[Suppressive effect of isoflurane on somato-sympathetic reflexes in cats]

Effects of isoflurane on the reflex activity of the sympathetic nervous system were investigated in 35 cats by observing alteration of the amplitudes of somato-sympathetic medullary reflex potentials. A and C-reflex potentials, which are induced from lumbar sympathetic trunk by electric stimulation of ipsilateral femoral nerve are known to arise from A-fiber group and C-fiber group, respectively. One or two percent isoflurane in oxygen was administered for 30 minutes, because it is known that I MAC of isoflurane in cats is 1.61%. The amplitude and latency of the potentials were measured during and after inhalation of the test drug, and the results were presented as percentage change from that at prior levels of inhalation as control. The results were as follows. (1) The amplitudes of A- and C-reflex potentials were suppressed dose-dependently in rats with intact brain. (2) The amplitudes of both potentials increased by decerebration at midbrain level. (3) In decerebrate cats, the amplitudes of both potentials at each inhaled concentration were suppressed to the similar levels at inhalation of 2% isoflurane in intact cats, although a slight difference between intact and decerebrated cats was observed with A-potentials during inhalation of 1% isoflurane. (4) The latencies were prolonged slightly in A-reflex potentials, only in intact cats at both concentrations. (5) Blood pressure decreased during inhalation of the test drug in every condition, but their dose-dependencies were evident only in intact cats. (6) Selective suppression of C-reflex potential by inhalation of isoflurane could not be demonstrated. These results suggest that isoflurane has a strong suppressive effect on sympathetic reflex center in medulla oblongata with was demonstrated by greater degree of suppression of the reflex responses in decerebrate cats than that in intact animals. It was concluded that the effect of isoflurane on sympathetic nervous system is similar to that of halothane.