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Keywords:

  • anesthesia;
  • dogs;
  • fentanyl;
  • minimum alveolar concentration;
  • sevoflurane

Abstract

Objective

The objectives of this study were to determine the effects of fentanyl on the end-tidal concentration of sevoflurane needed to prevent motor movement (MACNM) in response to noxious stimulation, and to evaluate if acute tolerance develops.

Study design

Randomized cross-over experimental study.

Animals

Six healthy, adult (2–3 years old), intact male, mixed-breed dogs weighing 16.2 ± 1.1 kg.

Methods

Six dogs were randomly assigned to receive one of three separate treatments over a 3 week period. After baseline sevoflurane MACNM (MACNM-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 MACNM was defined as the minimum end-tidal sevoflurane concentration preventing motor movement. The first post-treatment MACNM (MACNM-I) determination was initiated 90 minutes after the start of the CRI, and a second MACNM (MACNM-II) determination was initiated 3 hours after MACNM-I was established.

Results

The overall least square mean MACNM-B for all groups was 2.66%. All treatments decreased (p < 0.05) MACNM, 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. MACNM-I was not significantly different from MACNM-II within treatments.

Conclusions and clinical relevance

Fentanyl doses in the range of 3–12 μg kg−1 hour−1 significantly decreased the sevoflurane MACNM. Clinically significant tolerance to fentanyl did not occur under the study conditions.