GROWING CONCERN Ongoing physiologic changes throughout pregnancy produce a wide variety of unique challenges.

When a pregnant patient needs surgery that's unrelated to her pregnancy, understanding and managing the unique risks to both the mother and the unborn child are critical, even in the very early stages of pregnancy.

Mother and child
Every year, about 80,000 pregnant women in the United States — that's about 1 in 50 — require non-obstetric surgeries during pregnancy, with many taking place in outpatient settings. No currently used anesthetic agents have been linked to any teratogenic effects in humans when used at standard concentrations, according to the American Society of Anesthesiologists' statement on non-obstetric surgery during pregnancy, but you should take several other factors into consideration.

"? Respiratory system. Numerous physiologic changes in the airways of pregnant women underscore how important adequate pre-oxygenation is and how important it is to closely monitor oxygen levels during surgery.

As pregnancies progress and women gain weight, capillaries become engorged, leading to upper airway edema and decreased internal tracheal diameter. One result is that intubation failure rates for pregnant patients (1:280) are 8 times higher than those for non-pregnant patients (1:2,240). It's important to choose an appropriately sized endotracheal tube.

Pregnant patients also have decreased functional residual capacity, which increases the risk of desaturation. Though they consume more oxygen, their PaO2 levels stay largely unchanged. However, due to an increase in alveolar ventilation, they have an uncompensated respiratory alkalosis, with PaCO2 values ranging between 28 and 32mmHg. This relative hypocapnea is important to maintain during surgery, especially laparoscopic procedures, because maternal hypercapnea can result in fetal acidosis.

"? Cardiovascular system. Beginning at the very early stages of gestation, several factors can combine to require more aggressive resuscitation during surgical hemorrhage. Additionally, beyond the first trimester, place pregnant patients in lateral tilt during surgery.

Cardiac output begins to increase at 6 weeks and ultimately increases by 50%, with the uterus receiving about 20% of cardiac output at term. Meanwhile, blood pressure typically goes down, due to a decrease in systemic vascular resistance and placental shunting. There's also an increase in blood volume during the first trimester, but with no equal increase in red blood cell mass, which results in physiologic anemia. The combination of increased cardiac output and decreased reserve requires special diligence if hemorrhage occurs.

Aortocaval compression can begin during the second trimester, as the gravid uterus compresses the vena cava when the patient is lying down. This reduces venous return to the heart, which may reduce cardiac output up to 20%, causing not only symptomatic hypotension for the mother, but possibly fetal distress, secondary to decreased placental perfusion. As such, it's generally accepted that by 18 weeks, all pregnant patients should be placed in lateral tilt during surgery.

"? Gastrointestinal system. Pregnant women have a tendency toward reflux, due to anatomical changes in the gastrointestinal system. To reduce the risk of aspiration, after 18 to 20 weeks, it's a good idea to administer antacid prophylaxis.

"? Nervous system. Pregnancy increases sensitivity to inhaled and local anesthetics, but decreases sensitivity to vasopressors. The increased sensitivity to volatile anesthetics may require up to a 30% reduction in the minimum alveolar concentration, even as early as the first trimester. The decreased sensitivity to vasopressors requires larger doses.

PRACTICAL PEARLS
Keep Mother and Baby Safe

If you can't delay elective surgery until after pregnancy, here are tips for optimizing maternal and fetal well-being during surgery:

Multidisciplinary planning

• Arrange for pre- and post-operative fetal heart rate monitoring.
• If intraoperative fetal heart rate monitoring is used, arrange for additional personnel and instruments for emergency cesarean delivery.

Optimize maternal oxygenation

• Ensure adequate pre-oxygenation if general anesthesia is indicated.
• Choose an appropriately sized endotracheal tube.
• Avoid intraoperative hypoxemia, which can result in fetal hypoxemia.

Optimize maternal acid/base status

• Avoid intraoperative hypercapnea, which can result in fetal acidosis.
• Avoid intraoperative hypocapnea, which can result in decreased uterine blood flow and fetal hypoxemia.

Maintain maternal hemodynamics

• Maintain left uterine displacement after 18 weeks gestation.
• Use increased doses of vasopressor (phenylephrine is preferred) to manage hypotension.
• Use antacid prophylaxis after 18 weeks gestation to reduce the risk of aspiration.

Avoid teratogens

• Avoid hyperthermia, which is a known teratogen.
• Shield the abdomen during radiologic procedures.

Prevent preterm labor

• Prophylactic tocolytics are not indicated pre-operatively.
• Avoid dehydration.

— Paloma Toledo, MD, MPH

Additional considerations
Though elective surgery should always be postponed until after pregnancy, indicated surgery can be done during any trimester; however, the risks of spontaneous abortion and preterm delivery are lowest during the second trimester.

All pregnant patients should have fetal heart rate monitoring. The American College of Obstetrics and Gynecology (ACOG) recommends pre- and post-operative fetal monitoring for pre-viable fetuses, and simultaneous pre- and post-operative fetal and uterine contraction monitoring for viable fetuses.

Whether to use intraoperative fetal monitoring depends on fetal viability, the type of surgery being performed, and whether facilities and staff are available for cesarean delivery and neonatal care.

Remember also that many patients are concerned about how medications used during surgery might affect their unborn babies. Embryogenesis is the highest-risk period for structural changes, and there are several known teratogens, such as hyperthermia, radiation and certain medications, such as Coumadin. Pre-operative patient counseling is important to dispel any misconceptions or fears about properly administered anesthetics.