Transcript THE E E G James Peerless April 2012 Objectives Physics and Clinical Measurement Anaesthesia for neurosurgery, neuroradiology and neurocritical care Demonstrates knowledge of: • PC_BK_52: Amplification of biological signals: including ECG,

THE
E
E
G
James Peerless
April 2012
Objectives
Physics and Clinical Measurement
Anaesthesia for neurosurgery, neuroradiology and
neurocritical care
Demonstrates knowledge of:
• PC_BK_52: Amplification of biological signals: including
ECG, EMG, EEG, BIS, CFM, CFAM
• NA_IK_04: Explains the indications for using
neurophysiological monitoring [including EEG, evoked
potentials and ICP measurement] to benefit patients
requiring neurosurgery/neuro-critical care
History
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1875 – electrical activity from animals’ brains
1890 – electrical activity altered by stimuli
1924 – first human EEG described
1934 – epileptiform activity demonstrated
Introduction
• Recording of electrical activity of the brain
• Signals from ~20 scalp electrodes are collated
and presented as 16 traces
• 10-30 minutes; recorded with video to
correlate brain activity with clinical picture
• Characteristics of the traces, i.e. shape,
distribution, incidence and symmetry are
analysed
What is it?
• There are millions of nervous action potentials
firing at any one time
• ‘Brain waves’ are the summation of
synchronous activity of neurons detected at
the scalp
• Brain activity shows oscillation at various
frequencies
Method
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Electrodes
Amplifier
Filter
Microprocessor
Output Monitor
Biological Signal Transduction
• Heart – ECG
– 0.05 – 100 Hz
– 1mV
• Brain – EEG
– 0 – 13 Hz
– 50 – 200 μV
• Muscle – EMG
– 1 – 20 000 Hz
– 1 mV
Rhythms
Wave
Symbol
Frequency Comments
(Hz)
Delta
δ
<4
Abnormal;
May be normal in children
during sleep
Theta
θ
4-8
Sometimes abnormal
Alpha
α
8-12
Prominent at the parietooccipital area; at rest with eyes
shut
Beta
β
13-30
Prominent over the frontal area
Current Uses in Medicine
• Clinical medicine
– Distinguishing between seizure types
• Monitoring of depth of anaesthesia
– BIS
– indicator of cerebral perfusion in carotid endarterectomy
• Intensive & Neurocritical care
– brain function monitoring
– to monitor for non-convulsive seizures/ status epilepticus
– to monitor levels of sedation
• Research
Anaesthesia & The EEG
• Why don’t we use it much?
– Expensive equipment
– Skilled operators
– Dissimilar anaesthetic agents generate different
EEG patterns or signatures
• Increasing depth of anaesthesia  signal
amplitude is decreased, frequency increases
Causes of EEG Depression
• EEGs change with age, state of consciousness
(incl. GA)
• Metabolic states (e.g. hypoglycaemia, hepatic
coma)
• Hypotension, hypoxia, hypercarbia, cerebral
oedema
• Encephalitis
• CJD
• Brain death  isoelectric (flat line)
BIS
• Bispectral index analysis
• Monitors electrical activity and quantifies level
of sedation
• Aims: to reduce awareness; reduce over/underdosing of drugs
• Works best with hypnotic agents
• Doesn’t work with ketamine; and less
sensitive to sedative effect of opioids
BIS
• Displayed as a continuous trend
– Facial electromyogram (EMG)
– BIS
– Signal Quality Index (SQI)
• Forehead sensor
– 4 tines
Summary
• EEG measures electrical activity from the brain
• Complex analysis limits its use in mainstream
anaesthetic practice
• BIS monitoring incorporates EEG and
quantifies depth of anaesthesia
MCQ
Concerning electroencephalography (EEG):
• Voltages are in the range of 10-100 millivolts
• Spontaneous EEG activity is lost when the
body temperature drops below 25 °C
• β waves are enhanced by sedatives
• δ waves only occur in brain injury
• θ waves occur at a frequency of 4-7 Hz
MCQ
Concerning electroencephalography (EEG):
• Voltages are in the range of 10-100 millivolts
• Spontaneous EEG activity is lost when the
body temperature drops below 25 °C
• β waves are enhanced by sedatives
• δ waves only occur in brain injury
• θ waves occur at a frequency of 4-7 Hz
MCQ
Regarding the BIS monitor:
• It uses a dimensionless scale from 0 to 100 Hz
• Hypothermia can increase the BIS value
• The BIS value is not accurate during ketamine
anaesthesia
• Interference can occur due to EMG or diathermy
• BIS can measure the concentration of a particular
drug
MCQ
Regarding the BIS monitor:
• It uses a dimensionless scale from 0 to 100 Hz
• Hypothermia can increase the BIS value
• The BIS value is not accurate during ketamine
anaesthesia
• Interference can occur due to EMG or diathermy
• BIS can measure the concentration of a particular
drug