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

ABSTRACT 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 ( C max ), the time to reach C max ( T max ), the terminal half‐life, the area under the plasma concentration‐time curve extrapolated to infinity (AUC infinity ), and the percentage of AUC infinity extrapolated (AUC extrapolated ). The bupivacaine C max ranged from 38.4 to 151.8 ng/mL, T max from 1.5 to 4 h, and the terminal half‐life from 9.2 to 46 h. AUC infinity ranged from 1351.9 to 2463.3 h·ng/mL, and AUC extrapolated 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 AUC infinity and terminal half‐life. Further research on RAS block safety in anesthetized horses is needed before use in colic surgeries.