Action of the stereoisomers of medetomidine, in halothane-anesthetized dogs.

This study was designed to investigate the anesthetic-reducing and cardiovascular effects of the stereoisomers of medetomidine in halothane-anesthetized male beagles. Anesthesia was induced by mask inhalation of halothane in oxygen, followed by 2h equilibration period. The minimum alveolar anesthetic concentration of halothane which prevented the animals from moving in response to a painful stimulus (MAC of halothane) was determined and baseline hemodynamic function was assessed. Dl-Medetomidine, at 3 doses (1, 3 and 10 micrograms.kg-1) was administered via the right atrial port over 15 minutes while maintaining the dog at its individual MAC for halothane. Ten minutes later, hemodynamic parameters were reassessed. MAC determination was then repeated. In two separate sets of experiments, dogs (n = 5 each) were administered either the d- or the 1-enantiomer of medetomidine and the same procedure followed as described above. MAC for halothane significantly decreased following dl-medetomidine administration in a dose-dependent fashion. The d-isomer displayed similar MAC-reducing effects while the 1-isomer was without effect. Neither isomer changed the mean arterial pressure while only and d-isomer significantly decreased heart rate and cardiac output.