Isoflurane depresses motoneuron excitability by a direct spinal action: An F-wave study

Isoflurane decreases motoneuron excitability as measured by the F wave. It is unknown how much of this effect is direct and how much occurs as the result of an indirect, supraspinal effect. Seven goats were anesthetized with isoflurane, and the carotid arteries and jugular veins were isolated to permit cranial bypass. Isoflurane was delivered to the head via a bypass-oxygenator unit and to the torso via the lungs. Evoked gastrocnemius muscle potentials were measured after supramaximal electrical stimulation of the ischiatic nerve. F- and M-wave amplitudes and F-wave latencies were determined from 20 evoked responses obtained at each of the following head/torso isoflurane concentrations: 0.8%/0.3%, 1.3%/0.3%, 3%/0.3%, 0.3%/0.8%, and 1.3%/0.8%. When the torso isoflurane was 0.3%, increasing the cranial isoflurane concentration from 0.8% to 1.3% did not significantly affect the F/M amplitude ratio (from 0.188 +/- 0.166 to 0.194 +/- 0.124; P > 0.05), but the ratio decreased approximately 50% when the cranial isoflurane was 3% (to 0.088 rf: 0.078; P < 0.05). F-wave latency was not affected by changing the cranial isoflurane concentration. The F/M amplitude ratio decreased 80%-85% when isoflurane 0.8% was administered to the torso and was not measurable in five animals; at this torso concentration, changing the cranial concentration from 0.3% to 1.3% had no effect on the F/M ratio (from 0.042 +/- 0.065 to 0.030 +/- 0.041; P > 0.05). In goats, motoneuron excitability is very sensitive to the direct action of isoflurane, and supraspinal effects occur between cranial isoflurane concentrations of 13% and 3%. Implications: We studied the effect of isoflurane action in the brain on motoneuron excitability, using the F wave. We found that the F wave is very sensitive to the direct action of isoflurane and that cranial isoflurane has indirect effects between concentrations of 1.3% and 3%.