Epinephrine: effect on LA duration

Vasoconstrictors, such as epinephrine, can be added to local anesthetics. The addition of epinephrine causes multiple effects. Blood vessel constriction causes decreased blood flow to the site of drug administration or infiltration, leading to reduced systemic absorption of LAs, lower systemic blood levels, and thus reduced risk of LA toxicity. Additionally, administration with epinephrine results in increased intraneural concentration of LAs as reduced systemic absorption allows more local anesthetic to enter into the nerve membrane and remain for a longer period of time. This results in improved depth and increased duration of action of most LAs. The increased duration of action is only partially explained by decreased blood flow to the area, as the alpha-adrenergic effects of epinephrine will wear off prior to anesthetic effect. The extent of prolongation in duration of anesthesia is also dependent on the site of injection and the local anesthetic used for the block. Shorter-duration LAs, such as lidocaine, have significantly longer duration of action when combined with epinephrine compared to LAs such as bupivacaine which have a more modest prolongation of nerve block.

Epinephrine can also be added as a marker for inadvertent intravascular injection and to help decrease bleeding at the site of administration. Norepinephrine and phenylephrine have also been used for their vasoconstrictive properties, but have not been shown to be superior to epinephrine.