Time-frequency spectral representation of the EEG as an aid in the detection of depth of anesthesia.

A time-frequency spectral representation (TFSR) has been used to study the nonstationary information in the EEG as an aid in determining the anesthetic depth. This paper uses a TFSR with an exponential weighting function for the purpose. Raw EEG data were collected form 10 mongrel dogs at various levels of halothane anesthesia. Depth of anesthesia was tested by observing the response to tail clamping, which is considered a supramaximal stimulus in dogs. A positive response was graded as awake (depth 0), and a negative response was graded as asleep (depth 1). The EEG obtained during a period of 30 sec tail clamp was processed into TFSRs. It was observed that at depth 0, the spectrum becomes localized in time and frequency. The percentage of energy in the delta (1-3.5 Hz) and theta (3.5-7.5 Hz) frequency bands increased. At depth 1, the spectrum remained unchanged throughout the period of tail clamp. The performance of the TFSR in detecting the patient's awareness was also compared with the power spectrum. It was concluded that under certain anesthetic conditions, the TFSR is superior to the power spectrum.