51.3.1 What Are the Airway Considerations in Pregnant Women?
Pregnancy is associated with fluid retention and weight gain.2 Mallampati Class III and IV seem to be more prevalent in parturients at the beginning of labor (28%) than in the general adult population (7%-17%), suggesting that tongue volume increase maybe one of the physiologic changes of a normal pregnancy.3 Structurally, the pharyngeal airway is surrounded by soft tissues, such as the tongue and soft palate, which are enclosed by bony structures, such as the mandible and spine. Size of the airway space is determined by the balance between the bony enclosure space and soft tissue volume, when pharyngeal muscles are inactivated by general anesthetics and muscle relaxants. Pharyngeal edema, presumably due to fluid retention during pregnancy, and pharyngeal swelling acutely developed during labor, increases the soft tissue volume surrounding the airway, narrowing the pharyngeal airway in parturients.2 Many have hypothesized on predictors of this event including weight gain during the pregnancy, fluid administration during labor, and the length of the first and second stage of labor. A recent study from France by Boutonnet et al demonstrated an increase in the incidence of Mallampati Class III and IV from the eighth month of pregnancy to the beginning of labor, and during labor, and this incidence was not fully reversed up to 48 hours after delivery.4
Moreover, these changes occurred irrespective of any increase in body weight, duration of first and second stages of labor, or volume of IV fluid administered. In their study, they made observation at four time points: at 8 months of gestation (not in labor); when the epidural was placed; 20 minutes after delivery; and finally at 48 hours after delivery. They found no changes in score in 38.8% of their patients, however, in the remaining women, significant changes were observed both in the interval between the first nonlaboring assessment, and at placement of the epidural, and between placing the epidural and delivery. As illustrated in Figure 51-1, a progressive increase in Mallampati Class III and IV airway was seen.
Progression of Mallampati changes seen from late pregnancy to delivery in a study by Boutonnet et al.4
Similar results were observed by Kodali et al.5 As with the Boutonnet study,4 no correlation was observed between airway changes during labor and duration of labor, or fluids administered during labor.
Recent extensive research on the pathophysiology of upper airway obstruction revealed a significant role of lung volume reduction in pharyngeal narrowing.6 Obese parturients, a high-risk group for perioperative airway catastrophe, are prone to develop progressively narrower pharyngeal airways due to increase of soft tissue volume surrounding the pharyngeal airway, and decrease of lung volume during pregnancy. Lung volume reduction during general anesthesia is known to be more prominent and prolonged in obese patients.
The assessment of the pregnant patient must specifically address features that increase the risk of difficult laryngoscopic intubation, including receding mandible, limited mouth opening, short neck, limited neck movement, and high Mallampati Grade (III and IV). Taken together, these features are known to increase the likelihood of a difficult laryngoscopic intubation.7 Using the airway assessment strategies as described in Chapter 1 (MOANS, LEMON, RODS, and SHORT), this patient's airway assessment suggests possible difficult bag-mask-ventilation (BMV), possible difficult use of extraglottic device (EGD), and possible difficult surgical airway. But there are no other predictors of a difficult laryngoscopy and intubation.8,9
The standard of care in obstetrical anesthesia demands that the airway of this patient be secured in such a manner that the risk of aspiration is minimized, leaving the airway practitioner with two choices in this case: rapid-sequence induction (RSI) or an awake technique. An awake technique reduces the risk of a failed airway in an anesthetized and paralyzed patient. The decision to perform an awake intubation technique, rather than an RSI, should be based on clinical findings and the experience of the practitioner. In either case, contingency plans (Plans B and C) must be in place in the event that these techniques fail. One of the contingency plans must be a surgical airway, or cricothyrotomy.