title: Effect of propofol anaesthesia on the event-related potential mismatch negativity and the auditory-evoked potential N1.
abstract: BACKGROUND
  Studies on the effects of anaesthesia on event-related potentials and long latency auditory-evoked potentials (AEP) are sparse. Both provide information on cortical processing and may have potential as monitors of awareness. We studied the effect of propofol on the event-related potential mismatch negativity (MMN) and the long-latency AEP NI.
  
  
  METHODS
  Twenty-one patients received 1 microgram ml-1 stepped increases in the target concentration of propofol using Diprifusor until a maximum of 6 micrograms ml-1 was achieved or the patient had lost consciousness. Neurophysiological responses (MMN and N1) and the patients' level of consciousness were recorded before the administration of propofol and at a target effector site concentration of propofol of 1, 2, 3, 4, and 6 micrograms ml-1. Grand average evoked potentials were computed at baseline, before the administration of propofol (A); at the highest propofol concentration at which each patient was responsive (B); and at the concentration of propofol at which the patient became unconscious (C).
  
  
  RESULTS
  Patients lost consciousness at different target concentrations of propofol, all being unresponsive by 4 micrograms ml-1. The response to the deviant stimuli used to elicit duration-shift MMN was significantly more negative than to the standard stimuli at A (mean difference 2.58 microV, P = 0.0011) but this difference was virtually abolished at point B, before the patients lost consciousness (mean difference 0.63 microV, P = ns). The amplitude of N1 evoked by standard stimuli was negative compared with electrical baseline at both point A (mean amplitude -3.81 microV, P < 0.001) and at point B (mean amplitude -2.2 microV, P = 0.002), but was no longer significantly different to baseline at point C (mean amplitude 0.51 microV, P = ns). The change in the mean amplitude of N1 from last awake (point B) to first unconscious (point C) was also significant (mean difference in amplitude 1.69 microV, P = 0.02).
  
  
  CONCLUSIONS
  MMN is unlikely to be a clinically useful tool to detect awareness in surgical patients. In contrast, the loss of N1 may identify the transition from consciousness to unconsciousness and deserves further study.
authors.affiliation: Department of Anaesthesia, Frenchay Hospital, Frenchay Park Road, Bristol BS16 1LE, UK.
authors.forename: T P
authors.initials: TP
authors.lastname: Simpson
authors.forename: A R
authors.initials: AR
authors.lastname: Manara
authors.forename: N M
authors.initials: NM
authors.lastname: Kane
authors.forename: R L
authors.initials: RL
authors.lastname: Barton
authors.forename: C A
authors.initials: CA
authors.lastname: Rowlands
authors.forename: S R
authors.initials: SR
authors.lastname: Butler
mesh_terms: D000328:Adult; D000368:Aged; D018686:Anesthetics, Intravenous; D001364:Awareness / Q000502:physiology; D003243:Consciousness / Q000502:physiology; D005071:Evoked Potentials / Q000502:physiology*; D005072:Evoked Potentials, Auditory / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D015742:Propofol
chemical_list: D018686:Anesthetics, Intravenous; D015742:Propofol