The airway presents special concerns in obstetric patients. Attention to a few details, particularly in the setting of maternal cardiac arrest, may reduce morbidity and mortality. Airway management and its associated problems represent the greatest risk factor for anesthetic-related maternal deaths, the majority of which occur in the setting of an emergent cesarean section. Maternal death related to anesthesia most commonly results from aspiration of gastric contents or failure to intubate the trachea, resulting in hypoxia, which ultimately can result in cardiac arrest. 6 Mask ventilation can be difficult and ineffectual in obstetric patients because of low FRC, elevated diaphragms, and raised intraabdominal pressure. The incidence of failed intubations in pregnant patients is approximately 1 in 300 to 500 patients undergoing general anesthesia.7 Complicating the fact that pregnant patients develop hypoxia more quickly and are less tolerant of apneic periods, the airway poses more difficulties than in the general population. The potential factors accounting for these difficulties should be assessed prior to attempting an intubation.
Parturients in general are in an edematous state, which effects the tongue and supraglottic soft tissues. 8 The edema may compromise the airway lumen, making mask ventilation, laryngoscopy, and endotracheal intubation more challenging. Smaller endotracheal tubes may be needed to achieve a successful intubation and should be readily available. Mucosal engorgement and increased friability make the airway more likely to bleed and swell. This in itself can cause rapid deterioration. Decreased gastric emptying and diminished lower esophageal sphincter tone allow for increased gastric insufflation and result in a higher risk of aspiration during intubation. The intubating physician should make as few attempts as possible, to avoid making a difficult airway worse. Blind nasotracheal intubation is relatively contraindicated, and nasogastric tubes should generally be avoided, given the engorgement and friability of the mucosa in pregnant patients. Orogastric tubes may be used with caution in pregnant patients in cardiac arrest.
Several other physical conditions should be considered prior to intubating a parturient. Pregnant patients are likely to have full and intact dentition, and there may be little interdental distance in which to maneuver a laryngoscope. Obesity is relatively common in pregnant patients, causing relative neck extension when patients are supine, which results in greater anterior placement of the larynx. The neck is foreshortened, and there are often redundant pharyngeal and palatal folds in the airways of obese gravid women. In addition, enlarged and engorged breasts may obstruct placement of the laryngoscope in the mouth and the hand of an assistant attempting to maintain cricoid pressure.
The technique for intubating pregnant patients may require several modifications. As in any intubation, adjunctive equipment, including small endotracheal tubes, short laryngoscope handles, and stylets, should be readily available and familiar to the physician managing the airway. The patient should be placed in the supine position with the right hip elevated 10 to 12 cm to minimize aortocaval compression. The head and shoulders can be elevated with a pillow or folded sheets to achieve the sniffing position. This maneuver is particularly important in obese patients.
Use of rapid-sequence induction with cricoid pressure has become the standard of care for intubating pregnant women, particularly unstable patients with airway compromise. Administration of an induction agent, such as thiopental or etomidate, is followed by administration of succinylcholine, the muscle relaxant of choice, unless there is a contraindication to its use. It is helpful to allow a sufficient amount of time for muscle relaxants to take effect and for adequate preoxygenation prior to attempting laryngoscopy, given its potential hazards in pregnant patients. Preoxygenation prior to intubation is important because of the parturient's decreased FRC. However, hyperventilation may lead to respiratory alkalosis, which, in addition to shifting the oxyhemoglobin dissociation curve to the left, causes decreased uterine blood flow. Cricoid pressure must be carefully applied throughout the intubation procedure. In the case of failed intubation, more invasive maneuvers, such as percutaneous transtracheal jet ventilation or cricothyrotomy, may need to be performed in order to maintain oxygenation and prevent hypercarbia. Ventilator settings for pregnant patients are similar to those for nonpregnant patients, with minor modifications. Minute ventilation should aim to maintain a P co2 of approximately 30 mmHg. Significant respiratory alkalosis must be avoided to prevent decreased uterine blood flow.
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