AoA/ Neurological/ Muscular Flashcards
BIS/ Entropy Neuromuscular blockade testing Sensory evoked potentials
What are Sensory Evoked Responses?
SERs are electric CNS responses to electric, visual or auditory stimuli.
How is noise removed from sensory evoked response signals?
To remove background noise (eg EEG, EMG, ECG), the signal is digitised, signal averaging is applied, and recording is time-locked to the stimulus. The SER signal is a constant time from the stimulus, whereas noise is at random intervals. The random signals are removed, and the signal-to-noise ratio improves proportionally to the square root of the number of responses added.
What are Sensory Evoked Responses?
SERs are electric CNS responses to electric, visual or auditory stimuli.
What are near-field potentials?
Near-field potentials are SERs recorded from electrodes close to (3-4cm) the neural generators – therefore the morphology depends on the electrode positioning. Eg – Cortical SERs.
What are far-field potentials?
Far-field potentials are SERs recorded from electrodes a greater distance from the signal generator, therefore the signal is conducted through the body (eg CSF, brain). Eg – Subcortical SERs.
What are the two characteristics of an evoked potential?
Evoked potentials are described in terms of a) latency (ms) and b) amplitude. (mV, P or N)
a) What intraoperative changes in evoked responses can be seen,
b) what causes them?
a) decreased amplitude, increased latency, or loss
b) surgical issues (retractors, ischaemia),
or anaesthetic issues – BP, temp, drugs.
How are SSEPs delivered and measured?
SSEPs are recorded after electric stimulation of a peripheral mixed nerve. A square wave stimulus of 50-250μs is delivered, with intensity adjusted to give a trace of muscle contraction, or (if paralysed) at the maximal signal amplitude –usually around 20-50mA.
