There are differences among these standards, guidelines, algorithms, recommendations, and protocols, but in practice and in medicolegal cases, the distinction between terminologies can be blurred (Table 37-1).⁹ Published literature shows that the guidelines from the ASA Task Force on Management of the Difficult Airway have been discussed in 18% of nonobstetric medicolegal cases and were useful both in defending care (defense, 8%) and in criticizing care (plaintiff, 3%).¹⁰ Expert witnesses also have used these guidelines in litigated obstetric DA management cases.

TABLE 37-1 Definition of Terms and Degree of Obligation

Terms	Definitions	Degree of Obligation
Standards	Generally accepted principles for patient management; exceptions are rare, and failure to follow is often difficult to justify	Mandatory
Strategy	A well-planned series of steps for achieving a goal	Voluntary
Guidelines	Systematically developed statements to assist practitioners for specific clinical circumstances; incorporates the best scientific evidence with expert opinion	Voluntary
Practice policies	Describe present recommendations issued to influence practitioners in reaching decisions about interventions	Voluntary
Recommendations	Suitable and useful strategy; not as strict as standards or guidelines	Voluntary
Options	Different possibilities are available; neutral assessment	Voluntary
Protocols and algorithms	Stepwise procedures or decision trees to guide practitioners through diagnosis and treatment of various clinical problems	Voluntary

From Henderson JJ, Popat MT, Latto IP, Pearce AC: Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 59:675–694, 2004.

II Definitions

There is no consensus on a standard definition of the DA in the literature. The ASA Task Force defined DA as a clinical situation in which a conventionally trained anesthesiologist experiences difficulty with face mask ventilation, difficulty with intubation, or both. The original ASA description of difficult intubation (DI) included a limit of 10 minutes or multiple attempts. The wisdom of this definition must be questioned in obstetrics especially, given the fact that GA is generally reserved for emergency C/D in which delivery of the baby is of the utmost urgency. The common practice in obstetric anesthesia is to use a single dose of succinylcholine when anesthetizing for C/D under GA. In the obstetric situation, difficult intubation is the inability of an experienced anesthesiologist to intubate within one dose of succinylcholine.¹¹ A more apt definition of failed intubation in obstetric patients is inability to secure the airway with two attempts using a conventional laryngoscope or an alternative airway device to assist with tracheal intubation.

III Anesthesia-Related Maternal Mortality

Although the total number of maternal deaths has been decreasing steadily in the last few decades both in the United States and United Kingdom (Table 37-2),^12,¹³ anesthesia-related complications rank seventh among the leading causes of maternal death, in both countries (Fig. 37-1).^14,¹⁵ Even in the developing countries, anesthesia is emerging as an additional risk for maternal mortality and remains largely under-reported.^16,¹⁷ Failures in airway management are a primary cause of anesthesia-related maternal deaths in the underdeveloped countries.¹⁸

TABLE 37-2 Anesthesia-Related Maternal Deaths in the United States and United Kingdom, 1979-2001

Year of Death	United States*	United Kingdom^†
1979-1981	4.3	8.7
1982-1984	3.3	7.2
1985-1987	2.3	1.9
1988-1990	1.7	1.7
1991-1993	1.4	3.5
1994-1996	1.1	0.5
1997-1999	1.2	1.4
2000-2002	1.0	3.0

^* Maternal deaths per million live births.

^† Maternal deaths per million maternities (live births, stillbirths, pregnancy terminations, ectopic pregnancies, and abortions).

(From Hawkins JL, Chang J, Palmer SK, et al: Anesthesia-related maternal mortality in the United States: 1979-2002. Obstet Gynecol 117:71, 2011.)

Figure 37-1 Maternal mortality in the United States.

(From Berg CJ, Callaghan WM, Syverson C, Henderson Z: Pregnancy-related mortality in the United States, 1998 to 2005. Obstet Gynecol 116:1302–1309, 2010.)

The first national study of anesthesia-related maternal mortality in the United States between 1979 and 1990 was published in 1997.¹⁹ Most of these deaths (82%) took place during C/D. Death rates for GA during C/D increased from 20 per million in 1979-1984 to 32.3 per million in 1985-1990. Conversely, the death rate for regional anesthesia (RA) during the same periods declined from 8.6 to 1.9 per million, respectively. The relative risk for GA increased to 16.7 from 1985 to 1990. The case fatality risk ratio for GA was. 2.3 times that of RA from 1979 to 1984 and increased to 16.7 times from 1985 to 1990.¹⁹ The majority of maternal deaths were related to difficult or failed intubation, pulmonary aspiration, or respiratory complications.

A follow-up study examined 12 years of anesthesia-related maternal deaths between 1991 and 2002 and compared them with data from 1979 to 1990 to estimate trends over time and to compare the risks of GA and RA during cesarean delivery.¹³ Results showed that 86 pregnancy-related deaths were associated with complications of anesthesia, accounting for 1.6% of the total. Case-fatality rates for GA declined from 16.8 per million in 1991-1996 to 6.5 per million in 1997-2002 (Table 37-3).

TABLE 37-3 Case-Fatality Rates and Rate Ratios of Anesthesia-Related Deaths during Cesarean Delivery by Type of Anesthesia, United States, 1979-2002

Complications related to anesthesia still occur, despite the decrease of almost 60% in anesthesia-related maternal mortality between 1979-1990 and 1991-2002. Although deaths from GA during C/D declined, about two thirds of the anesthesia-related deaths were caused by intubation failure or induction problems.¹³ The decline in GA-related case fatalities was explained by improvements in management of the DA and failed intubation and by increased expertise of anesthesiologists with the laryngeal mask airway (LMA) and other airway devices.¹³

A review of maternal mortality in Michigan from1985 to 2003 identified eight anesthesia-related deaths.²⁰ Seven of these deaths were due to anesthesia-related factors. Interestingly, there were no deaths during induction of GA. The deaths were caused by airway obstruction or hypoventilation during emergence, lapses in monitoring, or lack of supervision in the postoperative period. Other risk factors included African-American race and obesity. This study highlighted the importance of airway-related problems during emergence, particularly in obese patients and in the African-American population, as well as the importance of vigilance in monitoring and management in the postoperative period for prevention of airway-related complications.

In the United Kingdom, despite the decline in the total number of maternal deaths, anesthesia-related causes consistently accounted for approximately 10% of the total direct deaths. During the period 1982-1984, anesthesia was the third leading cause of maternal death, resulting in 19 of 243 deaths, of which 15 were due to airway-related difficulties.²¹ The confidential enquiry spanning 1994-1996 showed that anesthesia was responsible for only 1 of 268 maternal deaths. In the Confidential Enquiry into Maternal and Child Health (CEMACH) 2000-2002 study, there were six direct deaths, all related to GA.¹⁵ Maternal deaths from complications of GA included a risk of 1 maternal death in 20,000. These cardiopulmonary arrests and deaths were related to difficult or failed intubation, difficult pulmonary ventilation resulting in failure to oxygenate, pulmonary aspiration, or acute respiratory distress syndrome (ARDS). In all of these cases, the anesthesia care was considered substandard.¹⁵

IV Obstetric Anesthesia Closed Claims Studies

The 2009 closed claims study published in the United States revealed that obstetric anesthesia claims for injuries from 1990 to 2003 had declined compared with claims for injuries before 1990.¹ In the period 1990-2003, the proportion of maternal death or brain damage and that of newborn death or brain damage decreased. Respiratory causes of injuries also decreased, from 24% to 4% in claims from 1990 or later. Claims related to inadequate oxygenation or ventilation and those related to aspiration also decreased.

Despite the decrease in claims, one of the most common anesthesia-related causes of maternal death or damage associated with GA was difficult tracheal intubation. Airway-related claims involved multiple attempts at tracheal intubation leading to progressive difficulty with ventilation. In two of the claims, tracheal intubation was assessed to be difficult preoperatively with a backup plan to awaken the patient and perform fiberoptic intubation. However, progressive airway difficulties occurred while attempts were made to awaken the patients, resulting in adverse outcomes.

The number of claims related to difficult tracheal intubation after 1990, compared with pre-1990 claims, was unchanged.¹ However, the overall improvement in closed claims statistics and the decline in anesthesia-related maternal mortality in more recent years were probably attributable to implementation of a minimum standard of care requiring the use of respiratory system monitors (pulse oximetry and capnography) during anesthesia in modern practice, enhanced awareness of the risk of pulmonary aspiration of gastric contents in the obstetric patient, decreased use of GA in obstetric practice, and use of advanced airway management techniques.

V Why Is Advanced Airway Management Important?

There have been tremendous advances in airway management in recent years, including an explosion in the use of airway devices as adjuncts to airway management. There has been a vast increase in the body of knowledge and in publications worldwide in this field. Improvements in advanced airway management have led to a documented decline in the incidence of airway-related perioperative morbidity in the surgical population.²² Similarly, in the last 2 decades, anesthesiologists’ focus on improving management of the DA and of failed intubation, experience with the LMA, application of the ASA difficult airway algorithm (DAA), and advanced airway management strategies have helped decrease GA-related case fatalities in obstetric cases.¹³

Because GA for cesarean delivery is frequently reserved for true emergencies, these high-level stress situations may lead to inadequate airway assessment or preparation, which can contribute to the risk of difficult or failed tracheal intubation, leading to the possibility of morbidity and mortality in both mother and baby.

Because the medical-legal liability associated with airway-related adverse maternal outcomes is high,¹ it is essential that all those practicing obstetric anesthesia as part of airway management should do the following:

1. Assess risk factors that predispose to airway-related complications

2. Be cognizant of predictors of the DA

3. Have appropriate algorithms and airway devices and equipment immediately available in the labor and delivery suite and in the operating room to manage the DA

4. Have an airway rescue plan, within the framework of the ASA DAA, for managing the DA

5. Be cognizant of the twin goals of urgently delivering the baby and preventing pulmonary aspiration, which can be hard to balance

6. Acquire and maintain advanced airway management skills.

A heightened awareness of the difficult obstetric airway among all anesthesia practitioners and use of video laryngoscopes, alternative devices to assist with tracheal intubation, and supraglottic devices as a bridge to ventilation, oxygenation, and airway management may further minimize or even help eliminate airway-related maternal morbidity and mortality. Unfortunately, there is a learning curve associated with acquiring these advanced airway skills.

VI Risk Assessment

Anatomic and physiologic factors alter the airway during pregnancy, placing the parturient at risk for difficult laryngoscopy, difficult tracheal intubation, and difficult mask ventilation (DMV). There is no single factor that explains the high incidence of failed tracheal intubation and respiratory-related injury in obstetrics. Difficult laryngoscopy or DMV may be related to excessive weight gain and upper airway edema during pregnancy, compounded by breast enlargement and by additional changes in preeclampsia. Rapid onset of hypoxemia associated with DA may be caused by respiratory changes, cardiovascular impairment from aortocaval compression, gastrointestinal changes placing the parturient at risk for pulmonary aspiration, and acute respiratory distress syndrome secondary to respiratory-related complications.

A Airway

Airway changes occur during pregnancy, labor, and delivery.²³ Further, the incidence of Mallampati (MP) class III and class IV scores increases during labor compared with the prelabor period, and these changes are not reversed by 48 hours after delivery. Therefore, it is absolutely necessary to examine the airway of a parturient in labor before administering anesthesia for a cesarean delivery.²⁴

An increase in the ground substance of the airway connective tissue, caused by elevated levels of estrogen during pregnancy, an increase in total body water, and an increase in interstitial fluid and blood volume, results in hypervascularity and edema of oropharynx, nasopharynx, and respiratory tract. Excessive weight gain during pregnancy, preeclampsia, iatrogenic fluid overload, excessive bearing-down efforts during labor, and increase in venous pressure all lead to increased upper airway mucosal edema. Additional upper airway changes include tongue engorgement during pregnancy, which leads to decreased mobility of the floor of the mouth and changes in the MP score.²⁵ Several published reports have described development of airway edema during labor and delivery, in preeclampsia, and after massive fluid and blood transfusion resuscitation following postpartum hemorrhage.²⁶^–³⁰ In some of these reports, the associated difficulties in tracheal intubation were secondary to changes in the MP score.

Because of the increased vascularity and engorgement of the mucosa, the parturient is at increased risk for epistaxis after manipulation of the nasopharynx with nasotracheal intubation and for swelling of the airway and is vulnerable to increased trauma with repeated attempts at intubation.³¹ Avoidance of manipulation of the nasopharynx, use of smaller-sized tracheal tubes, and strict adherence to no more than two attempts at tracheal intubation³² are vital measures to avoid airway-related trauma, complications, and catastrophes.

B Respiratory Changes

The enlargement of the uterus during the course of pregnancy causes a 25% decrease in expiratory reserve volume and a 15% decrease in residual volume, resulting in an overall 20% decrease in functional residual capacity (FRC). In the supine position, the FRC is 70% of its normal capacity measured in the upright position. The supine position, particularly in an obese parturient, can result in airway closure and an increase in the alveolar-arterial gradient during normal tidal respiration, predisposing the parturient to lower partial pressure of oxygen. At the same time, oxygen consumption is increased by 20% secondary to the metabolic needs of the growing fetus, uterus, and placenta. The increase in oxygen consumption, along with decreased FRC, places the parturient at increased risk for hypoxemia and its consequent adverse neurologic effects (brain death), especially during DI, failed intubation, and DMV.

C Cardiovascular Changes

The gravid uterus compresses the inferior vena cava in the supine position, resulting in a decrease in venous return and cardiac output. The reduction in cardiac output, together with elevated oxygen consumption, can further decrease oxygen saturation. The decrease in cardiac output and the ensuing hypoxemia during a “cannot intubate, cannot ventilate” (CICV) situation predisposes the mother to myocardial hypoxemia, cardiovascular arrest, and compromised uteroplacental perfusion, which can also place the fetus’ well-being at risk. Maintaining left uterine tilt, establishing an airway with adequate ventilation and oxygenation in a timely manner, maintaining adequate perfusion in mother and baby, and ensuring cardiovascular stability become extremely important to ensure a safe outcome for both mother and bady.

D Gastrointestinal Changes

Other risk factors include pregnancy-related hormonal, anatomic, and physiologic gastrointestinal changes that place the parturient at risk for gastric regurgitation and pulmonary aspiration during GA. The gravid uterus shifts the stomach cephalad and changes the angle of the gastroesophageal junction, resulting in incompetence of the gastroesophageal pinchcock mechanism. The lower esophageal tone is decreased, leading to increased gastric reflux. There is a progesterone-mediated smooth muscle relaxant effect on the gastrointestinal mucosa. Furthermore, gastric emptying is prolonged during labor. Therefore, the parturient is at risk for gastric regurgitation, active vomiting, and pulmonary aspiration during GA, in case of a DA, or during failed intubation. Aspiration-related deaths during pregnancy occur from complications associated with induction problems such as DI, esophageal intubation, and inadequate attempts at ventilation.¹²

E Obesity

Weight gain during pregnancy results from the increasing size of the uterus and fetus, increased blood and interstitial fluid volumes, and deposition of new fat. There is a correlation between weight gain and an increase in the MP score.³⁰ Pregnancy also results in a significant increase in breast size and engorgement. In the supine position, the enlarged breasts can encroach into the neck area, impeding effective application of cricoid pressure and leading to difficulty with laryngoscope blade insertion.

Obesity has reached epidemic proportions in the United States, and the incidence of obesity in pregnancy has doubled in the last 10 years. A body mass index (BMI) greater than 25 kg/m² is considered overweight, and a BMI greater than 30 kg/m² is considered obese.³³ In the nonobstetric population, a BMI greater than 26 kg/m² results in a threefold increased incidence of DMV and a 10-fold increased incidence of difficult tracheal intubation.^34,³⁵ Both prepregnancy obesity and excessive weight gain during pregnancy are associated with comorbidities such as hypertension or preeclampsia with intrauterine growth retardation, diabetes and macrosomia, and dysfunctional labor, thus increasing the incidence of operative cesarean delivery. The incidence of postpartum hemorrhage is higher in these patients, leading to increased likelihood of a GA intervention.

Because weight gain and obesity are associated with an increase in the MP score, the incidence of a partially obliterated oropharyngeal space in an obese parturient is doubled compared with that in nonpregnant patients. The aforementioned changes, the breast engorgement, and anthropometric differences between patients create a risk for difficult laryngoscopy, DI, and DMV.^30,^36,³⁷ DI is encountered more frequently in morbidly obese parturients weighing more than 130 kg.³⁸ Mask ventilation tends to be difficult because of low chest wall compliance and increased intra-abdominal pressure. In obesity, the respiratory-related changes of pregnancy are even more significant, with a marked decrease in FRC such that the closing capacity exceeds the FRC during tidal breathing, leading to a decrease in arterial oxygen tension and predisposing the parturient to a much higher risk of hypoxemia during a DI or DMV encounter. Obesity compounds all the risk factors associated with normal pregnancy, including DA, difficult laryngoscopy, DI, DMV, and aspiration-related complications.

In the obese parturient, a thorough preoperative assessment, a review of comorbidities, and a previous anesthetic history for difficulty with tracheal intubation are essential to allow for proper preparation and appropriate interventions. Placing the obese parturient in the so-called ramped position before induction of GA is critical to facilitate ventilation and improve laryngoscopic visualization of the glottis for intubation. The aim is to achieve the best alignment of the three axes (oral, pharyngeal, and laryngeal) in obese patients, thereby enhancing the success rate of intubation at the first attempt.

VII Incidence of Difficult OR Failed Intubation IN Obstetrics

In 1998, the incidence of difficult laryngoscopy or tracheal intubation in the nonobstetric population was 0.1% to 13%.³⁹ In the obstetric population, the incidence of difficult tracheal intubation was found to be 1/250⁴⁰ to 1/280,⁴¹ 1/300,⁴² and 1/750.³⁶

A review of GA for cesarean deliveries from 1990 to 1995 showed the incidence of difficult tracheal intubation to range from a high of 16.3% to a low of 1.3%.² There was a sentinel CICV incidence of 1 in 536 cases; in this patient, multiple attempts at intubation, unsuccessful mask ventilation, failed Combitube placement, and unsuccessful cricothyroidotomy resulted in cardiopulmonary arrest followed by surgical tracheostomy. Resuscitation was accomplished, but the mother remained in a coma until death, and the baby suffered significant neurologic injury.

In a review of GA for cesarean deliveries from 2000 to 2005, the incidence of CICV was 1 in 98. The single case of CICV occurred after failed tracheal intubation, unsuccessful LMA placement, and hypoxemia; successful cricothyroidotomy resulted in a good outcome for both mother and baby.⁴³

VIII Prediction of Difficult Airway Management

The ASA DAA recommended an airway-related history to detect medical, surgical, and anesthetic factors that might indicate the presence of a DA. The guidelines also recommended an airway physical examination using assessment of multiple airway features before initiation of anesthetic care and airway management in all patients.⁴ The ASA closed claims analysis (2005) showed that 8% of patients did not have a preoperative history or airway physical examination.¹⁰

A retrospective audit was performed of all obstetric GAs, a total of 3430 cases over an 8-year period.⁴⁴ None of the patients had a failed or esophageal intubation. There were 23 DIs, an incidence of 1:156. Anticipated difficult tracheal intubation occurred in 9 patients, 3 of whom underwent awake fiberoptic intubation; in the remaining 6 patients, who were morbidly obese, the DI was managed by senior trainees or consultants. Unanticipated difficulties occurred in 14 patients (61%). The preoperative assessment was found to be inadequate in these cases, being either not recorded (6 cases) or poorly documented (8 cases).

The first step in management of the high-risk airway is recognition of its presence. Research involving the best practices in promoting patient safety in airway management with rapid-sequence induction (RSI) suggests performing an adequate preoperative evaluation and examining the airway so as to be able to better predict difficult laryngoscopy, DI, and DMV. Similarly, because of the airway-related changes that occur during pregnancy, the obstetric airway should be considered a high-risk airway and therefore a thorough preoperative airway assessment and documentation should be required.

A Preoperative Assessment

1 History and Evaluation

According to the ASA Task Force, an airway history and a focused review of medical records must be conducted when feasible.⁴ Obviously, airway evaluation and prediction of DA in an obstetric patient starts with a thorough, focused airway-related history and evaluation of specific airway-related examination findings.⁴⁵ A thorough history addresses any difficulty with previous GAs, obstructive sleep apnea (OSA) or snoring, head and neck abnormalities, and diseases that might impair the airway and result in difficult tracheal intubation. A history of DA management should be considered a strong predictor of problems unless the history was related to a specific reversible disease process such as a dental abscess. The history may be available from verbal recollections by the patient, previous anesthetic records, hospital notes, a letter describing DA management, or a Medic-Alert bracelet. The introduction of anesthesia information management systems and mandatory electronic medical records should be extremely useful in the future as “airway alerts” are built in to notify future practitioners of the specific details encountered with DA management in a particular patient.

Preexisting conditions that can lead to difficulties in managing the airway include bull neck (neck circumference >16 cm in women), large breasts, large tongue, limited cervical movement, limited mouth opening, prominent upper incisors, and receding jaw.

The physical examination should include assessment of facial and neck masses, quality of dentition, buck teeth, small or large chin, maxillary and mandibular position, pharyngeal structures and high arched palate, and any deformity resulting from trauma, tumor, or inflammation.

2 Predictors of Difficult Airway

Numerous investigators have attempted to predict DA by using a simple bedside physical examination. There are numerous publications describing univariate or multivariate predictors of DI in nonobstetric patients and a handful relating the use of multivariate predictors of the DA, such as MP classification.

Yentis described the problems with many studies examining the prediction of DA.⁴⁶ It is appropriate here to delineate the terms used to describe the accuracy and predictive power of these tests. A test to predict DI should have high sensitivity, so that it will identify most of the patients in whom intubation will be truly difficult. It should also have a high positive predictive value (PPV), so that only a few patients with airways actually easy to intubate will be subjected to the protocol for DA management. The test should also have a high specificity, so that it will identify most patients in whom tracheal intubation will be truly easy (Table 37-4).⁴⁶ Excellent interobserver reliability is essential for any test to have high specificity, sensitivity, and PPV in predicting a DA.

TABLE 37-4 Statistical Terminology and Calculation *

Term	Calculation
True positive (TP)	Difficult intubation that had been predicted to be difficult
False positive (FP)	Easy intubation that had been predicted to be difficult
True negative (TN)	Easy intubation that had been predicted to be easy
False negative (FN)	Difficult intubation that had been predicted to be easy
Sensitivity	Percentage of correctly predicted difficult intubations as a proportion of all intubations that were truly difficult—that is, TP/(TP+FN)
Specificity	Percentage of correctly predicted easy intubations as a proportion of all intubations that were truly easy—that is, TN/(TN+FP)
Positive predictive value	Percentage of correctly predicted difficult intubations as a proportion of all predicted difficult intubations—that is, TP/(TP+FP)
Negative predicted value	Percentage of correctly predicted easy intubations as a proportion of all predicted easy intubations—that is, TN/(TN+FN)

^* The sensitivity, specificity, and positive predictive value of each test are calculated using the Cormack-Lehane score as the constant variable.

Adapted from Yentis SM: Predicting difficult intubation: Worthwhile exercise or pointless ritual? Anaesthesia 57:105-109, 2002.

B Quantitative Evaluation of Difficult Intubation

Multiple external features are associated with difficult laryngoscopy and intubation. In an urgent or emergent obstetric situation, a practical, systematic, and rapid evaluation of the airway is necessary and important to predict a potentially difficult laryngoscopic view and DMV before initiation of RSI for GA in a cesarean delivery. The evaluation should dictate the management plan and the availability of airway rescue devices.

The LEMON mnemonic represents one such assessment that is simple and quick, can be performed on any emergency patient, and has proved to have high PPV.⁴⁷ The LEMON mnemonic represents the following five elements for preanesthetic assessment (Box 37-1):

L:Look externally—The initial impression of potential airway difficulty is based on assessment for any obvious anatomic distortions or external features that may make intubation difficult, such as facial and periorbital edema in a preeclamptic patient.

E:Evaluate the 3-3-2 Rule—Measuring the geometry of the airway can predict the anesthesia practitioner’s ability to align the oral, pharyngeal, and tracheal axes.

• Mouth opening—The interincisor distance between the patient’s incisor teeth or the mandibular opening in an adult should be at least 4 cm or 3 finger-breadths

• The distance from the hyoid cartilage to the chin should be at least 3 cm or 2 finger breadths (under the chin). The ability of the mandible to accommodate the tongue can be estimated between the mentum and the hyoid bone. A smaller mandible is less likely to accommodate the tongue, which can impair visualization during laryngoscopy. An unusually large mandible can elongate the oral axis.

• The distance between the thyroid notch and the floor of the mandible (top of the neck) should be at least 2 finger breadths.

M:Mallampati score—The degree to which the tongue obstructs the visualization of the posterior pharynx has some correlation with the ability to visualize the glottis. The MP score can be estimated by having the patient, in a sitting position, open the mouth fully and protrude the tongue as far as possible without phonation. The relationship of the base of the tongue to the oropharyngeal structures—uvula and tonsillar pillars and fauces—is assessed as follows:

• Class I : Soft palate, uvula, faucial, and tonsillar pillars visible (easy intubation)

• Class II: Soft palate, uvula visible

• Class III: Soft palate, base of uvula visible

• Class IV: Hard palate only visible (DI)

O:Obstruction—Pathologic conditions such as edema, glottic tumor, lingular tonsil, hyperplasia, and trauma can cause obstruction and can make laryngoscopy and ventilation difficult.

N:Neck mobility—This is a vital requirement for successful intubation. The sniffing position is the optimal, classic position of the head and neck for facilitating intubation. The extension of the atlanto-occipital (A-O) joint on the cervical spine so as to be able to align the three axes (oral, pharyngeal, and laryngeal) during laryngoscopy enhances the ease of laryngoscopy and tracheal intubation. Normal A-O joint extension of the head over the neck is 35 degrees. It can be assessed easily by getting the patient to place the chin down on the chest and tilt the head backward as far as possible. A reduction in A-O joint extension of 12 degrees (33%) or more correlates with intubation difficulty; complete joint immobility significantly compromises the laryngeal view. Limited A-O joint extension is present in certain pathologic states such as spondylosis, rheumatoid arthritis, and cervical spine stenosis, resulting in symptoms of nerve compression with cervical extension. Complete A-O joint immobility (e.g., hard-collar neck immobilization) can compromise the view of the glottis during laryngoscopy.

Box 37-1

LEMON Airway Assessment Method

From Reed MJ, Dunn MJ, McKeown DW: Can an airway assessment score predict difficulty at intubation in the emergency department? Emerg Med J 22:99–102, 2005.

C Additional Tests

Difficult laryngoscopy was defined by the ASA Task Force as the inability to visualize any part of the vocal cords despite multiple attempts at conventional laryngoscopy.⁴ The original Cormack and Lehane classification of laryngoscopic views was used to describe the visibility of the glottis during laryngoscopy with a conventional laryngoscope and to predict the ease of intubation.⁴⁸ The view at laryngoscopy was divided into four grades on the basis of a study in obstetric patients.⁴⁹ The entire glottis is visible in a grade 1 view, whereas in a grade 2 view only the posterior portion of the glottis is visible; in a grade 3 view, only the epiglottis is seen, and in grade 4 view not even the epiglottis is seen. Grades 3 and 4 are considered to indicate DI.

1 Thyromental Distance

The thyromental distance (TMD) is defined as the distance from the chin (mentum) to the top of the notch of the thyroid cartilage with the head fully extended; it must be measured with a ruler for accuracy. The TMD gives an estimate of the mandibular space and helps in determining how readily the laryngeal axis will fall in line with the pharyngeal axis when the A-O joint is extended ⁵⁰:

• A TMD measurement of 6.5 cm or greater with no other abnormalities indicates the likelihood of easy intubation.

• A TMD measurement of 6.0 to 6.5 cm indicates that alignment of the pharyngeal and laryngeal axes will be challenging and that difficulty with laryngoscopy may result. However, intubation is possible with the use of adjuncts such as an Eschmann introducer or an optical stylet.

• A TMD measurement of less than 6 cm indicates difficult laryngoscopy; specifically, intubation may be impossible.

TMD, in conjunction with other parameters such as the MP classification, has been used to predict DI.

2 Sternomental Distance

The sternomental distance (SMD) is measured from the sternum to the tip of the mandible with the head fully extended and the mouth closed. The normal measurement is 13.5 cm. The SMD and the corresponding laryngoscopic view were documented in 523 parturients undergoing elective or emergency C/D under GA.⁵¹ An SMD of 13.5 cm or less had a sensitivity of 66.7%, a specificity of 71%, and a PPV of only 7.6%. Eighteen patients (3.5%) had a Cormack-Lehane grade 3 or 4 laryngoscopic view and were classified as having potentially difficult tracheal intubations. The SMD on its own as a sole indicator of DI was not useful, and the suggestion was to incorporate it with other tests in the preoperative airway examination.

3 Jaw Protrusion

Jaw protrusion (also termed prognathism or subluxation) is assessed by the mandibular protrusion test, which demonstrates the extent to which the lower incisors can be slid in front of the upper ones ⁵²:

• Class A: The lower incisors can be protruded anterior to the upper incisors.

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

• Class C: The lower incisors cannot reach the top incisors.

Class C protrusion was associated with difficult laryngoscopy and DMV, whereas class A protrusion rarely produced any difficulty.

4 Studies Assessing Predictors of Difficult Airway in Obstetrics

a Mallampati Classification

A number of studies in both general surgical and obstetric patients have indicated that the MP classification is a simple, reproducible, reliable, and important parameter in evaluating and assessing the airway. It has been used as a single predictor and as a part of multivariate analysis to predict difficult tracheal intubation. However, studies have shown that the MP classification by itself has limited discriminative power in accurately predicting difficult tracheal intubation.

In obstetric patients, the MP score has been used as a single parameter to illustrate the dramatic airway changes that occur in pregnancy and to highlight the importance of preoperative assessment of the airway. Pilkington and coworkers (1995) evaluated the MP class at 12 weeks’ and 38 weeks’ gestation ³⁰; photographs taken at the two time periods demonstrated the increase in MP class in the same patient as gestation advanced. The MP score correlated with the increase in body weight, implying that oropharyngeal edema was responsible for the increase in the MP score.

b Multivariate Predictors

Rocke and colleagues (1992) were the first to use multivariate predictors to predict difficult tracheal intubation.³⁶ They evaluated the MP classification as modified by Samsoon and Young, referring to it as the Modified Mallampati Test (MMT), along with other predictors in 3440 patients undergoing elective or emergency C/D under GA. Data were collected on 1606 patients, representing 46.7% of the obstetric surgical patients. Of the patients studied 1500 underwent general anesthesia. Other risk parameters for DA that they assessed included short neck, which equates with decreased A-O joint extension; receding mandible or decreased TMD (<3 finger breadths); and protruding maxillary incisors indicating significant overbite, which would equate to the current class C jaw protrusion or class III upper lip bite test.⁵³

Rocke’s group made subjective assessments of the ease or difficulty of tracheal intubation according to the following scale (Table 37-5)³⁶:

• Grade 1: Easy—intubation at first attempt with no difficulty

• Grade 2: Some difficulty—insertion of tracheal tube not achieved at first attempt; no difficulty but successful intubation after adjustment of laryngoscope blade and/or adjustment of head position, not requiring additional equipment, removal and reinsertion of the laryngoscope, or senior assistance

• Grade 3: Very difficult—requiring removal of the laryngoscope, further oxygenation by mask ventilation, and subsequent intubation with or without the use of airway adjuncts (e.g., Eschmann introducer, alternative laryngoscope blade) or intubation by a senior colleague

• Grade 4: Failed intubation—several attempts at intubation or unrecognized esophageal intubation by resident, followed by subsequent tracheal intubation by senior anesthesiologist

TABLE 37-5 Association between Oropharyngeal Structures Visualized Preoperatively and Subsequent Difficulty at Tracheal Intubation*

Based on these various parameters, the relative risk of experiencing difficult tracheal intubation (compared with an uncomplicated MMT class I airway) was determined as follows: 3.23 for MMT class II, 7.58 for MMT class III, 11.3 for MMT class IV, 5.01 for short neck, 9.71 for receding mandible, and 8.0 for protruding incisors. The investigators analyzed the univariate individual risk factor (i.e., MMT class) and combinations of the various risk factors and showed that a patient with an MMT III or IV classification plus protruding incisors, a short neck, and a receding mandible would have a probability of difficult laryngoscopy greater than 90% (Fig. 37-2). This study highlighted the importance of preoperative airway assessment to determine the best anesthetic intervention and the importance of prospectively preparing for airway interventions in the true obstetric emergency C/D under GA.

Figure 37-2 Probability of experiencing difficult intubation for varying combinations of risk factors. Roman numerals refer to Modified Mallampati Test (MMT) class. PI, Protruding incisors; RM, receding mandible; SN, short neck. See text for details.

(From Rocke DA, Murray WB, Rout CC, Gouws E: Relative risk analysis of factors associated with difficult intubation in obstetric anesthesia. Anesthesiology 77:67, 1992.)

Gupta and colleagues evaluated the obstetric airway,⁵⁴ using the MMT and the Wilson risk sum, to assess the potential for DA in 372 patients undergoing elective or emergency C/D under GA. The Wilson risk sum score was ascertained by adding scores for five factors (weight, head and neck movement, jaw movement/jaw protrusion, receding mandible, and buck teeth). The investigators also compared the sensitivity, specificity, and PPV of the MMT score (60%, 97.6%, and 65%, respectively) with the data published by Rocke and coworkers³⁶ for laryngoscopy (59.2%, 74.1%, and 4.0%, respectively). The MMT was reproducible and was more sensitive than the Wilson risk sum score. As a screening test for prediction of DI, the Wilson risk sum score was less sensitive (36%) but had almost the same specificity (98.5%) and PPV (64%) as the MMT. When both tests were combined as predictors, the sensitivity was improved to 100%, the specificity was marginally decreased to 96.2%, and the PPV (64.8%) remained almost the same, compared with the MMT score alone. Gupta and colleagues concluded that, in obstetric patients, use of the Wilson risk sum score along with the MMT score resulted in high sensitivity, high specificity, and a high PPV.⁵⁴ This study highlighted the importance of incorporating multiple predictors rather than using single (univariate) predictors of difficult laryngoscopy and intubation. There was a significant relationship between increased weight and external laryngeal manipulation.

Merah and associates studied the potential of five airway measurements to predict a difficult direct laryngoscopy in 80 West African obstetric patients during C/D under GA.³⁷ The five bedside tests that were evaluated were MMT, TMD, SMD, horizontal length of mandible, and interincisor gap. Eight patients (10%) had difficult laryngoscopy. The MMT as a sole predictor had a sensitivity, specificity, and PPV of 87.1%, 99.6%, and 70%, respectively. In light of the current state of knowledge with respect to airway evaluation, the investigators concluded that this prediction tool would behave similarly in Caucasians as in West Africans. Weight contributed to the prediction of difficult laryngoscopy. The difference between the mean weights of difficult-to-intubate patients (109 ± 12.4 kg) and easy-to-intubate patients (81 ± 12.0 kg) was statistically significant. The combination of MMT and TMD yielded values of 100%, 93.1%, and 61.5% for sensitivity, specificity, and PPV, respectively. The researchers concluded that MMT can be used as the sole predictor of difficult tracheal intubation.

Kodali and colleagues performed a two-part study to evaluate airway changes during labor and delivery.²³ In part I, they used the conventional Samsoon modification of the MP airway classification. The airway was photographed at the onset of labor (“prelabor”) and at the end of labor (“postlabor”). Pregnant women with MP class IV airways were excluded from this initial part of the study. In part II, prelabor and postlabor upper airway volumes were measured by acoustic reflectometry. In part I (n = 61), there was a significant increase in MP class between prelabor and postlabor measurements (P < 0.0001). The airway increased by one class in 20 parturients (33%) and by two classes in 3 parturients (5%). At the end of labor, there were 8 parturients with MP class IV (P < 0.01) and 30 with MP class III or IV (P < 0.0001). In Part II (n = 21), there were significant decreases in oral volume (P < 0.05) and in pharyngeal area (P < 0.05) and volume (P < 0.001) after labor and delivery.

Boutonnet and associates methodically evaluated the changes in MP class at four time intervals in 87 pregnant patients: during the eighth month of pregnancy (T₁), at placement of the epidural catheter (T₂), at 20 minutes after delivery (T₃), and at 48 hours after delivery (T₄).²⁴ MP class did not change for 37% of the patients. The proportion of patients falling into MP class III or IV at the various times of assessment were as follows: T₁, 10.3%; T₂, 36.8%; T₃, 51%; and T₄, 20.7%. The differences in the percentages were all significant (P < 0.01). The incidence of MP classes III or IV increased during labor compared with prelabor period, and these changes were not reversed by 48 hours after delivery.

The studies by Kodali and Boutonnet and their colleagues confirmed the frequent increase in MP score during pregnancy and particularly during the course of labor.^23,²⁴ These findings suggest that it is imperative to evaluate the airway in early labor and to reevaluate it before anesthetic management for operative delivery.

c Meta-Analysis of Bedside Screening Test Performance

Shiga and colleagues systematically determined the diagnostic accuracy of bedside tests for predicting difficult tracheal intubation in patients with no airway pathology.⁵⁵ Thirty-five studies comprising 50,760 patients, including both surgical and obstetric patients, were selected from electronic databases (Table 37-6).

TABLE 37-6 Pooled Estimates of Bayesian Statistics from Six Bedside Tests for Difficult Intubation (DI)*

The overall incidence of difficult tracheal intubation was 5.8% (95% confidence interval [CI], 4.5% to 7.5%). Screening tests included the MP oropharyngeal classification, TMD, SMD, mouth opening, and Wilson risk score. Each test yielded poor to moderate sensitivity (20% to 62%) and moderate to fair specificity (2% to 97%). The meta-analysis found that the most useful bedside test for prediction was a combination of the MP classification and TMD (positive likelihood ratio, 9.9; 95% CI, 3.1 to 31.9). The study concluded that in surgical patients, this combination of tests added some incremental diagnostic value compared with the value of each test alone.⁵⁵

The meta-analysis showed that in the obstetric population (2155 patients), the prevalence of difficult tracheal intubation was 3.1% (95 % CI, 1.7 to 5.5). The diagnostic performance of the MP classification in obstetric and obese populations was similar to that in the overall surgical population. The diagnostic odds ratios in these populations were similar, and the trend toward poor sensitivity and fair specificity remained. In the obstetric patients, the MP classification yielded a sensitivity of 56%, a specificity of 81%, and a likelihood ratio of 0.6%. The meta-analysis data in the obstetric patients remained inconclusive because of the small number of studies and heterogeneity. Whereas in the obese patients (BMI > 30 kg/m²), the incidence of difficult tracheal intubation was 15.8 % or three times higher than in the overall population. Obese patients with a 15% pretest probability of DI had a 34% risk of difficult tracheal intubation after a positive MP class result, which is twice the risk of the overall population with a 5% pretest probability. Similarly, obese pregnant patients also had a higher incidence of difficult tracheal intubation. Because of the higher incidence of difficult tracheal intubation, the MP classification may yield higher post-test probability of difficult tracheal intubation in obese patients than in the overall population.⁵⁵

5 Trends in Anesthesia in Obstetrics

Cesarean delivery is the commonest major surgical procedure carried out in the United States (U.S. Centers for Disease Control and Prevention National Center for Health Statistics Report 2010) and the incidence is increasing worldwide.⁵⁶ The increased incidence of maternal morbidity and mortality associated with GA in obstetric patients and the heightened awareness among anesthesia practitioners of the risk for DA and failed intubation in obstetrics has led to an increased use of RA techniques. A 2009 closed claims analysis found that use of RA was 65% before 1990 and increased to more than 80% since then, whereas the use of GA declined from 33% before 1990 to 17% after 1990.¹ National Health Service maternity statistics show that the number of obstetric GAs for C/D administered in the United Kingdom fell from over 50% in 1989-90, to as low as 5% in 2004-2005.⁵⁷ Johnson and colleagues similarly found a marked decline in GA for C/D, from 79% to less than 10% over the same period.⁵⁸

The current trend in the United States and the United Kingdom is to use GA mainly for the true emergency C/D, if there is insufficient time for a regional technique, or in cases in which there is a contraindication to RA. Despite the decline in the use of GA for C/D, the incidence of difficult tracheal intubation was not found to be significantly different (5% before 1990 versus 3% after 1990).¹ Encountering a DA in the obstetric population is not uncommon; therefore, the emerging problem of declining airway skills is important, as evidenced by a rise in the rate of failed tracheal intubation, from 1 : 300 between 1978 and 1983 to 1 : 250 in 1994.^40,^59,⁶⁰ It is uncertain whether these concurrent trends are inter-related. The risk of failed tracheal intubation is considerably higher for emergency than for elective C/D, with 80% of airway-related fatalities occurring during emergency C/D (nights and weekends) and usually involving trainees.⁴⁰ A review of GA for C/D at a tertiary hospital during the period 1990-1995 showed a decline in the use of GA, from 7.2% to 3.6%.² A further review of GA for C/D from 2000 to 2005 in the same institution showed that GA use had further declined to a low incidence of about 1%.⁴³

Because of the decreased use of GA for C/D, anesthesia trainees’ experience of GA in obstetric patients is also on the decline. The lack of opportunities for training and maintenance of airway skills is an emerging concern.^58,^61,⁶²

IX Recommendations for Management of the Difficult Airway

Guidelines for the management of anticipated and unanticipated DA have been published by various national societies. The guidelines of the ASA Task Force were originally developed as a consequence of the high number of perioperative respiratory adverse events during airway management.^63,⁶⁴ The updated version of the DAA was published in 2003 (Fig. 37-3).⁴ The ASA DAA guidelines include evaluation of the airway, physical examination (as described earlier), basic preparation for the DA, and follow-up care. Although these points are applicable to the obstetric patients as well, some elements of the ASA DAA and their adaptability to the obstetric DA cannot be assessed due to the lack of strong evidence. The available evidence on management of the DA in the obstetric patient is mainly in the form of case reports. The ASA DAA provides a framework for management of the DA and should be modified and adapted to the obstetric patient where applicable.

Figure 37-3 Difficult airway algorithm (DAA). LMA, Laryngeal mask airway.

(From American Society of Anesthesiologists Task Force on Management of the Difficult Airway: Practice guidelines for management of the difficult airway: An updated report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Anesthesiology 98:1269–1277, 2003.)

A key point of the ASA DAA and its strength is the recommendation of a clear strategy for management of both the anticipated and the unanticipated DA. This strategy, however, depends on the skills and preferences of the anesthesia practitioner. The ASA Task Force strongly recommended the use of strategies to maintain oxygenation throughout the process of airway management. It is the cornerstone of airway management in situations including the nonemergency pathway, the emergency pathway, and increasing hypoxemia during the CICV critical airway situation. Crucial elements of the ASA DAA include an early attempt at insertion of the LMA if face mask ventilation is not adequate (strengthened in the updated version), the possibility of awakening the patient, and simple “call for help” at an early stage. These are all applicable in the obstetric DA scenario and add to the strength of the algorithm. As mentioned earlier, the ASA DAA offers many options; however, it is not specific for an urgent obstetric situation.

X Equipment (Difficult Airway Cart)

The ASA DAA recommended that anesthesia personnel familiarize themselves with the patients and the airway equipment in advance. Likewise, the ASA Practice Guidelines for Obstetric Anesthesia recommended that airway equipment should be readily available in the labor and delivery area.⁴⁵ Based on our own experience, we believe that equipment and anesthesia personnel should actually be immediately available in the labor and delivery area, throughout the 24-hour shift, on a daily basis. Our current practice is to have a designated DA cart immediately available inside our obstetric operating rooms for use in airway emergencies, and this policy has proved to be invaluable on many occasions.⁶⁵^–⁶⁷

The fiberscope is located in a predesignated slot on the side of our DA cart. The top shelf has all the equipment and local anesthetics required for an awake tracheal intubation as well as an optical stylet for optimizing the glottic view during a DI under GA. The next three drawers are organized to follow the sequence of clinical scenarios that one might encounter during an unanticipated DA:

• Drawer A: Supraglottic airways of sizes 3 and 4 (LMA Excel, Fastrach LMA, ProSeal LMA) for use in the nonemergency pathway

• Drawer B: Specialized supraglottic airways (Combitube 37 F, King LTS-D sizes 3 and 4) for use in the emergency pathway

• Drawer C: Equipment available for invasive airway access (cricothyroidotomy kit, transtracheal jet cannula with adapter) for use in the emergency pathway, critical airway situation

XI Anticipated Difficult Intubation

Because the incidence of difficult tracheal intubation is significantly higher in the obstetric population, especially during emergency C/D and on nights and weekends,⁴⁰ it is only prudent to avoid GA for C/D if other safe anesthetic choices are available. In modern obstetric anesthesia practice, neuraxial anesthesia is administered to some patients who would have received GA in the past, such as patients with severe preeclampsia with lower platelet count, placenta previa without active bleeding, or umbilical cord prolapse in a parturient with a functioning epidural catheter.

The use of neuraxial anesthesia is based on current evidence in the literature, which includes the following:

1. An increased use of epidural anesthesia during labor in high-risk parturients

2. A heightened awareness that an in situ functioning epidural catheter may decrease the necessity for GA in an urgent situation

3. Understanding that the presence of a functioning epidural catheter allows analgesia to be converted to epidural anesthesia, if necessary for a C/D

4. Appreciation of the risks of airway complications associated with GA for C/D

5. Improvement in the quality of intraoperative and postoperative pain management, with the addition of opioids to the local anesthetic, during spinal and epidural blocks

6. Keeping parturients awake and allowing for bonding with the baby during C/D

The current practice of placing epidural catheters prophylactically, early in labor, in high-risk parturients who have clinical conditions such as morbid obesity, DA, or obstetric comorbidities or complications has reduced the risk of an unanticipated GA. This is considered best practice and is in keeping with the most recent ASA guidelines for obstetric anesthesia.⁴⁵ The practice guidelines advocate this concept of using a prophylactic epidural in high-risk parturients, which basically involves the placement of a catheter and the confirmation of functionality with small amounts of local anesthetic, early in labor, and possibly before a request for analgesia. This provides a readily available neuraxial route of anesthesia, should an emergent operative delivery be necessary. If the epidural catheter is not functioning well, it should be replaced with either a functioning continuous epidural catheter or a continuous spinal catheter.

Current evidence confirms the efficacy of dosing in situ epidural catheters in achieving decision-to-delivery intervals (DDI) for C/D and is comparable to that achieved with GA.^68,⁶⁹ Evidence suggests that it is necessary to allow the shortest DDI for the true fetal distress situation; additionally, the recommended 30-minute DDI for emergency cesarean deliveries is appropriate¹⁸ and allows the administration of a spinal anesthetic. Holcroft and colleagues showed the outcomes classified by type of anesthesia.⁷⁰ DDI for GA was almost 15 minutes shorter than for spinal anesthesia and 13 minutes shorter than for epidural anesthesia (P = 0.0002). The epidural group had functioning labor epidurals that were converted to surgical anesthesia for C/D, whereas the spinal anesthesias were performed after the decision for C/D was made. There were no differences in the incidence of 1-minute Apgar scores lower than 7 based on anesthesia type, but there was a significant increase in 5-minute APGAR scores lower than 7 in the GA group. There was no difference in the umbilical arterial pH between the groups, but the base excess was significantly worse in the GA group. The investigators concluded that although GA is considerably faster than either spinal or epidural anesthesia, it is well established that GA poses greater maternal risk.^13,⁷¹ Therefore, the benefits of obtaining anesthesia sooner versus the risk of a maternal complication must be weighed and determined by the obstetrician and anesthesiologist working in consultation together in each individual case. The American Congress of Obstetricians and Gynecologists (ACOG) Committee Opinion entitled, “Anesthesia for Emergency Deliveries” endorsed the following stance: “Cesarean deliveries that are performed for a nonreassuring fetal heart rate pattern do not necessarily preclude the use of regional anesthesia.”⁷² These guidelines require both familiarity and knowledge for application in an urgent C/D situation.

Another option for neuraxial techniques for C/D is deliberate continuous spinal analgesia using the Tuohy needle and an epidural catheter. A recent study by Palanisamy and colleagues confirmed that (1) the rate of use of GA for cesarean delivery between 2000 and 2005 was low, approximately 1%; (2) 85% of GAs were administered for emergent deliveries; (3) most were performed because of a perceived lack of time, particularly for the emergency deliveries; (4) very few GA cases resulted from failure of neuraxial anesthesia techniques; and (5) there was a very low incidence of GA-related morbidity with no cases of mortality.⁴³ This study confirmed first, that the policy of placing epidural catheters prophylactically in high-risk parturients, particularly obese parturients with potential DA, has reduced the risk of unanticipated GA. Second, the adoption of a more aggressive approach toward management of inadequate neuraxial block (by replacing epidural catheters for suboptimal analgesia during labor) may have reduced the incidence of intraoperative conversion to GA. Third, the willingness to perform emergent spinal anesthesia, including intentional continuous spinal techniques, especially in patients with certain comorbid conditions (e.g., severe preeclampsia)^73,⁷⁴ or morbid obesity,⁷⁵ may have been partly responsible for the reduction in GAs.

The use of a combined spinal/epidural technique for labor analgesia in a high-risk parturient may not be the most prudent choice. Even though several studies have found a lower incidence of failed epidural analgesia after initiation of analgesia with a combined spinal/epidural technique,^76,⁷⁷ there is a disadvantage in that the correct placement of the epidural catheter in the epidural space cannot be verified until the spinal anesthesia wanes. Therefore, if a functioning epidural is important, so as to be able to convert to surgical anesthesia for emergency C/D, especially in a parturient who has a high probability of a DA or a nonreassuring fetal status, then a combined spinal/epidural technique is not the preferred technique for labor analgesia.

If neuraxial anesthesia is contraindicated due to maternal hemorrhage, coagulopathy, or umbilical cord prolapse, one may have to resort to GA. The DA cart should be immediately available in the operating room and should include other advanced airway devices such as a cricothyroidotomy kit and transtracheal jet ventilation (TTJV) equipment. It is prudent to inform the ear, nose, and throat surgeon or the general surgeon to remain readily available in these situations.

XII Airway Management Strategies

A Cesarean Delivery

The anesthesia practitioner must have a clear and effective plan for managing a patient with DA, a difficult or failed intubation, or a CICV in a parturient undergoing C/D under GA. The ASA DAA is a beginning for an organized approach to DA problems.⁴ All anesthesia practitioners must be familiar with the ASA DAA. Although this algorithm is commendable in providing a logical, well-organized overview of the DA, it is not easily memorized and is not easy to implement in emergency situations, especially when two lives (mother and baby) are at risk. The United Kingdom DAS basic guidelines⁵ are more linear and emphasize a four-point approach with each plan consequent on failure of the previous plan (Fig. 37-4):

• Plan A: Initial intubation attempt

• Plan B: Secondary intubation attempt

• Plan C: Maintain oxygenation and ventilation using rescue techniques with supraglottic devices and bail out

• Plan D: Invasive techniques for the CICV situation

Figure 37-4 Airway management strategies for patients undergoing cesarean delivery. ILMA, Intubating laryngeal mask airway; LMA, laryngeal mask airway.

(Adapted from Henderson JJ, Popat MT, Latto IP, Pearce AC: Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 59:675–694, 2004.)

Two principles are emphasized in these guidelines: (1) maintenance of oxygenation during the execution of each plan and (2) seeking the best assistance available as soon as difficulty with laryngoscopy is experienced.

For purposes of this discussion, we have adapted our recommendations for airway management strategies in the obstetric patient with due consideration to both the ASA DAA guidelines and the U.K. DAS guidelines.

B Awake Fiberoptic Intubation

In a parturient who has a known history of DA, an anticipated DA, morbid obesity, certain anatomic features indicating that tracheal intubation by conventional means is likely to be difficult or impossible, or contraindications to RA and is undergoing C/D, a safe option is to secure the airway with the tracheal tube while the patient remains awake. If RA is not an option and time is not an issue, an awake tracheal intubation can be performed safely with the use of either a flexible fiberscope or a video laryngoscope.^78,⁷⁹ The following paragraphs describe the logical approach to awake tracheal intubation.

Adequate preparation is crucial to the success of an awake tracheal intubation and has several aspects:

• Psychological preparation of the patient.

• Administration of an antisialagogue such as glycopyrrolate, 0.2 to 0.3 mg intravenously, 10 to 15 minutes before the application of topical anesthesia to dry the oropharyngeal secretions. Excessive secretions interfere with the effects of local anesthetics. Glycopyrrolate, being a quaternary ammonium compound, does not cross the placenta.

• Judicious sedation with midazolam (1 mg) and fentanyl (50 to 100 µg) intravenously, so that the patient is calm and cooperative, without risking respiratory depression in the mother or neonate. Opioids produce analgesia and depress the airway reflexes to facilitate smooth oropharyngeal and laryngeal instrumentation.

• Adequate topicalization of the airway with local anesthetic to make it nonreactive to physical stimulation. Topical anesthesia of the oropharynx is achieved with 4 or 5 short sprays of 4% lidocaine to the palate, base of the tongue, and lateral and posterior pharyngeal walls. Two minutes after the spray of local anesthetic, 2% lidocaine gel is spread on the base of the tongue with a tongue blade, to supplement the topical anesthesia and to prevent gagging. Use of a good topical anesthetic before endoscopy prevents coughing, gagging, and laryngeal spasm and contributes to the success of the technique. Depressed gag and airway reflexes can make the parturient more susceptible to pulmonary aspiration of gastric contents. Ovassapian and colleagues ⁸⁰ presented the concept of topicalization of the lower airway and awake fiberoptic tracheal intubation in patients at high risk for pulmonary aspiration and have helped diffuse that controversy.

• The “Spray as you go” technique with local anesthetic spray of 2% lidocaine. This is used for application of the topical anesthetic and for anesthetizing the upper airway from the tip of the tongue to the trachea.

• The advanced airway management skills of the operator. The skill of the anesthetist is equally important for success of the technique.

Research has documented the successful use of awake fiberoptic tracheal intubation in 60 parturients with DAs undergoing C/D.⁸¹ These parturients received judicious sedation and underwent full topicalization, including transtracheal injection of lidocaine (except that those parturients with coagulopathy received nebulized lidocaine), and there were no cases of pulmonary aspiration. Lower esophageal tone is maintained in patients who are not oversedated and prevents pulmonary aspiration regardless of the extent of topicalization. In our own institution, we are occasionally faced with similar clinical situations and have successfully used awake, oral fiberoptic intubation to secure the airway before administration of GA for cesarean delivery.⁸²

The nasal route for awake, fiberoptic tracheal intubation has been used occasionally in pregnant women. However, it should be used with caution because of the hyperemic nasal mucosa and the potential for inducing epistaxis.

XIII Unanticipated Difficult Tracheal Intubation

A Step 1: Initial Tracheal Intubation Attempt

We propose a five-step flow chart for unanticipated difficult tracheal intubation after induction of GA in the obstetric patient (Fig. 37-5). It is based on the ASA DAA and the U.K. DAS algorithm and is modified for the obstetric patient. Each step should not take longer than 30 to 45 seconds, so no more than 5 minutes should elapse before the decision is made to use invasive airway access in the emergency pathway—critical airway scenario.

Figure 37-5 Five-step approach to unanticipated difficult tracheal intubation during rapid-sequence induction (RSI) of anesthesia in the obstetric patient. The BURP maneuver consists of backward, upward, and rightward pressure on the thyroid cartilage. CICV, “Cannot intubate, cannot ventilate”; CVS, cardiovascular system; G.I., gastrointestinal; Gr., Cormack-Lehane grade (laryngoscopic view); LUD, left uterine displacement; N, newtons (unit of force).

If the decision is made to proceed with GA, and before induction of anesthesia and intubation, one must strictly adhere to certain basic principles: use of gastrointestinal prophylaxis to prevent aspiration; optimal positioning, including the placement of a wedge under the right hip to offset any aortocaval compression; and use of the ramped-up position in obese parturients to facilitate laryngoscopy, tracheal intubation, and ventilation.

Induction of anesthesia is preceded by preoxygenation to effectively denitrogenate the lungs; effective use of a preoxygenation technique that maximizes oxygen stores (preoxygenation for 3 to 5 minutes or 4 deep breaths over 30 seconds)⁸³; and the first attempt at direct laryngoscopy should always be performed in optimal conditions. After succinylcholine administration and confirmation of adequate muscle relaxation, appropriate head and neck position, and use of effective cricoid pressure, proceed with RSI and tracheal intubation. The application of optimal external laryngeal manipulation to optimize the laryngoscopic view may be critical. All these elements are important in managing the DI scenario (see Fig. 37-5, Step 1) and are discussed in detail in the following sections.

1 Aspiration Prophylaxis

Because of the risk for gastric regurgitation and pulmonary aspiration, all pregnant patents are considered to have a “full stomach” regardless of their preoperative fasting status. Aspiration risk can be mitigated by oral administration of triple pharmacologic prophylaxis that includes sodium citrate to neutralize the acidic contents of the stomach; intravenous (IV) administration of a histamine (H₂) receptor antagonist to reduce gastric acid secretion; and IV administration of a gastrokinetic drug, such as metoclopramide, to speed up gastric emptying and increase the tone of the lower esophageal sphincter.

2 Positioning

Proper positioning of the patient is an often missed critical step in facilitating laryngoscopy, tracheal intubation, and mask ventilation, if needed. The optimal sniff position (slight flexion of the neck and extension of the head on the neck), which aligns the oral, pharyngeal, and laryngeal axes into a straight line, is mandatory to facilitate tracheal intubation.⁵² The neck should be flexed on a pillow and the A-O joint extended to achieve the optimal sniffing position. However, in morbidly obese patients including parturients, one should utilize the head-elevated laryngoscopy (HELP) position and create a ramp, using folded towels or blankets under the shoulders and head to ensure that the head and shoulders are higher than the chest.⁸⁴ The HELP position is determined by drawing an imaginary horizontal line that connects the patient’s sternal notch with the external auditory meatus, so that the head and neck are at a slightly higher elevation than the chest. The Troop Elevation Pillow (Mercury Medical, Clearwater, FL) is shaped like a ramp and is designed to optimize the HELP and sniffing positions.

3 Cricoid Pressure

Cricoid pressure has played an important role in prevention of pulmonary aspiration since its introduction by Sellick.⁸⁵ It is an integral part of the flow chart for the patient having RSI for cesarean delivery. Sellick first described a neck maneuver to compress the esophagus between the cricoid cartilages anteriorly and the body of the sixth cervical vertebra posteriorly in order to prevent regurgitated gastric contents from entering the hypopharynx during induction of GA.⁸⁵ Typically, a cephalad and posteriorly pointing force of 10 newtons (N), applied by the thumb and index finger of the assistant, is required in the awake patient; this force increases up to 30 N in the unconscious patient.^86,⁸⁷ On the other hand, as important as cricoid pressure is to prevent regurgitation of gastric contents into the oropharynx,⁸⁸ excessive cricoid pressure may obscure the glottic view by displacing the vocal cords anteriorly or laterally.⁸⁹ The assistant may be asked to transiently reduce or release cricoid pressure, with suction at hand, so that the glottic area and vocal cords can be visualized, despite the possible risk of pulmonary aspiration in the event of gastric regurgitation.^90,⁹¹ Aspiration is treatable, whereas irreversible hypoxia is not.

4 External Laryngeal Manipulation

Various steps taken to optimize the laryngoscopic view with external laryngeal manipulation, referred to as the BURP maneuver (i.e., backward, upward, rightward pressure on the thyroid cartilage) and the use of an Eschmann introducer can maximize the success of tracheal intubation. The use of optimal external manipulation (i.e., the BURP maneuver) involves pressure on the thyroid cartilage and displacement of the larynx in three specific directions—posteriorly against the cervical vertebrae, as far superiorly as possible, and slightly laterally to the right.⁹² Because the thyroid cartilage is the surface marking for the laryngeal aperture, proper application of the BURP maneuver improves the laryngoscopic view. In a study comparing glottic views with and without use of the BURP maneuver, it was shown that the BURP maneuver improved the glottic view by at least one whole grade and reduced the incidence of failure to view any portion of the glottis from approximately 9.2% to 1.6%.⁹²

B Step 2: Second Tracheal Intubation Attempt

1 Consider Calling for Help, Returning to Spontaneous Ventilation, and Awakening the Patient

In all but the most urgent situations, one can consider awakening the mother and reassessing the fetus before proceeding with an alternative plan. However, in urgent and emergent C/D situations under GA, the goal should be to balance maternal oxygenation, prevention of pulmonary aspiration, and expeditious delivery of the fetus. Such a situation dictates addressing management of the DA after a failed tracheal intubation attempt. Appropriate initial management and procedures after failed attempts at tracheal intubation can influence and even ensure the final optimal and best outcome for both mother and baby. Calling for help is critical; it should be done sooner rather than later. Immediate access to the DA cart is critical as well.

Despite use of optimal positioning and the BURP maneuver, if the first attempt at intubation fails because of a poor view, particularly a grade 3 laryngoscope view, other alternative intubation devices (i.e., Eschmann introducer, optical stylet, video laryngoscope) should be considered to assist in the second attempt at intubation (see Fig. 37-5, Step 2). There should be no more than two attempts at tracheal intubation in an obstetric patient. The second attempt at laryngoscopy should be considered the “best attempt at tracheal intubation.” To increase the success rate, it should be performed by a reasonably experienced anesthesiologist, the optimal sniff or ramped position should be used, and external laryngeal manipulation should be applied. Additionally, the laryngoscope blade type and handle may need to be changed. If necessary, cricoid pressure may be transiently released to optimize the glottic view.

Persistent attempts during emergency tracheal intubation have shown that there was a significant increase in the rate of airway-related complications as the number of laryngoscopic attempts increased (comparing ≤2 with >2 attempts), resulting in hypoxemia (11.8% versus 70%), regurgitation of gastric contents (1.9% versus 22%), aspiration of gastric contents (0.8% versus 13%), bradycardia (1.6% versus 21%), and cardiac arrest (0.7% versus 11%) (P < 0.001).³² We also recommend limiting the intubation attempts to two in an emergent obstetric case.⁹³^–⁹⁵

2 Role of Eschmann Introducer (Gum Elastic Bougie)—Grade 3A Laryngoscopic View

The Eschmann introducer, commonly referred to as the gum elastic bougie, is used universally to facilitate difficult tracheal intubation.⁹⁶ The original Cormack and Lehane classification of laryngoscopic view⁵⁰ was recently modified by Cook,⁹⁷ who proposed subdividing grade 3 into grades 3A and 3B (Fig. 37-6). In grade 3A, the glottic aperture cannot be seen but the epiglottis can be visualized and elevated; hence, a role for indirect methods, such as the Eschmann introducer, is indicated. Success rates have been shown to be similar between the Eschmann introducer and optical stylets in the grade 3A airway (31 versus 29.2 seconds).⁹⁸

Figure 37-6 Cormack and Lehane’s laryngeal grades of the airway, as modified by Cook.

(Adapted from Cook TM: A new practical classification of laryngeal view. Anaesthesia 55:274–279, 2000.)

3 Role of Optical Stylets—Grade 3B/4 Laryngoscopic View

In grade 3B, when the glottic aperture cannot be seen and the epiglottis is visualized but cannot be elevated, other alternative methods may be used (see Fig. 37-6).⁹⁹ Optical stylets have been used successfully to facilitate rapid tracheal intubation in the grade 3B laryngoscopic view and to help confirm tracheal tube placement.⁹⁸ The success rate has been shown to be higher with optical stylets than with the Eschmann introducer, and the time required is shorter with optical stylets in the grade 3B view (31 versus 45.6 seconds).⁹⁸

4 Role of Video Laryngoscopy

Video technology has become widespread across medical/surgical disciplines and allows improved visualization of anatomic detail. Besides allowing management of the DA, video laryngoscopy can be a useful teaching tool during both direct and indirect laryngoscopy.¹⁰⁰ The portability of video laryngoscopes should facilitate their use in the operating room and also in managing airway calls in emergency departments, intensive care units, endoscopy suites, and radiology suites. It seems only natural that video laryngoscopy will become the norm in managing and teaching the DA in the near future.

Perhaps, in situations in which traditional laryngoscopy fails during the first attempt, the next attempt (best attempt at tracheal intubation) should be the use of a video laryngoscope with which the operator is familiar. Some of the devices available include the Berci Kaplan DCI, Glidescope, McGrath Scope, and CMAC Storz. The Glidescope has been successfully used in large studies and case series.^78,¹⁰¹ Potential difficulty can be overcome by using a rigid stylet and shaping the tracheal tube before attempting insertion.

Channeled video laryngoscopes include the AirTraq, Pentax, and KingVision. These scopes are designed to provide an easy glottic view without aligning the oral, pharyngeal, and tracheal axes and have a channel for holding the tracheal tube. There has been one report of two cases of rapid tracheal intubation using Airtraq in morbidly obese parturients undergoing emergency cesarean delivery after failed tracheal intubation.¹⁰²

Some centers have already implemented the practice of video laryngoscopy as the “first-look” airway device for immediate C/D under GA, and for the anticipated difficult tracheal intubation. The use of alternative devices such as the Eschmann introducer, optical stylets, and video laryngoscopes can enhance the intubation success rate during the second tracheal intubation attempt.

C Step 3: Maintenance of Oxygenation and Ventilation

The goals and priorities in airway management strategies after a failed second tracheal intubation should include (1) maintenance of maternal oxygenation, (2) prevention of gastric regurgitation, (3) airway protection, and (4) expeditious delivery of the fetus (see Fig. 37-5, Step 3).

Maintaining oxygenation in the parturient is of paramount importance. Pregnancy-related changes can result in rapid development of hypoxemia and acidosis.¹⁰³ Because adequate oxygenation is critical, mask ventilation in the presence of cricoid pressure is attempted. If mask ventilation is difficult, an optimal or best attempt at ventilation (i.e., two-person mask ventilation via a conventional face mask) is initiated while cricoid pressure (30 N) is maintained.⁹⁵

The algorithm then divides into two pathways, determined by the presence or absence of adequate face mask ventilation.

After a failed second attempt at tracheal intubation, if oxygen saturation is maintained, the patient is considered to be on the nonemergency pathway (cannot intubate, but can ventilate), and use of the classic LMA, the intubating laryngeal mask airway (ILMA), or the ProSeal laryngeal mask airway (PLMA) as a rescue device must be considered. The revised ASA DAA, based on evidence in the literature, supports the role of the LMA in airway management.^104,¹⁰⁵ The LMA not only is useful to provide maternal oxygenation, ventilation, and anesthesia but also serves as a conduit for intubation. Cricoid pressure may have to be reduced or released transiently to allow proper placement of the LMA.⁹¹ The LMA or ILMA may be used as a conduit for tracheal intubation, either blind or fiberoptically assisted (see Fig. 37-5, Step 3).

1 The Classic Laryngeal Mask Airway

The classic LMA has been widely used for the difficult obstetric airway without any episodes of gastric regurgitation or pulmonary aspiration.^40,¹⁰⁶^–¹⁰⁹ Han and colleagues successfully used the classic LMA as a ventilatory device in 1060 of 1067 parturients undergoing elective cesarean delivery.¹¹⁰ No episodes of hypoxia, regurgitation, or aspiration were reported. Multiple case reports in the literature have described successful use of the classic LMA after failed tracheal intubation in obstetric patients.^107,¹¹¹^–¹¹⁴ In patients requiring tracheal intubation, fiberoptic-guided tracheal intubation through the classic LMA is reliable.¹¹⁵ However, a longer tracheal tube (e.g., Endotrol, Microlaryngeal, or Nasal Ring-Adair-Elwyn) is needed. The Aintree exchange catheter (Cook Medical, Bloomington, IL) may also be used to facilitate intubation through the classic LMA.

The fiberscope/Aintree catheter exchange technique is extremely useful in switching from an LMA to a tracheal tube.¹¹⁶ The Aintree catheter is 56 cm long and, once preloaded on the fiberscope, allows the distal 3 to 4 cm of the fiberscope to be available for manipulation and passage via the LMA into the trachea. The inner diameter of the Aintree catheter is 4.8 mm, allowing it to be preloaded on no larger than a 4.0-mm pediatric fiberscope. Its removable Rapi-fit connector allows for oxygen insufflation, if necessary, during the airway exchange. Its external diameter is 6.5 mm, allowing it to be used as an airway exchange catheter (AEC) for tracheal tubes that have an internal diameter of 7 mm or more. This airway exchange technique involves the following steps (see Fig. 35-2 in Chapter 35):

1. Insertion of LMA and cuff inflation

2. Confirmation for easy ventilation and oxygenation

3. Passage of fiberscope, preloaded with Aintree catheter, via the LMA into the trachea under vision

4. Removal of fiberscope, leaving the Aintree catheter in the trachea, with the LMA in situ

5. Deflation of LMA cuff, followed by removal of LMA

6. Passage of tracheal tube of at least 7 mm internal diameter over the Aintree catheter into the trachea

2 The Intubating Laryngeal Mask Airway

The LMA Fastrach or ILMA is designed to specifically overcome the problems associated with blind tracheal intubation through the classic LMA.¹¹⁷ The ILMA is particularly useful during failed intubation in an emergency C/D because it provides oxygenation and a conduit for blind tracheal intubation and prevents pulmonary aspiration. Several studies have shown successful use of the ILMA to help visually unassisted tracheal intubation in patients with a DA.^115,^117,¹¹⁸ The ILMA was used successfully after failed tracheal intubation during an emergency C/D in a patient who was morbidly obese and eclamptic and in a second case in which RA had failed and was followed by GA resulting in failed tracheal intubation.^66,⁶⁷ The ILMA proved to be a life-saving device in both parturients.

3 The ProSeal Laryngeal Mask Airway

The PLMA is a unique device that represents a substantial change in LMA design. The PLMA offers several advantages over the classic LMA for failed tracheal intubation in obstetrics: (1) the seal is 10 cm H₂0 higher, giving it greater ventilatory capability ¹¹⁹; (2) it enables correct positioning in which the glottis is isolated from the esophagus and therefore may provide airway protection and protection against pulmonary aspiration^120,¹²¹; (3) it facilitates gastric tube insertion to empty the stomach of fluid and air insufflated during difficult face mask ventilation. The PLMA has been used successfully after failed intubation during emergency C/D.^120,¹²²^–¹²⁴

After failed tracheal intubation, and once the anesthesia practitioner is able to successfully achieve pulmonary ventilation and oxygenation through the LMA, caution must be used in selecting a nonirritating inhalation anesthetic and in providing an adequate depth of anesthesia. Sevoflurane provides rapid, smooth induction and adequate depth of anesthesia; it is the least irritating volatile agent,¹²⁵ and it helps facilitate tracheal intubation through the LMA in patients with a DA.¹²⁶

If tracheal intubation is difficult via the LMA and the fetal or maternal condition warrants immediate surgery, the obstetrician should be asked to proceed with cesarean delivery while the anesthesia care team maintains cricoid pressure and provides oxygenation and anesthesia via the LMA. Communication with the obstetrician should include the need to avoid both exteriorization of the uterus and fundal pressure during cesarean delivery, because of the unprotected airway and the risk of regurgitation and aspiration (see Fig. 37-5, Step 3).

D Step 4: CICV—Noninvasive Rescue Ventilation

1 Emergency Pathway

If face mask ventilation was not adequate and the LMA was ineffective in providing oxygenation and ventilation, the patient is considered to be on the emergency pathway (CICV) and will require rescue ventilation with other noninvasive devices ⁴ such as the Combitube. King LTS/LTS-D may be a suitable alternative to the Combitube (see Fig. 37-5, Step 4).

2 Combitube

The Combitube should be considered in emergency airway situations, especially when patients are at risk for pulmonary aspiration, tracheal intubation has failed, and LMA is not effective.^127,¹²⁸ The ASA DAA suggests use of the Combitube after failed pulmonary ventilation with conventional face mask and LMA.⁴ The successful use of the Combitube has been described after failed tracheal intubation in an emergency C/D.¹²⁷

Even though the Combitube is losing its popularity in Canada and the United Kingdom, it is still a part of the ASA DAA, the American Heart Association guidelines, and the European Resuscitation Council guidelines. The ASA DAA incorporated use of the Combitube in the emergency pathway (i.e., the life-threatening CICV situation) in which establishing ventilation and oxygenation is critical. It may also be particularly useful for difficult or failed tracheal intubations in obstetric patients, who are at increased risk for gastric regurgitation and pulmonary aspiration.

When properly positioned, the Combitube allows ventilation with a higher seal pressure than the classic LMA, protects against regurgitation, and allows a subsequent attempt at fiberoptic intubation while the inflated esophageal cuff maintains airway protection.^127,¹²⁹ Although there have been failures,¹³⁰ the Combitube has been used successfully in cases of difficult tracheal intubation^127,¹³¹ and CICV,^132,¹³³ including failure with the LMA.¹³⁴ The Combitube may offer significant advantages over the LMA in parturients. These advantages include isolation of the stomach from the glottic area and minimal need for preparation. Oxygenation and pulmonary ventilation can be achieved rapidly, especially because the parturient is prone to rapid arterial oxygen desaturation. The Combitube has been shown to prevent pulmonary aspiration during cardiopulmonary resuscitation and to protect the airway from pulmonary aspiration of gastric contents during obstetric anesthesia.^129,¹³⁵

The Combitube is a disposable double-lumen tube with two cuffs and two pilot balloons that is designed for blind insertion. If blind insertion is difficult or unsuccessful, the Combitube may be placed in the upper esophagus under direct vision with the use of a laryngoscope. Ventilation is initially attempted through lumen 1, which forces air into the trachea (see Fig. 35-3 in Chapter 35). Causes of failed ventilation with the Combitube include inadvertent tracheal placement (5%), deep insertion laryngospasm, and bronchospasm. The steps in troubleshooting Combitube use are important to understand, especially when one is confronted with a critical airway. The Combitube is designed to enter the esophagus after blind insertion. If ventilation is difficult through the blue lumen 1, the Combitube could be in the trachea, and ventilation must be switched to the clear lumen 2, allowing air to enter the trachea directly through the open end. If the Combitube is inserted too deep (see Fig. 35-5A in Chapter 35), ventilation is impossible and the pharyngeal balloon must be deflated, the Combitube pulled back 1 to 2 cm, and ventilation switched back to the blue lumen 2 (see Fig. 35-5B in Chapter 35). The original Combitube is a large, bulky device and can cause esophageal damage; therefore, the small adult (SA) size is recommended.¹³⁶

3 Other Alternative Airway Devices

The King LT (known as the laryngeal tube in Europe) differs from the Combitube by being smaller, shorter, and softer and having a non-latex oropharyngeal cuff. The design of the King LT allows easy insertion, provides vertical positioning latitude, uses low-pressure cuffs,^119,¹³⁷ and allows passage of an AEC while simultaneously providing an esophageal barrier (Fig. 37-7A).¹³⁸ It is a versatile airway device that can be used in elective or emergent conditions or DA, and ventilation can be spontaneous or controlled.¹³⁹

Figure 37-7 King LTS-D supraglottic airway. A, Correct position in the esophagus. B, Correct position with orogastric tube (OGT) in situ. C, Tracheal intubation using Aintree catheter and fiberscope. ETT, Endotracheal tube.

(Courtesy of Baylor College of Medicine, Houston, TX.)

The King LTS/LTS-D is the double-lumen version. It has dedicated ventilation and gastric access lumens and allows passage of an 18-F orogastric tube via the gastric drainage lumen (see Fig. 37-7B). Ventilatory seal with the King LT and airway pressure of 40 cm H₂O was found to be possible without gastric inflation in 30 patients.¹⁴⁰ The LMA was compared with King LT in 22 patients, and the mean leak pressure was significantly greater for the King LT¹⁴¹; gastric insufflation did not occur with the King LT. The King LTS/LTS-D may be useful in situations in which the patient is at risk for aspiration. In a case report, the King LTS was used successfully to establish ventilation and oxygenation after failed intubation in an emergency C/D.¹⁴²

Exchange of the King LT for a tracheal tube can be accomplished with a fiberscope and an AEC (see Fig. 37-7C).¹³⁸ The incidence of complications and adverse airway events is minimal with the use of this device.¹¹⁹

If oxygenation and ventilation are not established with either the LMA, the Combitube, or the King LT and the patient develops increasing hypoxemia (associated with bradycardia), the patient is considered to be in the emergency pathway—critical airway situation, which is a life-threatening emergency requiring immediate invasive intervention and rescue ventilation with either percutaneous cricothyroidotomy, TTJV, or surgical tracheostomy (see Fig. 37-5, Step 5).^4,⁵

E Step 5: CICV with Increasing Hypoxemia—Invasive Rescue Ventilation

1 Emergency Pathway—Critical Airway

All current airway guidelines ⁴^–⁶ recommend management of the CICV situation using either cannula cricothyroidotomy with percutaneous TTJV or surgical cricothyroidotomy.

The scenario of failed intubation, increasing hypoxemia, and difficult ventilation often occurs secondary to repeated unsuccessful attempts when a “can ventilate” situation rapidly develops into a CICV situation, resulting in significant maternal morbidity, including hypoxic brain damage, and maternal mortality. The fetus is also at risk for severe neurologic injury. Rapid development of severe hypoxemia, particularly if associated with bradycardia, is an indication for imminent intervention with an invasive airway rescue technique.

Once the decision is made to perform an invasive procedure, it is essential to use an effective technique. Rapid reoxygenation is critical and is best achieved with a combination of an invasive airway device and a ventilation technique that is capable of delivering effective ventilation and high minute ventilation with a fraction of inspired oxygen (FIO₂) of 1.0.

2 Cricothyroidotomy and Transtracheal Jet Ventilation

The risks of invasive rescue techniques must be constantly weighed against the risks of hypoxic brain damage or maternal death.¹⁴³ In Palanisamy’s study,³ among the 98 parturients who received GA for C/D, there was a sentinel case of DI, resulting in a critical airway incident (CICV) that necessitated a surgical cricothyroidotomy. The investigators reported that the total time taken from RSI of anesthesia to surgical cricothyroidotomy was less than 5 minutes.³

The anesthesia practitioner must be prepared to use invasive techniques to secure the airway via the cricothyroid membrane. Success depends on understanding the anatomy of the cricothyroid membrane and factors that determine efficacy of ventilation airway devices.^144,¹⁴⁵

a Cannula Cricothyroidotomy with Percutaneous TTJV

This technique involves the combination of insertion of a cannula or catheter through the cricothyroid membrane and use of a high-pressure source for ventilation. TTJV can provide effective ventilation ^146,¹⁴⁷; however, it is fraught with barotrauma hazards and is associated with a low success rate.¹⁴⁸^–¹⁵⁰ It is important to keep the upper airway patent and open to allow for deflation of the lungs and exhalation through the upper airway. If an LMA has been used, it can be left in place to allow for exhalation.

The needle cricothyroidotomy entails insertion of a catheter (usually an intravenous catheter) through the cricothyroid membrane; the needle through the catheter is then removed, and the anesthetist advances the guidewire through the catheter using the Seldinger technique. Next, a dilator is advanced over the guidewire, then removed, and a tracheostomy tube is inserted.

b Surgical Cricothyroidotomy

A simplified cricothyroidotomy technique can be performed in 30 seconds.¹⁵¹ This technique consists of the following four steps:

Step 1—Identification of the cricothyroid membrane

Step 2—Horizontal stab incision (no. 20 scalpel) through skin and membrane

Step 3—Caudal traction on the cricoid membrane with a tracheal hook

Step 4—Intubation of the trachea

Cricothyroidotomy can be particularly difficult in the obese patient. Insertion of the tube can be facilitated by passage of an introducer (bougie) through the incision or by the use of a tracheal retractor.¹⁵²

Guidewire techniques of cricothyroidotomy can be successful. The Melker guidewire intubation set is now available with a cuffed tube. It has been shown that the technique can be performed in 40 seconds after practice in a mannequin.¹⁵³

3 Extubation

The ASA Task Force regarded the concept of an extubation strategy as a logical extension of the extubation process.⁴ The closed claims analysis for DA management in surgical patients concluded that development of management strategies covering emergence and the recovery phase after extubation may improve patient safety.¹⁰

An emerging problem in obstetric patients is respiratory-related complications after emergence and maternal mortality after extubation.²⁰ The data from the Confidential Enquiries into Maternal Death from the United Kingdom and a study from the United States on anesthesia-related deaths in Michigan indicated that airway management of the obstetric patient during induction of GA,^20,¹⁵⁴ including intubation, was without complications; however, critical airway and ventilation incidents including hypoventilation or airway obstruction occurred during emergence, extubation, or recovery.

Current literature does not provide a sufficient basis for evaluating the merits of an extubation strategy.⁴ The focus has been mainly on addressing strategies, safety concerns, and techniques for intubating the trachea after induction of anesthesia, with relatively little attention paid to extubation strategies. The U.K. data specifically mentioned lack of training as partly responsible for the undesirable outcomes after extubation. The review from Michigan identified eight anesthesia-related deaths between 1985 and 2003. All of these deaths occurred as a result of airway obstruction or hypoventilation during emergence and recovery. Lapses in standard postoperative monitoring and inadequate supervision by an anesthesiologist were identified. Obesity and African-American race also seemed to be important risk factors for anesthesia-related maternal mortality.

Proposals to improve patient safety in obstetrics must require identification of preexisting comorbidities that can lead to postoperative respiratory complications. Standard monitoring in the postoperative period that includes pulse oximetry must be mandatory. As perioperative physicians, anesthesiologists are uniquely qualified to supervise the anesthesia care team; to manage and minimize anesthesia-related maternal risk; to provide peripartum medical diagnosis and treatment; to facilitate life-saving airway management interventions; and to lead prompt, coordinated, and effective resuscitation efforts to prevent airway catastrophes in obstetric patients.

Further attention is required for high-risk extubation patients, such as obese parturients and patients with an edematous airway, obstructive sleep apnea, or known or suspected airway management difficulties. Development of a strategy to maintain continuous access to the airway, so as to facilitate potential rescue of the airway, must be considered. Postextubation hypoventilation, airway compromise, ventilation-perfusion inequalities, and airway obstruction due to respiratory fatigue or apneic episodes may affect the patient in the operating room during emergence or in the immediate postoperative recovery period. Maintenance of continuous access to the airway after extubation with an AEC can be an important component of an extubation strategy in these selected patients.¹⁵⁵ The AEC is well tolerated by most patients (90%) and therefore is a valuable option.¹⁵⁵ Currently, there are no evidence-based guidelines regarding the optimal period of time for which to maintain airway access after extubation via an indwelling AEC. Experts have suggested at least 30 to 60 minutes or until the likelihood of reintubation is minimized.¹⁵⁶^–¹⁵⁹ Patients with the potential for periglottic edema may benefit from extending the duration of the in-dwelling AEC to 60 to 120 minutes.¹⁵⁵

The presence of an AEC to assist in reintubating and resecuring the airway is another major step toward improving patient safety and must be considered for obstetric patients who are potentially at high risk for airway-related catastrophes during emergence and after extubation.