Physostigmine Reverses Propofol-induced Unconsciousness and Attenuation of the Auditory Steady State Response and Bispectral Index in Human Volunteers

Abstract

Background It is postulated that alteration of central cholinergic transmission plays an important role in the mechanism by which anesthetics produce unconsciousness. The authors investigated the effect of altering central cholinergic transmission, by physostigmine and scopolamine, on unconsciousness produced by propofol. Methods Propofol was administered to American Society of Anesthesiologists physical status 1 (n = 17) volunteers with use of a computer-controlled infusion pump at increasing concentrations until unconsciousness resulted (inability to respond to verbal commands, abolition of spontaneous movement). Central nervous system function was assessed by use of the Auditory Steady State Response (ASSR) and Bispectral Index (BIS) analysis of electrooculogram. During continuous administration of propofol, reversal of unconsciousness produced by physostigmine (28 microgram/kg) and block of this reversal by scopolamine (8.6 microgram/kg) were evaluated. Results Propofol produced unconsciousness at a plasma concentration of 3.2 +/- 0.8 (+/- SD) microgram/ml (n = 17). Unconsciousness was associated with reductions in ASSR (0.10 +/- 0.08 microV [awake baseline 0.32 +/- 0.18 microV], P < 0.001) and BIS (55.7 +/- 8.8 [awake baseline 92.4 +/- 3.9], P < 0.001). Physostigmine restored consciousness in 9 of 11 subjects, with concomitant increases in ASSR (0.38 +/- 0.17 microV, P < 0.01) and BIS (75.3 +/- 8.3, P < 0.001). In all subjects (n = 6) scopolamine blocked the physostigmine-induced reversal of unconsciousness and the increase of the ASSR and BIS (ASSR and BIS during propofol-induced unconsciousness: 0.09 +/- 0.09 microV and 58.2 +/- 7.5, respectively; ASSR and BIS after physostigmine administration: 0.08 +/- 0.06 microV and 56.8 +/- 6.7, respectively, NS). Conclusions These findings suggest that the unconsciousness produced by propofol is mediated at least in part via interruption of central cholinergic muscarinic transmission.