoea in pregnancy: an investigation using physiological modelling. Anaesthesia . 2008;63:264–269.)

Cormack and Lehane Grade

Cormack and Lehane devised a glottic view grading system in 1984. The purpose of the system was to grade the glottic view obtained with direct laryngoscopy and use the grade as a means of training for general anesthesia in the obstetric patient. Therefore, the Cormack and Lehane grade is not a preoperative assessment tool but rather a classification method to describe the relative difficulty with subsequent tracheal intubation. The original description includes four grades of laryngoscopy ( Fig. 29.2 ):

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  Grade 1: Full view of glottis

  
  
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  Grade 2: Partial view of glottis or arytenoids

  
  
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  Grade 3: Only epiglottis visible

  
  
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  Grade 4: Neither glottis nor epiglottis visible

Cormack and Lehane laryngoscopic view grades. Grade I is visualization of the entire laryngeal aperture. Grade II is visualization of only the posterior portion of the laryngeal aperture. Grade III is visualization of only the epiglottis. Grade IV is visualization of only the soft palate.

(From Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia. 1984;39:1105–1111.)

Subsequent modifications have been proposed. Grade 2 may be divided into 2A (part of vocal cords visible) and 2B (only arytenoids or very posterior origin of vocal cords visible). Further, Grade 3 may be divided into those in whom the epiglottis is visible and is able to be lifted, such as with a gum elastic bougie (Grade 3A), and those in which the epiglottis is visible but not able to be lifted (Grade 3B). Increasing difficulty with intubation is to be expected with each progressive grade of the Cormack and Lehane classification.

Because of the widespread acceptance of the Cormack and Lehane grading system, some useful information can be gained by reviewing the anesthetic records of patients who have a previous history of direct laryngoscopy; the Cormack and Lehane glottic view grade is often documented. However, prior reports should be treated with caution because grades given in the nonpregnant state will likely differ from those determined during pregnancy, and the potential for interobserver and intraobserver variability exists.

Mallampati Class

In 1985 Mallampati et al. described a three-point scale of the oropharyngeal view of the open mouth based on concealment of the faucal pillars, soft palate, and uvula by the base of the tongue; the more the view was obscured, the greater the difficulty with laryngoscopy and intubation. Samsoon and Young later modified the scoring system into a four-point scale ( Fig. 29.3 ):

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  Class I: visualization of soft palate, uvula, and tonsillar pillars

  
  
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  Class II: visualization of soft palate and base of uvula

  
  
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  Class III: visualization of soft palate only

  
  
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  Class IV: visualization of hard palate only

Modified Mallampati classification of the oropharynx. Classification of the upper airway in terms of the size of the tongue and the pharyngeal structures that are visible with the mouth open. In class I, the soft palate, uvula, and anterior and posterior tonsillar pillars can be seen. In class II, the soft palate and uvula can be seen; the tonsillar pillars are hidden by the tongue. In class III, the soft palate and the base of the uvula can be seen. In class IV, only the hard palate can be seen.

(From Mallampati SR, Gatt SP, Gugino LD, et al. A clinical sign to predict difficult tracheal intubation: a prospective study. Can Anaesth Soc J. 1985;32:429–434.)

The test should be performed with the patient sitting upright with her head in the neutral position. The patient is instructed to open her mouth as wide as possible and protrude her tongue as far as possible without phonation. Increasing difficulty with laryngoscopy and tracheal intubation has been demonstrated with greater Mallampati scores in both obstetric and nonobstetric populations.

Mallampati scores are frequently used as part of an assessment to predict difficult intubation. It is important to remember that scores change during pregnancy and during labor. When used as the sole predictor of a difficult airway, the incidence of both significant false-positive and false-negative results is high. This poor predictive value may be explained by the use of phonation, poor patient positioning, involuntary arching of the tongue, and interobserver variability in interpretation. A meta-analysis of the Mallampati score concluded that the test had limited accuracy for predicting a difficult airway and was not a useful screening test. Consequently, the Mallampati score is best used in combination with other tests.

Thyromental Distance

During laryngoscopy, the tongue is normally pushed into the mandibular space. The thyromental distance, the distance from the tip of the chin to the notch of the thyroid cartilage, can be used to estimate the size of this space and, therefore, whether the tongue can easily be displaced to facilitate laryngoscopy. In the absence of other abnormalities, if the thyromental distance is more than 6.5 cm and the horizontal mandibular length more than 9 cm, intubation should proceed without difficulty. A thyromental distance of less than 6 cm suggests an increased risk for difficulty. However, lack of detail in various studies regarding precisely how the thyromental distance was measured (whether it was performed from the inner or outer border of the mandible) make interpretation of this test difficult.

Anatomically, if the mandibular space is small and unable to accommodate the tissues displaced by the laryngoscope blade, few alterations will improve the line of vision during direct laryngoscopy. When the mandibular space is small, the larynx lies relatively anterior and the tongue must be pulled forward maximally and compressed to expose the larynx.

Atlanto-Occipital Joint Extension

Extension of the atlanto-occipital joint is necessary for the patient to be in the ideal intubating position in which the oral, pharyngeal, and laryngeal axes are aligned (see later discussion). Movement can be assessed with the patient seated with the head and neck in the neutral position facing forward and then with the joint maximally extended ( Fig. 29.4 ). Normal extension should be 35 degrees or more; difficulty with intubation can be expected when joint movement is decreased. The accuracy of this assessment is subject to interobserver variability, making its role in routine airway assessment questionable.

Clinical method for quantifying atlanto-occipital joint extension. When the head is held erect and faces forward, the plane of the occlusal surface of the upper teeth is horizontal and parallel to the floor. When the atlanto-occipital joint is extended, the occlusal surface of the upper teeth form an angle with the plane parallel to the floor. The angle between the erect and the extended planes of the occlusal surface of the upper teeth quantifies the atlanto-occipital joint extension. A normal person can produce 35 degrees of atlanto-occipital joint extension.

(From Bellhouse CP, Dore C. Criteria for estimating likelihood of difficulty of endotracheal intubation with Macintosh laryngoscope. Anaesth Intensive Care. 1988;16:329–337.)

Mandibular Protrusion

The patient’s ability to extend the mandibular teeth anteriorly beyond the line of the maxillary teeth may predict adequate visualization of the larynx during direct laryngoscopy. In the mandibular protrusion test, patients are asked to protrude their mandible as far as possible ( Fig. 29.5 ); one of three classes is assigned:

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  Class A: The lower incisors can protrude anterior to the upper incisors

  
  
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  Class B: The lower incisors can be brought edge to edge with the upper incisors

  
  
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  Class C: The lower incisors cannot be brought edge to edge with the upper incisors

Mandibular protrusion test. Three classifications are based on the test, which is also referred to as the upper lip bite test.

(Redrawn from Munnur U, de Boisblanc B, Suresh MS. Airway problems in pregnancy. Crit Care Med. 2005;33:S259–S268.)

Class A is a good predictor of a good glottic view with direct laryngoscopy, whereas class C is associated with poor glottic view.

The upper lip bite test (ULBT) is similar to mandibular protrusion. In class I, the lower incisors can bite the upper lip above the vermillion border (i.e., the normally sharp demarcation between the lip and the adjacent normal skin); in class II, the lower incisors can bite the upper lip below the vermillion border; and in class III, the lower incisors cannot bite the upper lip. The ULBT has been shown to be a better predictor than a Mallampati score for predicting ease with laryngoscopy and intubation. The ULBT cannot be assessed in edentulous patients.

Other Assessments

Sternomental distance has been suggested to predict difficult laryngoscopy. This distance is measured between the chin and sternum with the head fully extended on the neck and the mouth closed. Unfortunately, the assessment has extremely weak predictive power, and consequently it has largely been abandoned.

Limited mouth opening impedes the introduction of a laryngoscope blade as well as other airway devices; an interincisor distance of less than 5 cm may predict difficult intubation. Mouth opening of less than two fingerbreadths has been shown to reduce the prevalence of easy intubation from 95% to 62%. Mouth opening also is influenced by cervical spine movement; if movement is limited, mouth opening may also be restricted. Protruding maxillary incisors, a single maxillary incisor, and missing maxillary incisors have been shown to be predictive of difficult intubation in obstetric patients.

Comorbidities, including those not related to pregnancy, may influence airway management and should be considered before anticipated airway management. Most notably, maternal obesity is associated with an increased incidence of airway problems (see earlier discussion). Similarly, difficulties in airway management should be anticipated in patients with severe preeclampsia.

Multivariable Assessments

Individual tests are poorly predictive of airway difficulty; therefore, investigators have combined assessments in an effort to improve specificity. Wilson et al. assessed five risk factors (weight, head and neck movement, jaw movement, presence or absence of a receding mandible, prominent teeth). Each variable was scored from 0 to 2, giving a Wilson risk sum. Although 75% of cases of difficult laryngoscopy could be predicted, 12% were falsely predicted to be difficult. Subsequent work using the Wilson risk sum found a positive predictive value of only 9%, and consequently it is now rarely used in clinical practice.

Frerk demonstrated that a combination of the Mallampati score and thyromental distance was more predictive than either test alone; the combined assessment had a sensitivity of 81% and a specificity of 98% in predicting a difficult airway. However, owing to the rarity of difficult intubation, the positive predictive value was only 64%. Realizing that there was an absence of a clear description and agreement as to the method of performing individual tests, Lewis et al. assessed different methods of grading the oropharyngeal view and the mandibular space as predictors of difficult laryngoscopy. Twenty-four different oropharyngeal assessments were considered using two body positions, three head positions, and two tongue positions, each with and without phonation. Similarly, the mandibular space was measured in 24 ways using two body positions, three head positions, and two distal and two proximal endpoints. The results were subject to logistic regression analysis. Although most difficult intubations could be predicted, one-half of those that were anticipated to be difficult were ultimately found to be easy, even with the most predictive combination of tests.

Tse et al. combined the angle at full head extension (in an upright position, the angle made by a line joining the ear tragus [apex] and the corner of the mouth to a line parallel to the floor [horizontal]), thyromental distance, and Mallampati classification in an attempt to predict difficult intubation in a general surgery population. Although these tests were likely to identify easy intubations, they had low sensitivity for predicting those in whom intubation was difficult.

In a study of 400 pregnant women scheduled for elective cesarean delivery, Honarmand and Safavi evaluated Mallam­pati class score, ratio of height to thyromental distance, and the ULBT, both in isolation and combination. A total of 8.75% of patients had a Cormack and Lehane grade 3 or 4 laryngoscopic view; the ratio of height to thyromental distance was the best predictor of this outcome.

Recommendations

The thoroughness of the airway assessment often depends on the urgency with which surgery needs to be performed. For emergency procedures, relatively little time is available; thus it is prudent to assess all women in the labor and delivery suite soon after their arrival, focusing on those with the greatest risk for intervention. However, changes in assessment during the course of labor must be anticipated, and reevaluation before inducing anesthesia is vital to the safe care of these patients.

The assessment should attempt to identify the patients who will be difficult to ventilate and whose tracheas will be difficult to intubate. It should start with a history to detect factors that may indicate the presence of a difficult airway, as well as the potential risk for pulmonary aspiration. Examination of previous anesthetic records, if available, may indicate problems with ventilation or intubation. The presence of comorbidities such as obesity and preeclampsia should be considered. The ASA Practice Guidelines for Management of the Difficult Airway list 11 airway examination components that can be assessed ( Table 29.3 ), acknowledging the absence of a single test that can reliably predict who is likely to present difficulty with airway management. Consequently, a combination of assessments is generally considered preferable.

TABLE 29.3

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