- 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
- 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
- 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
- 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
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 ...