THE

EEG

James PeerlessApril 2012

Objectives

Physics and Clinical MeasurementAnaesthesia 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

• 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

• 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

RhythmsWave Symbol Frequency

(Hz)Comments

Delta δ <4 Abnormal;May be normal in children during sleep

Theta θ 4-8 Sometimes abnormal

Alpha α 8-12 Prominent at the parieto-occipital 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