Synovial butorphanol concentrations and mechanical nociceptive thresholds after intravenous regional limb perfusion in standing sedated horses.

Objective: To determine synovial butorphanol concentrations and mechanical nociceptive threshold (MNT) changes after butorphanol intravenous regional limb perfusion (IVRLP).

Study Design: Experimental ANIMALS: Six adult horses.

Methods: Cephalic IVRLP was performed with 10 mg butorphanol in sedated horses with a wide rubber tourniquet and a total volume of 30 mL. Radiocarpal synovial fluid and serum concentrations along with MNT were evaluated prior to and 0.5, 1, 2, 4, and 6 hours after IVRLP. Butorphanol concentrations were determined with liquid chromatography coupled to tandem mass spectrometry positive electrospray ionization.

Results: Butorphanol concentrations reached mean (SD) peak concentrations of 9.47 ng/mL (±12.00) in synovial fluid and 3.89 ng/mL (3.29) in serum 30 minutes after IVRLP. Concentrations remained above baseline for 4 hours in synovial fluid (P ≤ .017) and for 2 hours in serum (P ≤ .016). The only difference in MNT was detected 1 hour after IVRLP, when MNT were higher in controls than in treated horses (P = .047).

Conclusion: Butorphanol IVRLP seemed well tolerated and resulted in measurable levels of butorphanol in the radiocarpal synovial fluid of five of six horses.

Clinical Significance: Intravenous regional limb perfusion appears to be a viable alternative to administer butorphanol, but additional investigation is required to evaluate the dose and local concentrations required for analgesia.