As many as 2% of pregnant women undergo anesthesia for non-obstetric surgery [Rosen MA. Anesthesiology 91: 1159, 1999]. Routine pregnancy testing should be mandatory for all women of child-bearing age. Note that organogenesis occurs from 15-56 days of gestation. Early concerns for the pregnant patient revolve around the potential for altered organogenesis, but as the fetus matures concerns shift towards fetal hypoxia (as metabolic demands increase) and accidental induction of premature labor.
A total of five studies have attempted to understand the complex relationship between surgery and anesthesia and pregnancy [Smith BE. Anesth Analg 42: 521, 1963; Shnider SM et al. Am J Obstetr Gynecol 92: 891, 1965; Brodsky JB et al. Am J Obstetr Gynecol 138: 1165, 1980; Duncan PG et al. Anesthesiology 64: 790, 1986; Mazze RI and Kallen B. Am J Obstetr Gynecol 161: 1178, 1989]. Taken as a whole, there does not appear to be a relationship between surgery or anesthesia and congenital abnormalities, but there is a significant relationship between surgery and/or anesthesia and fetal death [Barash, PG. Clinical Anesthesia, 5th ed. (Philadelphia), p. 1176, 2006]. The Duncan study, of 2565 pregnant Canadian women (matched based on age, region, etc.), was notable in that it showed a statistically significant increase in the risk of spontaneous abortion in both the first and second trimesters (from 6.5 to 7.1%) [Duncan PG et al. Anesthesiology 64: 790, 1986]. The Mazze and Kallen study is analyzed 5,405 Swedish births and found no increased incidence of congenital anomalies in children whose mothers had gestational surgery, although they did find an increased incidence of VLBW infants as well as mortality within 168 hours of surgery, both of which the authors postulated might have been due to the illness and not the surgery/anesthesia [Mazze RI and Kallen B. Am J Obstetr Gynecol 161: 1178, 1989]
Reviewed elsewhere (Physiologic Changes of Pregnancy). Critical points include decreased MAC and decreased local anesthetic requirements, lower BP, increased CO, aortocaval compression, engorged mucous membranes, lower baseline pCO2, decreased FRC and increased metabolic rate, reduced LES pressure.
While virtually every drug available for any indication can be shown to be teratogenic in some animal model at some dose, there is no evidence that any current anesthetic agents are teratogenic in humans at doses used [Stoelting RK. Basics of Anesthesia, 5th ed. Elsevier (China), p. 498, 2007]. Unfortunately, the anesthetic risks of volatile agents is based on non-optimal data, which is unlikely to change given the ethical implications of conducting such a study. [Rosen MA. Anesthesiology 91: 1159, 1999]
Drugs thought to be safe at doses used in humans include volatiles (but not N2O?), opioids, paralytics, barbiturates, and propofol.
Drugs thought to be dangerous include midazolam (cleft palate) and cocaine (growth retardation).
Fetal Hypoxia (and Acidosis)
Maintain uterine oxygenation by avoiding hypotension (left side positioning) and maintaining oxygenation. High maternal oxygen values do not predispose to retrolental fibroplasia because by the time maternal oxygen makes it through the placenta (which has a high metabolic rate), it has dropped to safe levels – a maternal PaO2 of 500 mm Hg will result in a fetal PaO2 of 60 mm Hg.
According to Rosen’s review, the uterine circulation is not autoregulated, represents 10% of cardiac output by term, and remains sensitive to vasopressors. Alpha-adrenergic agents, dopamine, and epinephrine can potentially reduce uterine blood flow, thus uterine displacement, fluid bolus, Trendelenburg position, leg elevation, compression stockings, or any combination of these, remain the agents of choice in the initial management of maternal hypotension. [Rosen MA. Anesthesiology 91: 1159, 1999]
Left uterine displacement can be helpful. Avoid excessive peritoneal pressures during laparoscopy (and maintain pCO2 at 32 mm Hg if possible).
The pathology leading to surgery, and not the operation itself, usually leads to preterm labor. If surgery must occur during a pregnancy, and if the patient and her obstetrician are willing to section intraoperatively, one should monitor uterine activity and fetal heart rate intraoperatively. Intraoperative preterm labor can potentially be treated with terbutaline (ß2-agonist, can cause hypokalemia and arrhythmias), magnesium, indomethacin (can close the ductus), or CCBs.
Defer elective surgery until after delivery. Surgery, if mandatory, should be attempted in the second trimester, as the combined risks of abnormal organogenesis and premature labor are minimized (but still non-zero). First trimester surgery should be done under regional or central neuraxial (esp. spinal) if possible. Volatile agents do not compromise uterine blood flow as long as blood pressure is maintained [Stoelting RK. Basics of Anesthesia, 5th ed. Elsevier (China), p. 499, 2007]. Uterine displacement should be utilized in all patients after the first trimester. Policies about fetal heart rate monitoring before and after surgery, or during surgery, varies between institutions. Generally, it is not done prior to viability (somewhere around 24 weeks gestation). Coordination with the obstetric team and confirmation of policies is advised if not known.