Chapter 29: Maternal & Fetal Physiology & Obstetric Anesthesia

The editors would like to acknowledge that this chapter is abridged from a chapter originally written by Dr. Michael A. Frölich.

PHYSIOLOGICAL CHANGES DURING PREGNANCY

Central Nervous System Effects

The minimum alveolar concentration (MAC) progressively decreases during pregnancy—at term, by as much as 40%—for all general anesthetic agents; MAC returns to normal by the third day after delivery. Progesterone, which is sedating when given in pharmacological doses, increases up to 20 times normal at term and is at least partly responsible for this phenomenon. A surge in β-endorphin levels during labor and delivery also likely plays a major role.

Pregnant patients display enhanced sensitivity to local anesthetics during regional anesthesia and analgesia, and neural blockade occurs at reduced concentrations of local anesthetics. The term minimum local analgesic concentration (MLAC) is used in obstetric anesthesia to compare the relative potencies of local anesthetics and the effects of additives; MLAC is defined as the local analgesic concentration leading to satisfactory analgesia in 50% of patients (EC₅₀). Local anesthetic dose requirements during epidural anesthesia may be reduced as much as 30%, a phenomenon that appears to be hormonally mediated but may also be related to engorgement of the epidural venous plexus. Obstruction of the inferior vena cava by the enlarging uterus distends the epidural venous plexus and increases epidural blood volume. The latter has three major effects: (1) decreased spinal cerebrospinal fluid volume, (2) decreased potential volume of the epidural space, and (3) increased epidural (space) pressure. The first two effects enhance the cephalad spread of local anesthetic solutions during spinal and epidural anesthesia. Bearing down during labor further accentuates all these effects. Positive (rather than the usual negative) epidural pressures have been recorded in parturients. Engorgement of the epidural veins also increases the likelihood of placing an epidural needle or catheter in a vein, resulting in an unintentional intravascular injection.

Respiratory Effects

Oxygen consumption and minute ventilation increase progressively during pregnancy. Tidal volume and, to a lesser extent, respiratory rate and inspiratory reserve volume also increase. By term, both oxygen consumption and minute ventilation have increased up to 50%. PaCO₂ decreases to 28–32 mm Hg; significant respiratory alkalosis is prevented by a compensatory decrease in plasma bicarbonate concentration. Hyperventilation may also increase PaO₂ slightly. Elevated levels of 2,3-diphosphoglycerate offset the effect of hyperventilation on hemoglobin’s affinity for oxygen. The P₅₀ for hemoglobin increases from 27 mm Hg to 30 mm Hg; the combination of the latter with an increase in cardiac output (see next section on Cardiovascular Effects) enhances oxygen delivery to tissues.

The maternal respiratory pattern changes as the uterus enlarges. In the third trimester, the elevation of the diaphragm is compensated by an increase in the anteroposterior diameter of the chest; diaphragmatic motion, however, is not restricted. Both vital capacity and closing capacity are minimally affected, but ...