DOES THE BRAIN INFLUENCE SOMATIC RESPONSES TO NOXIOUS STIMULI DURING ISOFLURANE ANESTHESIA

Background: Recent evidence suggests that anesthetic action within the spinal cord is important in suppressing somatic responses to painful stimuli. Whether the brain influences this response is not clear. This study was designed to test the hypothesis that the brain affects anes thetic requirements. Methods: Six goats were anesthetized with isoflur ane. After tracheal intubation and femoral arterial cannulation, bilat eral neck dissections were performed to isolate the external carotid a rteries and external jugular veins. The occipital arteries were ligate d bilaterally. Control isoflurane requirements as defined by the minim um alveolar concentration (MAC) were determined by using a dew-claw cl amp as a painful stimulus. Cranial venous blood was drained into a bub ble oxygenator in which an isoflurane vaporizer was placed in line wit h the gas flow, and arterial blood was infused into a carotid artery w ith a roller pump. This arrangement permitted selective control of the delivery of anesthetic to the head and to the systemic circulation. I soflurane concentration in the arterial blood delivered to the head wa s estimated from the isoflurane concentration in the oxygenator exhaus t. While isoflurane concentration in the head was maintained at approx imately 0.2-0.3%, MAC for the body was determined. After return to the native circulation, MAC was determined again. Results: During bypass with cranial isoflurane concentration at 0.2-0.3%, all animals showed varying, intermittent degrees of light anesthesia, including spontaneo us head movement, chewing, swallowing, and eye opening. Isoflurane MAC was 1.4 +/- 0.2% (mean +/- SD) at baseline, decreased to 0.8 +/- 0.1% during bypass (P < 0.05), and increased to 1.2 +/- 0.2% after bypass (P not significant compared with baseline). Conclusions: These results verify the importance of volatile anesthetic action at an extracrania l site vis a vis purposeful movement in response to a noxious stimulus . Furthermore, the results confirm that the brain affects anesthetic r equirements.