TOPICS

INTRODUCTION

The term difficult airway most commonly refers to difficulty in placing an endotracheal tube through the vocal cords with the use of direct laryngoscopy. However, it can also refer to difficulty in providing adequate mask ventilation.¹ Despite advances in airway management and rescue, the incidence of airway difficulty encountered in the pregnant population resulting in inability to intubate is still estimated by some to be 1 in 300, a value eight times higher than that seen in the general population.^2–4 In the past, aspiration was considered a major cause of maternal morbidity and mortality.⁵ It follows that complications of airway management and failed or difficult intubation after induction of general anesthesia in near term pregnant women may be significant contributors to anesthesia-related maternal complications, and avoidance of difficult airway scenarios be of paramount concern to the obstetric anesthesiologist. Indeed, failure to intubate was the leading cause of anesthesia related maternal mortality from 1979 to 1990.⁶ Historically, this has led anesthesiologists to reduce the use of general anesthesia and thus the rate of airway catastrophe at the time of induction of general anesthesia for cesarean delivery. More recently, however, because of improved difficult airway protocols and rescue equipment, including the use of the laryngeal mask airway, increased use of regional anesthesia, and overall increased awareness, the maternal death rate from airway complications, particularly at induction of general anesthesia, appears to be decreasing.^7,8 The following discussion focuses on airway and gastrointestinal changes in pregnancy, managing the difficult airway, and aspiration.

UPPER AIRWAY AND RESPIRATORY MECHANICS

Pregnancy, labor, delivery, and the puerperium induce significant changes in upper airway anatomy and respiratory mechanics (Table 14-1). Higher levels of estrogen and an increase in maternal blood volume contribute to capillary engorgement, mucosal edema, and tissue friability in the parturient’s airway. This airway edema can be significantly worsened by preeclampsia, respiratory tract infection, expulsive efforts during the second stage of labor, and excessive fluid administration. Mallampati score increases during gestation and more so during labor.⁹ Because of these airway changes, it is generally recommended to use smaller sized endotracheal tubes for general anesthesia in pregnant women.⁴ Direct laryngoscopy should be performed carefully so as to minimize trauma and subsequent bleeding. Nasal endotracheal intubation should be used cautiously and with careful attention to vasoconstriction of the nasal mucosa.¹⁰

Table 14-1. Risk Factors for Airway Complications During Pregnancy^a

Airway edema

Decreased FRC

Increased oxygen consumption

Weight gain

Enlarged breasts

Full dentition

Decreased lower esophageal sphincter tone

Decreased gastric emptying during labor

^aAdapted from Chestnut DH, Polley LS, Tsen LC, Wong CA, eds. Obstetric Anesthesia. 4th ed. Philadelphia, PA: Mosby-Elsevier; 2009:651.

Respiratory physiologic changes during pregnancy relevant to airway management include an increase in minute ventilation, an increase in oxygen consumption, and a decrease in functional residual capacity (FRC) to 80% of the nonpregnant value by term gestation. The decreased FRC brings the lung closer to closing capacity, which further predisposes parturients to atelectasis. For these reasons, pregnant women near term become hypoxemic more rapidly than do nonpregnant women during episodes of apnea. For example, during rapid sequence induction of general anesthesia, the PaO₂ of parturients decreases at more than twice the rate of that in nonpregnant women (139 versus 58 mm Hg/min).¹¹

As is the case before any anesthetic, it is critical to perform a thorough airway assessment in obstetric patients. Assessment of Mallampati classification, atlanto-occipital extension, thyromental distance, and mandibular protrusion are four useful examinations for predicting difficulty with laryngoscopy. However, because no single test is sufficient to identify a patient with a difficult airway, anesthesia providers should perform a complete airway evaluation in obstetric patients, as outlined in the American Society of Anesthesiologists (ASA) Practice Guidelines.¹ Regardless of when the airway was first examined, labor and pushing may all affect the Mallampati score, and it should be rechecked when closer to anesthetic intervention.

NEURAXIAL ANALGESIA AND ANESTHESIA AS PROPHYLAXIS

The widespread acceptance of neuraxial analgesic techniques for obstetric patients has improved maternal and fetal outcome by reducing the need for general anesthesia and airway manipulation.⁸ Neuraxial techniques provide a mechanism by which anesthesia for cesarean delivery can be delivered in a safe and predictable manner, even in urgent situations. Advances in the delivery of neuraxial anesthesia that have made it safer include (1) administration of an epidural test dose, (2) use of a dilute solution of local anesthetic for epidural analgesia, (3) administration of the therapeutic dose of local anesthetic in incremental boluses through an epidural catheter, (4) maintenance of adequate left uterine displacement to prevent aortocaval compression, and (5) prompt and aggressive treatment of hypotension.⁸

There is also a greater acceptance of early or prophylactic placement of an epidural catheter in high-risk parturients. A prophylactic epidural catheter is one that is placed and tested with a small dose of local anesthetic; analgesia is not established until active labor occurs, the patient requests analgesia, and/or an operative delivery is required. Such a catheter provides a readily available conduit for providing neuraxial analgesia or anesthesia, especially if rapid onset (eg, emergency operative delivery) is desirable, thus avoiding airway manipulation. An early epidural catheter is placed in a controlled setting and allows time for catheter manipulation and replacement, if necessary, before further pathophysiologic changes (eg, decrease in platelet count, worsening airway edema) occur.¹²

Given that a failed neuraxial anesthetic may require conversion to general anesthesia, it is generally recommended that some form of pharmacologic prophylaxis should be administered to alter gastric pH in all parturients requiring surgical intervention, whether or not they receive general or neuraxial anesthesia.¹² The anatomic and physiologic changes of pregnancy (eg, reduced lower esophageal sphincter tone, delayed gastric emptying) increase the risk of aspiration in labor (although likely not prior to the onset of labor).¹³ Failed or difficult intubation is also associated with aspiration (see Pulmonary Aspiration in Obstetrics, below).

In light of the increased utilization of neuraxial anesthesia, one might expect the frequency of failed endotracheal intubations to have decreased. Yet, despite the dramatic increase in use of neuraxial techniques, Rahman and Jenkins demonstrated that over an 11-year period while the number of cesarean deliveries doubled, the frequency of failed endotracheal intubation remained constant.³ This study, and others, suggests that the majority of cases of failed tracheal intubation occurred during emergencies, outside normal working hours, and involved anesthesia trainees. Unfortunately, modern day trainees are not well experienced in obstetric general anesthesia. Hawthorn et al suggested that trainees’ exposure to general anesthesia in obstetrics has decreased simply because there are more trainees.⁶ However, it is also clear that trainee exposure has been reduced as a result of there being fewer general anesthetics in obstetrics due to the overwhelming preference for regional techniques.¹⁴ These data suggest there may be room for improvement in obstetric anesthesiology training programs, perhaps in the area of anesthesia simulation.¹⁵

The etiology of airway-related mortality appears to be shifting. In a review of anesthesia-related mortality in Michigan, Mhyre and colleagues reviewed 850 maternal deaths and were unable to find a single case of failed intubation during elective cesarean delivery. They demonstrated that anesthesia-related deaths from airway obstruction or hypoventilation took place during emergence, extubation, or recovery—not during the induction of general anesthesia—as previously observed. Also of note in this landmark study, system errors contributed to the majority of maternal deaths—specifically, lapses in standard postoperative monitoring and inadequate supervision by an anesthesiologist.⁸ The ASA guideline for postoperative care¹⁶ suggests that pulse oximetry is associated with early detection of hypoxemia and recommends periodic assessment of airway patency, respiratory rate, and oxygen saturation measured by pulse oximetry during emergence and recovery. The data from this study suggest also that the rate of failed intubation is now overstated and that the presence of an emergency itself is a significant risk factor for failed intubation.⁸

The most recent Confidential Enquiries into Maternal Deaths¹⁷ (2006-2008) reported that of the seven maternal deaths in the United Kingdom during this period as a result of direct anesthetic causes, two were the result of failure to ventilate the lungs, one was an aspiration of gastric contents on emergence from general anesthesia, and one was the result of postoperative “opiate toxicity.” The first case of “failure to ventilate” involved an unrecognized esophageal intubation through an intubating laryngeal mask airway after it was utilized for rescue during induction of general anesthesia. The second failure to ventilate case occurred in a patient with a known difficult airway that suffered dislodgement of her preexisting tracheostomy appliance postoperatively in the intensive care unit. This most recent survey suggests that vigilance around airway management should be extended to the entire perioperative period, not only during induction of general anesthesia and extubation.

Several authors have warned that a higher incidence of failed intubation in obstetric patients should be expected in the future.^3,6,15 One reason for this concern is that there is a significant increase in the use of neuraxial analgesic and there are limited opportunities to teach and practice the skills necessary for obstetric airway management. Another reason is the changing demographics of the obstetric population. The prevalence of obesity is rising at an alarming rate in both developed and developing countries worldwide. Obesity itself is an independent risk factor associated with airway management problems during pregnancy.¹⁷ In addition, anesthesiologists today provide care for older parturients having more comorbidities, in part as a result of delayed childbearing and the use of assisted reproductive technologies. These comorbid conditions may exacerbate the effects of hypoxemia, hypercarbia, and acidosis during a delayed or failed intubation.^15,17 Clearly, more work needs to be done, and many experts believe that the obstetric anesthesia community has a responsibility to become more aggressive in encouraging research and in developing clinical protocols that specifically address airway issues in the obstetric population.

THE UNANTICIPATED DIFFICULT AIRWAY

Even with the best attempts to adequately assess an airway preoperatively, it is possible that unanticipated difficulty with airway management can still occur. This may be due to an unrecognized anatomic variation, airway edema from preeclampsia, airway changes during labor, failure of bedside airway examination techniques, or some other unforeseen cause. Adequate equipment suitable for a variety of airway techniques should be available in all obstetric anesthetizing locations (Table 14-2). Initial management consists of repositioning the patient to achieve the proper sniffing position. Use of a different laryngoscope blade, video-assisted laryngoscope, a gum elastic bougie (Eschmann stylet), and/or a smaller diameter endotracheal tube should also be considered. In experienced hands, the authors suggest that subsequent laryngoscopy attempts should be performed only by the most experienced operator available, be limited to no more than two or three attempts, and that the second or third attempt should be performed only in those cases in which a portion of the laryngeal anatomy is visible (grade III or better). If a grade IV laryngoscopic view is identified with the initial laryngoscopy, the anesthesia provider should immediately focus on ensuring adequate oxygenation and ventilation of the mother (Figure 14-1).¹⁸

Table 14-2. Suggested Airway Equipment to Maintain for the Induction of Obstetric Anesthesia

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