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Spontaneous electrical activity in the superficial layers of the cerebral cortex may be recorded from the scalp, surface, or the substance of the brain by electrodes. When a complex waveform is obtained from the scalp, it is called the electroencephalogram (EEG). In contrast, the waveform obtained from the surface of the brain is known as the electrocorticogram. Empiric observations have been well described and consistent to distinguish gross clinical states such as consciousness, unconsciousness, sleep, anesthesia, epilepsy, and coma. However, the correlations of the EEG with drug effects, anesthetic depth, and awareness are less clear and consistent. Though efforts have been made about the EEG in terms of the clinical status and outcome of patients, the interpretation of the EEG is still largely operator dependent, that is, largely dependent on the experience and expertise of the operator.
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EEG RECORDING AND PROCESSING
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The classic “10-20 system” montage is used to record the electroencephalogram (Figure 25-1). This system is a symmetric array of 20 scalp electrodes: 10% of the circumferential distance above the inion, nasion, and the external auditory meatus, and 20% of the circumferential distance apart. Additional outside electrodes are used as reference electrodes. Each electrode–reference pair forms a recording channel. Anywhere from 2 to 16 channels may be used at the discretion of the operator.
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The strength of the EEG signal reaches a maximum of 50 μV at the scalp surface. When the recording electrode is placed properly, the impedance is approximately 5000 ohms. Increasing impedance will increase the random electrical noise and obscure the EEG signal. Though variations are present in individuals and individual generators, certain EEG patterns appear to be common and normal. Interpretation of the EEG depends on the assessment on the frequency, amplitude, form, and distribution of the electrical activity (Figure 25-2). EEG frequencies are classified as Alpha (8–13 Hz), Beta (>13 Hz), Delta (<4 Hz), and Theta (4–8 Hz) (Table 25-1).
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