BASICS OF LOCAL ANESTHESIA

Mechanism of Action

Local anesthetics block Na+ influx during depolarization of the nerve cell membrane. The result is the blockade of the action potential and subsequent anesthesia.

Order of Blockade

The blockade proceeds from pain through temperature, touch, pressure, vibration, propioception, and motor function:

• First affected are the small, unmyelinated C-type nerve fibers (which transmit pain and temperature).

• Last affected are the largest, myelinated A-type fibers (which transmit pressure sensations and motor fibers).

Patients may have anesthesia but still feel pressure and may have the ability to move because the A-type fibers are unblocked.

Composition

Anesthetics are weak organic bases that exist in two forms:

• The ionized form is an active form that blocks nerve conduction. The ionized form is water-soluable, therefore allowing injection. At physiologic pH, 80% is in the ionized form.

• The nonionized form is the lipid-soluable form that facilitates diffusion into tissues and nerve cell membranes.

Chemical Structure

The chemical structure has three components:

• The aromatic portion is usually composed of a benzene ring (lipophilic).

• The intermediate chain is either an ester or an amide linkage (determines class).

• The amine is hydrophilic (water solubility).

Lipid solubility is important because it enables diffusion through the lipophilic nerve membrane. Lipophilicity is directly related to potency.

Duration of Anesthesia

Duration is directly related to the degree of protein binding of the anesthetic receptors along the nerve cell membrane (determined by amine structure).