eeg in the icu: part i teneille e. gofton july 2012
TRANSCRIPT
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EEG in the ICU: Part ITeneille E. Gofton
July 2012
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Objectives To outline the importance of EEG monitoring
in the ICU To briefly review the neurophysiological
basis of EEG To introduce formal EEG and subhairline EEG
Lead placement To present normal features of formal and
subhairline EEG To discuss the limitations of subhairline EEG
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Why is continuous EEG monitoring in the ICU useful? To detect nonconvulsive seizures
19-50% of patients with acute neurological conditions have seizures
To characterise fluctuating or paroxysmal spells e.g. posturing, tremors, agitation
To identify silent neurological events e.g. ischemic infarction, vasospasm
Jette and Hirsch. Curr Neuro Neurosci Rep 2005:;5:312.
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What are some common conditions to be monitored? Convulsive and nonconvulsive status or
seizures Coma or encephalopathy of unknown
etiology Intracerebral hemorrhage Ischemic stroke Subarachnoid hemorrhage Traumatic brain injury
Jette and Hirsch. Curr Neuro Neurosci Rep 2005:;5:312.
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How long should patients be monitored with cEEG? Varies based on clinical situation If no particular events then suggest 48
hours 50% of seizures (convulsive or
nonconvulsive) are detected within one hour of cEEG
87% within 24 hours 93% within 48 hours
Claassen et al. Neurology 2004;62:1743.
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What EEG features are most helpful in the ICU? Presence of seizures or spikes Focal abnormalities (e.g. unilateral
suppression or PLEDs) Variability
Spontaneous variation in waveforms seen Reactivity
Change in EEG pattern in response to afferent stimulus
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Neurophysiological Basis of EEG EEG reflects the electrical activity
emanating from the surface of the cortex
Scalp electrodes detect the sum of neuronal activity The EEG displays the sum of neuronal
post-synaptic potentials These post-synaptic potentials consist of
either excitatory or inhibitory post-synaptic potentials
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Neurophysiological Basis of EEG Many neurons must be active together
in order to detect their electrical activity on the scalp The electrical activity is dampened as it
passes through the CSF, the skull and the soft tissues of the scalp
The electrical signal is then amplified by the EEG machine so that it can be read on the digital display
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Neurophysiological Basis of EEG Not all cortical potentials can be well-
detected on the scalp surface This is due to the convoluted surface of
the brain
Sulcus = cortical surface “hidden” from scalp electrodes
Gyrus = cortical surface “seen” by scalp electrodes
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Neurophysiological Basis of EEG The most important contributor to the
EEG activity seen on the scalp is the portion of the cortex parallel with the scalp surface The activity within the sulci is largely
‘hidden’ from the scalp electrodes
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Neurophysiological Basis of EEG The most important neurotransmitters
responsible for generating neuronal activity include Excitatory = glutamate Inhibitory = GABA
GABA = gamma-aminobutyric acid
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Differential Amplifiers The electrical signal recorded on the scalp
is generated using a differential amplifier Differential amplifiers have two input
electrodes Input 1 Input 2
Any signal common to both inputs is rejected by the amplifier
Any signal that is different in the two inputs is amplified and displayed on the monitor
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Differential Amplifiers
Input 1
Input 2
Amplification by EEG machine
EEG output displayed on the monitor
Differential amplifier:Rejects signals common to both inputs and amplifies signals that are different in both inputs
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Differential Amplifiers By convention
All negative amplifier outputs result in an upward deflection on the EEG
All positive amplifier outputs result in a downward deflection on the EEG
+
- Negative potentials
Positive potentials
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Sampling Rate Sampling rate: how often the digital
recording apparatus records a data point each second Important for accurate waveform
representation (also called aliasing) Formal EEG samples data at 250
samples per second Subhairline EEG samples data at 100
samples per second
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If the same waveform is sampled at two different sampling rates it will look different
If the sampling rate is too low then the output waveform will appear slower than it really is this is aliasing
This waveform is alpha, but looks like delta
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Formal EEG In order to understand the subhairline
EEG, a basic understanding of the formal EEG is required
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EEG electrodes EEG electrodes are adhered to the scalp
using a special conducting electrode gel There are several important steps:
1. The skin must be cleaned to remove any debri
2. An abrasive gel is used to prepare the skin area
This ensures that the electrodes are as closely apposed to the skin as possible which eliminates artifacts
NuPrep is used in the ICU
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EEG Electrodes3. Electrode paste is applied to the
electrodes4. Electrodes are placed onto the scalp in a
specific position Formal EEGs use the 10-20 system The 10-20 system is a precise measurement
of the head with specific subdivisions that guide electrode placement
This ensures that EEGs can be compared from recording to recording
e.g. before and after removing electrodes for and MRI
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10-20 SystemFp1 Fp2
C3
F3
T3
P3
F7
O1
F4
T5P4
C4
O2
T4
F8
T6
Cz
Fz
Pz
A1 A2
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Left parasagittal
Right parasagittal
Left temporal
Right temporal
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EEG Electrodes Subhairline EEG This EEG montage is designed for ease
of application with a minimum of EEG expertise
It uses sticker electrodes on non-hairy surfaces instead of the metal electrodes applied to the full scalp surface using the 10-20 system
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EEG Electrodes The skin is prepared in the same way for
the subhairline EEG, but electrode paste is not required because the stickers are self-adhesive
9 electrodes are placed symmetrically on the forehead, anterior to the ear and immediately posterior to the ear Placement is based on anatomical
landmarks
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Subhairline EEG First channel = left
frontal Second channel =
right frontal Third channel =
left temporal Fourth channel =
right temporal
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Subhairline EEG Young et al 2009
Continuous subhariline EEG monitoring detects 70% of non-convulsive seizures when compared to formal EEG
But, 98% specificity when seizures are seen
The lower sensitivity for seizures is due to the fact that only some of the cerebral lobes of the brain are covered by the subhairline electrodes
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Subhairline EEG
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Subhairline EEG
Left frontal
Right frontal
Left temporal
Right temporal
EKG
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10-20 System Subhairline EEG
Number of electrodes
Minimum 21 electrodes
9 electrodes
Lobes coveredAll lobes
(except the cerebellum)
Frontal lobesAnterior
temporal lobes
Electrode application
conductive paste
Self-adhesive electrodes
Ease of useRequire certified
technologist
Many staff can be trained
Sensitivity for seizures
Gold standard 68%
Specificity for seizures
Gold standard 98%
Sensitivity for spikes/PLEDs
Gold standard 39%
Specificity for spikes/PLEDs
Gold standard 92%
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Advantages of Subhairline EEG Available when formal EEG may not be Allows for a very rapid assessment of
cerebral function Greater capacity for continuous
continuous EEG recording only 2 formal EEG machines available Subhairline EEG available at each bedside
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Limitations of Subhairline EEG Poor scalp coverage Detects only 68% of non-convulsive
seizures Continuous recording NOT continuous
monitoring Data only stored for previous 24 hours Poor sampling rate
Degraded quality of waveforms on display Potential for misrepresentation of
waveforms (aliasing) and misinterpretation of displayed information
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Looking at the Subhairline EEG
Left frontal
Right frontal
Left temporal
Right temporal
EKG
1 second
amplitude
duration
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Normal Features of EEG EEG waveforms are divided infrequency
bands1. Beta – >13Hz2. Alpha – 8-13Hz3. Theta – 4-7Hz4. Delta – <4Hz
fastest
slowest
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Normal Features of EEG Dominant rhythm
This is the most prominent posterior rhythm seen in the EEG
It is most consistent in the occipital regions and usually falls within the alpha range
This will not be seen on the subhairline EEG because the subhairline EEG does not cover the posterior portion of the head
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Alpha Rhythm
10Hz
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Beta Most commonly seen in the frontal
regions or diffusely May be caused by medication
Barbiturates, benzodiazepines May be seen in patients with an overdose
This is not seen often on the subhairline EEG because the sampling rate is too low and there is likely aliasing of beta into other waveforms
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Beta1 second
~20Hz
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Beta – Subhairline EEG
1 second
14Hz
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Theta May be generalised or focal Not seen in normal adults Generalised theta is associated with
coma or light sedation Focal theta is associated with a
structural abnormality such as ischemia, infarction, tumour or hemorrhage
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Theta
1 second
6 Hz
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Theta –Subhairline EEG
1 second7Hz
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Delta May be generalised or focal Not seen in normal adults Generalised dela is associated with
coma or anaesthesia Focal delta is associated with a
structural abnormality such as ischemia, infarction, tumour or hemorrhage
Delta is slower than theta and usually reflects more severe cerebral dysfunction
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Delta
The dominant rhythm is delta (prior to stimulation).
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Delta – Subhairline EEG
1 second2.5Hz
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Variability This reflects the number of different
waveforms spontaneously produced by the cerebrum
Greater variability usually reflects a better prognosis
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Variability
Note the spontaneous presence of waveforms of higher and lower frequencies.
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Variability – Subhairline EEG
This patient shows good variability: delta, theta and rare beta frequencies can be seen.
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Reactivity This reflects a change in the EEG
following afferent stimulus The presence of reactivity suggests that
the brain is processing the stimulus at some level
The presence of reactivity is prognostically favourable
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Reactivity
Note the change in amplitude and frequency of the EEG when the patient hears their name being called.
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Reactivity – Subhairline EEG
There is a change in the voltage and frequency of cerebral waveforms after the stimulus, which indicates reactivity. This is a favourable sign on EEG.
The patient’s name is called out loud.