Chapter 161: The Parturient for Nonobstetric Surgery

INTRODUCTION

Nonobstetric surgery during pregnancy occurs in approximately 1%–2% of pregnancies. This affects more than 80,000 female patients each year. Possible indications include obstetrical, nonpregnancy related, or fetal. The most common obstetrical surgery is cerclage to treat cervical incompetence. Nonobstetric surgeries are most often appendectomy, cholecystectomy, malignancy, or trauma. Fetal indications, including fetal surgery or ex utero intrapartum treatment (EXIT), are more common than previously.

Because there are a vast amount of alterations that occur during pregnancy to a woman’s body, both mechanically and physiologically, it is essential to understand the implications on anesthetic management. The anesthesiologist must also consider the impact to the fetus when a parturient undergoes surgery and anesthesia during pregnancy.

MATERNAL CONSIDERATIONS

Every organ system is affected by pregnancy either physiologically by hormones or by its impact from an enlarging uterus. The heart and cardiovascular system change significantly. Cardiac output increases by 50%, secondary to increases in both stroke volume and heart rate. There is an increase in blood volume with a decrease in systemic vascular resistance. Later in pregnancy, the enlarged uterus can compress the inferior vena cava and aorta. Particularly in the supine position, this leads to a decrease in venous return and hypotension and may also lead to supine hypotension syndrome. Therefore, it is important to use a right hip roll to facilitate left uterine displacement.

The respiratory system is also greatly impacted by pregnancy because of the mechanical compression of the uterus on the lungs and also due to hormonal changes. There is an increase in minute ventilation of 40% due to an increase in respiratory rate and tidal volume secondary to progesterone. The consequence is chronic respiratory acidosis with metabolic compensation. A normal arterial blood gas (ABG) of a parturient has a pH of 7.4, PaCO₂ of 30, and bicarbonate of 20. During times of controlled ventilation, minute ventilation should be increased to maintain a more physiologic PaCO₂ and thus normal pH. Also, due to the decrease in functional residual capacity and increase in oxygen consumption, parturients must be well pre-oxygenated because of the increased risk of desaturation during periods of hypoventilation and apnea. Parturients also have an increased incidence of difficult bag mass ventilation and intubation.

Patients should also be considered to have a full stomach and to be at increased risk of aspiration, regardless of their nil per os (NPO) status, after 18 weeks gestation.

Parturients also have a decrease in minimum alveolar concentration (MAC) of inhalation agents, as well as decrease local anesthetic requirements for neuraxial techniques and peripheral nerve blocks.

FETAL CONSIDERATIONS

Fetal Oxygenation and Uteroplacental Perfusion

The greatest determinants of fetal well-being are maternal blood pressure and oxygenation. Hypoxia and hypotension are of particular concern. Fetal oxygenation relies on maternal oxygenation; maternal arterial ...