In one embodiment, EEG signal data entropy is determined (120) as an indication of the depth of anesthesia. A frequency domain power spectrum quantity (126) is obtained fom the patient's EMG signal data (124), which can be updated more frequently than the EEG entropy due to its higher frequency. The EEG entropy indication (122) and the EMG power of spectrum indication (126) can be combined into a composite indicator (128) that provides an immediate indication of patient state changes. In a further embodiment, a set of frequency components obtained from selected frequency ranges (406) is utilized in determining entropy. The patient signal data (400) from which the frequency components are extracted are obtained from time windows of differing lengths (402) so that the resulting entropy indication (410,412) will have optimally fast response times.
METHOD AND APPARATUS FOR DETERMINING THE CEREBRAL STATE OF A PATIENT WITH FAST RESPONSE