The effect of fentanyl on the end‐tidal sevoflurane concentration needed to prevent motor movement in dogs

ObjectiveThe objectives of this study were to determine the effects of fentanyl on the end-tidal concentration of sevoflurane needed to prevent motor movement (MAC_NM) in response to noxious stimulation, and to evaluate if acute tolerance develops.Study designRandomized cross-over experimental study.AnimalsSix healthy, adult (2–3 years old), intact male, mixed-breed dogs weighing 16.2 ± 1.1 kg.MethodsSix dogs were randomly assigned to receive one of three separate treatments over a 3 week period. After baseline sevoflurane MAC_NM (MAC_NM-B) determination, fentanyl treatments (T) were administered as a loading dose (Ld) and constant rate infusion (CRI) as follows: T1-Ld of 7.5 μg kg^?1 and CRI at 3 μg kg^?1 hour^?1; T2-Ld of 15 μg kg^?1 and CRI at 6.0 μg kg ^?1 hour^?1; T3-Ld of 30 μg kg^?1 and CRI at 12 μg kg^?1 hour^?1. The MAC_NM was defined as the minimum end-tidal sevoflurane concentration preventing motor movement. The first post-treatment MAC_NM (MAC_NM-I) determination was initiated 90 minutes after the start of the CRI, and a second MAC_NM (MAC_NM-II) determination was initiated 3 hours after MAC_NM-I was established.ResultsThe overall least square mean MAC_NM-B for all groups was 2.66%. All treatments decreased (p < 0.05) MAC_NM, and the decrease from baseline was 22%, 35% and 41% for T1, T2 and T3, respectively. Percentage change in T1 differed (p < 0.05) from T2 and T3; however, T2 did not differ from T3. MAC_NM-I was not significantly different from MAC_NM-II within treatments.Conclusions and clinical relevanceFentanyl doses in the range of 3–12 μg kg^?1 hour^?1 significantly decreased the sevoflurane MAC_NM. Clinically significant tolerance to fentanyl did not occur under the study conditions.