5. Evoked potentials Flashcards
EPs
one means of monitoring the depth of anaesthesia, and they
are also used to assess spinal cord function during surgery.
AEP
VEP
SSEP
MEP
How
An evoked potential (EP), also known as an evoked response (ER) or event-related
potential (ERP), is an aspect of EEG monitoring.
The signal in the EEG is produced when an individual receives a visual, auditory or somatosensory stimulus, and the EPs are detected by an electrode which is positioned over the primary receiving area for that sensory modality.
Somatosensory evoked potentials (SSEPs), for example, are recorded from electrodes positioned over the sensory cortex.
These low potentials
are extracted from the EEG by a process of computer averaging. The patient is
subjected to a large number of repeated stimuli, and the EEG is recorded during a
fixed period after each one. It is then amplified before the EPs are extracted by taking
the average of this large number of responses
Somatosensory evoked potentials (SSEPs)
Somatosensory evoked potentials (SSEPs): as described, these are potentials of
very small amplitude which are measured by electrodes over the sensory cortex.
Traditionally it was assumed that if sensory pathways were intact, as in spinal surgery
for example, then motor pathways will not have been damaged. This does not always
hold true.
Motor evoked potentials (MEPs):
these are large amplitude potentials which are generated by
stimulation of the motor cortex and measured by needle electrodes
placed in selected muscles.
Their absence suggests damage to the corticospinal tract
whose integrity is essential for the motor response.
In contrast to SSEPs, motor evoked responses are suppressed by volatile anaesthetic agents at MAC values greater than 0.5, and so total intravenous anaesthesia may be indicated for spinal surgery.
An alternative is to test cord function by means of epidural motor EPs, which are
relatively unaffected by anaesthetic agents
Anaesthetic depth
Anaesthetic depth: EPs have been investigated as indicators of the depth of anaesthesia.
Visual and somatosensory EPs show less promise than auditory evoked responses
Spinal surgery:
Spinal surgery:
EPs are used to monitor spinal cord function,
which can be compromised by distraction during scoliosis surgery.
Historically, patients were subjected
to the intraoperative wake-up test,
during which anaesthesia was lightened (with appropriate analgesia) to the point at which the subject could respond to a request to move both arms and legs.
(Arm movement excludes the possibility of neuromuscular blockade.)
This technique actually worked better than it may sound to those who
have never seen it done,
but it is nonetheless crude in comparison to somatosensory and motor EPs.
As described earlier, the potentials are of very low amplitude, and the
signal is averaged.
The latency and amplitude are measured, usually by electrodes
which monitor the cerebral cortex.
Somatosensory potentials are also depressed by high-concentration volatile agents,
although not at normal concentrations,
and by high-dose opioids (such as fentanyl in doses greater than 50 μg kg1),
but the normal clinical use of these drugs does not compromise the technique.
Hypoxaemia and hypoperfusion of the cord are confounding factors which may influence the response.
They decrease its amplitude but do not have any effect on its waveform.
Neurology:
Neurology: EPs are used to aid the diagnosis of a number of neurological conditions.
These include multiple sclerosis and other demyelinating diseases, tumours in the
posterior fossa, in which auditory EPs are useful, and global head injury.
