Preoxygenation is done because functional reserve lung capacity is reduced and the pregnant woman becomes hypoxemic more rapidly during periods of apnea. Obesity exacerbates this tendency (McClelland, 2009). To minimize hypoxia between the time of muscle relaxant injection and intubation, oxygen is introduced into the lungs in place of nitrogen. 1bis preoxygenation is accomplished by administering 100-percent oxygen via face mask for 2 to 3 minutes before anesthesia induction. In an emergency, four vital capacity breaths of 100-percent oxygen via a tight breathing circuit will provide similar beneit (Norris, 1985).
Almost all parturients are considered to have a full stomach, which necessitates a rapid-sequence induction. Namely, an intravenous anesthetic and rapid-onset muscle relaxant are simultaneously administered while cricoid pressure is applied by an assistant.
Of anesthetics, intravenous propofol or etomidate is widely used and ofers a smooth, rapid induction. Propofol is associated with a quick onset and recovery, and it may lower the incidence of nausea and vomiting. Since thiopental is no longer available, propofol is used as the primary agent for induction of general anesthesia with a reasonable safety record. Etomidate is the induction agent of choice for hemodynamically unstable parturients. Alternatively, ketamine can be used but is avoided in hypertensive women. For muscle relaxation, succinylcholine is an ultrafast-onset, short-acting agent commonly used in obstetrics. It ofers intense muscle relaxation to aid endotracheal intubation but also allows for the rapid return of spontaneous respiration in the case of failed intubation. Rocuronium is an alternative muscle relaxant if succinylcholine is contraindicated or unavailable. Its duration is much longer than succinylcholine unless its efect is reversed by sugammadex (Bridion), a specific binding agent recently approved by the FDA. To decrease the incidence of fetal respiratory depression, an intermediate or long-acting opioid is usually avoided upon induction of general anesthesia. The intense stimulation from direct laryngoscopy may worsen hypertension and tachycardia in certain women. Remifentanil, an ultrashort-acting narcotic, has been used during induction for cesarean deliveries with favorable maternal hemodynamics and fetal outcome (Heesen, 2013).
During induction and intubation, cricoid pressure is applied by a trained assistant to occlude the esophagus and thereby minimize regurgitation of the gastric contents-the Sellick maneuver. Positive mask ventilation during rapid sequence induction is typically avoided to lower the risk of increased intragastric pressure, which raises the risk of vomiting. Surgery should begin only after an airway is secured or, depending on the status of the mother and fetus, efective ventilation has been established.
Although uncommon, failed intubation is a major cause of anesthesia-related maternal mortality. A history of prior difficult intubation and a careful anatomical assessment of the neck and maxillofacial, pharyngeal, and laryngeal structures may help predict intubation complications. Even in cases in which the initial airway assessment was unremarkable, edema may develop intrapartum and present considerable challenges. Morbid obesity is another major factor for failed or diicult intubation. The American Society of Anesthesiologists Task Force on Obstetrical Anesthesia (2016) stresses the importance of appropriate preoperative preparation. This includes the immediate availability of specialized equipment such as diferent-shaped laryngoscopes, laryngeal mask airways, a fiberoptic bronchoscope, and a transtracheal ventilation set, as well as liberal use of awake oral intubation techniques.