How are the EEG and EMG used in anaesthesia?

Performing a complete EEG with 20–22 electrodes in the intraoperative environment is not practical and the raw EEG is complex to interpret. Modified EEG-based depth of anaesthesia monitors such as bispectral (BIS) analysis and compressed spectral array (computer-processed EEG) are more commonly used in the intraoperative setting. The BIS monitor analyses EEG data obtained from four electrodes placed on the forehead. This involves Fourier’s analysis to determine the frequency components of the waveform, with subsequent comparison of the phase of each wave relative to others. A dimensionless number between 0 and 100 is displayed, 100 indicating fully awake and 0 the absence of cortical activity. A suggested suitable depth for general anaesthesia lies within the 40–60 range.

The EMG can be used to distinguish between disorders of muscle, nerve, and the neuromuscular junction.

Monitoring motor-evoked potentials during spinal surgery can be used to alert the surgical team of impending spinal cord compromise. Scalp electrodes stimulate the motor cortex at regular intervals, and the EMG response is measured through needle electrodes in the tibialis anterior, abductor hallucis or vastus medialis. Reduction in EMG activity indicates potential compromise of the corticospinal tract.

EMG can be used to assess the presence of residual block following neuromuscular blockade. A supramaximal stimulus with a unipolar square waveform current of 20–60 mA for 0.2–0.3 ms is applied to a peripheral nerve (commonly the facial nerve or ulnar nerve). The EMG response at the corresponding muscle is then measured. The EMG is considered the gold standard when assessing neuromuscular blockade, as it is able to detect muscle action potentials even when mechanical contraction is undetectable.