**2. Anatomic and physiologic risk factors for airway complications in obstetric patients**

Pregnancy results in multiple anatomic and physiological changes, which impact airway management. While respiratory changes have the most significant effect, there are also gastroesophageal and other pregnancy-induced changes that increase the risk for difficult airway management.

### **2.1 Respiratory changes**

Increases in the renin-aldosterone system result from augmented estrogen and progesterone production during normal pregnancy cause a physiological fluid retention [11, 12]. In the respiratory tract, this may lead to narrowing of the airway, increasing risk for airway obstruction during ventilation, poor laryngoscopic views, and difficulty with tracheal intubation [2, 3]. Although airway changes develop gradually from first to third trimester of pregnancy, even more drastic changes may be observed at delivery and during labor. Studies have shown an increase in the Mallampati score and a decrease in upper airway volume on acoustic reflectometry during labor, presumably due to increasing soft tissue edema [13, 14]. Rather than relying solely on prelabor assessment, it is necessary to reevaluate the airway immediately prior to induction of general anesthesia as the airway edema may be exacerbated during the second stage of labor following fluid resuscitation [13, 15, 16]. Capillary engorgement of airway mucosal lining in pregnant women can increase the risk for bleeding during manipulation of the upper airway, especially in the nasal cavity [2, 3]. Consequently, many practitioners are reluctant to perform a nasal intubation as it has higher risk for epistaxis during pregnancy [2]. However, a 2011 review from Arendt et al. suggests that nasal intubation is acceptable with proper preparation of the nasal mucosa using topical vasoconstrictors, and the intubation is performed by the fiberoptic nasotracheal intubation technique [17]. However, an individual assessment is required prior to this procedure, as topical vasoconstrictors may have an impact on maternal hemodynamic and uteroplacental perfusion [2].

Physical derangements of pregnancy increase the risk of maternal and fetal hypoxemia during induction of anesthesia and subsequent airway manipulation. As the gravid uterus expands toward diaphragm, functional residual capacity (FRC) decreases by approximately 20–30% at term. This reduction comprises a 25% reduction in expiratory reserve volume (ERV) and a 15% reduction in residual volume (RV) [2, 18, 19]. The reduction is more prominent during a supine position. Although FRC decreases significantly, the closing capacity (CC) remains unchanged in pregnancy, resulting in a reduced FRC/CC ratio. Consequently, more rapid closure of the small airways may occur, increasing the risk of atelectasis [2, 3, 20]. In addition to the upward movement of the diaphragm, there are also other diaphragm changes that occur as pregnancy advances: lengthening of muscle fibers, an increase in the zone of apposition, and an increase in the radius of curvature of the diaphragm [21–23]. These changes contribute to an increase in ribcage dimension and concurrent tidal volume increase of up to 45% at term [21, 22, 24]. Along with the increased respiratory rate that is observed during pregnancy, an approximately 48% increase is observed in minute ventilation during the first trimester. Due to increased minute ventilation, maternal PaCO2 decreases and arterial pH increases causing a mild respiratory alkalosis (typically 7.42–7.44) [3]. As the pregnancy progresses, oxygen consumption also increases significantly [2, 3, 25]. An approximately 60% increase in oxygen demand

is observed during pregnancy as the fetus grows. Consequently, pregnant women are more susceptible to developing hypoxemia during the induction of GA [25].

### **2.2 Gastroesophageal changes**

During pregnancy, the esophagus, stomach, and pylorus are displaced cephalad by the enlarging uterus, decreasing the competence of the lower esophageal sphincter (LES). Elevated levels of progesterone and estrogen during pregnancy further reduce LES tone [2, 3, 18]. In addition, gastrin secretion increases during pregnancy, leading to increased gastric hydrogen ion production, and thereby increasing gastric pressure. This increase in gastric pressure, plus the incompetence of the LES, increases risks of regurgitation, aspiration, and the development of esophagitis and acid pneumonitis [2, 3]. Although the increased risk of regurgitation and aspiration does not necessarily result in difficult laryngoscopy and intubation, it has been reported to cause death during general anesthesia [26].

Gastric emptying time is not prolonged during pregnancy compared with the nonpregnant women [27]. However, it begins to prolong with the onset of labor, due to pain, anxiety, and the administration of analgesics. It also further contributes to the increased risk of regurgitation and aspiration in pregnant women. Therefore, it is prudent to consider all pregnant women at increased risk for pulmonary aspiration during labor [2, 3].

### **2.3 Other changes**

Most women gain between 10 and 15 kg during pregnancy due to increase in uterine size, fetal mass, fat deposition, blood, and interstitial fluid volume [18]. This may increase the risk of difficult airway management, as high BMI is associated with more difficult mask ventilation, laryngoscopy, and tracheal intubation, especially in short stature women. Moreover, a more rapid oxygen desaturation during the induction of GA is also associated with people with higher BMI. In addition to pregnancyrelated weight gain, breast enlargement during pregnancy is also associated with more difficult laryngoscopy. Therefore, optimizing intubation position is necessary to facilitate correct placement of the laryngoscope blade [2, 3].
