How does an epidural work?

As mentioned, the nerve signals passing through the epidural space are blocked by local anaesthetic. The injection spreads up and down over a few centimetres. This causes the local anaesthetic to not only block the nerve that comes out of the spinal cord at the same level that the epidural injection took place, but also the nerves that leave the spinal cord higher up and lower down.

In practice, this means the following: if an anaesthetist performs an epidural block low down in the back, at the so-called lumbar level, the nerves that innervate the lower abdomen and the legs will be affected. This is suitable for e.g. surgery on the prostate, hysterectomies and hip and knee surgery.

If the epidural injection is performed higher up in the back (lower thoracic level), the nerves leaving and entering the spinal cord at a higher level will be blocked, i.e. those innervating the upper and lower abdomen, but not the legs. This is suitable for surgery on the upper abdomen (stomach, bowel, liver, etc.).

For chest surgery, even higher placed epidurals are necessary (between the shoulder blades on the higher thoracic level). This will block the innervation of the chest and upper abdomen, but not the lower abdomen or the legs.

Reviewed by Prof CL Odendal, senior specialist at the department of anaesthesiology at the University of the Free State, April 2010.

Read more:
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