2023 AEMV Grant Winner! 

Bupivacaine is routinely used for peripheral nerve blocks in veterinary practice. Despite its potential to cause local anesthetic toxicity, only a few studies have evaluated the systemic reabsorption of bupivacaine following locoregional anesthesia. This study therefore aimed to describe the pharmacokinetics of bupivacaine at two concentrations following administration via an ultrasound-guided transversus abdominis plane (TAP) block in rabbits undergoing ovariohysterectomy. Twelve healthy adult female shelter-owned rabbits were included in a prospective, randomized, blinded clinical study. The anesthetic protocol included methadone, midazolam, ketamine, alfaxalone, isoflurane, and meloxicam. Each rabbit received a bilateral two-point TAP block with 1 mL/kg of either bupivacaine 0.25% (low dose, TAP-BL, n=6) or 0.5% (high dose, TAP-BH, n=6). Plasma bupivacaine concentrations were determined by high-performance liquid chromatography-mass spectrometry immediately before and at 5, 15, 30, 60, 120, 240, 480, 720, and 1440 minutes after TAP block. A non-compartmental analysis was used to describe pharmacokinetic parameters. Bupivacaine remained detectable after 24 hours in 2/6 individuals in TAP-BL and 5/6 individuals in TAP-BH. Mean maximum plasma concentration was 750.3 ± 316.74 ng/mL in TAP-BL and 1810.5 ± 384.71 ng/mL in TAP-BH and was reached 15 minutes after TAP block. Mean half-life was 4.39 ± 0.6 h in TAP-BL and 6.96± 2.31 h in TAP-BH. Mean area under the curve from zero to infinity was 2933.75 ± 1050.43 ng/mL·h in TAP-BL and 6616.52 ± 1328.98 ng/mL·h in TAP-BH. High interindividual variability was noted. No clinical signs of neurotoxicity, i.e. seizures, were noted in the peri- and post-operative period.