THE EFFECT OF DEXMEDETOMIDINE IN COMBINATION WITH PETHIDINE ON ISOFLURANE MINIMUM ALVEOLAR CONCENTRATION FOR CANINE ANTINOCICEPTION

Abstract

The effects of dexmedetomidine combined with pethidine on the minimum alveolar concentration (MAC) of isoflurane and cardiorespiratory variables in dogs were evaluated in thirty client-owned, healthy adult male dogs scheduled for castration at the Small Animal Teaching Hospital, Faculty of Veterinary Science, Chulalongkorn University. The dogs were allocated into Group 1 (n=6) receiving sterilized water for injections (placebo) as a negative control group, Group 2 (n=12) receiving 5 µg kg-1 of dexmedetomidine IM as a positive control group, and, Group 3 (n=12) receiving 5 µg kg-1 of dexmedetomidine and 5 mg kg-1 of pethidine IM. At 15 minutes after injection of the tested drug, sedation and pain response to clamping of the 3rd or 4th digit of the hindimb were evaluated. Then, anesthesia was induced via a face mask with 4% isoflurane in 4 L min-1 of oxygen. The ET isoflurane had been initially set at 1% for at least 15 min for anesthetic equilibration before the noxious stimulation was carried out by clamping the 3rd or 4th digit of the hind limb using a 20-cm hemostat. A positive response was considered when there was gross purposeful movement of the head or extremities. Once the response was positive or negative, the ET isoflurane concentration was increased or decreased by 0.1 - 0.2%, respectively. The new ET concentration was maintained for at least 15 min for anesthetic equilibration before the noxious stimulation was repeated. The isoflurane MAC was the average concentration of isoflurane between the highest ET concentration at which the purposeful movement was detected and the lowest ET concentration at which the movement was not detected. After determining the isoflurane MAC, the animal was castrated. Respiratory rate, heart rate, systolic arterial pressure, and venous blood gas variables were monitored before and after injection of the tested drug. Sedation scores after dexmedetomidine given alone and in combination with pethidine were significantly higher than that after the placebo injection, and the score after the combination was significantly greater than that after dexmedetomidine given alone. All dogs responded to the noxious stimulation before anesthesia induction. The isoflurane MAC of Groups 2 and 3 were significantly less than that of Group 1 (p <0.05), and the isoflurane MAC of Group 3 was significantly less than that of Group 2. Cardiorespiratory variables except the heart rate were within the clinically acceptable limits. In conclusion, dexmedetomidine given with pethidine provided sedation and sparing effect on the isoflurane MAC significantly greater than dexmedetomidine given alone.