Reduction of the minimum alveolar concentration of isoflurane in dogs using a constant rate of infusion of lidocaine–ketamine in combination with either morphine or fentanyl

The objective of this study was to determine the effects of a constant rate of infusion of lidocaine and ketamine in combination with either morphine or fentanyl on the minimum alveolar concentration of isoflurane (MACISO) during ovariohysterectomy in dogs. Female dogs (n = 44) were premedicated with acepromazine and midazolam. Anaesthesia was induced with propofol and maintained with isoflurane. Dogs received ketamine (0.6 mg/kg/h) and lidocaine (3 mg/kg/h) together with morphine (0.24 mg/kg/h; MLK) or fentanyl (0.0036 mg/kg/h; FLK). The control group received Ringer’s lactate solution. A skin incision was used as the noxious stimulus. The MACISO value was obtained with Dixon’s up-and-down method. MACISO was 0.7 ± 0.0 vol.% in the control group, 0.3 ± 0.0 vol.% in the MLK group (45% MAC reduction) and 0.0 ± 0.0 vol.% in the FLK group (97% MAC reduction). A combination of fentanyl with lidocaine and ketamine decreased the MACISO in dogs; this decrease was more pronounced than that produced by morphine, lidocaine and ketamine.