Chapter 42

• The minimal 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; dose requirements may be reduced as much as 30%.
• Obstruction of the inferior vena cava by the enlarging uterus distends the epidural venous plexus and increases the risk of intravascular injection during epidural anesthesia.
• Up to 20% of women at term develop the supine hypotension syndrome, which is characterized by hypotension associated with pallor, sweating, or nausea and vomiting.
• The reduction in gastric motility and the tone of the gastroesophageal sphincter as well as hypersecretion of gastric acid place the parturient at high risk for regurgitation and pulmonary aspiration.
• Ephedrine, which has predominantly β-adrenergic activity, has traditionally been considered the vasopressor of choice for hypotension during pregnancy. However, clinical studies suggest that α-adrenergic agonists such as phenylephrine and metaraminol are just as effective in treating hypotension in pregnant patients and are 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 prelabor values.
• Current techniques employing very dilute combinations of a local anesthetic (eg bupivacaine 0.125% or less) and an opioid (eg, fentanyl 5 Âµg/mL or less) for epidural or combined spinal–epidural (CSE) analgesia do not appear to prolong labor or increase the likelihood of a cesarean section.
• Because the maturation of the lungs occurs later in fetal development, extrauterine life is not possible until after 24–25 weeks of gestation, when pulmonary capillaries are formed and come to lie in close approximation to an immature alveolar epithelium.

Pregnancy produces profound physiological changes that alter the usual responses to anesthesia. Moreover, anesthetic care of the pregnant patient is unique in that two patients are cared for simultaneously: the parturient and the fetus. Failure to take these facts into consideration can have disastrous consequences.

This chapter reviews the normal physiological changes associated with pregnancy, labor, and delivery. Uteroplacental physiology and its response to common anesthetic agents are also discussed. Much of this knowledge forms the basis for current anesthetic practices for labor and delivery (see Chapter 43). Lastly, care of the neonate in the obstetric suite or the intensive care unit requires an understanding of the physiological transition from fetal to neonatal life.

Pregnancy affects virtually every organ system (Table 42–1). Many of these physiological changes appear to be adaptive and useful to the mother in tolerating the stresses of pregnancy, labor, and delivery. Other changes lack obvious benefits but nonetheless ...

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