Chapter 2: Obstetric Anesthesia and the Neonate

INTRODUCTION

During birth, the fetal status can be influenced by obstetric analgesia and anesthesia. Care in choosing analgesic and anesthetic agents can often prevent complications such as respiratory depression in the newborn.

I. PLACENTAL TRANSFER OF DRUGS

Drugs administered to the mother may affect the fetus via placental transfer or may cause a maternal disorder that affects the fetus (eg, maternal drug-induced hypotension producing fetal hypoxia). All anesthetic and analgesic drugs cross the placenta to some degree, usually through flow-dependent passive diffusion. Most anesthetic and analgesic drugs have a high degree of lipid solubility, low molecular weight (<500), and variable protein-binding and ionization capabilities. These characteristics lead to rapid placental transfer.

II. ANALGESIA IN LABOR

1. Inhalation analgesia. Inhalation analgesia is not commonly used in the United States because neuraxial (regional) analgesia is widely available. Although not as effective, a mixture of 50% oxygen and 50% nitrous oxide, either premixed (Entonox in Europe) or administered with a blender device (Nitronox in the United States), can be used to lessen the perception of pain. Although placental transfer is rapid, there is no evidence of neonatal respiratory depression or altered neurobehavioral scores. Several problems with inhalational anesthesia limit its routine use:

   1. The need for specialized vaporizers

   2. Concern for environmental pollution

   3. Incomplete analgesia

   4. Maternal childbirth amnesia

   5. Possible loss of maternal protective airway reflexes

2. Pudendal block and paracervical block. Paracervical blocks are rarely used today because they may precipitate severe fetal bradycardia from reduced uteroplacental and/or fetoplacental perfusion. This occurs because of an increase in uterine activity and/or a direct vasoconstrictive effect of the local anesthetic. If a paracervical block is performed, the fetal heart rate (FHR) must be monitored continuously. Paracervical blocks are effective in the first stage of labor, and pudendal blocks are effective during the second stage. Pudendal blocks have little direct effect on the fetus.

3. Opioids. All intravenously (IV) administered opioids are rapidly transferred to the fetus, resulting in dose-related respiratory depression and alterations in Apgar and neurobehavioral scores.

   1. Meperidine. Meperidine can cause severe neonatal depression including respiratory acidosis, low oxygen saturation, decreased minute ventilation, and increased time to sustained respiration. The risk is least if administered intramuscularly (IM) within 1 hour of delivery and greatest if given 3 to 5 hours before delivery. Fetal normeperidine, a long-acting meperidine metabolite and significant respiratory depressant, accumulates after multiple doses or a prolonged dose-delivery interval.

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

   3. Butorphanol and nalbuphine. These are mixed agonist-antagonist opioid agents that may be safer than morphine because they demonstrate a ceiling effect for respiratory depression with increasing doses. Unlike butorphanol, maternal administration of nalbuphine can result in decreased FHR variability and, rarely, a sinusoidal FHR pattern.

   4. Fentanyl and remifentanil. These are synthetic opioids, best administered via patient-controlled analgesia. Both are short acting and have no active metabolites. Fentanyl ...