Question:

Why an infection area presents a poor site of local anesthesia?

Answer:

Regarding the difficulties of attaining profound local anesthesia in an area of infection. The answer to the question is related to very basic chemistry.

A common acid is vinegar and a common base is baking soda. When an acid is added to a base, the acid is neutralized and a chemical reaction typically results.
 
In the case of vinegar plus baking soda, bubbles of carbon dioxide gas are produced. If an acid is added to another acid, no reaction takes place.

Local anesthetics before injection are slightly acidic to make them dissolve in sterile water and, once the anesthetic is injected into the body, the anesthetic must depend on the slightly basic nature of the body tissues and body fluids to neutralize the acid nature of the anesthetic so that the anesthetic molecules can be transformed into fat-soluble molecules.
 
It is only when the anesthetic molecules are in their fat-soluble form that they can penetrate into the inside of the nerve to block pain impulses.
 
In the presence of infection, the body tissues and fluids in the area of infection lose their normal “basic” nature and become acidic. Infections interfere with normal activities of body tissues, so acids from metabolism are not taken away but rather build up at the infection site.

Therefore, if the local anesthetic is injected into an infected area, the water-soluble local anesthetic molecules cannot be neutralized because the infected body tissues are no longer basic, but rather acidic.

So the local anesthetic molecules stay in their water-soluble form and cannot penetrate into the nerve to block painful nerve impulses.

For more information:

Go to the Dental Anesthesia health topic.