Epinephrine as an adjuvant to amino‐amide local anesthetics does not prolong their duration of action in infraorbital nerve block in the rat

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Abstract
The effects of epinephrine as an adjuvant to local anesthetics were studied in the rat infraorbital nerve block (IONB) model, using solutions of 0.5% prilocaine, 0.5% mepivacaine, 0.125% bupivacaine or 0.125% ropivacaine in 50 mmol/l tris‐hydroxymethylaminomethane (THAM) tested both without and with epinephrine (EPI) added at 2, 4, 8 or 16 μg/ml. Solutions of THAM and EPI in normal saline did not induce IONB. THAM‐buffered solutions of bupivacaine induced IONB of longer duration than bicarbonate‐buffered solutions. Intensity of block during onset was increased only when EPI at 2 and 16 μg/ml was included in bupivacaine solutions. The duration of block induced by prilocaine, bupivacaine and ropivacaine was not significantly prolonged by addition of EPI at any of the concentrations tested. Only at a concentration of 16 μg/ml did EPI significantly prolong the duration of mepivacaine‐induced block (+ 48%). Low concentrations of EPI in solutions of bupivacaine and ropivacaine significantly reduced their duration of action by up to 22% and 57%, respectively. It is concluded that the duration of action of local anesthetics in buffered solutions is only moderately affected by the inclusion of EPI, the effects differing only slightly from one to another. The efficacy of EPI as an adjuvant would seem to be governed by factors affecting the local disposition of the main drugs, such as non‐specific binding, buffering of solutions and tissue pH.