Spinal anesthesia (subarachnoid block) produces extensive and profound anesthesia with a minimum amount of drug. The local anesthetic solution is introduced directly into the spinal fluid, where the nerves are not protected by a perineurium. This produces, in effect, a temporary cord transection such that no impulses are transmitted beyond the level that is anesthetized. The onset is rapid, and with proper drug selection, the anesthesia may last 1 to 4 hours.With careful technique, neurological complications are extremely rare. Procedures as high as upper-abdominal surgery can be performed under spinal anesthesia. Arterial hypotension produced by the local anesthetic is proportional to the degree of interruption of sympathetic tone, and it can produce pooling of blood in the lower extremities, which leads to decreased cardiac filling pressures. Knowing this allows blood pressure to be easily controlled, and hypotension is not usually a deterrent to spinal anesthesia. The sites of action of spinal anesthesia are the spinal nerve roots, spinal ganglia, and (perhaps) the spinal cord.
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