Re: "Hot" tooth - Myths or Reality?


"Dr Steve" <nospam@xxxxxxxx> wrote in message
news:BKJHe.230$646.52@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Are you sure?

Yes...sort of. Local anesthetics have pH's of approximately 7.9...making
them a weak base. But hydrogen ions are added to make them acidic:

" Local anesthetics are weak bases with pKa in the range of 7.9. This of
course means that at a pH of 7.9 a population of local anesthetic molecules
is equally divided between a charged and uncharged state. In order to keep
the local anesthetic molecules in solution, hydrogen ions are added (that
is, the solution is made acidic) or said another way, the pH of the solution
is lowered. If the local anesthetic solution is made with epinephrine, the
pH is made even lower.

Lets turn our discussion now to a consideration of nerves. As most junior
high school students know, nerve membranes are a lipid bilayer with protein
channels. Local anesthetics act in the sodium channel, entering from the
internal aspect. Now you may have noticed a bit of a problem. Positively
charged local anesthetic molecules soluble in an aqueous solution will have
trouble passing through a lipid membrane. This problem is overcome when the
tissue surrounding the nerve accepts (or buffers) the hydrogen ion and the
uncharged molecules are then free to pass through the axonal membrane. Once
in the cell, the molecules must be recharged before they can effect a block
of the sodium channel.

OK, so what? What difference does this make clinically? Local anesthetics
will not work in tissue that is unable to buffer the excess hydrogen ions.
This is why local anesthetics injected into the acidic environment of an
abscess will not cause numbness. Occasionally anesthesiologists will add
NaHCO3 to local anesthetic to speed the onset of the drug effect. This
additive is not a powerful enough base to overcome the acidity in abscessed
tissue, however. "

http://www.anesthesia.wisc.edu/med3/localanes/localhandout.html


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  • Re: "Hot" tooth - Myths or Reality?
    ... It changes the pH...which changes the polar charge. ... But hydrogen ions are added to make them acidic: ... >> " Local anesthetics are weak bases with pKa in the range of 7.9. ... >> order to keep the local anesthetic molecules in solution, ...
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