Measuring the Depth of Anesthesia Using Ordinal Power Spectral Density of Electroencephalogram

The decreasing trend in power spectral density (PSD) of electroencephalogram (EEG) has been shown to be related with the depth of anesthesia (DOA). However, conventional DOA indices, which utilize part of EEG frequency band, have a difficulty in quantifying the decreasing trend in the EEG PSD along the frequency axis. This paper proposes a method of effectively quantifying the characteristic change in the EEG PSD to measure the DOA. The method is based on the newly introduced ordinal PSD (O-PSD) which assigns ordinal indices to a series of values in the order of PSD magnitude. The O-PSD can capture the spectral trend along the overall frequency band robust to the EEG variation due to inter-subject dependency and measurement environment. We quantified the O-PSD pattern into a unitless index in the range 0 to 1. We compared the proposed O-PSD based index with the conventional indices for 15 subjects with injection rate of 12 mg/kg/h and 12 subjects with injection rate of 6 mg/kg/h. We evaluated the discriminative performance of the proposed index using prediction probability and Spearman's correlation coefficient. Also, we evaluated the steadiness and stability of the proposed index using the coefficient of variation. Our proposed index was shown to be superior in distinguishing consciousness and unconsciousness, and a stable and steady measure during unconsciousness. These results indicate that the O-PSD and the proposed index would be a reliable method for quantifying the DOA.