PSYCHOPHYSICAL AND ELECTROPHYSIOLOGICAL RESPONSES TO EXPERIMENTAL PAIN MAY BE INFLUENCED BY SEDATION - COMPARISON OF THE EFFECTS OF A HYPNOTIC (PROPOFOL) AND AN ANALGESIC (ALFENTANIL)

Sedation may influence the responses of some experimental pain models used to test analgesic efficacy. In this study we compared the effects of a sedative (propofol) and analgesic (alfentanil) on: nociceptive r eflex to single and repeated electrical stimulations; mechanical press ure pain;and evoked potentials elicited by nociceptive (electrical and laser) and non-nociceptive (acoustical) stimulation. We studied 12 he althy volunteers with two subanaesthetic concentrations of propofol an d two analgesic concentrations of alfentanil. Both propofol and alfent anil increased the threshold for nociceptive reflex to single electric al stimulations, but only alfentanil increased the threshold for nocic eptive reflex to repeated electrical stimulations. The pressure pain t olerance thresholds were increased significantly by alfentanil, wherea s propofol significantly decreased the thresholds (hyperalgesia). Prop ofol and alfentanil induced similar reductions in the amplitudes of th e evoked potentials elicited by nociceptive (electrical and laser) and non-nociceptive (acoustical) stimulation, whereas only alfentanil red uced the perceived pain to nociceptive stimulations. We have shown tha t sedation can influence both the psychophysical and electrophysiologi cal responses of some experimental pain tests used to measure analgesi c efficacy, and that propofol in subhypnotic doses, has no analgesic e ffect on painful electrical and heat stimulations, but has a hyperalge sic effect on mechanical pressure pain.