2: Obstetric Anesthesia and the Neonate

INTRODUCTION

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.

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. Flow-dependent passive diffusion is the usual mechanism.

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. Local anesthetics and narcotics (lipid-soluble, unionized) cross the placenta easily; neuromuscular blocking agents (highly ionized) do not.

II. ANALGESIA IN LABOR

1. Inhalation analgesia. Inhalation analgesia is rarely used in the United States as a result of the wide availability of regional anesthesia (Note: Entonox, a mixture of 50% oxygen and 50% nitrous oxide, is used outside the United States). In addition, several problems associated with inhalational anesthesia limit its routine use:

   1. The need for specialized vaporizers

   2. Potential pollution of the labor and delivery environment with waste anesthetic gases

   3. Incomplete analgesia

   4. Maternal childbirth amnesia

   5. Possible loss of maternal protective airway reflexes and pulmonary aspiration of gastric contents

2. Pudendal block and paracervical block. Paracervical blocks are rarely used today because they may precipitate severe fetal bradycardia or increased uterine activity or cause a direct vasoconstrictive effect of the local anesthetic, resulting in decreased uteroplacental/fetoplacental perfusion. If a paracervical block is performed, the fetal heart rate (FHR) must be monitored. 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; however, seizures have been reported after both.

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

   1. Meperidine. Meperidine can cause severe neonatal depression (measured by Apgar scoring) if the drug is administered 2–3 hours before delivery as a result of maximum fetal uptake. Depression is manifested as respiratory acidosis, decreased oxygen saturation, decreased minute ventilation, and increased time to sustained respiration. 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 agonist-antagonist narcotic 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, sinusoidal FHR patterns, fetal tachycardia, and fetal bradycardia.

   4. Fentanyl and remifentanil. These are synthetic opioids, best administered via patient-controlled analgesia during labor. Both are short acting and have no active metabolites. Fentanyl may cause low 1-minute Apgar scores, but neonatal neurobehavioral scores are normal. Remifentanil requires careful monitoring and one-on-one maternal nursing care.

4. Opioid antagonist ...