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Intravenous administration of 0.8 microgram/kg and 1.1 micrograms/kg fentanyl in low back pain patients reduced both sensory intensity and unpleasantness visual analogue scale (VAS) responses to experimental pain evoked by graded 5-sec nociceptive temperature stimuli (45-51 degrees C) as well as VAS-sensory and VAS-affective responses to clinical pain. Fentanyl produced similar decreases in VAS-sensory responses to experimental and clinical pain. Fentanyl produced nearly equal reductions in VAS-sensory and VAS-affective responses to experimental pain but greater reductions in clinical pain VAS-affective as compared to clinical pain VAS-sensory responses. This interaction of type of pain (experimental versus clinical) and pain dimension (sensory versus affective) results from either a steeper sensory intensity-unpleasantness relationship for clinical pain as compared to experimental pain or additional selective influences of opiates on affective factors uniquely related to clinical pain. These results indicate that low to moderate doses of opiates reduce both sensory and affective dimensions of pain and strongly suggest that changes in pain affect occur mainly as a direct consequence of reductions in pain sensation intensity.
Price et al. (Sat,) studied this question.