Differential effects of systemically administered ketamine and lidocaine on dynamic and static hyperalgesia induced by intradermal capsaicin in humans

We have examined the effect of systemic administration of ketamine and lido caine on brush-evoked (dynamic) pain and punctate-evoked (static) hyperalge sia induced by capsaicin. In a randomized, double-blind, placebo-controlled , crossover study, we studied 12 volunteers in three experiments. Capsaicin 100 mu g was injected intradermally on the volar forearm followed by an i. v. infusion of ketamine (bolus 0.1 mg kg(-1) over 10 min followed by infusi on of 7 mu g kg(-1) min(-1)), lidocaine 5 mg kg(-1) or saline for 50 min. I nfusion started 15 min after injection of capsaicin. The following were mea sured: spontaneous pain, pain evoked by punctate and brush stimuli (VAS), a nd areas of brush-evoked and punctate-evoked hyperalgesia. Ketamine reduced both the area of brush-evoked and punctate-evoked hyperalgesia significant ly and it tended to reduce brush-evoked pain. Lidocaine reduced the area of punctate-evoked hyperalgesia significantly. it tended to reduce VAS scores of spontaneous pain but had no effect on evoked pain. The differential eff ects of ketamine and lidocaine on static and dynamic hyperalgesia suggest t hat the two types of hyperalgesia are mediated by separate mechanisms and h ave a distinct pharmacology.