General vs. Local

Q. Please define general and local anesthesia.

A. General anesthesia is a controlled state of unconsciousness produced by anesthetic agents associated with an absence of response to pain and a degree of muscle relaxation. Local anesthesia is the loss of sensation in a localized area of the body caused by depression of nerve endings or an obstruction of the conduction process of the peripheral nerves. Local anesthetics are the only drugs that actually prevent pain from occurring as they block the nerve impulses from ever reaching the brain. Patients under general anesthesia still feel pain but since they are unconscious, they are unable to respond outwardly.

Q. What are the differences between the two?

A. In general anesthesia, drugs are administered that depress the central nervous system, which produces the loss of consciousness. In patients under general anesthesia, the brain still receives nerve impulses so although the pain registers, they are unable to respond. Local anesthesia prevents the stimulated nerve impulses from reaching the brain, thereby preventing the sensation of pain.

Q. How do local anesthetics work?

A. The local anesthetic diffuses from outside the nerve into the nerve, with the anesthetic molecule going into the sodium channel and blocking it. When stimulated, the sodium ions can’t enter into the nerve and, therefore, the nerve doesn’t conduct the impulse.

Q. Why are vasoconstrictors added to local anesthetics?

A. The local anesthetic is like a liquid dropped onto a paper towel—it diffuses from the area of highest concentration to the lowest. The drop of liquid on the paper towel diffuses outward and eventually evaporates. The local anesthetic diffuses from outside the nerve into the nerve—it blocks nerve conduction. As long as the anesthetic stays within the nerve in a high enough concentration, anesthesia persists. However, outside of the nerve are blood vessels and as blood circulates through the area, the anesthetic outside the nerve gets absorbed by the blood and is taken away. Eventually, more anesthetic exists within the nerve than outside the nerve so the diffusion process reverses and the anesthetic begins to leave. The local anesthetic enters into the cardiovascular system and leaves the area in which it was injected. At some point there is no longer enough anesthetic within the nerve to prevent the pain impulse from reaching the brain.

The local anesthetics currently used in dentistry are inherently vasodilators. When the local anesthetic is injected, the anesthetic diffuses into the nerve, blocking nerve conduction. Additionally, the capillaries, the veins, and the arterioles in the region dilate. With a plain local anesthetic (without a vasoconstrictor), more blood enters into the area, which leads to the local anesthetic being removed from the area more quickly. Local anesthetics without vasoconstrictors provide a relatively short duration of action and a lesser depth of anesthesia. Adding a vasoconstrictor, which produces the constriction of arterioles, retards the absorption of the local anesthetic, leading to a drug that is both longer acting and provides a more profound anesthesia. Without the addition of a vasoconstrictor, the local anesthetic blood level will also be higher, which increases the risk of negative effects. A plain local anesthetic has a higher risk of overdose (toxicity). Adding a vasoconstrictor decreases the blood flow into the region because aretioles are constricted. Decreased blood flow into the region where the anesthetic was placed, leads to the drug being absorbed away from the site more slowly. This results in the drug remaining in the nerve in higher concentration longer and since less blood flows into the region, bleeding is decreased. Since the drug stays within the nerve longer, the resulting blood levels are lower, thereby decreasing to a degree, the risk of overdose.

From Dimensions of Dental Hygiene. July 2006;4(7): 22.