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Providing straightforward information pertaining to drugs, drug use & drug policy. The Grey Pages promotes drug-related literacy and advocates a system of viable and tolerant drug policies. This is my personal collection of commentaries, essays, tid-bits, and other such writings on everything ranging from drug use, drug policy and drug-myths, to drug-science, addiction, human behavior, and the workings of the human brain. I started this blog with a particular focus on opioids, and over the past year have found my interest gravitate toward the intriguing, ever-changing world of designer intoxicants (i.e. "research chemicals" or "designer drugs").

Saturday, February 5, 2011

Direct CNS Route - Intraspinal Analgesia & Anaesthesia


Routes:

Intrathecal Route: Medication is injected directly into the "dura matter" of the subarachnoid space containing the cerebrospinal fluid coating the spinal cord. This procedure uses a much smaller needle than is used in epidurals, in order to avoid leakage of cerebrospinal fluid. Fluid leakage can result in "spinal headaches". This is an occasional occurrence in epidural procedures when the dura matter is accidentally punctured with the larger 'epi' needle.


Epidural Route: Medication is injected into the epidural space, between the bony walls of the spinal canal and the dura matter surrounding the spinal cord.


Sometimes, a catheter is inserted into the epidural space, or less often the intrathecal sac itself, to allow continuous infusions of medications; such as opioids, general anaesthetics or local anaesthetics; for those undergoing a long surgery or requiring round-the-clock analgesia.


Areas:


Dura Matter: A tube of tissue which coats the arachnoid matter/spinal fluid surrounding the spinal cord. The dura is the layer which separates the epidural space from the inner-most arachnoid membrane and spinal cord.


Arachnoid Matter: Membrane which is contained within the dura matter which encomasses the spinal cord and fluid.

Drugs:


Opioids are effective and may be given by the spinal route; by both intrathecal and epidural injection, or infusion using a catheter. Opioids which are administered into the CNS by either of these routes will provide analgesia with much fewer side effects such as sedation and pruritis. Differing opioids have differing efficacy by this route depending on their individual properties.

Lipophilic opioids are rapidly absorbed by spinal lipid tissues, usually most targeted near the area of the injection site/puncture. Fentanyl and its analogues all share this property of high solubility, and should generally be administered as close as possible to area of the spine mediating pain signals.


Less absorbed opioids such as morpine generally spread throughout more area of the spinal canal, often making their way up to higher regions of the CNS via cerebrospinal fluid. In both cases, the opioid given intraspinally will make it's way to the subarachnoid space containing cerebrospinal fluid, and be absorbed by neural tissue.

Midazolam (Versed) can be used in conjunction 
with analgesics, to provide sedation during 
procedures using regional anaesthesia.

Settings:


The intraspinal route is commonly used in anaesthesia; for procedures which general anaesthesia cannot be used or puts the patient at risk, etc. This is typically done with a local anaesthetic such as bupivacaine, and is called regional anaesthesia. A spinally administered anaesthetic depending on dose and concentration, will effectively block pain or all sensation in the lower area of the body (below the navel cavity is the general rule of thumb). With a sufficient level of anaesthesia, patients may remain conscious during the procedure if this is desired; however in the US, Anaesthetists usually administer sedatives for the procedure.


Regional anaesthesia is often done by the epidural route to women during labor, and for C-Section procedures. Most commonly in Western medicine, potent opioids are administered (spinally) along with the anaesthetic - this provides a 'smoother' and more effective anaesthesia and also serves to provide post operative analgesia for up to 12 hours after the procedure. Spinal anaesthesia is extremely prevalent in India, where it is common to administer an infusion of buprenorphine along with bupivacaine.


Side Note: A vasoconstrictor such as phenylephrine can be used in the spinal to prolong the effect of anaesthetic.



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