Lumbar epidural anesthesia affects the same area of the body as does spinal anesthesia. As the name implies, the drug is deposited outside the dura. In contrast to spinal anesthesia, this method requires a much larger amount of drug. This procedure makes segmental anesthesia possible, whereby the anesthetized area is bordered caudally and cephalad by unaffected dermatomes and myotomes.
The concentration and volume of the local anesthetic solution will affect the extent of the cephalad and caudad spread of the block. The anesthesia can be made continuous by maintaining a small catheter in the epidural space; prolonged effects are obtained by periodically injecting supplemental doses through the catheter or by attaching it to a computer pump. The site of anesthetic action is on the nerves as they leave the intervertebral foramina. However, effective drug concentrations may be found in the spinal fluid, probably gaining entrance through the arachnoid villi. Arterial hypotension occurs by the same mechanism and is managed as in spinal anesthesia.
Epidural anesthesia is especially useful in obstetrics. Excellent analgesia occurs and the patient remains awake. Analgesia by the epidural route can be provided for labor and delivery or for cesarean section. Bupivacaine in lower concentrations has the advantage of providing excellent analgesia while minimally reducing motor strength.
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