A quantitative double-blind evaluation of the antinociceptive effects of perineurally administered morphine compared...

In two double-blind, placebo-controlled investigations, morphine and lidocaine were administered perineurally to the ulnar nerve. Thresholds (warmth and pain) and pain-evoked brain potentials (amplitude and latency) to argon laser stimulation were measured up to 120 min after the injection. Hypalgesia to laser pain was detected 15 min after the injection of morphine and 5 min after the injection of lidocaine. The duration of hypalgesia and analgesia was less than 15 min for morphine and 85 min for the lidocaine injection. Both morphine and lidocaine increased the latency of the brain potentials, which indicates that the same blocking mechanisms could be involved. Pin-prick analgesia was obtained 5 min after the injection of lidocaine, but 15-30 min elapsed before the laser pain was inhibited maximally. Laser pulses can activate larger skin areas than needle pricks, indicating that a central summation of the activity from many cutaneous nociceptors is important in order to obtain a reliable indicator of adequate analgesia.