Detection of Consciousness by EEG and Auditory Evoked Potentials

During the past years, monitoring of anesthetic effects on the main target of anesthesia, the brain, has gained increasing attention. Monitoring of the spontaneous electroencephalogram (EEG) and of auditory evoked potentials (AEP) has been suggested. Even if EEG and AEP reflect effects of anesthetic drugs, a visual interpretation is not expedient. As a consequence, several processing methods have been suggested that reduce the electroencephalogram to a numerical value. In the current study, a set of EEG- and AEP parameters based on different analysis methods is tested with respect to the parameters ability in separating consciousness (reflected by responsiveness to command) from unconsciousness at the transition between these stages. Therefore data of 80 unpremedicated patients undergoing general anesthesia with remifentanil and either sevoflurane or propofol is used. The study period is from induction of anesthesia until patients follow command after surgery and includes a reduction of the hypnotic agent after tracheal intubation until patients follow command. The isolated forearm technique is applied before muscle relaxants are given to maintain the ability to squeeze hand to command.