Analgesic efficacy of i.m. alfentanil

We have studied the analgesic, respiratory, cardiovascular and sedative effects of alfentanil 30 micrograms kg-1 i.m. in two experiments in six healthy volunteers. In the first experiment, recordings were made up to 120 min after injection. Thresholds (sensory and pain) and pain-related evoked potentials (amplitude and latency) to cutaneous argon laser stimulation were used for quantitative assessment of onset, efficacy and duration of analgesia. Pain was significantly reduced from 5 to 60 min after injection. The onset phase of analgesia was faster than the recovery phase. In the second experiment, the alfentanil-induced analgesia was antagonized by naloxone 0.4 mg i.v. The reaction time to pain was prolonged significantly 15 min after injection, whereas the latency of the pain-related potential remained constant. No clinically significant changes were observed in respiratory and cardiovascular measurements.