THE EEGEEG 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 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 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 – Hz – 1 mV

Rhythms WaveSymbolFrequency (Hz) Comments Deltaδ<4Abnormal; May be normal in children during sleep Thetaθ4-8Sometimes abnormal Alphaα8-12Prominent at the parieto- occipital area; at rest with eyes shut Betaβ13-30Prominent 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 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 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