The aim of this study was to characterize within-patient
alfentanil dose-response curves for the relief of spontaneous
postoperative pain and to test the closeness of relationships 1) between
pain intensity and
alfentanil analgesic requirements, and 2) between
alfentanil requirements for
analgesic and nonanalgesic (
sedative and
miotic) effects. The effects of
alfentanil were studied in 23 patients after elective abdominal surgery. During a 40- to 60-min testing session, the patient received two intravenous (i.v.)
injections of saline (placebo) and up to six 3-micrograms/kg increments of
alfentanil at 5-min intervals. The patient rated the
pain intensity with a visual analog scale, and in a subgroup of 15 patients the blinded observer rated patients' sedation and measured pupil diameter. Spontaneous
postoperative pain was completely relieved in all patients with cumulative doses of
alfentanil ranging from 6 to 8 micrograms/kg. The within-patient
alfentanil dose-
analgesic response curves were primarily quantal in nature: a precipitous decrease in
pain intensity (representing > or = 2/3 of the total effect) after the injection of only one of the increments of
alfentanil was observed in 15 of 23 patients. When the
analgesic effect of
alfentanil was presented as the collective response of a group of individuals, the quantal nature of the response was concealed by the wide interindividual variability of the response. No statistically significant relationship was noted between predrug
pain intensity and the cumulative dose of
alfentanil necessary to produce
pain relief. A strong correlation was found between interpatient variabilities in the
analgesic and
sedative effects of
alfentanil (r = 0.75, P < 0.002). At the same time, the relationship between
alfentanil requirements for
pain relief and that for pupil constriction did not demonstrate any significant correlation. The results suggest that, in a population of patients with
postoperative pain, the intensity of spontaneous
pain cannot be the primary factor determining the dose of
alfentanil necessary for its relief.