Anesthesia for Nonobstetric Procedures During Pregnancy

Timing of Surgery

The ASA/ACOG joint practice statement on nonobstetric surgery during pregnancy addresses the optimal timing for procedures during pregnancy. First, it is clear that if a surgery is necessary, it should not be denied solely because of pregnancy. However, elective nonobstetric surgery (including cardiac surgery) should be postponed until the postpartum period. If surgery must be performed during pregnancy, the second trimester is the optimal time because the risks of spontaneous abortion and preterm labor are both at their lowest.

The evidence for timing is overall lacking in robust data, with the bulk of evidence in retrospective reviews and outcome studies. Some of the strongest data come from a large Swedish registry, in which more than 5000 women underwent surgery during pregnancy. From these data, there was an increase in low- and very-low-birth-weight infants because of prematurity and intrauterine growth restriction for procedures performed in any trimester. Regardless of the trimester, there was no observed increase in congenital anomalies, even in first trimester operations under general anesthesia, which accounted for more than 50% of cases.

Other subsequent reviews support this study and provide the basis for the ACOG guidelines. Overall, there is an increase in spontaneous abortion associated with first trimester surgeries and an increase in preterm labor with third trimester surgeries. Appropriate patient counseling should be done if procedures must be performed. As always, a multidisciplinary risk and benefit discussion should be undertaken to make these difficult decisions. If maternal surgery is necessary during the third trimester, consideration of whether to perform a cesarean section beforehand is warranted. This should be an individualized decision based on the risks and benefits in each particular case. If surgery is performed after 28 weeks, steroids may be administered to stimulate fetal lung maturity in the event of premature labor and delivery. Because of the increased risk of venous thromboembolism during pregnancy, these patients should be screened and receive appropriate prophylaxis.

Anesthetic Preevaluation

A thorough preoperative examination, with particular attention to the cardiac status, airway, and stage of gestation, should be performed. The physiologic changes of pregnancy have a significant effect on anesthetic management.

The upper airway experiences changes because of increased circulating progesterone. Increased blood flow leads to mucosal edema, capillary congestion, and tissue friability. Pregnant women are therefore at increased risk for nasal and oral mucosal bleeding, especially with airway instrumentation. Airway assessment is critical in parturients. The reported rate of failed intubation in obstetric patients is approximately 1 in 250 to 300, which is eight times higher than in the general population. Multiple studies have documented that changes in Mallampati score, a method of predicting difficulty of intubation, occur throughout pregnancy and during labor.

The physiologic changes to the cardiovascular system have been well reviewed elsewhere. For each particular patient, the specific nature of her cardiac lesion and her current functional status should be noted. Results of any recent cardiac testing, including electrocardiogram, echocardiogram, cardiac catheterization, or any other relevant information, should also be reviewed.

The gravid uterus and mechanical changes in the thorax result in alterations in lung volumes and capacities. In addition, because of the increased metabolic demand imposed by the fetus, increased minute ventilation is necessary. This is accomplished by both an increase in tidal volume and respiratory rate. Despite the observed diaphragmatic elevation, diaphragmatic excursion is actually increased. Vital capacity is maintained at prepregnancy levels. Whereas tidal volume and inspiratory reserve volume both increase, reserve volume and expiratory reserve decrease. This leads to a decreased functional residual capacity (FRC). The decrease in FRC becomes very important during intubation of a parturient because this contributes to rapid desaturation. The increased minute ventilation seen during pregnancy leads to characteristic changes in arterial blood gas values. PaO ₂ increases slightly to 100 to 106 mm Hg while PaCO ₂ decreases to approximately 30 mm Hg. This leads to a mild respiratory alkalosis, with an average pH of 7.44. These changes are usually present by the end of the first trimester.

Pregnant women undergo several physical and physiologic changes that increase their risk of aspiration. The expanding uterus displaces the stomach cephalad and increases pressure in the stomach. In addition, increased circulating progesterone relaxes the lower esophageal sphincter. Both of these changes increase the incidence of gastroesophageal reflux disease, which affects up to 50% of pregnant women. Because of these changes, the risk of aspiration during airway manipulation is increased. Most experts consider pregnant women to have a full stomach regardless of the time of last oral intake. Several options for premedication are available to decrease the risk of aspiration or mitigate the severity of aspiration if it occurs. Commonly used interventions include H2 antagonists and sodium citrate.

Selection of Anesthetic Method

The choice of anesthetic technique is highly dependent on the surgery being performed, with the goal of providing the safest patient care while optimizing procedural conditions. The risks and benefits should always be weighed to determine the best plan on a case-by-case basis. General anesthesia poses the highest potential risk because of the need for airway manipulation, aspiration risk, and increased systemic drug exposure to the fetus. If regional or neuraxial anesthesia is appropriate for the surgery, it is preferred. Both regional and neuraxial techniques rely entirely on local anesthetics to block sensory nerve conduction; therefore, minimal medication enters the maternal bloodstream, limiting the drug exposure to the fetus. However, many surgeries cannot be performed feasibly under regional anesthesia. In these cases, general anesthesia can be safely performed with attention paid to minimizing the risks.