Systemic and anti-nociceptive effects of prolonged lidocaine, ketamine, and butorphanol infusions alone and in combination in healthy horses

BACKGROUND Prolonged drug infusions are used to treat horses with severe signs of pain, but can be associated with altered gastrointestinal transit. The purpose of this study was to determine the effects of prolonged constant rate infusions (CRI) of lidocaine (L), butorphanol (B), and ketamine (K) alone and in combination on gastrointestinal transit, behavior, and thermal nociceptive threshold in healthy horses. METHODS Eight healthy adult horses were used in a randomized, cross-over, blinded, prospective experimental trial. Interventions were saline, L, K, B, LK, LB, BK, and LBK as an intravenous CRI for 96 hours. Drugs were mixed or diluted in saline; following a bolus, CRI rate was 0.15 mL/kg/hr with drug doses as follows: L - 1.3 mg/kg then 3 mg/kg/hr; B - 0.018 mg/kg then 0.013 mg/kg/hr; K - 0.55 mg/kg then 0.5 mg/kg/hr. Two-hundred plastic beads were administered intragastrically by nasogastric tube immediately prior to the bolus. Feces were collected every 2 hours, weighed, and beads manually retrieved. Behavior was scored every 2 hours, vital parameters every 6 hours, and thermal nociceptive threshold every 12 hours for 96 hours. Drug concentrations in the LBK solution were tested every 6 hours for 72 hours. RESULTS Four of 64 trials (3 LBK, 1 BK) were discontinued early due to signs of abdominal discomfort. There were no apparent differences between groups in vital parameters or thermal threshold. Transit time was delayed for LB and LBK with a corresponding decrease in fecal weight that was most severe in the final 24 hours of infusion. Significant changes in behavior scores, vital parameters, or thermal threshold were not observed. The concentration of each drug in the combined solution declined by less than 31% over the sampling period. CONCLUSIONS Drug combinations containing butorphanol cause an apparent delay in gastrointestinal transit in healthy horses without substantially affecting somatic nociception at the doses studied. Combinations of lidocaine and ketamine may have less impact on gastrointestinal transit than infusions combined with butorphanol. Further work is needed to determine the effects of these drugs in painful or critically ill patients.