Additives Commonly Mixed With Local
Anesthetics
Sodium Bicarbonate
Because the free base form of most local anesthetics is poorly
soluble in water, they are prepared as hydrochloride salts with
commercial solutions having a pH in the range of 4-6. Addition
of bicarbonate can increase the rate of onset of some local anesthetics
by shifting the equilibrium to the uncharged base form which readily
penetrates into nerve tissue. Neutralizing the pH of a local anesthetic
solution may also decrease the associated pain on injection. Using
an 8.4% sodium bicarbonate solution, one ml can be added to a
lidocaine 30 ml vial or 0.2 ml can be added to a bupivacaine 30
ml vial. Attempts to add further bicarbonate will result in precipitation
of the local anesthetic from its solution.
Preservatives
Antimicrobial preservatives are added to multi-dose vials of local
anesthetics. Preservative-containing local anesthetic solutions
must not be used in spinal, epidural anesthesia because of their
potentially cytotoxic effects. The most frequently used antimicrobials
are methyl- ethyl- and propyl-parabenzoates. Antioxidants or
ion chelating agents such as sodium ethylenediaminetetraacetic
acid (EDTA) are added to commercially available local anesthetics
to prevent oxidation, or to scavenge divalent cations and thus
retard their degradation. Preservative free preparations are available
for patients suspected of having allergies to any of these preservatives.
Epinephrine
Low concentrations of epinephrine can be added to local anesthetics
in order to retard their systemic absorption. This has the effect
of decreasing blood levels thus minimizing toxic reactions and
prolonging the duration of anesthesia they produce. Addition of
epinephrine 5 mg to each ml of local anesthetic produces the
commonly used 1:200,000 concentration. Caution is exercised with
patients known to suffer from coronary vascular disease; and epinephrine
containing solutions should never be injected into areas with
no collateral circulation such as fingers, toes, ears, penis and
nose.