The antinociceptive activity and respiratory effects of fentanyl in sheep

Abstract

The analgesic activity of fentanyl was measured in sheep using both thermal and mechanical test systems. Fentanyl at a dose rate of 5 μg/kg given intravenously (iv) produced significant analgesia to thermal pain for some 30 mins but no detectable mechanical antinociceptive activity. However, at a dose rate of 10 μg/kg the drug produced both thermal (for 60 mins) and mechanical (40 mins) antinociceptive effects.

In one sheep a dose of 20 μg/kg produced thermal analgesia for 110 mins and mechanical antinociception for 60 mins.

Following iv injection at 10 μg/kg in five of the sheep, there was a brief period of respiratory depression evidenced by a significant fall in PaCO₂ of the order of 25 per cent and an increase in PCO₂ levels, but these changes were short lived and levels were back to normal at 15 mins after injection.