Data Acquisition Period

An important consideration in the processed EEG is the element of time. The standard EEG is continuous in real time. The processed EEG samples data over a given period (i.e., epoch), processes the data, and then displays information in various formats. There is a relationship between epoch length and spectral resolution. If a long epoch length is chosen, the waveform can be described precisely, but the time required for data processing is long. If a short length of data is sampled, analysis may be done almost in real time, but the epoch chosen for analysis may not be representative of the overall waveform. There may also be insufficient data points for meaningful Fourier transformation. This issue, as related to the use of intraoperative EEG for analysis of anesthetic depth, has been studied by Levy.^[21] A longer epoch may produce less epoch-to-epoch variability and allow more precise description of frequency and power; however, the longer epoch increases the delay before new information is processed and displayed, thereby reducing the amount and timeliness of information available for clinical decision-making. In studying epochs of 2 to 32 seconds, Levy concluded that 2-second epochs are appropriate during general anesthesia. Many of the commercially available devices have used 2-second epoch lengths, which are updated at various user-selected intervals. With better and faster computers, continuous monitoring of 2-second epochs is possible.