Electroencephalography (EEG)
Electroencephalography used in monitoring neural function during surgery
Electroencephalography is the measurement of electrical activity in different parts of the brain and the recording of such activity as a visual trace. From millions of neurons to a single cell, the EEG provides an index of localization in conscious and unconscious tissue, multi-unit recordings provide spike data on small groups (2-5) of neurons, cross correlation methods between groups of neurons, scanning (CAT, PET & MRI) provide a snapshot at low to moderate resolution of the state of large areas. Anesthesia has a profound effect on receptive field organization. The concept of receptive field has continued to evolve; excitatory-inhibitory subregions in the classical receptive field domain.
NUWER
The physiologic range of human EEG is from 0.5 - 70 Hz.
The EEG recorded from human brain shows neuronal oscillations extending well beyond what was first reported by Hans Berger (1935), and beyond what is commonly recorded in clinical practice (0.5-70Hz). There is an increasing awareness of the physiologic and possibly clinically relevant, bandwidth of human intracranial EEG. The presence of low-frequency oscillations extending from 0.5Hz down to ~0.01 Hz, as well as spontaneous direct current (DC) voltage fluctuations, in human EEG recordings are well established and detectable from both scalp and intracranial EEG recordings. Similarly the high-frequency end of the EEG spectrum ~40 Hz up to 60 Hz, is an active area of research and clinical interest. In contrast to low-frequency oscillations by virtue of the low amplitude of high-frequency activity, the study of high-frequency EEG is almost exclusively from intracranial recordings.
EEG general equipment requirements
EEG monitoring requires the continuous observation of the EEG by an appropriately qualified clinical neurophysiologist and an experienced neurophysiology technician. The equipment used in the operating room has to conform to OR safety specifications. There should be an adequate common mode rejection to eliminate 50 or 60 Hz line interferences (at least 85 dB). there should also be automatic artifact rejection to minimize interference due to surgical diathermy. Special attention should be given to electrical safety of the patient. Any leakage of current through the patient exceeding 10 uA should be prevented. This implies that the patient should be properly electrically groundad at only one site. The diathermy return plate should be positioned as close as possible to the operating site. The application of defribilators and fibrilators may pose similar problems (introduction of implantable cardioverter defibrilator). The same ground is used for diathermy and other OR equipment. Neurophysiology equipment should have optical isolation of each patient contact. Biomedical engineers should check the equipment for proper grounding and for leakage current on a regular basis.
NUWER
The physiologic range of human EEG is from 0.5 - 70 Hz.
The EEG recorded from human brain shows neuronal oscillations extending well beyond what was first reported by Hans Berger (1935), and beyond what is commonly recorded in clinical practice (0.5-70Hz). There is an increasing awareness of the physiologic and possibly clinically relevant, bandwidth of human intracranial EEG. The presence of low-frequency oscillations extending from 0.5Hz down to ~0.01 Hz, as well as spontaneous direct current (DC) voltage fluctuations, in human EEG recordings are well established and detectable from both scalp and intracranial EEG recordings. Similarly the high-frequency end of the EEG spectrum ~40 Hz up to 60 Hz, is an active area of research and clinical interest. In contrast to low-frequency oscillations by virtue of the low amplitude of high-frequency activity, the study of high-frequency EEG is almost exclusively from intracranial recordings.
EEG general equipment requirements
EEG monitoring requires the continuous observation of the EEG by an appropriately qualified clinical neurophysiologist and an experienced neurophysiology technician. The equipment used in the operating room has to conform to OR safety specifications. There should be an adequate common mode rejection to eliminate 50 or 60 Hz line interferences (at least 85 dB). there should also be automatic artifact rejection to minimize interference due to surgical diathermy. Special attention should be given to electrical safety of the patient. Any leakage of current through the patient exceeding 10 uA should be prevented. This implies that the patient should be properly electrically groundad at only one site. The diathermy return plate should be positioned as close as possible to the operating site. The application of defribilators and fibrilators may pose similar problems (introduction of implantable cardioverter defibrilator). The same ground is used for diathermy and other OR equipment. Neurophysiology equipment should have optical isolation of each patient contact. Biomedical engineers should check the equipment for proper grounding and for leakage current on a regular basis.
*LH*
Electroencephalogram; EEG recorded from the scalp is a summation of excitatory and inhibitory postsynaptic potentials produced inthe pyramidal layer of the cerebral cortex (EPSP & IPSP)
What we see is the summation of these postsynaptic membrane potentials (not action potentials) in local cortical neurons
The continuous thythmical activity is a result of thalamocortical and cortical-cortical feedback systems
Intraoperative EEG applications; monitoring for cerebral ischemia during vascular surgeries; carotid endarterectomy, cardiac and aortic surgeries; monitoring for depth of anesthesia, maintenance of burst suppression during cerebral aneurysm clipping; electrocorticography; direct cortical recording used for seizure detection
EEG can be classified according to frequencies ranges; Delta 0-4 Hz, Theta 4-8 Hz, Alpha 8-13 Hz, Beta 13-30 Hz, During intraoperative monitoring of EEG you should see mostly theta and alpha activity with some beta
Types of EEG montages; referential EEG all leads are referenced to a common site usually A1/A2, don mostly for outpatient EEG testing, responses have a potential at a specific site with smaller amplitude signals in the channels near it (signals localized using amplitude); Bipolar EEG each lead is referenced to the next; Fp1-F7, F7-T3, T3-T5, used in our intraoperative recordings, signals may appear in two channels with phase reversal (signals localized using phase reversal)
Electrodes are placed according to the 10-20 system; processed EEG is a means of displaying the frequency content of the EEG to help detect changes, CSA, Compressed Spectral Array mountain range frequency spectra; DSA Density Spectral Array power indicated by density of dots, CDSA Color Density Spectral Array, power indicated by color scheme
Spectral Analysis of Raw EEG; 10 sec epochs of raw EEG are process through a fast fourier transform to produce the frequency spectrum (displayed as CSA or DSA)
Compressed Spectral Array; FFT is performed on a sample of raw EEG converting it into a frequency spectrum; each spectrum plots power or voltage vs. frequency, power is proportional to the voltage squared.
Power Spectral Edge; frequency at which a given percentage of the power exits in the EEG; PSE should be set to a value placing it at the upper edge of the alpha band; if the predominant frequency content of the EEG shifts to the slow frequencies with loss of the fast frequencies as sometimes seen with ischemia, the PSE should shift to the left
Bovie artifact, EEG during awake procedures, posterior alpha activity with the eyes closes
Recording Parameters for EEG; Filters 1-70 Hz bandpass, timebase given in terms of paper speed, default setting is 30mm/sec, faster speed not recommended, slower speed may help in detecting slow frequencies; sensitivity; 3 microvolt/mm, recommended to avoid 60 Hz notch filter
Anesthetic Effects on EEG
low dose anesthesia has an excitatory effect, increasing amplitude of fast frequencies. enhanced alpha activity can be observed with 0.5 mac halogenated agents; moderate to deep anesthesia shows progressive slowing of frequencies and amplitude decrease; deep anesthesia causes burst suppression a pattern of EEG showing periods of flat EEG interrupted by bursts of large amplitude EEG activity; as anesthesia continues to deepen the bursts become progressively shorter and less frequent.
Burst suppression is sometimes desired by the surgeon in procedures like cerebral aneurysm clipping or carotid endarterectomy; burst suppression is achieved by giving either a bolus or infusion of thiopental or propofol. at dosages that produce burst suppression, these drugs provide cerebral neuroprotection; when in burst suppression, EEG is not reliable for detecting ischemia, it is important to monitor MN SSEPs
Carotid Endarterectomy; barbiturate bolus given, note effect on mn response and on eeg (burst suppression)
effets of ischemia; mild increase in delta activity and mild decrease in alpha and beta activity; moderate increase in delta activity and loss of alpha and beta activity; severe complete loss of all activity, flat line; EEG begins to show changes at a cerebral blood flow value
Neuronal viability as a function of severity and duration of ischemia
should do EEG for up to 24 hours after surgery
Montages used in MCA territory; F7-C3, C3-P3; F8-C4, C4-P4
EEG's vs SSEPs
EEG monitors a larger area of cortex and may be more sensitive to focal ischemia, EEG changes may be detectable prior to SSEP changes and at a higher critical CBF threshold, is unreliable for ischemic detection during burst suppression anesthesia
SSEP while in the MCA region, the MN cortical response is localized in the post-central gyrus, SSEPs are subject to signal averaging delay and have a lower critical CBF threshold, can be monitored during burst suppression anesthesia, studies support SSEPs as having very high specificity for cortical ischemia
CEA Monitoring Protocol
Bilateral MN SSEP, CPc-FPz, Cs3-Fpz, Erb's point, EEG and CSA for the following minimum 4 channels; F7-C3, C3-P3, F8-C4, C4-P4
Anesthesia, request low dose isoflurane or other halogenated agent, 0.5 MAC with no nitrous, stay under 1 MAC, if a neuroprotective bolus of thiopental is planned emphasize the MN monitoring during burst suppression
Document critical events, heparin (usually 5 minutes before clamp), clamping and unclamping, shunt placement and removal; if clamp related changes are observed a shunt should be placed; blood pressure should be kept high to help maintain collateral perfusion. a drop in pressure could result in ischemic changes
Interpreting MN SSEP Ischemic changes
cerebral ischemia will result in changes in the cortical peaks of the MN response only, subcortical and peripheral responses should be unaffected; due to the thalamocortical contribution to the N20 peak the N20 may not be immediately affected by ischemia. The later cortical peaks (P23 and beyond) will decrease in amplitude or disappear; changes are most likely to occur in the MN contralateral to the operative side. EEG will most likely be affected on the ipsilateral side.
Ischemic changes with carotid clamping; ipsilateral hemispheric changes are most common, contralateral MN changes ipsilateral EEG changes
bilateral hemispheric changes occur less frequently, bilateral MN and EEG changes, stenosed carotid may be providing critical blood flow to both hemispheres but ipsilateral hemisphere still has adequate posterior communicating artery
Clamp related EEG changes, 80% occur in the first minute, 69% occur within the first 20 seconds
Electroencephalogram; EEG recorded from the scalp is a summation of excitatory and inhibitory postsynaptic potentials produced inthe pyramidal layer of the cerebral cortex (EPSP & IPSP)
What we see is the summation of these postsynaptic membrane potentials (not action potentials) in local cortical neurons
The continuous thythmical activity is a result of thalamocortical and cortical-cortical feedback systems
Intraoperative EEG applications; monitoring for cerebral ischemia during vascular surgeries; carotid endarterectomy, cardiac and aortic surgeries; monitoring for depth of anesthesia, maintenance of burst suppression during cerebral aneurysm clipping; electrocorticography; direct cortical recording used for seizure detection
EEG can be classified according to frequencies ranges; Delta 0-4 Hz, Theta 4-8 Hz, Alpha 8-13 Hz, Beta 13-30 Hz, During intraoperative monitoring of EEG you should see mostly theta and alpha activity with some beta
Types of EEG montages; referential EEG all leads are referenced to a common site usually A1/A2, don mostly for outpatient EEG testing, responses have a potential at a specific site with smaller amplitude signals in the channels near it (signals localized using amplitude); Bipolar EEG each lead is referenced to the next; Fp1-F7, F7-T3, T3-T5, used in our intraoperative recordings, signals may appear in two channels with phase reversal (signals localized using phase reversal)
Electrodes are placed according to the 10-20 system; processed EEG is a means of displaying the frequency content of the EEG to help detect changes, CSA, Compressed Spectral Array mountain range frequency spectra; DSA Density Spectral Array power indicated by density of dots, CDSA Color Density Spectral Array, power indicated by color scheme
Spectral Analysis of Raw EEG; 10 sec epochs of raw EEG are process through a fast fourier transform to produce the frequency spectrum (displayed as CSA or DSA)
Compressed Spectral Array; FFT is performed on a sample of raw EEG converting it into a frequency spectrum; each spectrum plots power or voltage vs. frequency, power is proportional to the voltage squared.
Power Spectral Edge; frequency at which a given percentage of the power exits in the EEG; PSE should be set to a value placing it at the upper edge of the alpha band; if the predominant frequency content of the EEG shifts to the slow frequencies with loss of the fast frequencies as sometimes seen with ischemia, the PSE should shift to the left
Bovie artifact, EEG during awake procedures, posterior alpha activity with the eyes closes
Recording Parameters for EEG; Filters 1-70 Hz bandpass, timebase given in terms of paper speed, default setting is 30mm/sec, faster speed not recommended, slower speed may help in detecting slow frequencies; sensitivity; 3 microvolt/mm, recommended to avoid 60 Hz notch filter
Anesthetic Effects on EEG
low dose anesthesia has an excitatory effect, increasing amplitude of fast frequencies. enhanced alpha activity can be observed with 0.5 mac halogenated agents; moderate to deep anesthesia shows progressive slowing of frequencies and amplitude decrease; deep anesthesia causes burst suppression a pattern of EEG showing periods of flat EEG interrupted by bursts of large amplitude EEG activity; as anesthesia continues to deepen the bursts become progressively shorter and less frequent.
Burst suppression is sometimes desired by the surgeon in procedures like cerebral aneurysm clipping or carotid endarterectomy; burst suppression is achieved by giving either a bolus or infusion of thiopental or propofol. at dosages that produce burst suppression, these drugs provide cerebral neuroprotection; when in burst suppression, EEG is not reliable for detecting ischemia, it is important to monitor MN SSEPs
Carotid Endarterectomy; barbiturate bolus given, note effect on mn response and on eeg (burst suppression)
effets of ischemia; mild increase in delta activity and mild decrease in alpha and beta activity; moderate increase in delta activity and loss of alpha and beta activity; severe complete loss of all activity, flat line; EEG begins to show changes at a cerebral blood flow value
Neuronal viability as a function of severity and duration of ischemia
should do EEG for up to 24 hours after surgery
Montages used in MCA territory; F7-C3, C3-P3; F8-C4, C4-P4
EEG's vs SSEPs
EEG monitors a larger area of cortex and may be more sensitive to focal ischemia, EEG changes may be detectable prior to SSEP changes and at a higher critical CBF threshold, is unreliable for ischemic detection during burst suppression anesthesia
SSEP while in the MCA region, the MN cortical response is localized in the post-central gyrus, SSEPs are subject to signal averaging delay and have a lower critical CBF threshold, can be monitored during burst suppression anesthesia, studies support SSEPs as having very high specificity for cortical ischemia
CEA Monitoring Protocol
Bilateral MN SSEP, CPc-FPz, Cs3-Fpz, Erb's point, EEG and CSA for the following minimum 4 channels; F7-C3, C3-P3, F8-C4, C4-P4
Anesthesia, request low dose isoflurane or other halogenated agent, 0.5 MAC with no nitrous, stay under 1 MAC, if a neuroprotective bolus of thiopental is planned emphasize the MN monitoring during burst suppression
Document critical events, heparin (usually 5 minutes before clamp), clamping and unclamping, shunt placement and removal; if clamp related changes are observed a shunt should be placed; blood pressure should be kept high to help maintain collateral perfusion. a drop in pressure could result in ischemic changes
Interpreting MN SSEP Ischemic changes
cerebral ischemia will result in changes in the cortical peaks of the MN response only, subcortical and peripheral responses should be unaffected; due to the thalamocortical contribution to the N20 peak the N20 may not be immediately affected by ischemia. The later cortical peaks (P23 and beyond) will decrease in amplitude or disappear; changes are most likely to occur in the MN contralateral to the operative side. EEG will most likely be affected on the ipsilateral side.
Ischemic changes with carotid clamping; ipsilateral hemispheric changes are most common, contralateral MN changes ipsilateral EEG changes
bilateral hemispheric changes occur less frequently, bilateral MN and EEG changes, stenosed carotid may be providing critical blood flow to both hemispheres but ipsilateral hemisphere still has adequate posterior communicating artery
Clamp related EEG changes, 80% occur in the first minute, 69% occur within the first 20 seconds
Electroencephalogram; synaptic potentials in cortical pyramidal cells
sensitive to the state of consciousness, cortical ischemia
general anesthesia; complex events depending on the agent
burst suppression; lowers CMRO2 if ischemia is anticipated
slowing of power spectrum with (regional) cerebral ischemia
sensitive to the state of consciousness, cortical ischemia
general anesthesia; complex events depending on the agent
burst suppression; lowers CMRO2 if ischemia is anticipated
slowing of power spectrum with (regional) cerebral ischemia