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Evoked potentials

Created: 20/7/2005

Evoked potentials (EP)

Evoked potential monitors measure electrical activity in certain areas of the brain in response to stimulation of specific sensory nerve pathways.

Somatosensory evoked potentials (SEP)

A supramaximal stimulus is applied to peripheral nerves while a recording scalp electrode is placed over the appropriate sensory area. In general, most anaesthetic agents increase the latency and decrease the amplitude in a dose-dependent manner. Etomidate consistently increases the amplitude.


i] Are SEPs the best predictor of outcome after severe brain injury? A systematic review. Intensive Care Med 2005; 31: 765-75

Visual evoked potentials (VEP)

Light-emitting diodes are incorporated into specialised goggles and the optic nerve is stimulated at 2 Hz. EEG electrodes take recordings from the occiput. Most anaesthetic agents increase the latency and decrease the amplitude of P100 in a dose-dependent manner.  Although VEP are considered less reliable than AEP, they have been used to monitor function during surgery for lesions involving the pituitary gland, optic nerve and chiasma.

Auditory evoked potentials (AEP)

This monitoring technique isolates the neurophysiological signal generated during stimulation of the 8th cranial nerve using a repetitive auditory stimulus at 6-10 Hz.
The repeated sampling allows the signal to be extracted from the background EEG noise. The signal is acquired using EEG electrodes located on the mastoid processes, a midline reference electrode and a ground electrode.

The auditory evoked potential (AEP) is a composite waveform that can be plotted against time. The brainstem AEP (1-10 ms latency) and the late cortical AEP (50-500 ms latency) do not correspond with depth of anaesthesia but the early or mid-cortical AEP does (MCAEP 10-50 ms latency). The mid-latency of the Pa and Nb components is analysed. Anaesthetic agents decrease the amplitude and increase the latency of the MCAEP in a dose-dependent, but agent-independent, manner.

The auditory evoked potential index (AEP index)

The raw waveform is difficult to analyse in real time and is subject to interobserver variability. The auditory evoked potential index (AEP index) is a parameter derived by summing the square root of the difference in amplitude between successive 0.56 ms segments of the MCAEP. This can be calculated online and presented as a running average that can be updated approximately every 30 seconds.

A rough guide to the AEP index

>80 in the awake patient.
<50 in anaesthetised patients.

Unlike the bispectral index (BIS), the transition from asleep to awake is characterised by a sudden increase in the AEP index.

AEP discriminate between the awake and the anaesthetised state better than BIS values, as there is less overlap between the ranges of values in conscious and anaesthetised patients. Nevertheless, interindividual variability in the MCAEP latency at which consciousness is lost is sufficient to make it difficult to define a sensitive and specific cut-off point. However, AEP appear to demonstrate minimal hysteresis within an individual, so that the MLAEP values at loss and recovery of consciousness are well conserved during repeated transitions. This raises the theoretical possibility of defining individualised thresholds for loss and recovery of consciousness. Monitors that process the amplitude and latency changes to the Pa and Nb waves of the AEP, using proprietary algorithms to generate an indexed score between 0-100, are being marketed. However, these have not yet gained wide popularity as a depth of anaesthesia monitor, partly due to problems relating to signal interference. 


[i] Awareness detected by auditory evoked potential monitoring. Br J Anaesth 2003; 91: 290-2

[ii] Comparison of bispectral EEG analysis and auditory evoked potentials for monitoring depth of anaesthesia during propofol anaesthesia. Br J Anaesth 1999; 82: 672-8

[iii] The auditory evoked response as an awareness monitor during anaesthesia. Br J Anaesth 2001: 86: 513-8
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