Section 4: Subspecialty Management
Chapter 57: Anesthesia for Obstetrics

ANESTHESIA FOR SURGERY DURING PREGNANCY

Pregnant women undergoing surgery require special attention in their anesthetic management if injury to the fetus is to be avoided. Goals include maternal safety, avoidance of teratogenic drugs, avoidance of intrauterine fetal asphyxia, and prevention of preterm labor. Many of the physiologic changes of pregnancy are due to hormonal influence and may occur early in pregnancy. Significant changes in minute ventilation, functional residual capacity, cardiac output, and anesthetic requirements occur during the second and third trimesters. Similarly, the hypotensive syndrome associated with the supine position begins to manifest itself early in the third trimester and may lead to decreases in cardiac output, blood pressure, and uterine blood flow.

Most commonly used anesthetic and sedative drugs are teratogenic in some animal species. The applicability of many of these animal studies to human subjects is not clear. Studies in humans have shown an association between increased risk of congenital anomalies and ingestion of minor tranquilizers, such as diazepam (Valium). 407  However, a direct link between the anomaly and the specific drug is difficult to confirm because of confounding issues (including the reason for taking the medication).

Long-term exposure to trace amounts of anesthetic gases or vapors had been suggested as having an adverse effect on reproductive outcome (including spontaneous abortion). However, a review of 14 studies from 1967 to 1982 concluded that no adverse effect of anesthetic gases on reproductive outcome could be accepted on the basis of the retrospective inquiries because of important methodologic problems, including unclear outcomes, poor survey response rates, selection bias, recall bias, and lack of control of confounding variables. 408 

All surveys of women who have received anesthesia for operations during pregnancy have failed to indict any anesthetic as a teratogen. 409, 410, 411, 412, 413, 414  However, in all studies to date, the number of pregnant women receiving an anesthetic for an operative procedure is, in fact, likely too small to state categorically that anesthetics are not teratogenic.

In a large survey, Mazze and Kallen 414  combined data from three Swedish health care registries for the years 1973 to 1981. Adverse outcomes examined were (1) congenital anomalies, (2) stillborn infants, (3) infants dead at 168 hours, and (4) infants with very low and low birth weights. There were 5,405 operations in the population of 720,000 pregnant women (operation rate, 0.75%). Of these, 2,252 operations were performed in the first trimester, and 65 percent received general anesthetics, almost all of which included nitrous oxide. The results are summarized in Figure 57–29 . The incidences of congenital malformations and stillbirths were not increased in the offspring of women undergoing operations. However, the incidences of very low and low-birth-weight infants were increased, as a result of both prematurity and intrauterine growth retardation. The incidence of infants born alive but dying within 168 hours was increased. No specific types of anesthesia or operation were associated with increased incidences of adverse reproductive outcomes.

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FIGURE 57–29 Total number of observed and expected adverse outcomes among women having nonobstetric operations during pregnancy. Incidence of infants with birth weights under 1,500 g and of infants born alive and dying within 168 hours of birth were significantly increased (P < .05) in the surgical patients. (From Mazze and Kallen414 )

Some reports have suggested that anesthesia and surgery during pregnancy are associated with the onset of preterm labor. 410  No one agent or technique has been associated with a higher incidence of premature delivery. However, the halogenated anesthetics decrease uterine contractility; if uterine manipulation is anticipated, the use of these agents would theoretically be more likely to minimize the possibility of preterm labor. Alternatively, tocolytic drugs can be given in conjunction with the anesthetic for surgery.

During the anesthetic visit before surgery during pregnancy, great effort should be made to allay maternal anxiety and apprehension. The lack of documented teratogenicity can be presented. The patient should understand that the likelihood of first-trimester miscarriage increases from 5.1 percent without surgery to 8 percent with surgery, 411  and the incidence of premature delivery increases from 5.13 percent without surgery to 7.47 percent with surgery. 414  Although it is clear that there is significant risk to the fetus when an operation is performed during pregnancy, it is not clear whether the hazard is due to the surgery, the pathology for which the surgery was necessary, or the anesthetic.

Elective surgery should be deferred until after delivery when the physiologic changes of pregnancy have returned toward normal. Women of child-bearing age who are scheduled for elective surgery should be carefully queried regarding the possibility of pregnancy. Urgent surgery (i.e., operations that are essential but can be delayed without increasing the risk of permanent disability) should be deferred until the second or third trimester. Despite the lack of proof that particular drugs should be avoided, we consider it prudent to minimize or eliminate fetal exposure to drugs during the vulnerable first trimester.

Emergency surgery (i.e., operations that cannot be delayed without increasing the risk of maternal morbidity or mortality) may be necessary at any time during pregnancy. The minimal drug exposure conferred by regional anesthesia (particularly spinal anesthesia) makes these techniques theoretically preferable; however, general anesthesia should not be withheld if it will accomplish the best surgical conditions. If general anesthesia is necessary during the first trimester, there is no proof that any well-conducted technique is superior to any other. Adequate oxygenation and avoidance of hyperventilation are mandatory. During pregnancy, patients may be at increased risk of aspiration, and the usual safeguards to prevent aspiration pneumonitis should be observed. Aortocaval compression during the second and third trimesters should be prevented by avoiding the supine position. Ideally, continuous FHR monitoring during surgery should be employed if possible after the middle of the second trimester. This may provide an indication of abnormalities in maternal ventilation or uterine perfusion. Uterine activity should be monitored continuously with an external tocodynamometer during the postoperative period to detect the onset of preterm labor.