Isoflurane minimum alveolar concentration sparing effects of fentanyl in the dog.

OBJECTIVE

To characterize the isoflurane-sparing effects of a high and a low dose of fentanyl in dogs, and its effects on mean arterial pressure (MAP) and heart rate (HR).

STUDY DESIGN

Prospective, randomized crossover trial.

ANIMALS

Eight healthy male Beagle dogs weighing 12.1 ± 1.6 kg [mean ± standard deviation (SD)] and approximate age 1 year.

METHODS

Dogs were anesthetized using isoflurane and minimum alveolar concentration (MAC) was determined in duplicate by the bracketing method using an electrical stimulus on the tarsus. Animals were administered fentanyl: low dose (33 μg kg loading dose, 0.2 μg kg minute) or high dose (102 μg kg loading dose, 0.8 μg kg minute) and MAC was re-determined (MAC). Blood was collected for analysis of plasma fentanyl concentrations before administration and after MAC determination. All values are presented as mean ± SD.

RESULTS

Isoflurane MAC (MAC) was 1.30 ± 0.23% in the low dose treatment, which significantly decreased to 0.75 ± 0.22% (average MAC reduction 42.3 ± 9.4%). MAC was 1.30 ± 0.18% in the high dose treatment, which significantly decreased to 0.30 ± 0.11% (average MAC reduction 76.9 ± 7.4%). Mean fentanyl plasma concentrations were 6.2 and 29.5 ng mL for low and high dose treatments, respectively. MAP increased significantly only in the high dose treatment (from 81 ± 8 to 92 ± 9 mmHg). HR decreased significantly in both treatments from 108 ± 25 to 61 ± 14 beats minute with the low dose and from 95 ± 14 to 42 ± 4 beats minute with the high dose.

CONCLUSIONS AND CLINICAL RELEVANCE

Fentanyl administration resulted in a dose-dependent isoflurane MAC-sparing effect with bradycardia at both doses and an increase in MAP only at high dose. Further evaluation is needed to determine the effects of fentanyl on the overall cardiovascular function.