Final answer:
A regional lumbar epidural block involves a local anesthetic, which is not listed among the options of intramuscular opioids, inhalational anesthetics, spinal anesthesia, or nitrous oxide administration.
Step-by-step explanation:
The anesthetic used in labor for a regional lumbar epidural block is not among the list provided, such as: intramuscular opioids, inhalational anesthetics, spinal anesthesia, or nitrous oxide administration. A regional lumbar epidural block involves the injection of a local anesthetic near the spinal cord in the lumbar region, which provides pain relief in the lower body. It is a type of local anesthetic that works by blocking nerve conduction to achieve a reversible loss of sensation in a specific region without affecting consciousness.