Nocifensive reactions during propofol-induced loss of consciousness are not based on the development of central hyperalgesia—A functional imaging study in humans

Abstract

Summary Background Barbiturates are widely held to have hyperalgesic properties. Using functional magnetic resonance imaging (fMRI) techniques we sought to investigate the effects of graded doses of propofol on central pain processing in the lateral and medial pain system in healthy volunteers. Methods Touch stimuli as well as phasic and tonic mechanical pain stimuli were applied to the right hand of 18 volunteers. fMRI was performed on an 1.5 T scanner using EPI-sequences. The experiments were repeated under targeted propofol plasma levels (TCI 0, 1.5, 2.5 and 3.5 μg/ml). Changes of the BOLD signal in different cortical areas (primary and secondary somatosensory cortical areas S1 and S2, insular, anterior cingulate cortex and frontal areas) were investigated during propofol anaesthesia using correlation and clustering analysis. Results Functional activation in cerebral areas belonging to the lateral pain system (S1, S2 and insular cortex) was significantly reduced with increasing propofol plasma levels, whereas activation of the medial system (anterior cingulate and prefrontal cortex) was nearly unaffected. Conclusion Propofol seems to preferentially interfere with pain processing at the cortical level in the lateral projection system while the medial system controlling affective components obviously remains unchanged. Therefore, the clinically observed nocifensive reactions during painful stimulation under propofol anaesthesia are most likely the result of differential effects of the drug on different central nervous system pathways leading to nocifensive hyperreflexia.