The Difficult Airway

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.¹ This definition includes difficulty in providing ventilation via a facemask 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 facemask ventilation of the upper airway, difficulty with tracheal intubation, or both.²

The prevalence of difficult facemask ventilation is dependent on the definition. In one study,³ 5% of 1502 nonpregnant patients experienced difficulty in facemask ventilation, which was defined as an oxyhemoglobin saturation value less than 92%.³ A multivariate analysis identified five independent risk factors for difficult facemask ventilation: (1) age older than 55 years, (2) body mass index (BMI) greater than 26 kg/m², (3) presence of a beard, (4) lack of teeth, and (5) a history of snoring. Impossible mask ventilation, defined as an inability to exchange air during bag-mask ventilation despite multiple providers, airway adjuncts, and neuromuscular blockade, was reported in 77 of 50,000 (0.15%) nonobstetric anesthetic procedures.⁴ Independent predictors of impossible mask ventilation were previous neck irradiation, male gender, diagnosis of sleep apnea, and a Mallampati class III or IV (see later discussion).⁴ 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 > 15 to 20 cm H₂O).¹ In a study of 11,910 nonobstetric patients,⁵ the incidence of difficult laryngeal mask ventilation was 0.19%.

Although failed tracheal intubation is a tangible endpoint, defining difficult intubation is 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 Confidential Enquiries into Maternal Deaths in the United Kingdom over the past 40 years, complications from general anesthesia, primarily complications of airway management, continue to be the leading cause of anesthesia-related maternal mortality.⁶ Similarly, data from the United States have demonstrated a higher case-fatality rate with general anesthesia compared with neuraxial anesthesia.⁷ Although the development of national guidelines^2,8,9 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 if neuraxial anesthesia has failed 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,^10-14 which is approximately eight times greater than that in the general population (Table 30-1).¹⁵ A number of reasons have been proposed to explain the increased difficulty with obstetric airway management. There are significant physiologic and anatomic changes of pregnancy (see later discussion) affecting the airway, oxygenation, and metabolism. The majority of obstetric general anesthetics are administered for emergency deliveries, often during off hours¹⁶; these anesthetic procedures may be conducted by inexperienced anesthesia providers with less proficiency in difficult airway management. Indeed, in one U.K. study,¹² the relative risk of failed intubation at emergency compared with elective cesarean delivery was 1.79 (95% confidence interval [CI], 0.61 to 5.26). Excessive cricoid pressure applied by a poorly trained assistant can worsen the glottic view at laryngoscopy,¹⁷ as can positioning the parturient with left lateral tilt. Marfin et al.¹⁸ proposed that the introduction of disposable airway equipment may adversely affect airway management; single-use, disposable gum elastic bougies are less reliable than reusable devices.

An increase in the incidence of airway-related complications in obstetric patients has been predicted.^19,20 The change 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. 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.^21-23 Changes in anesthesia training, notably the reduction in trainee working hours and the advent of supraglottic airway (SGA) devices mean that, overall, 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. In an editorial, Russell¹⁹ suggested that failure to intubate during emergency cesarean delivery may be a self-fulfilling prophecy as practitioners draw on less experience with intubating the trachea during general anesthesia for an emergency cesarean delivery.

The increasing prevalence of maternal obesity is of significant concern. Obese women are at increased risk for obstetric interventions requiring anesthesia²⁴ and are more likely to have unsuccessful neuraxial anesthesia, necessitating the use of general anesthesia for emergency delivery (see Chapter 50). Difficulty with intubation has been reported to occur in 15.5% of the nonobstetric obese population.²⁵ A large Danish cohort study of more than 90,000 nonobstetric patients found that BMI of greater than 35 kg/m² was a significant risk factor for difficult intubation (odds ratio, 1.34)²⁶; BMI was a more accurate predictor of difficult intubation than weight alone. Data collected from one U.K. region from 1993 to 1998 identified 26 parturients with failed intubation at cesarean delivery; the mean BMI was 33.1 kg/m².¹² Poor head and neck positioning at induction of anesthesia, inappropriate cricoid pressure, and operator anxiety may be responsible for a higher incidence of difficult airway management in obese patients.²⁷

In contrast to some experts, others have questioned whether the rate of difficult and failed intubation is increasing in obstetric anesthesia practice.^28-30 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.²⁹ The ethics of using a technique potentially associated with greater risk for individual harm to reduce the overall incidence of anesthesia-related mortality associated with the technique presents an interesting dilemma. 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.¹⁶ It is hoped that the introduction and widespread acceptance of simulation training in obstetrics³¹ will lead to improvement in staff performance during critical events such as difficult airway management.

Maternal Morbidity and Mortality

For many years the U.K. Confidential Enquiries into Maternal Deaths have reported thromboembolism, hypertensive disease, and hemorrhage as the leading causes of maternal mortality. In the most recent report, covering the 2006 to 2008 triennium, pregnancy-related mortality from anesthetic causes was the 11th most common cause, accounting for 2.7% of maternal deaths.⁶ 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.^7,32 Experience from both countries demonstrates dramatic improvements in anesthesia-related maternal mortality in the past three decades. This improvement likely reflects the tremendous efforts that been made 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 30-2, see Chapter 40).⁷ Using data from the Centers for Disease Control and Prevention (CDC), the estimated case-fatality risk ratio for general compared with neuraxial anesthesia was 16.7 between the years 1985 and 1990.³² However, the estimated risk ratio for the period between 1997 and 2002 was only 1.7 (95% CI, 0.6 to 4.6, P = .2).³² Improvements in monitoring and the publication of algorithms for difficult airway management have been suggested to 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.³³

Unfortunately, maternal deaths directly attributable to general anesthesia are still reported.⁶ Although protocols for the management of a difficult airway are now ubiquitous, they are not always followed.^12,34

Hypoventilation and airway obstruction after extubation are now increasingly recognized as causes of maternal mortality.^6,35 In Michigan between 1985 and 2003, eight maternal deaths were believed 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.³⁵ 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 30-1).

Cormack and Lehane Grade

Cormack and Lehane⁵⁰ devised a glottic view grading system in 1984. The purpose of the system was to grade the glottic view obtained with direct laryngoscopy 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 (Figure 30-2):

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).^51,52 Further, Grade 3 may be divided into those in which the epiglottis is visible and 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).^52,53 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 inter-observer and intra-observer variability exists.

Mallampati Class

In 1985 Mallampati et al.³⁷ 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 Young¹⁵ later modified the scoring system into a four-point scale (Figure 30-3):

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 obstetric²⁸ and nonobstetric populations.³⁷

Mallampati scores are frequently used as part of an assessment to predict difficult intubation. It is important to remember that scores change during pregnancy³⁸ and during labor.^39-41 When used as the sole predictor of difficult airway, the incidence of both significant false-positive and false-negative results is high.⁵⁴ 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.⁵⁵ 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.⁵⁶ In the absence of other abnormalities, if the thyromental distance is more than 6.5 cm and the horizontal mandibular length more the 9 cm, intubation should proceed without difficulty. A thyromental distance of less than 6 cm suggests an increased risk for difficulty.⁵⁴ 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 (Figure 30-4).⁵⁶ 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 (Figure 30-5). Normal extension should be 35 degrees or more; difficulty with intubation can be expected when joint movement is decreased.⁵⁶ The accuracy of this assessment is subject to inter-observer variability, making its role in routine airway assessment questionable.

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 (Figure 30-6); one of three classes is assigned^57,58:

Class A is a good predictor of a good glottic view with direct laryngoscopy whereas class C is associated with poor glottic view.⁵⁷

The upper lip bite test (ULBT) is similar to mandibular protrusion. In class 1 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 2, the lower incisors can bite the upper lip below the vermillion border; and in class 3, the lower incisors cannot bite the upper lip.⁵⁹ The ULBT has been shown to be a better predictor than a Mallampati score for predicting ease with laryngoscopy and intubation.⁵⁹ 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 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%.⁶⁰ Mouth opening also is influenced by cervical spine movement; if movement is limited, mouth opening may also be restricted.⁶¹ Protruding maxillary incisors, a single maxillary incisor, and missing maxillary incisors have been shown to be predictive of difficult intubation in obstetric patients.²⁸

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).^24,25,27 Similarly, difficulties in airway management should be anticipated in patients with severe preeclampsia.

Multivariable Assessments

Individual tests are poorly predictive of airway difficulty; therefore, a number of investigators have combined assessments in an effort to improve specificity. Wilson et al.⁶² 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.⁶² 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.⁶³

Frerk⁶⁴ 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%.⁶⁴ Realizing that there was an absence of a clear description and agreement as to the method of performing individual tests, Lewis et al.⁶⁵ 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, half of those that were anticipated to be difficult were ultimately found to be easy, even with the most predictive combination of tests.⁶⁵

Tse et al.⁶⁶ 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.⁶⁶

In a study of 400 pregnant women scheduled for elective cesarean delivery, Honarmand and Safavi⁶⁷ evaluated Mallampati class score, ratio of height to thyromental distance, and the ULBT, both in isolation and combination. A total of 8.75% 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.⁶⁷

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; and so 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.⁶⁸ 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 Airway² list 11 components that can be assessed (Table 30-3), 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.

Performing and documenting mouth opening, the Mallampati class, atlanto-occipital mobility, thyromental distance, and mandibular protrusion may be performed relatively quickly and should identify most patients who will present difficulties with airway management. The preanesthesia evaluation should seek to identify risk factors for difficulty with mask ventilation, laryngoscopy, airway device insertion (including intubation), and performance of a surgical airway. When risk factors are identified, appropriate plans for airway management, such as the ready availability of additional equipment and personnel (e.g., individuals experienced with airway management and the creation of a surgical airway) should be made. The proposed plan should consider that the administration of a neuraxial anesthetic technique may be the safest option for both mother and infant, even in the presence of nonreassuring fetal status. The plan must also include alternatives for situations in which the initial plan is not possible. The risks and benefits of various alternatives should be discussed with the patient and the obstetric, neonatal, and nursing teams and documented in the patient's medical record.

Neuraxial Labor Analgesia

The widespread acceptance and use of neuraxial analgesic and anesthetic techniques for obstetric patients has significantly reduced the need for general anesthesia and airway manipulation. In obstetric patients in whom difficulty in airway management or neuraxial technique administration is anticipated or when risk factors for an urgent or emergent cesarean delivery are present, early or prophylactic placement of an epidural catheter should be encouraged. A prophylactic epidural catheter is one that is placed and tested with a small dose of local anesthetic; analgesia is not established until active labor begins, the patient requests analgesia, and/or an operative delivery is required. Such a catheter provides a readily available conduit for providing neuraxial analgesia or anesthesia, especially if rapid onset (e.g., an emergency cesarean delivery) is desirable. Early epidural catheter placement also allows the procedure to take place in a controlled setting and allows time for catheter manipulation and replacement, if necessary, before further pathophysiologic changes (e.g., decreased platelet count, worsening airway edema) occur. The correct placement of the epidural catheter in the epidural space should be tested with the injection of a local anesthetic test dose and careful bilateral sensory testing to confirm the presence of bilateral neuroblockade.

Unfortunately, labor analgesia cannot always be successfully converted to surgical anesthesia for an operative delivery; reported failure rates are as high as 8%.⁶⁹ A 2012 meta-analysis has demonstrated the need for conversion to general anesthesia in 5% of women who receive epidural analgesia in labor⁷⁰; higher failure rates are observed among women requiring more physician interventions for inadequate epidural labor analgesia, in settings of need for urgent delivery, and when an anesthesiologist without specialty training or experience in obstetric anesthesia is providing care. Consequently, women receiving labor epidural analgesia must be evaluated at regular intervals; if analgesia is inadequate, re-siting the epidural catheter must be considered. A meta-analysis of studies comparing different local anesthetics for conversion of epidural analgesia to anesthesia found that lidocaine with epinephrine has a faster onset than bupivacaine, levobupivacaine, or ropivacaine.⁷¹ The addition of bicarbonate to chloroprocaine or lidocaine with epinephrine further hastens the onset of local anesthetic action (see Chapter 26).

In situations in which conversion of epidural analgesia to anesthesia is not possible, general anesthesia may still be avoided if time permits the initiation of spinal or combined spinal-epidural anesthesia. However, care should be taken when performing a spinal anesthetic after a failed epidural top-up dose of local anesthetic, because cases of high and total spinal anesthesia have been reported in this setting (see Chapter 26). An airway management plan must always be in place, even if the primary plan is for the administration of neuraxial anesthesia.

Fasting and Antacid Prophylaxis

All obstetric patients requiring surgical anesthesia are at risk for pulmonary aspiration of gastric contents, particularly if airway difficulties are encountered (see Chapter 29). Because conversion from neuraxial to general anesthesia may be required either before or during surgery, strategies must be adopted to minimize this risk. The ASA and the American College of Obstetricians and Gynecologists (ACOG) recommendations allow modest amounts of clear liquid with uncomplicated labor, but they recommend the avoidance of solid foods in laboring women.^72,73 Clear liquids and solids are allowed up to 2 hours and 6 to 8 hours, respectively, before an elective operative procedure.^72,73 However, more liberal policies on oral intake in labor have become increasingly widespread as maternal death from aspiration becomes less common. The U.K. National Institute for Health and Clinical Excellence suggests that women be allowed to drink isotonic fluids in labor and eat a light diet unless they develop risk factors that make general anesthesia more likely.⁷⁴ Consequently, all laboring women should be assessed and oral intake restricted if surgical intervention appears likely.

Once surgical intervention is required, antacids such as intravenous histamine-2 (H₂)-receptor antagonists or proton-pump inhibitors should be administered in case general anesthesia is necessary. However, these drugs take up to 30 minutes to become effective. If emergency general anesthesia is required, oral administration of a nonparticulate antacid such as sodium citrate is used to increase the pH of gastric contents. A dose of sodium citrate (0.3 molar) 30 mL is effective for approximately 30 minutes and should be administered shortly before the induction of general anesthesia.⁷⁵

Metoclopramide promotes gastric emptying and increases lower esophageal sphincter tone, although its efficacy is decreased by concurrent use of opioids. It may be given orally in labor or intravenously before general anesthesia.

A number of deaths from presumed aspiration after tracheal extubation were reported in the most recent Confidential Enquiries into Maternal Deaths in the United Kingdom report.⁶ The report's authors recommended that when general anesthesia is administered to woman with a potentially full stomach, consideration should be given to passing an “in and out” orogastric tube before extubation.

Patient Positioning

The optimal view during laryngoscopy, which yields the best chance for a successful intubation, requires appropriate patient positioning. The sniffing position, with 35 degrees of neck flexion and 15 degrees of head extension, has been considered the ideal position for facilitating a view of the glottis by aligning the oral, pharyngeal, and laryngeal axes (Figure 30-7).⁷⁶ Although use of the sniffing position has recently been questioned, most studies find it superior to other positions. The correct use of the sniffing position requires that the external auditory meatus and sternum are in horizontal alignment. Some video laryngoscopes (see later discussion) do not require the patient's head and neck to be in the sniffing position for successful device use.

For obese patients, a ramped position is preferable (Figure 30-8). The anteroposterior chest diameter is increased in obese patients, making 35 degrees of neck flexion unachievable in the supine position. Consequently, the shoulders and upper torso need to be raised; this position can be achieved with the use of blankets or pillows or one of the many commercially available, wedge-shaped positioning cushions. Optimal elevation is verified by checking that the external auditory meatus and sternoclavicular joint are in horizontal alignment. Elevating the back of the operating table by 25 degrees may make laryngoscopy easier and also aids preoxygenation (discussed later). The operating room table should be elevated to a height at which the laryngoscopist is most comfortable, with space at the head of the bed to accommodate access for the anesthesia team and necessary equipment.

Left uterine displacement to minimize aortocaval compression should be maintained during preparation for, and induction of, general anesthesia. This may be achieved by tilting the operating table or by placing a wedge under the right hip.

Denitrogenation (Preoxygenation)

In pregnancy, the decrease in FRC and increase in oxygen requirement result in rapid oxygen desaturation during periods of apnea (e.g., during induction of general anesthesia). The FRC is the primary reservoir for oxygen during apnea. Therefore, effective denitrogenation, or preoxygenation, of the FRC is vital to delay the onset of hypoxemia.

The standard technique for preoxygenation has been to breathe 100% oxygen through a tight-fitting facemask at normal tidal volumes for 3 to 5 minutes. Given the urgent nature of obstetric general anesthesia, attention has focused on whether several maximal deep breaths over a shorter period can be as effective. Chiron et al.⁷⁷ compared a traditional 3-minute technique with either eight deep breaths over 1 minute (8 DB/1 min) or four deep breaths over 30 seconds (4 DB/30 sec). By monitoring with end-tidal fractional oxygen concentration (FETO₂), which is probably the best marker of lung denitrogenation, the authors found that 3 minutes of tidal volumes or the 8 DB/1 min technique was more effective than the 4 DB/30 sec technique. They suggested using the 8 DB/1 min technique in the setting of emergency obstetric anesthesia.

The use of maximal deep breaths to achieve denitrogenation may cause maternal hypocarbia and, therefore, should be limited. However, preoxygenation with the 4 DB/30 sec technique leads to more rapid desaturation than standard normal tidal volume breathing or the 8 DB/1 min technique, indicating that a longer period of time is required to maximize oxygen storage in tissue and vascular body compartments. Indeed, during apnea, the time to desaturation depends on (1) the amount of oxygen stored in the lungs, tissue, and blood; (2) the mixed venous oxyhemoglobin saturation; and (3) the presence of intrapulmonary shunting.

A tight-fitting facemask is necessary to prevent air entrainment, which reduces the efficiency of preoxygenation. With normal tidal volume breathing, preoxygenation is best achieved with oxygen flow rates in excess of 10 L/min for 3 minutes,⁷⁸ although this may still be inadequate due to air entrainment; some authors suggest FETO₂ should be greater than 0.8 before anesthesia is induced.⁷⁹ A 20 degree to 30 degree head-up tilt increases the FRC and delays the time to desaturation, especially in obese patients.^80,81

Rapid-Sequence Induction and Cricoid Pressure

In an attempt to minimize the risk for aspiration, the rapid-sequence induction has become the standard technique for induction of obstetric general anesthesia. It usually consists of preoxygenation, rapid intravenous injection of a predetermined dose of induction agent followed immediately by succinylcholine administration, application of cricoid pressure, and avoidance of positive-pressure ventilation before tracheal intubation with a cuffed endotracheal tube. The relative urgency of intubation, in a patient in whom ventilation is avoided, may increase the likelihood of failure.

Although in widespread use, the conduct of rapid-sequence induction is not uniform. Induction agents should provide rapid loss of consciousness with minimal hemodynamic instability while improving the quality of intubating conditions.⁸² Thiopental has traditionally been the induction agent of choice for obstetric general anesthesia.⁸³ When used with succinylcholine, it provides intubating conditions that are as good or better than other agents. An additional advantage is that it can be reconstituted and stored for future use. Furthermore, some evidence suggests that, compared with propofol, thiopental leads to less maternal hypotension and fewer detrimental effects on the neonate.⁸³ Propofol, however, appears to offer better intubating conditions when combined with rocuronium; compared with thiopental in nonobstetric patients, visualization of the vocal cords was significantly more likely with propofol.⁸⁴ Its superiority to thiopental is most likely related to its greater ability to suppress pharyngeal and laryngeal reflexes. However, when succinylcholine is used, the choice of induction agent has no significant effect on intubating conditions.⁸²

Opioids have traditionally not been part of rapid-sequence induction because of concerns of respiratory depression should intubation fail. Moreover, in the obstetric population, the potential for neonatal depression is greater if opioids are used because of rapid placental transfer of these drugs. In nonobstetric patients, the addition of opioids produces better intubating conditions when combined with rocuronium; this improvement has not been demonstrated with succinylcholine.⁸²

Succinylcholine is associated with a number of undesirable side effects, most notably a prolonged duration of action in patients with cholinesterase deficiency and a trigger for malignant hyperthermia and anaphylaxis. However, because of its rapid onset, succinylcholine has traditionally been the muscle relaxant of choice for rapid-sequence induction of anesthesia in obstetric patients. Despite reduced levels of plasma pseudocholinesterase in pregnancy, the duration of action of succinylcholine remains clinically unchanged in the obstetric patient.⁸⁵ The ideal dose of succinylcholine, traditionally 1 mg/kg, remains controversial⁸⁶; when combined with opioids in nonobstetric patients, succinylcholine 1 mg/kg fails to produce good intubating conditions at 1 minute in up to 8% of cases.⁸⁷ Naguib et al.⁸⁷ found that succinylcholine doses as high as 2 mg/kg still do not guarantee excellent intubating condition in all patients; however, the authors found little extra benefit from using doses above 1.5 mg/kg.

The potential disadvantage of increasing the succinylcholine dose is delayed return of spontaneous respiration; return of spontaneous ventilation is vital should intubation not be achieved. Reducing the succinylcholine dose to 0.5 to 0.6 mg/kg does not appear to compromise intubating conditions, at least when administered in the nonobstetric population with propofol and fentanyl; the lower dose slightly shortens the recovery time.⁸⁸ In pregnancy, however, the reduced maternal FRC and greater oxygen demands make significant desaturation likely before the return of spontaneous respiration, no matter the succinylcholine dose. Because thiopental remains a widely used induction agent, and opioids are not commonly administered, continued use of succinylcholine 1 to 1.5 mg/kg is recommended.

The side effect profile of succinylcholine has resulted in consideration of alternative muscle relaxants for the rapid-sequence technique. Rocuronium is often used when succinylcholine is contraindicated; however, its prolonged duration of action is a significant concern when failure to ventilate or intubate occurs. Sugammadex, with its ability to rapidly reverse the effects of rocuronium, may help resolve this controversial issue; however, the drug is not available in the United States and there have been no clinical trials in pregnancy. When used with propofol for rapid-sequence induction, a 2008 meta-analysis indicated that succinylcholine (1 mg/kg or greater) produced superior intubating conditions compared with rocuronium (0.6 to 0.7 mg/kg) (relative risk [RR], 0.88; 95% CI, 0.80 to 0.97).⁸⁹ No significant difference was observed between the two agents when a larger dose of rocuronium (1.2 mg/kg) was used.⁸⁹ Therefore, succinylcholine remains the preferred muscle relaxant for use in rapid-sequence induction.

Cricoid pressure as a method to decrease pulmonary aspiration during induction of anesthesia was first described in 1961.⁹⁰ Interest in the technique was promoted by the reports of deaths due to aspiration under general anesthesia.⁹¹ Although the effectiveness of cricoid pressure in preventing pulmonary aspiration of gastric contents has recently been challenged,^92,93 it is frequently used during the induction of obstetric general anesthesia (see Chapters 26 and 29). However, the use of cricoid pressure can adversely affect the ease of ventilation, laryngoscopy, and intubation. In a comparison of cricoid pressure with 20 N, 30 N, and 44 N of force, increasing pressure was more likely to lead to cricoid deformity and esophageal occlusion, particularly in women.⁹⁴ Difficulty with ventilation is less likely when 30 N is applied (currently accepted practice) than with 44 N (the previously suggested optimum value).⁹⁵ When correctly applied, with an increase in force from 10 N to 30 N with the induction of general anesthesia, there is little evidence of harm. However, when difficulty with intubation or ventilation arises, pressure may need to be reduced or released (see later discussion).

Although the cricoid pressure technique originally described by Sellick⁹⁰ was a one-handed technique, the placement of a second hand behind the patient's neck to prevent excessive neck flexion has been observed to provide a superior laryngoscopic view.⁹⁶ However, it should also be remembered that the two-handed cricoid pressure technique does not allow the anesthetic assistant to assist with other procedures, such as holding additional equipment necessary for difficult airway management.

Planning

The approach to the difficult airway in the obstetric patient depends on the situation as well as the skill set of the anesthesia provider. In Figure 30-9 a suggested approach is outlined for management of obstetric patients with an anticipated difficult airway. After an initial assessment of the patient, an airway management plan should be created and shared with the patient and other members of the multidisciplinary team. In extreme cases, the anesthesia considerations may influence the mode and timing of delivery. Despite a thorough airway assessment and management plan, unanticipated or unrecognized airway issues and complications may arise; alternative algorithms and equipment should be readily available to ensure oxygenation and ventilation. Emergence and extubation should also be planned in advance. Lack of forethought and planning can lead to poor decision-making in crisis situations.

Neuraxial Anesthesia

The value of establishing and confirming a functional epidural catheter during labor in patients with an anticipated difficult airway has been described (see earlier discussion). A neuraxial anesthetic technique may also be preferable in patients with an anticipated difficult airway undergoing urgent or elective cesarean delivery. The choice of anesthetic technique (e.g., single-shot spinal, combined spinal-epidural, epidural, continuous spinal techniques) depends on the circumstances and preferences of the anesthesia provider. Neuraxial techniques do not obviate the necessity of planning airway management. High spinal anesthesia necessitating urgent airway intervention is a complication of all neuraxial techniques. Epidural anesthesia may be complicated by unintentional intravascular or intrathecal injection. Despite optimal planning and execution, a neuraxial anesthetic technique may fail to provide a surgical blockade of adequate density or duration.^33,58,97 Therefore, plans for securing the airway must always be preformulated, and standard and alternative airway equipment should be readily available.

Awake Intubation before General Anesthesia

Performing an awake intubation may be the safest option for the patient with an anticipated difficult airway, particularly if very difficult or impossible mask ventilation is anticipated or if neuraxial anesthesia is contraindicated or fails. Even patients with an advanced upper airway pathologic process have the ability to breathe when awake. However, the induction of general anesthesia with paralysis can distort the airway anatomy by allowing soft tissue relaxation and movement of the larynx in an anterior direction; this distortion can make attempts at direct laryngoscopy more difficult. Therefore, an appropriate sequence of events includes securing the airway of these patients while they are awake and spontaneously breathing, before the induction of general anesthesia.⁹⁸ There is a perceived notion, particularly among practitioners with limited experience with the technique, that an awake intubation is time consuming, results in patient discomfort and anxiety, and is often difficult. In skilled hands, the technique can be accomplished quickly and comfortably with a high success rate.⁹⁹

Awake intubation can be performed with a number of airway management devices, but the flexible fiberoptic bronchoscope offers unique advantages (Box 30-2). Proper planning and execution, with attention to detail, are keys to patient cooperation and a high success rate. Appropriate equipment must be readily available and experienced assistance is desirable. It is useful to have two anesthesia providers: one to perform the endoscopy and another to monitor the patient.¹⁰⁰ Pulse oximetry, capnography, continuous electrocardiography, and blood pressure monitoring are mandatory. The level of conscious sedation must be constantly monitored to obtain the desired level for the procedure (see later discussion). Supplemental oxygen should be administered.

An unhurried, thorough explanation of the technique to the patient helps to allay anxiety. Pharmacologic premedication should include prophylaxis for pulmonary aspiration and an antisialagogue such as intravenous glycopyrrolate 0.2 mg. A dry mouth improves topical oral anesthesia by ensuring better contact between the local anesthetic and the mucosa.¹⁰¹ Secretions may also cause internal reflection from the light source and distort the fiberoptic view. Performing the procedure with the patient in the upright, rather than supine, position minimizes airway obstruction and aortocaval occlusion, enhances drainage of secretions, and allows better acceptance of topical anesthesia by the patient.

Indirect Optical/Video Laryngoscopy

Since the introduction of the Bullard laryngoscope in the late 1980s, a number of rigid indirect-optical laryngoscopes have been developed. Frequently referred to as video laryngoscopes because of a video camera eye positioned near the tip of the blade, these devices can capture an image of the glottis in real time and transmit it to a video screen.¹¹⁷ Video laryngoscopes offer several advantages over conventional direct laryngoscopy. To obtain a good view of the glottis with a direct laryngoscope, a line of sight from the oral opening to the glottis must be obtained by neck flexion and head extension (see earlier discussion). With video laryngoscopes, a direct line of sight to the glottis is unnecessary. In patients with an anteriorly positioned larynx, an assistant is frequently required to apply pressure on the thyroid cartilage to move the larynx posteriorly and improve the view; with the use of video laryngoscopes, the assistant may watch the screen image to more directly witness the effect of the pressure. Randomized controlled studies have found higher success rates for intubation with several types of video laryngoscopes compared with the Macintosh laryngoscope blade in adult nonobstetric patients with predicted difficult airways.^118-122 Video laryngoscopes are perceived to cause less trauma and stress to patients owing to less need to reposition the head and neck, less pressure on the neck, and less frequent use of ETT introducers^118,123; however, data are currently insufficient to confirm this perception.¹¹⁷ Data are conflicting as to whether the elapsed time required to intubate using video laryngoscopy is longer or shorter than that required using conventional laryngoscopy.¹¹⁷

The indirect-optical and video laryngoscopes may be classified into three categories¹¹⁷:

There are few studies that compare the use of different video laryngoscopes in patients with an anticipated difficult airway.^129,130 Furthermore, it not known whether the preoperative assessments used to predict difficult direct laryngoscopy are valid predictors of difficult video laryngoscopy. Additional investigation is necessary to compare various devices and to ascertain whether specific devices or device types are better for specific airway variations.

Dhonneur et al.¹³¹ reported the successful use of a difficult airway algorithm in which the Airtraq device was used in parturients as a rescue device if tracheal intubation failed after 2 minutes of direct laryngoscopy. During a 6-month period, 69 parturients underwent emergency cesarean delivery under general anesthesia; 2 morbidly obese parturients required the Airtraq device for successful tracheal intubation. The investigators suggested that the device might be an acceptable primary airway management tool in cases of emergency cesarean delivery in parturients with an anticipated difficult airway. Aziz et al.¹³² retrospectively analyzed 180 tracheal intubations over a 3-year period in their obstetric unit. Traditional direct laryngoscopy resulted in 157 of 163 successful intubations on first attempt, with one failed intubation (95% CI, 92% to 99%). Video laryngoscopy with a GlideScope resulted in 18 of 18 successful intubations on the first attempt (95% CI, 81% to 100%) and a successful intubation in the patient with the failed direct laryngoscopy. Of note, the patients whose tracheas were intubated with the video laryngoscope were more likely to require urgent or emergency surgery and/or have predictors of difficult direct laryngoscopy than the patients whose tracheas were intubated using direct laryngoscopy.

The GlideScope has also been used for awake intubation or to assist fiberoptic laryngoscopy intubation.^133,134

Awake Tracheostomy or Surgery Standby

It is possible to perform an awake tracheostomy with local anesthesia, a technique that may be required in some situations in which airway management is anticipated to be extremely difficult and dangerous.^103,135,136 In some cases, particularly if there is a known airway pathologic process, it is prudent to request that a surgical team proficient in emergency surgical airway management be immediately available before the induction of anesthesia for cesarean delivery.

Local Anesthesia for Cesarean Delivery

Rarely, the infiltration of local anesthesia may be used as a primary anesthetic technique for emergency cesarean delivery in the patient with an anticipated difficult airway. This technique, which has been well described, is most often used in developing countries, where contemporary anesthetic techniques may not be readily available.^137-139 Few obstetricians today are familiar or proficient with this technique, but some resident training programs still provide instruction on its use.⁵⁸ A large volume (i.e., 75 to 100 mL) of a dilute local anesthetic solution, such as 0.5% lidocaine, is often required. Administration of such a large volume entails a risk for systemic local anesthetic toxicity.

Mei et al.¹⁴⁰ described four cases in which cesarean delivery was performed with bilateral transversus abdominis plane (TAP) block and ilioinguinal-iliohypogastric (IIIH) nerve blocks using 0.5% ropivacaine 40 mL.

In some cases it is possible to perform the entire surgical procedure with local infiltration, provided the obstetrician makes a midline abdominal incision, makes minimal use of retractors, and does not exteriorize the uterus. Alternatively, the obstetrician might begin surgery and deliver the infant with the aid of local infiltration. Temporary hemostasis may be achieved until the airway is secured and then surgery completed after the induction of general anesthesia.

Cesarean delivery performed with local infiltration, if successful, has the advantages of preserving maternal hemodynamic stability and a patent airway while allowing emergency delivery of the infant. However, the technique requires a skilled and patient obstetrician. Maternal anesthesia is typically incomplete and often inadequate, a fact that subsequently presents significant management issues, given that the abdomen has been opened, positioning options are limited, and the consequences of the surgical procedure such as hemorrhage may require immediate attention.

Features of the Obstetric Patient

Despite attempts to adequately assess parturients preoperatively, cases of unanticipated difficulty with airway management do occur (Figure 30-12).¹⁴¹ Therefore, the anesthesia provider and the entire operating team should have a plan to manage unanticipated difficulties in airway management before administering general anesthesia to obstetric patients. Although national guidelines exist for this scenario in the nonobstetric patient,^2,8 none exist specifically for the obstetric patient. The lack of guidelines likely reflects the unique and often difficult conflict posed by the failure to intubate the mother's trachea: the optimal and safe management of the mother may threaten the life of the fetus and vice versa. A classification schema for the degree of urgency of cesarean delivery¹⁴² may help clarify the risks to the mother and fetus, thereby improving communication among providers, particularly between anesthesia providers and team members whose primary focus may be directed toward the fetus (obstetricians, midwives, neonatologists). Communication between anesthesia providers and other members of the multidisciplinary team is vital,⁷³ especially if the anesthesia provider(s) believes it is in the best interest of the mother to delay delivery of the fetus to manage the difficult airway. The entire operating room team should be familiar with local protocols to manage this infrequent but life-threatening situation. Examples of failed intubation guidelines are available on the website of the Obstetric Anaesthetists' Association (OAA).¹⁴³

The safest method to prevent difficulty in airway management is by making the first attempt at laryngoscopy the best attempt. This is achieved by using optimal head and neck position, applying cricoid pressure correctly, and waiting for an adequate dose of muscle relaxant to act (see earlier discussion). If an adequate view of the larynx is not achieved, a second attempt should be made using a gum elastic bougie and/or a different laryngoscopy blade or device. In experienced hands, laryngoscopy attempts should be limited to no more than three, and the second or third attempt should be performed only in those cases in which a portion of the laryngeal anatomy is visible (Cormack and Lehane grade 3A or better). If a grade 3B or 4 laryngoscopic view is identified with the initial laryngoscopy attempt, the anesthesia provider should immediately focus attention on ensuring adequate oxygenation and ventilation of the mother.¹⁴⁴

The fact that the trachea cannot be intubated should be conveyed loudly to the entire operating room team and help should be summoned immediately. Maintenance of maternal and fetal oxygenation becomes the anesthesia provider's first priority. Facemask ventilation should be attempted with maintenance of cricoid pressure. Placement of an oropharyngeal airway may be helpful. If difficulty is encountered, a two-person mask technique should be employed, with one person firmly holding the mask in place with two hands while providing a jaw lift as the second person squeezes the reservoir bag.^58,141 Once facemask ventilation is attempted, several scenarios may arise, which are discussed next.

Cannot Intubate but Can Ventilate

In the “cannot intubate but can ventilate” scenario, maternal and fetal status should be rapidly assessed in consultation with the obstetrician. If the mother and fetus are not in immediate jeopardy, then the safest course is to awaken the mother. Once this is accomplished, other anesthetic options, such as an awake intubation or a neuraxial anesthetic technique, should be considered. The risk for proceeding with surgery in this scenario represents an elective commitment to the possibility of mask airway failure, more airway manipulation, aspiration, and progression to a “cannot ventilate” scenario.

If the situation is immediately life threatening to the mother secondary to hemorrhage (e.g., uterine rupture, placental abruption), it may be necessary to proceed with cesarean delivery to optimize outcome for both the mother and infant. Significant angst and controversy often accompany decision-making in the management of a stable mother with evidence of life-threatening fetal compromise, such as fetal bradycardia as a result of a prolapsed umbilical cord. In such cases, if mask ventilation is easy and adequate, the risk-benefit ratio of proceeding with an unsecured airway and an increased risk for aspiration should be weighed against the benefits of prompt delivery of the infant. In cases in which the maternal risk for aspiration is considered low and mask ventilation is easy, it may be reasonable to continue mask ventilation and avoid further intubation attempts. It is unclear as to whether continued mask ventilation or repeated intubation attempts represent the greater risk to the mother; even insertion of an LMA may further traumatize the airway or precipitate regurgitation.

The anesthesia provider should carefully consider the maternal risks of proceeding with cesarean delivery in a mother with an unsecured and unprotected airway, especially if no urgency exists and/or mask ventilation is difficult. Some obstetric anesthesiologists argue that even a nonreassuring (but non–life-threatening) fetal heart rate tracing does not always justify proceeding with cesarean delivery under general anesthesia in a patient with an unsecured airway. Alternatively, in some of these cases, proceeding with cesarean delivery via mask ventilation or with an SGA may be a better option than awakening the patient, especially in those in whom neuraxial techniques are contraindicated. In these cases, the importance of communication between the obstetric and anesthesia teams cannot be overemphasized (see Chapter 11).

Cannot Intubate and Cannot Ventilate

In the “cannot intubate and cannot ventilate” scenario, the first objective remains the maintenance of maternal and fetal oxygenation. This scenario indicates that facemask ventilation has failed despite optimizing head and neck position, inserting an oropharyngeal airway, and using the two-person mask technique. If partial ventilation exists and succinylcholine has been given, it may be possible to allow the neuromuscular blockade to resolve and spontaneous ventilation to return. If ventilation is not possible, insertion of an SGA (e.g., LMA, laryngeal tube) should be considered (see next section). Initial SGA insertion should be attempted with the application of cricoid pressure¹⁴⁴; however, pressure may need to be reduced or removed to facilitate insertion. If SGA insertion or ventilation fails, the management team should secure an airway using an anterior neck technique (i.e., jet ventilation via a cannula inserted through the cricothyroid membrane, or surgical cricothyrotomy or tracheostomy). If these methods prove successful, the risks and benefits of proceeding with surgery should be discussed among team members; both maternal and fetal health should be considered. A 14- or 16-gauge cannula placed through the cricothyroid membrane is inherently unstable; thus, ideally, a more definitive airway should be established before surgery is started.⁵⁸

Laryngeal Mask Airway

The LMA is arguably the SGA with which anesthesia providers are most familiar. The introduction of the LMA into anesthetic practice was a significant advance in airway management that resulted in major alterations to the difficult airway algorithms of the ASA and other societies.^2,8 The insertion of an LMA in an obstetric patient who can easily be ventilated by facemask is controversial, because little additional ventilation benefit is obtained and the placement of an LMA can induce vomiting and aspiration in this setting. However, in any situation in which conventional facemask ventilation is difficult or impossible, an SGA is the rescue device of choice.

The LMA has many advantages, most notably its ease of use and a very high initial success rate.¹⁴⁵ Moreover, the LMA need not be perfectly positioned over the larynx to allow adequate ventilation. When assessed by flexible fiberoptic endoscopy, radiography, and magnetic resonance imaging, the placement of the LMA around the larynx is variable¹⁴⁵; however, 94% to 99% of patients with an LMA have little or no difficulty with ventilation.

In a prospective study, an LMA was inserted by experienced users in 1067 healthy parturients undergoing elective cesarean delivery under general anesthesia.¹⁴⁶ The investigators demonstrated that a clinically effective and acceptable airway was obtained on the first attempt in 98% of the patients and on the second or third attempt in an additional 1%. Fewer than 1% of patients required intubation for failure to obtain satisfactory LMA placement within 90 seconds, or for an SpO₂ less than 94%, or an end-tidal CO₂ greater than 45 mm Hg. Moreover, the airway management (which was accomplished with the LMA, maintenance of cricoid pressure until delivery, and mechanical tidal-volume ventilation of 8 to 12 mL/kg) was associated with no episodes of hypoxemia (SpO₂ < 90%), regurgitation, aspiration, laryngospasm, bronchospasm, or gastric insufflation. The investigators concluded that, in experienced hands, an LMA is effective and “probably safe” for ventilation and the administration of a volatile anesthetic agent for general anesthesia in selected healthy patients undergoing elective cesarean delivery.

A number of reports have described the use of an LMA as a rescue device for obstetric patients in whom conventional methods of securing the airway have failed.^147-152 In a national case control study performed in the United Kingdom from 2007 to 2009, 39 of 57 patients with a failed intubation were managed with a classic LMA.¹⁵³

An LMA may also act as a conduit for intubation; however, passage of an ETT without visualization has a low success rate.¹⁵⁴ By contrast, fiberoptic-guided intubation through the LMA has a success rate that has approached 100% in some studies.¹⁴⁵

Despite these benefits, the LMA has been associated with the following disadvantages: (1) placement can induce vomiting; (2) aspiration of gastric contents is not prevented; (3) improper positioning can lead to gastric insufflation; (4) multiple insertion attempts may be required for correct placement, which may result in airway trauma; and (5) use of positive-pressure ventilation may be limited. In 0.4% to 0.6% of patients with normal airway findings, the placement of an LMA leads to inadequate ventilation¹⁴⁵; reasons for this outcome have been reported to include (1) backfolding of the distal cuff, (2) occlusion of the glottis by the distal cuff, (3) complete downfolding of the epiglottis, and (4) 90- to 180-degree rotation of the mask around its long axis.

Most of the data just cited pertain to the use of the original classic LMA; several variations in LMA design have since become available. Updated designs that have been found to be useful in difficult airway management of parturients are the ProSeal LMA (LMA North America San Diego, CA, USA) and the Fastrach LMA (LMA North America, San Diego, CA, USA). The ProSeal LMA has a specialized high-volume/low-pressure cuff that allows the device to achieve a better fit over the glottis than a classic LMA (Figure 30-13).¹⁵⁵ This design allows the use of higher ventilation pressures (up to 30 to 40 cm H₂O) with less air leakage around the cuff and a lower risk for air entry into the stomach. The ProSeal LMA also contains a specialized drainage conduit that bypasses the bowl of the LMA to minimize the entry of gastric fluid into the glottis. The drainage conduit has been shown to be effective in venting both passive and active regurgitation^156,157 and can accommodate the passage of a gastric tube, which can assist in decompressing or emptying the stomach.

Cook et al.¹⁵⁸ randomly assigned 180 nonparalyzed, nonpregnant anesthetized patients to airway management with a ProSeal or classic LMA. The ProSeal more reliably allowed positive-pressure ventilation than the classic LMA. Halaseh et al.¹⁵⁹ described the use of the ProSeal LMA in 3000 patients undergoing cesarean delivery who had fasted for more than 4 hours and were not thought to have a difficult airway. They established an “effective” airway on the first attempt in 2992 (99.7%) women, with only 8 patients (0.3%) requiring a change to a different LMA size. None of the patients required tracheal intubation, and only one patient experienced regurgitation of gastric contents into the mouth.¹⁵⁹ Minor side effects such as sore throat occurred in 21 patients (0.7%). A number of case reports have described the successful use of the ProSeal LMA after failed intubation in obstetric patients.^160-162

A disadvantage of use of the ProSeal LMA in an emergency is that it requires practice and experience to use correctly. Further, because the gastric drainage conduit traverses the LMA bowl, the passage of an ETT may be more difficult.¹⁶³ A disposable, single-use version of the ProSeal LMA—the LMA Supreme—is now available.

Although designed specifically to facilitate blind tracheal intubation,¹⁶⁴ the Fastrach or intubating LMA can also be combined with fiberoptic bronchoscopy (Figure 30-14).^165,166 Both reusable and disposable versions are available. When properly placed, the intubating LMA allows ventilation similar to that with the original LMA; however, a more rigid J-shaped design improves the alignment of the mask over the glottic opening and better accommodates a special soft-tipped tracheal tube for blind intubation (see Figure 30-11). This special silicone ETT minimizes the risk for airway trauma and is available in diameter sizes 6.0 to 8.0 mm. In addition, the intubating LMA has an epiglottis elevator bar at its distal end that acts to lift the epiglottis anteriorly as the ETT exits the intubating LMA into the glottis.¹⁶⁴ When a fiberoptic bronchoscope is used with an intubating LMA, the ETT should be advanced far enough to partially elevate the epiglottis elevator bar to assist the passage of the fiberoptic bronchoscope into the trachea. A variation of the intubating LMA, the Air-Q LMA (Mercury Medical, Clearwater, FL, USA), has a different mask tip, a keyhole-shaped airway outlet, and a removable circuit connector that accommodates an 8.5-mm ETT (Figure 30-15). The successful use of the intubating LMA during a failed intubation at emergency cesarean delivery has been reported.¹⁶⁷

Laryngeal Tube and Esophageal-Tracheal Combitube

The laryngeal tube (VBM Medizintechnik GmbH, Sulz, Germany) is another SGA device.^179,180 Laryngeal tubes are manufactured from either silicone or polyvinylchloride (PVC) and have ventilation apertures between a proximal oropharyngeal cuff and a distal esophageal cuff. The laryngeal tube is inserted into the oropharynx until resistance is met, which should result in positioning of the ventilation apertures directly above the glottic opening. These devices are reported to provide seal pressures similar to those with the ProSeal LMA (40 cm H₂O) and insertion times and success rates comparable to those of the LMA.¹⁸¹ The Laryngeal Tube-S (LTS) contains a second lumen that can be used for drainage of the stomach.¹⁸² The LTS has been used successfully after a failed intubation and ventilation in a patient undergoing emergency cesarean delivery.¹⁸³

The esophageal-tracheal Combitube (ETC) (Sheridan Catheter Corporation, Argyle, NY, USA) is a twin-lumen plastic tube with an outer diameter of 13 mm. One lumen has an open distal end and thus resembles a tracheal tube (i.e. the tracheal lumen), and the other (esophageal) lumen has a closed distal end. The ETC has a 100-mL proximal pharyngeal balloon; when the ETC is correctly positioned, the pharyngeal balloon fills the space between the tongue base and soft palate. When inflated, the proximal balloon seals the oral and nasal cavities. Distal to the pharyngeal balloon, but proximal to the level of the larynx, are eight perforations in the esophageal lumen. A smaller 15-mL distal cuff, similar to that on an ETT, seals either the esophagus or trachea when inflated. The ETC is inserted, with or without the aid of a laryngoscope, but its insertion does not require visualization of the larynx. Indeed, in the usual clinical context, the larynx cannot be visualized. The ETC enters the esophagus 96% of the time, allowing ventilation through the esophageal lumen perforations.¹⁸⁴ If the ETC enters the trachea, the patient's lungs can be ventilated directly through the tracheal lumen. Therefore, regardless of whether the distal end of the ETC enters the trachea or esophagus, the anesthesia provider can ventilate the lungs, assuming correct identification of which lumen should be used for ventilation.

The ETC allows adequate ventilation while preventing aspiration of gastric contents. If the distal end of the ETC enters the esophagus, the ETC can assist in removing gastric fluids through suction applied to the “tracheal” lumen. When long-term ventilation is anticipated or required, the ETC should be exchanged for an endotracheal tube.

Use of the ETC in the out-of-hospital setting has been associated with a notable incidence of serious complications, including upper airway bleeding, esophageal laceration and perforation, and mediastinitis.¹⁸⁵ Although a lower incidence of serious complications would be expected in the more controlled operating room environment with an anesthesiologist using a laryngoscope to facilitate placement, the stiffness and the anterior curvature of the ETC, as well as the potential for balloon overinflation, still represent potential sources of airway and esophageal injury.

Cannula and Surgical Cricothyrotomy

The anesthesia provider must diagnose the failure of facemask and alternative devices to oxygenate and ventilate the patient and decide that direct tracheal access is necessary. A delay in performing cricothyrotomy results in greater morbidity and mortality than complications resulting from the attempt.^35,186 Three techniques for cricothyrotomy are available¹⁸⁷:

Narrow-bore cannula cricothyrotomy requires the use of a high-pressure ventilation source (e.g., Manujet) for transtracheal jet ventilation (TTJV). The other two techniques use standard ventilation circuits. Most TTJV systems have an in-line regulator that allows the delivery pressure to be controlled to between 0 and 50 psi. A minimum pressure of 20 to 30 psi is required in the majority of patients to inflate the chest and provide appropriate tidal volumes and minute ventilation.¹⁸⁸ During TTJV, it is critical that the operator allow time for exhalation of inspired gas to avoid hyperinflation of the lungs, potential air trapping, and barotrauma.^189-191 Exhalation can be facilitated by keeping the airway patent by some means (e.g., nasal/oral airway, jaw thrust, LMA). There are numerous reports of the use of TTJV as a means to prevent hypoxemia during life-threatening airway emergencies^192-194 and as a temporizing measure before a more definitive surgical airway can be established.

Although cannula cricothyrotomy is faster and carries significantly fewer risks, its success rate is much lower than that of surgical cricothyrotomy.¹⁸⁷ Skill fade associated with increased length of time since training is likely to have a more significant impact on outcome than choice of device.¹⁸⁷ The choice of technique can therefore be determined by local protocols. The anesthesia provider should evaluate and practice various airway techniques to maximize success during an emergency.

General Principles

Tracheal extubation is a critical step during emergence from general anesthesia. Airway condition at time of tracheal extubation may be less favorable than at induction of anesthesia. The patient's disease process (e.g., preeclampsia) may contribute to worsening airway edema, as do the fluid and blood products that have been administered during the procedure. Comorbidities, such as obesity and obstructive sleep apnea, may contribute to an increased risk for airway compromise after extubation of the trachea. Although the majority of extubations occur without incident, a number of serious adverse events, including hypoxic brain injury, occur during emergence from general anesthesia and tracheal extubation or in the postoperative period.^35,195-198

After publication of the ASA guidelines for management of the difficult airway,² there was a statistically significant reduction in airway claims arising from injury at induction of anesthesia.¹⁹⁹ However, the number of claims arising from intraoperative events and those at extubation and during recovery has not changed. Death and brain injury occur more commonly after extubation and during recovery than during induction of anesthesia.¹⁹⁹ Not surprisingly, extubation problems are more common in obese patients and in those with obstructive sleep apnea.¹⁹⁹ In the fourth National Audit Project (NAP4) of the Royal College of Anaesthetists and the Difficult Airway Society (DAS), major airway complications in patients receiving anesthesia occurred during emergence or recovery in approximately one third of the reported cases.¹⁰⁰

Extubation is an elective procedure that should always have a management strategy.^200-202 The goals of extubation management are to ensure uninterrupted oxygen delivery, avoid airway stimulation, and allow ventilation and possibly reintubation with minimum difficulty and delay should extubation fail. The U.K. Difficult Airway Society Extubation Guidelines⁹ describe a four-step approach to the safe management of tracheal extubation:

The planning and preparation steps allow the anesthesia provider to stratify risk and categorize the patient as either at low risk or at risk for extubation complications. A fasted patient with an uncomplicated airway is at low risk, whereas a patient at risk for aspiration in whom the ability to oxygenate and reintubate is potentially difficult is at risk.⁹ By definition, all obstetric patients fall into the “at-risk” group owing to the risk for pulmonary aspiration.⁹ A crucial decision is to determine whether it is safe to extubate the trachea or to postpone the extubation. Unfortunately, studies attempting to identify risk factors that can reliably predict difficulty with extubation, performed almost entirely in the critical care population, have been inconclusive.²⁰³ The leak test, in which the ETT cuff is deflated, the proximal end of the tube is occluded, and the patient is evaluated to determine if she can spontaneously ventilate around the tube, has not been uniformly demonstrated to predict success with extubation.²⁰⁴

If the surgical procedure was long, or massive bleeding and significant fluid replacement has occurred, consideration should be given to transferring the patient to the intensive care unit and delaying tracheal extubation. The decision to remove the ETT should follow a full assessment of the patient (i.e., ability to follow commands, appropriate level of consciousness, recovery from neuromuscular blockade). The head of the bed can be elevated or the patient positioned on her left side. Patients for whom oxygenation and/or reintubation is predicted to be difficult may benefit from the insertion of an airway exchange catheter before tracheal extubation (see later discussion).

Airway Exchange Catheters

In situations in which the patient appears ready for extubation but concerns exist regarding potential difficulty with—and/or need for—re-intubation (e.g., difficult intubation), establishing continuous airway access may be of benefit; such access can be achieved with an airway-exchange catheter (AEC) inserted through the ETT before extubation.^205,206 There are many commercially available AECs: the Cook AEC (Cook Critical Care, Bloomington, IN, USA) is a long, thin, hollow tube made from semi-rigid thermostable polyurethane. It has a blunt end with distal terminal and side holes; it is radiopaque and has length markings on its outer surface. The catheter is supplied with a removable 15-mm connector that is compatible with anesthetic circuits for oxygenation and Luer-Lok connectors for use with high-pressure source (jet) ventilation. AECs are available in various sizes; the most appropriate sizes for extubation are of sufficient length (e.g., 83 cm) and diameter (e.g., 11- to 14-gauge) to remain between the vocal cords while removing and/or reinserting an ETT. These catheters are compatible with tracheal tubes of internal diameters greater than 4 and 5 mm, respectively. Successful reintubation can be enhanced by the use of a smaller tracheal tube and rotation of the ETT 90 degrees during insertion to facilitate passage of the bevel through the vocal cords.¹¹⁵

Mort²⁰⁷ evaluated 354 patients with potentially difficult airways who were extubated over an AEC; 51 required reintubation. The reintubation success rate was greater than 90%. Complications during airway management, including low oxygen saturation, bradycardia, hypotension, esophageal intubation, and the need for accessory airway adjuncts, were less common when an AEC was used to assist re-intubation. Successful outcomes have been reported in other similar studies.^205,208 Visualization of the larynx, either by direct or video laryngoscopy, during AEC use can also increase the success of reintubation and reduce the incidence of complications.²⁰⁹

Morbidity associated with AEC use is predominantly a result of inappropriate positioning and failure of oxygenation. The distal tip of the AEC is ideally positioned in the mid trachea; oxygen insufflation and jet ventilation through an AEC should only be undertaken with extreme caution, because barotrauma and death have been reported.^206,210,211