SYNERGISM BETWEEN SEVOFLURANE AND INTRAVENOUS FENTANYL ON A-DELTA ANDC-SOMATOSYMPATHETIC REFLEXES IN DOGS

In this study, we defined the nature of the interactions between sevof lurane and fentanyl on spontaneous and reflex-evoked sympathetic activ ity, resting heart rate (KR), and mean arterial pressure (MAP). Sponta neous renal sympathetic nerve activity (RSNA) and A delta- and C-fiber -mediated somatosympathetic reflexes, evoked by electrical stimulation of radial nerves, and HR and MAP were recorded in anesthetized dogs. In one group, the effects of incremental doses of 2-64 mu g/kg fentany l IV were observed. It had a greater inhibitory effect on C than on A delta reflexes, which were abolished by mean cumulative doses of 64 mu g/kg and approximately 128 mu g/kg, respectively. Although 1.5% sevof lurane reduced C reflexes by 28% and A delta reflexes by only 12%, it reduced the total doses of fentanyl required for their abolition to 32 mu g/kg and 64 mu g/kg, respectively. Mean RSNA, HR, and MAP values w ere reduced by 46%, 54%, and 30%, respectively, by fentanyl alone and by 23%, 11%, and 17%, respectively, in response to 1.5% sevoflurane. T he combination of fentanyl and sevoflurane caused reductions of 44%, 5 4%, and 41%, respectively, which indicates a less than additive effect . These results indicate that sevoflurane interacts synergistically wi th fentanyl to depress A delta and C somatosympathetic reflexes, where as for RSNA, HR, and MAP, their effects were less than the additive. I mplications: Although fentanyl caused a greater depression of C than o f A delta reflexes to the point of abolition, the maximal depression o f spontaneous sympathetic activity, heart rate, and arterial pressure occurred at smaller doses. The combined depressant effects of sevoflur ane and fentanyl were synergistic on somatosympathetic reflexes but we re less than additive on spontaneous sympathetic activity, heart rate, and arterial pressure.