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.
Pregnant patients display enhanced sensitivity to local anesthetics during regional anesthesia and analgesia, and neural blockade occurs at reduced concentrations of local anesthetics; dose requirements may be reduced by as much as 30%.
Obstruction of the inferior vena cava by the enlarging uterus distends the epidural venous plexus and increases epidural blood volume.
Approximately 5% of women at term develop supine hypotension syndrome, which is characterized by hypotension associated with pallor, sweating, or nausea and vomiting.
The reduction in gastric motility and gastroesophageal sphincter tone places the parturient at high risk for regurgitation and pulmonary aspiration.
Ephedrine, which has considerable β-adrenergic activity, has traditionally been considered the vasopressor of choice for hypotension during pregnancy. However, clinical studies suggest that the α-adrenergic agonist phenylephrine is more effective in treating hypotension in pregnant patients and is associated with less fetal acidosis than ephedrine.
Volatile inhalational anesthetics decrease blood pressure and, potentially, uteroplacental blood flow. In concentrations of less than 1 MAC, however, their effects are generally minor, consisting of dose-dependent uterine relaxation and minor reductions in uterine blood flow.
The greatest strain on the parturient’s heart occurs immediately after delivery, when intense uterine contraction and involution suddenly relieve inferior vena caval obstruction and increase cardiac output as much as 80% above late third-trimester values.
Current techniques employing dilute combinations of a local anesthetic (eg, bupivacaine, ≤0.125%) and an opioid (eg, fentanyl, ≤5 mcg/ml) for epidural or combined spinal–epidural (CSE) analgesia do not appear to prolong the first stage of labor or increase the likelihood of an operative delivery.
This chapter reviews the normal physiological changes associated with pregnancy, labor, and delivery. It concludes with a description of the physiological transition from fetal to neonatal life.
PHYSIOLOGICAL CHANGES DURING PREGNANCY
Pregnancy affects most organ systems (Table 40–1). Many of these physiological changes appear to be adaptive and useful to the mother in tolerating the stresses of pregnancy, labor, and delivery.
Table Graphic Jump Location TABLE 40–1Average maximum physiological changes associated with pregnancy.1 ||Download (.pdf) TABLE 40–1 Average maximum physiological changes associated with pregnancy.1
|Parameter ||Change |
+20 to 50%
Systolic blood pressure
Diastolic blood pressure
+30 to 250%