The use of medetomidine/fentanyl combinations in dogs.

The sedative effects of medetomidine at doses of 20 and 40 micrograms/kg im given alone or followed 16-18 min later by fentanyl (2 micrograms/kg iv) was investigated in 6 bitches of mixed breeds. The higher dose of medetomidine alone caused the greater degree of sedation, but two bitches were only lightly sedated with either dose. Side effects noted in some cases included apparent pain on injection, vomiting on induction of sedation, bradycardia, slowing of respiratory rate, cyanosis and muscular twitching. The intravenous injection of fentanyl caused a marked increase in depth of sedation in all animals, inducing a condition similar to neuroleptanaesthesia in which the eyes were rotated downward and the pedal reflex abolished. Slight twitching and sensitivity to sound occurred immediately after fentanyl injection, but this was transient. The cardiopulmonary effects of medetomidine (40 micrograms/kg im) followed 20 min later by either fentanyl (2 micrograms/kg iv) or a saline placebo were investigated in 4 beagle dogs. Medetomidine caused bradycardia, hypotension and reduced respiratory rate, inducing an intermittent respiratory pattern. The iv injection of fentanyl did not further alter the heart or respiratory rate or blood pressure. However there was a small but significant decrease in arterial oxygen tension and rise in arterial carbon dioxide tension. indicating some respiratory depression. We conclude that the use of intravenous fentanyl to dogs already sedated with medetomidine could prove useful in clinical cases where the initial sedation with medetomidine has proved inadequate.