Isoflurane depresses electroencephalographic and medial thalamic responses to noxious stimulation via an indirect spinal action

Anesthetics such as isoflurane act in the spinal cord to suppress movement in response to noxious stimulation. Spinal anesthesia decreases hypnotic/sedative requirements, possibly by decreasing afferent transmission of stimuli. We hypothesized that isoflurane action in the spinal cord would similarly depress the ascending transmission of noxious input to the thalamus and cerebral cortex. In six isoflurane-anesthetized goats, we measured electroencephalographic (EEG) and thalamic single-unit responses to a clamp applied to the forelimb. Cranial bypass permitted differential isoflurane delivery to the torso and cranial circulations. When the cranial-torso isoflurane combination was 1.3% ± 0.2%-1.0% ± 0.4% the noxious stimulus did not evoke significant changes in the EEG or thalamic activity: 389 (153-544) to 581 (172-726) impulses/min, (median, 25th-75th percentile range, P > 0.05). When the cranial-torso isoflurane combination was 1.3% ± 0.2%-0.3% ± 0.2%, noxious stimulation increased thalamic activity: 804 (366-1162) to 1124 (766-1865) impulses/min (P < 0.05), and the EEG 'desynchronized': total EEG power decreased from 25 ± 20 μV² to 12 ± 8 μV² (P < 0.05). When the cranial-torso isoflurane was 1.7% ± 0.1%-0.3% ± 0.2%, the noxious stimulus did not significantly affect thalamic: 576 (187-738) to 1031 (340-1442) impulses/min (P > 0.05), or EEG activity. The indirect torso effect of isoflurane on evoked EEG total power (12.6 ± 2.7 μV²/vol%, mean ± SE) was quantitatively similar to the direct cranial effect (17.7 ± 3.0 μV²/vol%; P > 0.05). These data suggest that isoflurane acts in the spinal cord to blunt the transmission of noxious inputs to the thalamus and cerebral cortex, and thus might indirectly contribute to anesthetic endpoints such as amnesia and unconsciousness.