An "intrathecal" is a subarachnoid block. Just a different medical term. The medication dose is much smaller than an epidural because it is injected directly into the subarachniod space (cauda equina and CSF). Usually, the intrathecal will last for 2-3 hours. If Duramorph is used, the effect can be longer. The epidural space is a "potential" space, made up of blood vessels and fat. The epidural space lies just posterior to the dura. Therefore, the epidural space is entered first with the needle, and only a few more mm further, you go through the dura and enter the intrathecalor subarachnoid space.
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