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Module 4: Other Findings of Intraoperative EEG Pathological States and Artifacts (ICETAP)
Transcript of Module 4: Other Findings of Intraoperative EEG Pathological States and Artifacts (ICETAP)
Other Findings of Intraoperative EEG: Pathological States and Artifacts
Uses of EEG under pathological states
In the previous units, we learned how to distinguish between different wave forms.
In this unit, we will learn how to use that knowledge to apply to pathological states.
In seizure, ischemia, hypothermia, hypercapnia and brain death, EEG can be either diagnostic or prognostic.
It can be a valuable tool in predicting outcomes.
1. Seizures and EEG
1. Seizure 2. Ischemia 3.Hypothermia 4. Hypercapnia 5. Brain death
18-lead (or greater) EEGs are used to diagnose seizure types and to differentiate them from states that look like seizures.
EEG patterns may be able to identify the area of the brain that is generating the seizure.
Absence seizures, which do not have motor symptoms, are diagnosed by characteristic 3 Hz spike and wave discharges on the EEG (below).
Epileptiform discharge under anesthesia
Sevoflurane and enflurane induction are associated with an epileptiform pattern on the EEG.
1: Delta waves with spikes turning into rhythmic polyspikes
2: Periodic epileptiform discharges and the beginning of suppression with spikes
3: Suppression with spikes
Especially in patients with a history of convulsions, monitor the EEG carefully on induction.
May induce seizure in children with epilepsy and without.
2. EEG During Ischemia
EEG is an important tool to distinguish between reversible and irreversible brain damage.
As cerebral perfusion decreases, lower frequency waves predominate in brain.
When cerebral blood flow is decreased to the point of permanent cell death, all EEG frequency stops.
Delta and theta waves are associated
with bad outcome.
A higher ratio of delta and theta waves compared with beta and alpha activity (focal or generalized slowing) is associated with unfavorable stroke outcome.
Stroke-induced ischemic EEG changes are both diagnostic and prognostic.
EEG asymmetry suggests ischemia
A. Theraputic Hypothermia
B. EEG during Hypothermia
Intraoperative EEG to detect brain ischemia
Ischemic EEG changes (increased delta and theta power; decreased alpha and beta) correlate well with cerebral ischemia during carotid endarterectomy.
Look for a difference in the EEG, or in a processed EEG index, between the two hemispheres
May complement other monitoring (e.g. patient response to commands)
Hypothermia improves the probability of surviving out-of-hospital cardiac arrest.
Moderate hypothermia in surgeries when cardiac arrest is planned
Deep hypothermia when full circulatory arrest is planned
May be induced following spontaneous cardiac arrest
Hypothermic cells have better tolerance to reperfusion and are less self destructive in this setting.
Characteristic EEG changes seen with increasing hypothermia include the following:
1. Total power decreases
2. Spectral edge frequency usually decreases
Following cardiac arrest, an EEG that is nonreactive to painful stimuli portends a poor prognosis
4. Acute Hypercapnia and EEG
1. Blink 2. EMG 3. Cautery 4. EKG
During hypercapnia, EEG alpha activity decreases and delta activity increases
Hypercapnia can occur clinically with sleep apnea, postictal or stroke states, congestive heart failure, and ischemia.
5. EEG changes in coma and brain death
Higher delta wave percentage is associated with worse outcomes (brain death and persistent vegetative states) in comatose patients
Good prognostic value for recovery likelihood in coma
Good diagnostic value for distinguishing between brain death and near brain death
Absence of brain function in the case of brain death may be determined by two isoelectric EEGs 24 hours apart
May affect the appearance of EEGs and should be taken into consideration during analysis.
1. Blink Artifacts
The eyeball is a dipole, with a positive pole (cornea) and a negative pole (retina)
Rotating the eye, as with a blink, moves the dipole and creates a characteristic EEG trace
A blink artifact is characterized by a distinct positive plateau (where the voltage is above the display limit) which then sharply drops off
Extraocular muscles may also contribute some EMG artifact
2. EMG and Muscle Movement
EEGs often pick up signals from nearby muscles, often frontalis or temporalis
Usually short-lasting bursts with high frequency and amplitude
Parkinson’s essential tremor may be mistaken for brain activity due to constant 3-6 Hz tremor
Muscle relaxants decrease the amount of muscle artifact
May also affect (lower) processed EEG indices without affecting consciousness
3. Cautery Artifacts
Electrocautery uses electric current to cut or burn tissue
Causes impressive EEG artifact (similar to electrocardiogram cautery artifact)
Saturates EEG and produces a flat line in tracings.
<Insert Cautery artifact from Mark’s Video>
4. Electrocardiogram Artifact
Occurs when the heart’s conduction system discharges are picked up by the EEG.
More common in patients with short and wide necks.
Show up as small recurrent spikes that coincide with those found on the EKG.
<Bahaa’s Video-still goes in here>
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