For which of the following local anesthetics is the duration of action MOST prolonged by the addition of epinephrine?
Vasoconstrictors are added to local anesthetic solutions to decrease the rate of vascular absorption of local anesthetic deposited near the nerve. The added vasoconstrictor stimulates α-adrenergic receptors on vascular smooth muscle, causing contraction and reduced local blood flow, and thereby slowing local clearance of the local anesthetic. Epinephrine in a concentration of 1:200,000 has been shown to provide the most optimal degree of vasoconstriction compared to norepinephrine or phenylephrine in similar concentrations.
The extent to which epinephrine prolongs the duration of local anesthesia depends on the specific local anesthetic used and the injection site. Epinephrine will significantly extend the duration of action of a short-acting local anesthetic (e.g., lidocaine) but will only slightly to modestly prolong the duration of action of a longer-acting local anesthetic such as etidocaine or bupivacaine.
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