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Keywords:

  • horse;
  • systemic lidocaine;
  • visceral;
  • somatic;
  • analgesia

Summary

Reasons for performing study: Commonly used analgesics (nonsteroidal anti-inflammatory agents, opioids and α2-agonists) have unwanted side effects. An effective alternative with minimal adverse effects would benefit clinical equine pain management.

Objectives: To compare the effect of lidocaine or saline on duodenal and rectal distension threshold pressure and somatic thermal threshold in conscious mature horses.

Hypothesis: Systemically administered lidocaine would increase somatic and visceral nociceptive thresholds.

Methods: Lidocaine (2 mg/kg bwt bolus followed by 50 μg/kg bwt/min for 2 h) or saline was administered to 6 horses each carrying a permanently implanted gastric cannula, in a randomised, blinded cross-over design. Thermal threshold was measured using a probe containing a heater element placed over the withers which supplied heat until the horse responded. A barostatically controlled intraduodenal balloon was distended until a discomfort response was obtained. A rectal balloon was inflated until extruded or signs of discomfort noted.

Results: Thermal threshold was increased significantly 30 and 90 mins after the start of lidocaine infusion. There was no change in duodenal distension pressure and a small but clinically insignificant change in colorectal distension pressure in the lidocaine group.

Conclusions: At the dose used, systemically administered lidocaine produced thermal antinociception but minimal changes in visceral nociception.

Potential relevance: At these doses, lidocaine may play a role in somatic analgesia in horses.