Pharmacokinetics of 2 mg/kg Bupivacaine in a Rectus Abdominis Sheath Block in Horses

Administering large volumes of bupivacaine in the rectus abdominis sheath (RAS) block in horses could cause local anesthetic toxicity. This study aimed to evaluate the pharmacokinetics of 2 mg/kg bupivacaine in RAS blocks and the associated toxicity risk. Six healthy adult horses were sedated with xylazine and received an ultrasound-guided bilateral 2-point RAS block. Plasma samples were collected at baseline and from 5 min to 24 h after the block and analyzed using liquid chromatography-tandem mass spectrometry. A noncompartmental analysis was performed to determine the maximum concentration (Cmax), the time to reach Cmax (Tmax), the terminal half-life, the area under the plasma concentration-time curve extrapolated to infinity (AUCinfinity), and the percentage of AUCinfinity extrapolated (AUCextrapolated). The bupivacaine Cmax ranged from 38.4 to 151.8 ng/mL, Tmax from 1.5 to 4 h, and the terminal half-life from 9.2 to 46 h. AUCinfinity ranged from 1351.9 to 2463.3 h·ng/mL, and AUCextrapolated ranged from 8.6% to 61.4%. One sample showed plasma levels consistent with neurotoxicity in humans, but was deemed an outlier. The 24-h sampling period was considered insufficient for accurate determination of AUCinfinity and terminal half-life. Further research on RAS block safety in anesthetized horses is needed before use in colic surgeries.