ANALGESIC EFFICACY OF LOW-DOSE KETAMINE - SOMATOSENSORY-EVOKED RESPONSES IN RELATION TO SUBJECTIVE PAIN RATINGS

Background: Low-dose ketamine has been shown to exert analgesic effect s. Whether ketamine-induced pain relief may be quantitated by somatose nsory evoked cerebral potentials has not been established. Methods: Th irty healthy volunteers were assigned randomly to one of three groups. Subjects of group 1 (n = 10, control) were given saline as placebo. I n groups 2 (n = 10) and 3 (n = 10), intravenous ketamine (0.25 mg . kg (-1) and 0.50 mg . kg(-1), respectively) was administered. The followi ng variables were recorded at baseline and for 50 min after drug admin istration: electroencephalographic (EEG) data, somatosensory-evoked la te cortical responses (SEP) elicited by intracutaneous stimulation of the fingertip (2-3 fold pain threshold), heart rate, mean arterial blo od pressure, and end-tidal PET(CO2) via a tightfitting mask. Electroen cephalographic spectral power in selected frequency bands and frequenc y percentiles were calculated from the spontaneous EEG segment precedi ng each somatosensory stimulus. Somatosensory-evoked late cortical res ponse parameters were calculated from the respective poststimulus EEG segments. After recording of each EEG response, subjects were asked to rate the individual pain sensation. Results: In group 1, all variable s did not change over time. Ketamine administration resulted in dose-d ependent decreases in alpha-activity and increases in theta power (gro up 2: 190% group 3: 440%). Electroencephalographic changes were not re lated to changes in pain perception. For the first 30 min after ketami ne injection, a dose-dependent decrease of the long-latency N-150-P-25 0 somatosensory-evoked late cortical response component was observed ( group 2: 15-20%; group 3: 25-30%). Subjective pain ratings were also d ifferent between groups, with a higher degree of pain relief in group 3 for the first 30 min. At the end of the observation period, pain rel ief and the N-150-P-250 amplitude were comparable in both ketamine gro ups. Conclusions: These data indicate that pain relief induced by low- dose ketamine is dose-dependent for the first 30 min after bolus injec tion. Changes in pain perception may be quantitated by somatosensory-e voked cortical responses. Also, EEG changes are not specific for chang es in nociception, but the increase in theta power may reflect the hyp notic effect of low-dose ketamine.