EFFECT OF DIFFERENTIAL DELIVERY OF ISOFLURANE TO HEAD AND TORSO ON LUMBAR DORSAL HORN ACTIVITY

Background: The spinal cord appears to be the site where anesthetic ag ents prevent movement in response to noxious stimuli. When isoflurane is differentially delivered to the head and torso (with low torso conc entrations), cranial anesthetic requirements increase compared with sy stemic administration. The aim of the current study was to test the hy pothesis that isoflurane action in the brain has descending influences on spinal cord dorsal horn neurons. A secondary aim was to determine the association, if any, of high cranial concentrations of isoflurane (>6%) with dorsal horn activity. Methods: Ten goats were anesthetized with isoflurane and the carotid arteries and jugular veins isolated an d cannulated for cerebral bypass. A laminectomy was performed for reco rding from single lumbar dorsal horn neurons with hind limb mechanical receptive fields (one cell per goat). A standard noxious mechanical s timulus was applied to the den clan or hoof bulb during a control peri od with end-tidal isoflurane at 1.3% and during bypass with the follow ing head/torso isoflurane concentrations: 1.3%/1.3%, 3.2%/1.3%, 9.4%/1 .3%, 1.3%/0.2%, 3.0%/0.2% and 8.8%/0.3%. Results: When torso isofluran e concentration was 1.3%, increasing cranial isoflurane concentration to 3% or 3% had no significant effect on the activity of dorsal horn u nits. When torso isoflurane was 0.2-0.3%, spontaneous activity increas ed; however, at these torso concentrations, evoked responses were sign ificantly decreased (-60%) only when cranial isoflurane concentration was increased to 9%. Conclusions: Isoflurane action in the brain had a n inhibitory effect on dorsal horn activity with the combination of su praclinical cranial and low torso concentrations.