Contribution of the spinal cord to arousal from inhaled anesthesia: comparison of epidural and intravenous fentanyl on awakening concentration of isoflurane.

UNLABELLED: To investigate the contribution of modulation of afferent nociceptive inputs by an opioid in the spinal cord to arousal from inhaled anesthesia, we determined the awakening concentration of isoflurane in 50 unpremedicated patients scheduled for abdominal hysterectomy. Patients were assigned randomly to three groups. Group I received bolus injections of both epidural and intravenous (I.V.) saline, followed by both epidural and I.V. infusions at the rate of 0.2 mL x kg(-1) h(-1). Group II received an I.V. injection of fentanyl 2 microg/kg, followed by an infusion at the rate of 25 ng x kg(-1) x min(-1), and Group III received an epidural injection and infusion in the same administration regimen as Group II. Anesthesia was induced with and maintained by isoflurane in an air/oxygen mixture (fraction of inspired oxygen = 0.5) with no adjuvant drugs. The study drug was administered at the start of retroperitoneal suturing. The awakening concentrations of isoflurane in Groups I, II, and III (mean +/- SD) were 0.32% +/- 0.07%, 0.31% +/- 0.06%, and 0.24% +/- 0.06%, respectively. At that time, plasma fentanyl concentrations in Groups II and III were 1.12 +/- 0.09 ng/mL and 0.65 +/- 0.04 ng/mL, respectively. Epidural fentanyl infusion reduced the awakening concentration of isoflurane more (P < 0.01) than I.V. fentanyl infusion, despite the lower plasma concentration (P < 0.01) in the epidural group. These findings suggest that epidural fentanyl delays arousal from inhaled anesthesia by modulating the afferent nociceptive inputs in the spinal cord. The spinal cord may contribute to arousal from inhaled anesthesia through the regulation of afferent inputs by opioids along with the supraspinal region of the central nervous system (CNS), even if the effects of subarachnoid fentanyl on the higher CNS via the cephalad migration is taken into consideration.

IMPLICATIONS: The present study revealed that the spinal cord, the lower level of central nervous system, contributed to arousal from general anesthesia, along with the higher central nervous system, by comparing the concentrations of an inhaled anesthetic, isoflurane, in the expiration of patients receiving systemic or regional administration of an opioid, fentanyl.