Plasma histamine concentrations in horses administered sodium penicillin, guaifenesin–xylazine–ketamine and isoflurane with morphine or butorphanol

Abstract Objectives Various drugs administered to horses undergoing surgical procedures can release histamine. Histamine concentrations were evaluated in horses prepared for surgery and administered either butorphanol or morphine intraoperative infusions. Study design Prospective studies with one randomized. Animals A total of 44 client-owned horses. Methods In one study, anesthesia was induced with xylazine followed by ketamine-diazepam. Anesthesia was maintained with guaifenesin–xylazine–ketamine (GXK) during surgical preparation. For surgery, isoflurane was administered with either intravenous (IV) morphine (group M: 0.15 mg kg−1 and 0.1 mg kg−1 hour−1; 15 horses) or IV butorphanol (group B: 0.05 mg kg−1 and 0.01 mg kg−1 hour−1; 15 horses). Histamine and morphine concentrations were measured using enzyme-linked immunoassay before opioid injection (Time 0), and 1, 2, 5, 30, 60 and 90 minutes afterwards. In a subsequent study, plasma histamine concentrations were measured in 14 horses before drug administration (baseline), 15 minutes after IV sodium penicillin and 15 minutes after starting GXK IV infusion. Statistical comparison was performed using anova for repeated measures. Pearson correlation compared morphine and histamine concentrations. Data presented as mean ± standard deviation. Significance assumed when p ≤ 0.05. Results With histamine, differences occurred between baseline (3.2 ± 2.4 ng mL−1) and GXK (5.2 ± 7.1 ng mL−1) and between baseline and Time 0 in group B (11.9 ± 13.4 ng mL−1) and group M (11.1 ± 12.4 ng mL−1). No differences occurred between baseline and after penicillin or between groups M and B. Morphine concentrations were higher at 1 minute following injection (8.1 ± 5.1 ng mL−1) compared with 30 (4.9 ± 3.1 ng mL−1) and 60 minutes (4.0 ± 2.5 ng mL−1). Histamine correlated with morphine at 2, 30 and 60 minutes. Conclusion and clinical relevance GXK increased histamine concentration but concentrations were similar with morphine and butorphanol.