Modulation of remifentanil-induced analgesia,hyperalgesia, and tolerance by small-dose ketamine in humans

Adding a small dose of ketamine to opioids may increase the analgesic effect and prevent opioid-induced hyperalgesia and acute tolerance to opioids. In this randomized, double-blinded, placebo-controlled crossover study, we investigated the effect of remifentanil combined with small concentrations of ketamine on different experimental pain models. Pain detection thresholds to single and repeated IM electrical stimulation and to repeated transcutaneous electrical stimulation, pressure pain tolerance threshold, and sedative, respiratory, and cardiovascular side effects were assessed in 14 healthy volunteers. Saline, remifentanil alone, and remifentanil combined with ketamine at target plasma concentrations of 50 or 100 ng/mL were administered in four study sessions. The ketamine infusion was started after baseline testing at a constant target concentration. Remifentanil was started after testing with ketamine alone at an initial target concentration of 1 ng/mL and then increased to 2 ng/mL and decreased to 1 ng/mL. The last test series were started 10 min after discontinuation of remifentanil. Acute remifentanil-induced hyperalgesia and tolerance were detected only by the pressure pain test and were not suppressed by ketamine. Remifentanil alone induced significant analgesia with all pain tests. Ketamine further increased the remifentanil effect only on IM electrical pain. Remifentanil at a 2 ng/mL target concentration induced a slight respiratory depression that was antagonized by ketamine. We conclude that ketamine effects on opioid analgesia are pain-modality specific. IMPLICATIONS: Coadministration of ketamine and morphine for pain relief is still controversial. Our experimental pain study with volunteers showed that ketamine enhances opioid analgesia without increasing sedation and reduces respiratory depression. Opioid-induced hyperalgesia and tolerance were not affected by ketamine and depended on the type of nociceptive stimulus. This may explain the conflicting results on opioid tolerance in previous studies.