EFFECT OF RACEMIC-MIXTURE AND THE (S-ISOMER OF KETAMINE ON TEMPORAL AND SPATIAL SUMMATION OF PAIN())

We have compared the analgesic efficacy of the racemic mixture and the stereoisomer (S+) of the NMDA antagonist ketamine. In a double-blind, three-way crossover, placebo-controlled study, we assessed the follow ing: pain evoked by small/large area pressure stimuli, pain detection threshold and pain ratings to small/large area of heat stimuli, pain d etection threshold and pain rating to heat stimuli of brief/long durat ion, summation pain threshold and pain ratings to repeated heat/electr ical stimuli, side effects and reaction time. Plasma concentrations of 350 ng ml(-1) for ketamine (racemic) and 180 ng ml(-1) for ketamine ( S+) were tried. We found that ketamine (racemic) prolonged the reactio n time more than ketamine (S+). Both drugs affected pain caused by rep eated stimuli or stimuli of long duration equally or more than a singl e stimulus of short duration. They also affected pain evoked from larg e areas equally or more than pain evoked from small areas. The (S+)-is omer was approximately twice as potent as the racemic mixture of ketam ine in inhibiting central summation.