Airway Changes during Labor and Delivery

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Abstract
THE incidence of failed tracheal intubation in the pregnant population is perhaps eight times higher than in the nonpregnant population.1 Difficult or failed intubation after induction of general anesthesia for cesarean delivery remains the major contributing factor to anesthesia-related maternal complications.2,3 The first national study of anesthesia-related maternal mortality in the United States revealed that 52% of the deaths resulted from complications of general anesthesia predominantly related to airway management problems.4 Despite decreases in the number of obstetric general anesthetics and better awareness of obstetric airway difficulties, a recent survey has shown that the incidence of difficult intubation and subsequent complications have not diminished with time.5 Furthermore, a critical evaluation of anesthesia-related maternal deaths in Michigan, 1985–2003, showed that airway obstruction or hypoventilation during emergence and extubation were the cause of five maternal deaths.6 Obvious factors such as enlarged breasts have been implicated, but simple maneuvers for dealing with these problems did not seem to decrease the incidence of difficult intubation.5,7,8 There is no known bone or joint abnormalities in pregnancy that could pose difficulties during intubation. However, soft tissue changes such as airway edema are an invariable association of pregnancy, and this may contribute to difficult intubation.8 Pilkington et al. 8 demonstrated that airway edema can increase during the course of pregnancy and result in increases in Mallampati score. There is speculation and an anecdotal report that airways can also change during labor and delivery.9 There is no comprehensive study that has prospectively evaluated airway changes in pregnant women undergoing labor and delivery. To evaluate airway changes associated with labor and delivery, we undertook two studies that used two different approaches of airway evaluation. In the first study, we used the conventional Samsoon modification of the Mallampati score to evaluate airway changes, and in the other, we used acoustic reflectometry to objectively analyze airway changes. Acoustic reflectometry offers a distinct advantage of studying the concealed portion of the upper airway extending from the uvula to the glottis (pharyngeal volume), in addition to the visible portion (oral volume). The Samsoon modification of the Mallampati airway classification is based on the visibility of the soft palate, faucial pillars, and uvula.1,10 The visibility of structures is dependent on the relative capacity of the oropharyngeal cavity and the volume of the base of the tongue.10 If the base of the tongue is disproportionately large, it can mask the visibility of faucial pillars and the uvula (increasing airway class) by encroaching into the oropharyngeal cavity.10 This leads to a relative decrease in the oropharyngeal volumes. Therefore, the outcomes of the two studies, airway evaluation by the Samsoon modification of the Mallampati airway classification and airway volume determinations by acoustic reflectometry, can complement each other in determining changes in the airway during labor.

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