Effect of fentanyl constant-rate infusions with or without medetomidine on the minimum infusion rate of propofol required to prevent motor movement in dogs

Propofol is a potential injectable anesthetic agent used in total intravenous anesthesia. However, the sparing effect of fentanyl and medetomidine on the required propofol dose in dogs remains unclear. We aimed to investigate the effect of fentanyl constant-rate infusion (CRI) with or without medetomidine on the minimum infusion rate of propofol required to prevent motor movement (MIRNM) in dogs. Six healthy purpose-bred dogs were anesthetized on three occasions with propofol alone (loading dose [LD], 8 mg/kg to effect; initial infusion rate [IR], 0.70 mg/kg/min); propofol (LD, 6 mg/kg to effect; IR, 0.35 mg/kg/min) and fentanyl (LD, 2 pg/kg; IR, 0.10 pg/kg/min); or propofol (LD, 4 mg/kg to effect; IR, 0.25 mg/kg/min), fentanyl (LD, 2 pg/kg; IR, 0.10 pg/kg/min), and medetomidine (LD, 2 pg/kg; IR, 0.5 pg/kg/hr) under controlled ventilation. The MIRNM was determined by observing the response to a noxious electrical stimulus. Heart rate, blood pressure, and blood gas analyses were performed at 1, 2, 3, and 4 hr after initiating CRI. The MIRNM (mean [range]) was significantly lower in the propofol-fentanyl-medetomidine group (0.16 [0.10-0.27] mg/kg/min) than that in the propofol-alone group (0.63 [0.47-0.82] mg/kg/min) (P=0.0004). Fentanyl combined with medetomidine did not significantly decrease the mean arterial pressure in dogs receiving propofol CRI 1-3 hr after initiating CRI compared with propofol CRI alone (P>0.9999, P=0.1536, and P=0.0596, respectively), despite inducing a significantly lower heart rate.