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Chapter 2

During birth, the status of the fetus can be influenced by obstetric analgesia and anesthesia. Care in choosing analgesic and anesthetic agents can often prevent respiratory depression in the newborn, especially in high-risk deliveries.

1. Placental transfer of drugs. Drugs administered to the mother may affect the fetus via placental transfer or, less commonly, may cause a maternal disorder that affects the fetus (eg, maternal drug-induced hypotension may cause fetal hypoxia). All anesthetic and analgesic drugs cross the placenta to some degree. Flow-dependent passive diffusion is the usual mechanism.

Most anesthetic and analgesic drugs have a high degree of lipid solubility, a low molecular weight (<500), and variable protein-binding and ionization capabilities. These characteristics lead to a rapid transfer across the placenta. Local anesthetics and narcotics (lipid-soluble, un-ionized) cross the placenta easily, whereas neuromuscular blocking agents (highly ionized) are transferred slowly.

2. Analgesia in labor

1. Inhalation analgesia. Inhalation analgesia is rarely used in the United States due to the availability of regional anesthesia and because several problems limit its routine use:

• The need for specialized vaporizers.

• The concern regarding pollution of the labor and delivery environment with waste anesthetic gases.

• Incomplete analgesia.

• The potential for maternal amnesia.

• The potential for the loss of protective airway reflexes and pulmonary aspiration of gastric contents.

Note: Entonox (a mixture of 50% oxygen and 50% nitrous oxide) is widely used outside the United States.

1. Pudendal block and paracervical block. Paracervical block may be associated with severe fetal bradycardia caused by decreased uteroplacental/fetoplacental perfusion from increased uterine activity or a direct vasoconstrictive effect of the local anesthetic, and it is now rarely used. If a paracervical block is performed, the fetal heart rate (FHR) must be monitored. Pudendal blocks have little direct effect on the fetus. However, seizures have been reported after both pudendal and paracervical blocks. Paracervical blocks are used in the first stage of labor, and pudendal blocks during the second stage.

1. Opioids. All intravenously administered opioids are rapidly transferred to the fetus and cause dose-related respiratory depression and alterations in the Apgar and neurobehavioral scores.

• Meperidine (Demerol) can cause severe neonatal depression (measured by Apgar scoring) if the drug is administered 2–3 h before delivery. Depression is manifested as respiratory acidosis, decreased oxygen saturation, decreased minute ventilation, and increased time to sustained respiration. Fetal normeperidine (a meperidine metabolite that may cause significant respiratory depression) increases with longer intervals between drug administration and delivery. Levels are highest 4 h after intravenous administration of the drug to the mother. The half-life of meperidine is 13 h in neonates, whereas that of normeperidine is 62 h.

• Morphine has a delayed onset of action and may cause greater neonatal respiratory depression than meperidine.

• Butorphanol (Stadol) and nalbuphine (Nubain) are agonist-antagonist narcotic agents that cause less respiratory depression than morphine because they demonstrate a ceiling effect for respiratory depression with increasing doses. Decreases in FHR variability, sinusoidal FHR patterns, fetal tachycardia, and fetal bradycardia have all been reported following maternal administration of nalbuphine, unlike butorphanol.

1. Opioid antagonist (naloxone [Narcan]).Naloxone should never be administered to neonates of women who have received chronic opioid therapy because it may precipitate acute withdrawal symptoms. Naloxone may be used to ...

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