Physiology:
Pain relief from uterine contractions and cervical dilation in stage 1 of labor involves thoracic nerve roots, T10 to T12. Pain relief from perineal distention in stage 2 of labor involves sacral nerve roots, S2 to S4.
- Pregnancy predisposes to hypoxia because of decreased functional residual capacity.
- Placental transfer of medications exposes the fetus to lipid-soluble anionic substances.
- Antacids should be given prophylactically because of delayed gastric emptying time in pregnancy.
- Uterus should be laterally displaced to avoid inferior vena cava compression in the supine position.
Anesthetic options during labor
Intravenous agents
Intravenous agents include sedatives, which are frequently given in the active phase of labor. Advantages include ease of administration and inexpensive cost. Disadvantages include neonatal depression if given close to delivery. The neonate may need administration of naloxone to reverse the effect.
Paracervical block
Paracervical block is a mode of conduction anesthesia that involves bilateral transvaginal local anesthetic injection to block Frankenhauser’ ganglion lateral to the cervix. It is administered in the active phase of labor. Disadvantages include temporary high levels of local anesthetic in the uterus that may lead to transitory fetal bradycardia, which is managed conservatively.
Pudendal block
Pudendal block is a mode of conduction anesthesia that involves bilateral transvaginal local anesthetic injection to block the pudendal nerve as it passes by the ischial spines.It is administered in stage 2 of labor to provide perineal anesthesia.
Epidural block
Epidural block is a mode of conduction anesthesia that involves injection of local anesthetic into the epidural space to block the lumbosacral nerve roots during both stages 1 and 2 of labor. Advantages include use for either vaginal delivery or cesarean section. Disadvantages include patchy block from nonuniform spread of the local anesthetic around the nerve roots. Complications include hypotension from peripheral vascular dilation owing to sympathetic blockade and spinal headache from inadvertent dural puncture, as well as CNS bleeding or infection (rare). Hypotension is treated with IV fluids and IV ephedrine. Spinal headache is treated with IV hydration, caffeine, or blood patch.
Spinal block
Spinal block is a mode of conduction anesthesia that involves injection of local anesthetic into the subarachnoid space to block the lumbosacral nerve roots. It is used as a saddle block for stage 2 of labor and for cesarean delivery. Advantages are complete predictable anesthesia. Complications include hypotension from peripheral vascular dilation because of sympathetic blockade (common) and spinal headache
(rare), as well as CNS bleeding or infection (rare).
General anesthesia
General anesthesia is seldom used for vaginal delivery and rarely for cesarean section. Indications include need for rapid emergency delivery and maternal medical conditions in which conduction anesthesia is unsafe (e.g., blood dyscrasia, thrombocytopenia). Complications include aspiration pneumonia, atelectasis, and uterine atony (associated with inhalation agents, e.g., halothane, enflurane).
