Long answer questions: Details & Discuss
Long answer questions:
You record an EEG which appears to be featureless and of generalised low
which may be responsible
Braids of electrods must be earthed
CMRR and groundings?
Amplifiers output might be faulty
use square wave check
faulty reference electrodes
Wires of electrods might be cracked, and they might be connected to crucially placed electrodes
check the wirings and be careful not rubbing them together
silver chloride plating of crucial electrodes might have 'come off'
low pass and high pass filters might be in place and the bandwiths might be too narrow
sensitivity too low or too high
Signal gain and signal loss: remember that physiological signals are usually smaller than amplified noise!
are the leads even bunched?
check connections like the sockets and see if every electrod is in place
skin preparation has been done properly? Cleaned? Gel?
if electrodes have been montaged on the wrong surface area, you might have been measuring something different from what you'd expect
Discuss with reference to a VEP how the pattern stimulator contributes to a
successful evoked potential recording.
correct distance is linked to the 20° cones of vision in the retina (macula). It can be adjusted in relation to patient's visual acuity, but not an individual basis: a range of possible choices is often used
width of screen and angle is helpful to know
it has to be adjusted
20°, usually. More or less.
usually a refresh time of two seconds
learning when to trigger
at the right time is the best thing one can do to improve quality of a VEP
stimulus trigger the start of the recording
this helps you at understanding which is the best moment to trigger the averager
sensitivity might affect VEP, even if the equipment has in-built automatic correction
Discuss how the normal rhythmic activity may alter if an alert patient
becomes very drowsy.
open, closed eye alpha and certain kind of artefacts (breathing artefacts come to mind) might also signal imply and evolution of the patient alert recording into a sleep EEG.
Drowsiness happen to end slowly also, not instantaneously. Same for Alertness into Drowsiness.
eye closed patient, alpha rhythm
before converting into beta
Presence of POST and Vertex might imply
as POST are, for example, present in NREM and lead to sleep
location of waveforms and their frequency is extremely indicative of a changing of states
Age might also be indicative of different waveforms pertaining to alertness and drowsiness
Discuss the clinical and technical factors that may affect the quality of a VEP
talk to patient
don't let them get too comfortable. Cold environment helps
help patient relax
Lie patient down
dark adapting and luminance regulation
have a range of options, remembering the 20° angle
Time of averaging start (pros let a few minutes pass before starting the averager)
Discuss with illustrations how a square wave can assist in a machine check.
to get better scores
: explain consequences of unattended square wave corrections
Frequency response distortion
discuss how it can assist means:
calibrating the machine can help to understand if there
is one single large abnormality on a single area
or if there are multiple abnormalities, it all goes back to time coding
which you might mistake for two or more abnormalities, due to time errors
all square waves have the same time codes, if something seems mismatched there is a problem with the amplifiers
Square wave shape:
top? Low pass filter might be faulty
ending? high pass filter is faulty
"duration of the decline" has to be looked at
if square waves have the same height it is a good sign:
it means amplifier are all producing signals with the same gain
You record an EEG which appears to be obscured by 50Hz mains noise.
Discuss the technical factors which may be responsible
frequency filters higher of lower
low pass: 10 Hz
you will mistakenly include in the EEG higher frequency artefacts from muscles and even satellites, cellphones and such
high pass: 0.5 Hz
cut out of delta and bottom-end theta
, it might filter away precious stuff
electromagnetic field from nearby electronical equipments
from patient's end
amplifiers turned on and left 'open' along with missing
electrodes (empty sockets active,
put amplifier inputs for extra marks
skin not properly rubbed, impedence
an unused grounding lead, completely missing or faulty
such as EMG
, he might be as antisemitic as you or be anxious from your rampant racism.
Make the patient lie down or otherwise relax
For both low-pass and high-pass filters, discuss the consequences of altered
settings on the EEG recording.
A low pass filter is a filter that blocks a certain range of frequencies below a set point from appearing the EEG recordings. An high pass filter excludes a range of freqeuencise
above a set point; As it can be inferred, both of these filters have benefits and contro-indications: While an high/low pass filter may be useful in filtering away interference from artefacts some of these might be useful (i.e. alertness: jaw muscles contractions, myoclonic jerks). They can also erroneously filter useful waveforms: such as fluctuating (squeaking) alpha
amplitudes* that are slowly becoming beta waves in a drowsy patient. A possible solution, in particular cases it is the use of a Notch filter instead, to filter annoying interferences such as 50-60 Hz mains noise, while leaving the normal background EEG relatively intact
Discuss the features of an evoked response and the background signal which
are required for the process of signal averaging to work.
[STEEL YOUR HEART]
Background signal of random EEg recordings, which is spontaneous and random
imagine the usually scrolling of an EEG recording being 'cut' at fixed time-frames and superimposed
phase-locked, not random anymore