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ANESTHESIA ALERT
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.
