Evidence of auditory processing during postoperative propofol sedation

Objectives: Our aim was to characterize cerebral event-related responses, which index the detection of auditory stimuli during postoperative sedation. Methods: We monitored auditory event-related potentials (ERPs) before and after elective cardiac operation in 29 patients. Sedation levels, induced with propofol, were evaluated clinically with Ramsay score (RS). Results: During deep sedation (RS 6), patients could be categorized into 3 groups according to ERP responses evoked by a standard ‘oddball’ paradigm. In one group, there were no cortical responses indexing the detection of a sound (N100), another group showed clear responses, and the third group was characterized by a later P300a component which was taken to reflect orienting to a novel stimulus in adults who were awake. However, in deep sedation, P300a did not show behaviour characteristic to an orienting reaction. In moderate sedation (RS 4), all the patients showed a visible N100. Total sedative propofol dose, hemodynamics and the spontaneous electroencephalography (EEG) were not connected to the category of an individual's responses. Conclusions: The results of our study suggest that the detection and automatic auditory information processing function during postoperative sedation, and may signal the individual stages of awakening in a sensitive way. In addition, the findings suggest a deficit in sensory memory function during sedation.