EFFECTS OF SUBANESTHETIC CONCENTRATIONS OF NITROUS-OXIDE ON COLD-PRESSOR PAIN IN HUMANS

Nitrous oxide (N2O) has analgesic properties as determined in both ani mal and human research. In the present study, we sought to determine w hether N2O given in subanesthetic concentrations would reduce cold pre sser (CP)-induced pain. A crossover, double-blind study was conducted in 10 healthy volunteers. Each subject participated in four separate s essions, and in each session the effects of one of four concentrations of N2O in oxygen (0, 20, 30, and 40%) were assessed. The duration of inhalation was 40 min, and within each session, subjects immersed thei r nondominant arm in water (2-3 degrees C) twice for 3 min (at 10 and 30 min intrainhalation). Pain intensity, the degree to which the pain was bothersome (measured on a verbal scale of 0-10, 0 = ''not at all'' and 10 = ''extremely'' painful/bothersome), and pain quality [measure d by the short-form McGill Pain Questionnaire (SF-MPQ)] were assessed during the forearm immersion. Mood effects were measured with the use of visual analogue scales (VAS) in the presence and absence of pain. S elf-reported pain intensity and bothersomeness, SF-MPQ ratings of ''sh arp pain'' and ''throbbing pain,'' and VAS rating of ''unpleasant bodi ly sensations'' were significantly reduced by N2O (p < 0.05) in a conc entration-dependent manner. Nitrous oxide had a number of effects on m ood (e.g., increased VAS ratings of ''stimulated,'' ''high,'' ''coasti ng,'' ''carefree,'' and ''having pleasant bodily sensations''). The co ld-water immersion also influenced mood, but had little impact on modu lating N2O effects. Results from our study indicate that the CP test i s a sensitive assay to measure the analgesic properties of subanesthet ic concentrations of N2O in humans.