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ANESTHESIA FOR NONOBSTETRIC SURGERY DURING PREGNANCY

It is estimated that approximately 2% of pregnant women undergo nonobstetric surgery in the United States annually. Most of these procedures are nonelective and sometimes life-threatening. Such situations are challenging to both the anesthesiologist and the surgeon. Anesthetic issues to be considered include maternal risk factors resulting from the physiologic and anatomic changes of pregnancy, the teratogenic potential of anesthetic agents, maintenance of adequate uteroplacental blood flow, and the direct and indirect effects of maternally administered agents on the fetus. Surgical management of such patients is also more complicated than in the nonpregnant state. The diagnosis of abdominal pathology is compounded by anatomic displacement of abdominal organs secondary to the gravid uterus. Abdominal tenderness and leukocytosis are often normal findings during pregnancy.

A review of combined data from three Swedish health care registries from 1973 to 1981 by Mazze and Kallen detailed the types and rates of surgery during each trimester.^[275] The most common abdominal procedures included appendectomy, cholecystectomy, and adnexal surgery ( Table 58-12 ). Other less frequent, but more challenging situations include laparoscopic surgery, neurosurgery, cardiac surgery requiring cardiopulmonary techniques, and more recently, fetal surgery.

Specific risks for the mother and fetus undergoing surgery include fetal loss, fetal asphyxia, premature labor, premature rupture of membranes, the potential for failed intubation, and thromboembolic phenomena. Surgery for obstetric indications is associated with a higher risk of perinatal mortality.

Trauma

Injury related to trauma occurs in up to 6% to 7% of all pregnancies and is perhaps the most common cause of nonobstetric maternal mortality. Motor vehicle accidents are responsible for most injuries, followed by domestic abuse and, to a lesser extent, falls. In contrast to nonpregnant women, abdominal injury during pregnancy is more likely than head injury.^{[276] [277]}

Rapid assessment, hemodynamic stabilization, and treatment of maternal injuries are essential for fetal survival. It is important to remember that the anatomic and physiologic changes associated with pregnancy may cause the clinician to underestimate the true extent of hypovolemia. For instance, shock in a pregnant patient may not be clinically evident until 25% to 30% of maternal blood volume is lost; at this point, the fetus may already be in jeopardy. In hemorrhagic shock, maternal blood is shunted away from the uterus to preserve perfusion to vital maternal organs at the expense of the fetus; such a physiologic response causes fetal hypoxemia and even death.^[278]

Regardless of the clinical situation, a preoperative assessment that includes airway evaluation should be performed. The choice of regional or general anesthesia techniques should be based on the clinical status, surgical procedure, experience of the anesthesiologist, and the psychological condition of the patient.

Aspiration prophylaxis should be administered to all patients over 14 weeks' gestation because physiologic changes at the lower esophageal sphincter enhance the risk of aspiration. An H₂ -antagonist should be given 1 hour before surgery if possible and a nonparticulate antacid such as sodium citrate just before induction of anesthesia. Use of a prokinetic agent such as metoclopramide, 10 mg intravenously, may also enhance gastric emptying. It is imperative to position the patient correctly after the second trimester to avoid aortocaval compression by the gravid uterus; correct positioning may be accomplished by placing a wedge under the right hip.

Monitoring of the fetus perioperatively is important, but not always feasible, especially during abdominal surgery. External FHR monitoring is usually possible from 18 weeks onward. Whether intraoperative FHR monitoring can affect fetal outcome remains controversial. A recent literature review questioned the need to monitor FHR changes during surgery because of the lack of alterations in fetal outcome.^[279] However, alterations in FHR may indicate adverse maternal conditions before they become apparent with standard monitoring. Such alterations should therefore encourage evaluation of maternal oxygenation, hemodynamics, acid-base status, and activities at the surgical field for compromise of uterine perfusion. It is advisable to document FHR before and after institution

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If general anesthesia is necessary, a rapid-sequence technique with adequate preoxygenation, cricoid pressure, and endotracheal intubation should be used to minimize the risk of aspiration for any woman after 14 to 16 weeks' gestation. Drugs administered should be chosen for their known safety in pregnancy. Such agents include thiopental, depolarizing and non-depolarizing muscle relaxants, opioids (fentanyl, morphine, and meperidine), inhaled agents, and 50:50 O₂ /N₂ O mixtures. Maternal PaCO₂ should be maintained in the normal range for pregnancy (30 mm Hg) because maternal hyperventilation may reduce placental blood flow. The patient should not be extubated until awake because there is still a risk of aspiration at the end of the procedure. Uterine activity should be monitored into the postoperative period, and tocolytic drugs may be required.

Nonsteroidal anti-inflammatory drugs should be avoided after the first trimester because some of these agents may constrict or close the fetal ductus arteriosus in the later stages of pregnancy.^[280]