How do local anaesthetics stay local for several hours instead of transferring around your body through the bloodstream?
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Usually epinephrine is added to the solution. This causes your blood vessels to tighten so it takes longer for the anesthetic solution to get absorbed into your bloodstream.
When they inject local anesthetics, it’s usually only skin deep. The blood vessels in the skin are super tiny. Much smaller than the needle they’re using. The anesthetics just pool in a spot in the skin. Some of it does get absorbed through the small blood vessels, but the amount they typically use is so small it doesn’t affect the rest of the body.
Now, if they use a lot of anesthetics in a larger area, more of it will get absorbed and can be toxic.
Short answer: they don’t. The medication is constantly being taken into the blood stream by the capillaries. There is a list of locations throughout your body where it is absorbed into the bloodstream fastest that you have to memorize in anesthesia school to avoid local anesthetic toxicity.
However in the dental locations there is a combo of lots of extra medication being injected (it only takes a drop to cause the nerve to stop conducting, but that would only last a few minutes before being cleared, so they usually inject several cc’s) and the injection site having slow clearance of the local (as opposed to next to a major nerve bundle in the shoulder or upper leg, which is next to major arteries and nerves).
The other half of the equation, is that the local anesthetic penetrates the layers of the nerve, so even when the anesthetic is cleared away from around the nerve, it still has to be cleared from the nerve itself which can take awhile.
A local anesthetic will not usually last several hours but I get what you’re asking. I think of it as dropping a drop of food coloring into a bowl of water. It slowly spreads out. Any chemical like anesthetic injected under your skin does this same thing called diffusion where the molecules slowly move further and further away. It does end up in the bloodstream eventually but not nearly as fast as a drug injected directly into a vein.
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