Synergistic antinociceptive interaction between sevoflurane and intrathecal fentanyl in dogs

Abstract

This study determined the nature of the antinociceptive interaction between sevoflurane and intrathecal fentanyl on somatosympathetic reflexes in anaesthetized dogs. Afferent Aδ- and C-fibre-mediated somatosympathetic reflexes, evoked by supramaximal electrical stimulation of tibial nerves, were recorded from renal sympathetic nerves. The effect of fentanyl alone, administered intrathecally (i.t.) in incremental doses from 2 to 64 μg, was compared with the effect of the same doses during the administration of 1.5% sevoflurane. The mean ED50s for the depressant effect of fentanyl (i.t.) on Aδ and C reflexes were 35.6 μg and 14.2 μg while 1.5% sevoflurane, when administered alone, depressed them by 15.5% (P < 0.05) and 27.5% (P < 0.01) respectively. During the administration of 1.5% sevoflurane, the mean ED50s of fentanyl (i.t.) for the depression of Aδ and C reflexes were reduced by 76% and 75%, to 8.5 μg and 3.5 μg respectively. The combined antinociceptive effects of sevoflurane and intrathecal fentanyl were not additive but exhibited a high degree of synergistic interaction.