[Bispectral index decreased to "10" during propofol anesthesia despite the appropriate serum concentration of propofol].

We report a case showing a bispectral index (BIS) value "10" during anesthesia maintained with propofol. Vital signs of the patient including blood pressure and heart rate were stable and the depth of anesthesia was considered as adequate. The serum concentration of propofol measured later was 2.5 and 2.2 microg ml(-1) when BIS indicated 10 and 35, respectively. The age of patient was 81 and the BIS values in elderly might decrease in accordance with the reduction of brain function.