A Rapid and Effective Method to Prolong Pain Relief in Sheep Using Bupivacaine and Intralipid® Combination

Background: To compare the duration of action of a local anesthetic block using a lipid formulation of bupivacaine to the commercially available aqueous formulation. Bupivacaine 0.5% was mixed with an equal volume of either lipid emulsion (Intralipid, Fresenius Kabi) or normal saline resulting in a final concentration of 0.25% bupivacaine. Eighteen sheep were administered a nerve block of either control or treatment at the metacarpal region of each forelimb to compare the efficacy of the injected formulations. The nociceptive test was determined by applying a blunt noxious stimulus to the foot below the nerve block at multiple time intervals until the sheep responded by withdrawing its foot. The person assessing the response to the noxious stimulus was blinded to the treatment. Results: The Intralipid formulation significantly extended the duration of the nerve block compared to the control group. The mean analgesic period (mean±SD) in the control legs was 4.23±1.8 hr. compared to 5.81±1.78 hr. in the Intralipid injected legs (p=0.013). Conclusions: In conclusion, an Intralipid®-based formulation provided a more prolonged duration of local anesthesia after nerve blocks in the sheep metacarpal region compared to aqueous bupivacaine. Keywords: Bupivacaine; Sheep; Sustained; Release; Intralipid®; Pain; Animal welfare