EEG and ICP Flashcards
What is normal cerebral blood flow? How much O2 does this deliver?
50 ml/min/100g tissue
Delivers 150 ml O2/min
What is the energy requirement of the brain?
3-5 ml O2/min/100g tissue
How much O2 does the brain extract?
35-50%
What is the equation of cerebral perfusion pressure?
CPP = MAP - ICP
Cerebral blood flow is impaired
Post trauma
Auto regulation of cerebral blood flow
Increases at the lower limit
Graph - as MAP increases, so does CBF… But plateaus in the middle…?
CBF is reduced by
Head injury Intracranial hypertension Hypotension Vasospasm Hyperventilation
Intracranial pressure can be directly monitored by
Subdural/subarachnoid bolts
Epidural transducers
Intraparenchymal fiber optic devices
Ventricular catheters
What is the Monroe-Kellie hypothesis?
The skull is a fixed volume and changes in one unit is balanced by the others.
What are the three components of the Monroe-Kellie hypothesis?
Blood, brain, cerebral spinal fluid
What are the percentages of the units that make up ICP?
Brain mass - 80%
Blood flow - 10%
CSF - 10%
T or F. ICP monitoring does not require a watertight fluid interface?
False
How is brain activity converted to a waveform?
Deformation of transducer membrane
Converted to electrical pulsations
Amplified
Displayed as a waveform
T or F. Catheter tip transducers need to be zeroed prior to insertion.
True
External transducers are zeroed
Anytime
Monitoring ICP is important in
Head injury Poor grade subarachnoid hemorrhage Intracerebral hematoma Meningitis Stroke
What are normal ICP values? Abnormal?
Normal: 7-15 mmHg
Abnormal: >20 mmHg
Aggressive management of ICP is indicated at what value?
> 25 mmHg
Elevated ICP causes
Herniation of internal and external brain
Distortion of cranial nerves and vital neurological centers
Impeded cerebral perfusion
Loss of CSF
Reduced venous blood flow
What are the pros of intraventricular drain and transducer?
Gold standard
ICP control by CSF draining
External zeroing
What are the cons of intraventricular drain and transducer?
Bleeding
Blockage
Infection
What is the correct placement of intraventricular catheters?
Lateral ventricle (frontal horn)
What is the position of the transducer?
Level with the meatus of the ear
What are the pros of the intraparenchymal pressure monitor?
Less infection risk
Less risk of hemorrhage
Excellent metro logical properties (less drift)
What are the cons of the intraparenchymal pressure monitor?
Underestimates very high ICP
Drift becomes a problem after several days
What are contraindications for using intraparenchymal pressure monitoring?
Intracranial infections
Coagulopathies
Severe skull fracture
CSF drainage necessary
What are the three phases of ICP waveforms?
P1: percussion wave (arterial pulsations)
P2: rebound wave (intracranial compliance)
P3: dichrotic wave (venous pulsations)
How is ICP managed?
Decrease brain water
Reduce CSF volume
What are ways to decrease brain water?
Hyperosmolar diuretics: mannitol w/ intact BBB
Loop diuretics: Lasix
Corticosteroids
What is the dosage of mannitol?
.25-1g per kg
CSF can be reduced from drainage by
Ventricular
Lumbar subarachnoid
Head elevation
Auto regulation is impaired by
Inhaled anesthetics
Direct acting vasodilators
What are examples of direct acting vasodilators?
Adenosine Prostacyclin Calcium channel blockers Nitroglycerin Nitroprusside
Transcranial Doppler Ultrasonography
Continuous or intermittent monitoring of CBF velocity
What is Transcranial Doppler Ultrasonography most useful for?
Vasospasm post subarachnoid hemorrhage
Does vasospasm cause increased or decreased CBF velocity?
Increased (due to Posseiulle’s Law)
For TCD, what allows for monitoring independent of rising ICP?
ICA:MCA flow
Ratio of internal carotid artery to middle cerebral artery flow
What is electroencephalogram monitoring?
Summation and recording of postsynaptic potentials from the pyramidal cells of the cerebral cortex
Reflects the metabolic activity of the brain
EEG was first used by
Hans Berger in 1924
A tracing of voltage fluctuations versus time recorded from electrodes placed over scalp in a specific array
EEG
EEG
Represents fluctuating dendritic potentials from superficial cortical layers
Required amplification
Part of the brain not sampled well by EEG
Deep parts of the brain
International 10-20 system of electrode placement
Electrodes spaces at ten or twenty percent of distances between specified anatomical landmarks
More than 21 electrodes can be added on an EEG to
Increase spatial resolution
Record from specific areas
Monitor electrical activity
Odd electrodes are placed
Even electrodes are placed
On the left
On the right hemisphere
Disadvantages of EEG
Detects cortical dysfunction, but not etiology
Low sensitivity and low specificity
Subject to electrical and physiologic artifacts
Influenced by alertness, hypoglycemia, and drugs
Small or deep lesions might not produce EEG abnormality
Limited time sampling and spatial sampling
Indications for EEG
Craniotomy for cerebral aneurysm clipping when a temporary clip is used
Carotid endarterectomy under GA
Cardiopulmonary bypass
Extra cranial intracranial bypass
Pharmacological depression of brain for “cerebral protection”
Beta waves
13-30 Hz
Awake and alert
Alpha waves
8-13 Hz
Closed eyes, relaxed
Theta waves
4-7 Hz
Young children
Drowsiness in older children or adults
Delta waves
0-4 Hz
Deep sleep, deep sedation
Suppression event
Very deep sedation, hypothermia, and ischemia
EEG artifact
Eye induced - blinks, movement, extra ocular muscle activity Gloss kinetic artifacts Poor grounding IV drips Body movement EKG artifact
Relaxation with eyes closed
Alpha waves predominance
Light anesthesia
Increase in beta
Decrease in alpha
Deepening of anesthesia
Increase in slow wave activity, delta 5 and theta 8
Cortical silence
Burst suppression
EEG is not
An output of the spinal cord or predictor of movement
Activation
High frequency
Low voltage
Light anesthesia
Surgical stimulation
Depression
Low frequency
High voltage
Deep anesthesia
Cerebral compromise
Most anesthetics produce
A biphasic pattern… Initial activation… Followed by dose dependent depression
Agents that activate EEG
Sub anesthetic inhalationals Low dose barbs and benzos Small doses of etomidate N2O Ketamine
Agents that depress EEG
1-2 MAC gases
Barbs/propofol/etomidate
Narcotics - dose dependent
Other things that influence - activate
Mild hypercapnia
Surgical stimulation
Early hypoxia
Other things that depress
Hypocapnia
Hypothermia
Late hypoxia
