During a spinal, medication is given through a needle or small plastic tube (catheter) directly into the subarachnoid space, which is the part of the spinal column that contains the cerebral spinal fluid. Numbing medication given via the spinal route is very fast acting, and the patient starts to feel numb immediately.

During an epidural, medication is given through a needle or catheter directly into the epidural space that is the part of the spinal column just outside the space that contains the spinal fluid. Numbing medication given via the epidural route is slower acting than the spinal. It usually takes about 15 minutes for the patient to feel numb.

A combination of local anesthetics and narcotics are used in both spinal and epidural regional anesthesia. The drugs prevent pain messages from being sent from the lower region of the spinal cord to the brain. Pain relief is achieved by the drugs acting on the sensory nerve fibers. The local anesthetics that are most commonly used are bupivacaine or ropivacaine. The narcotics that are most commonly used are fentanyl and sufentanil.

A "walking epidural" is actually a combined spinal epidural. During a combined spinal epidural, two needles are used. The needle used for the epidural is inserted into the epidural space first. Then a longer, narrower needle containing medication is inserted through the epidural needle into the spinal (subarachnoid) space. After the medication is injected, the spinal needle is removed. The numbing effects of the medication are felt immediately. The epidural needle remains in place until after the epidural catheter is inserted. A continuous infusion of medication can be run through the epidural catheter to keep the patient comfortable throughout her labor.

During a combined spinal epidural, the dose of the drug used in the subarachnoid space is much smaller than that used during a regular epidural. Dr. Hawkins says that all labor epidurals are walking epidurals: "All that means is that they are designed not to affect the muscle strength in the legs," she says.

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