RESEARCH PAPER Comparison of the effects of the alpha-2 agonists detomidine, romifidine and xylazine on nociceptive withdrawal reflex and temporal summation in horses Helene Rohrbach* DVM, Toni Korpivaara DVM, Urs Schatzmann* DVM, Diplomate ECVAA, PhD & Claudia Spadavecchia* DVM, Diplomate ECVAA, PhD *Anaesthesiology Section, Vetsuisse Faculty, University of Bern, Bern, Switzerland  Orion Pharma, Espoo, Finland Correspondence: Helene Rohrbach, Anaesthesiology Section, Department of Veterinary Sciences, Vetsuisse Faculty, University of Berne, Laenggassstrasse 124, 3012 Berne, Switzerland. E-mail: helene.rohrbach@knp.unibe.ch Abstract Objective To evaluate and compare the antinoci- ceptive effects of the three alpha-2 agonists, detom- idine, romifidine and xylazine at doses considered equipotent for sedation, using the nociceptive with- drawal reflex (NWR) and temporal summation model in standing horses. Study design Prospective, blinded, randomized cross- over study. Animals Ten healthy adult horses weighing 527– 645 kg and aged 11–21 years old. Methods Electrical stimulation was applied to the digital nerves to evoke NWR and temporal sum- mation in the left thoracic limb and pelvic limb of each horse. Electromyographic reflex activity was recorded from the common digital extensor and the cranial tibial muscles. After baseline measurements a single bolus dose of detomidine, 0.02 mg kg )1 , romifidine 0.08 mg kg )1 , or xyla- zine, 1 mg kg )1 , was administered intravenously (IV). Determinations of NWR and temporal sum- mation thresholds were repeated at 10, 20, 30, 40, 60, 70, 90, 100, 120 and 130 minutes after test-drug administration alternating the thoracic limb and the pelvic limb. Depth of sedation was assessed before measurements at each time point. Behavioural reaction was observed and recorded following each stimulation. Results The administration of detomidine, romifi- dine and xylazine significantly increased the current intensities necessary to evoke NWR and temporal summation in thoracic limbs and pelvic limbs of all horses compared with baseline. Xylazine increased NWR thresholds over baseline values for 60 min- utes, while detomidine and romifidine increased NWR thresholds over baseline for 100 and 120 minutes, respectively. Temporal summation thresholds were significantly increased for 40, 70 and 130 minutes after xylazine, detomidine and romifidine, respectively. Conclusions and clinical relevance Detomidine, romifidine and xylazine, administered IV at doses considered equipotent for sedation, significantly increased NWR and temporal summation thresh- olds, used as a measure of antinociceptive activity. The extent of maximal increase of NWR and temporal summation thresholds was comparable, while the duration of action was drug-specific. Keywords alpha-2 agonist, electromyographic mea- surements, horse, nociceptive withdrawal reflex, sedation. 384 Veterinary Anaesthesia and Analgesia, 2009, 36, 384–395 doi:10.1111/j.1467-2995.2009.00466.x