1.11 Regional Neuraxial Anesthesia (Spinal, Epidural), indications, complications, and local anesthetics (classification, pharmacology and indications)

Neuraxial anesthesia is an alternative to general anesthesia. It is classified into two types:

Okay, we used one of these techniques for anesthesia; now what? Well, now the anesthetic starts acting on the nerve fibers:

1. The thin unmyelinated or myelinated fibers which is the sympathetic fibers.
2. The A-Delta and C Fibers for pain sensation.
3. Next, the thick mechano-receptors are blocked. The patient feels pressure but not pain.
4. The motor neurons. The patient will have heavy legs for example.

Next are some examples of anesthetics used in this way.

1. Amides, which are metabolized in the liver. They have two i in thier name so are more complex and can only be broken down by the liver.
   1. Bupivacaine, Ropivacaine, Lidocaine etc
2. Esters, which are metabolized by blood esterases. They have a single i in their name so are simpler and can be degraded in the blood.
   1. Cocaine, Procaine, Tetracaine, Benzocaine etc.

But when might these anesthetics not work?

Local anesthetics are weak bases. At high pH (basic pH), they are non-ionized. But, at low pH (acidic) they become ionized. Now, they must be non-ionized to cross the cell membrane of neurons, otherwise they can't perform their function. This is important for example in an infected tooth. Lidocaine will not work as there is an inflammation and infection meaning there is an acidic environment and it will become ionized.

Contraindications

1. If the patient refuses,
2. inflammation or infection at site,
3. Coagulopathy: a hematoma at the epidural site can cause paralysis
4. they can cause cause a sudden drop in bp in patients with cardiac complications