| DABNM EXAM | IOM STUDY HOME |
EEG
|
GUIDELINES FOR INTRAOPERATIVE NEUROMONITORING USING RAW (ANALOG OR DIGITAL WAVEFORMS) ANDQUANTITATIVE ELECTROENCEPHALOGRAPHY A POSITION STATEMENT BY THE AMERICAN SOCIETY OF NEUROPHYSIOLOGICAL MONITORING : http://www.asnm.org/EEG41.pdf |
Surgical Procedures
|
Notes:
The primary clinical outcomes for which modern EEG technology has made significant intraoperative contributions include:
Assessing cortical perfusion and oxygenation- recognizing and/or preventing perioperative ischemic insults.
EEG patterns associated with anesthesia - monitoring of brain function for anesthetic drug administration in order to determine depth of anesthesia (and level of consciousness), including the tailoring of drug levels to achieve a predefined neural effect (e. g., burst suppression).
Raw(rEEG) (analog or digital) and Quantitative Electroencephalography (qEEG)
Typically, artifact-minimized, pre- and post-induction baselines are established, and efforts are made to preserve those baseline waveforms.
the EEG is used to establish if there is a change in the baseline of ongoing brain activity as a result of any expected or unexpected anesthetic- or surgically-induced alteration
Current Recommendations for EEG:
Patient Preperation
Electrode Montage
Full array of scalp electrodes is recommended when possible
Standard 8-, 10-, 16-, and 18-channel montages for diagnostic rEEG using transverse bipolar and referential derivations
No less than 8 channels of simultaneous recording be used
A larger number of channels is encouraged
ASET intraoperative guidelinesdeclared that “aminimumof 16 channels should be used whenever possible;” however, 21 or more channels are optimal for “EEG surgical testing” since it allows recording of additional parameters such as electrocardiographic and muscle potentials.
Specifically addressing intraoperative monitoring, only 4 channels may be adequate if a “lateralize change” or interhemispheric asymmetry is the “only desired information
The adequate number of channels deemed necessary for intraoperative monitoring remains controversial and unresolved, and has been based primarily on the recommendations used in the diagnostic setting.
Reduces the risk for internal and external noise interference, and distorted signals
Great care should be paid to achieving low electrode impedances prior to the start of surgery and should be rechecked whenever there is any artifact present in the signal.
Metal disk or "cup" electrodes (gold, silver, or tin) applied using the method of collodion-soaked gauze and filled with a conductive gel is the preferred for long-term recordings allowing for routine acquisition of pre-surgical baselines where detection of pre-existing asymmetries or interhemispheric differences may be important in the intraoperative management of vascular (e. g., carotid endaterectomy) and cardiac (e. g., particularly cardiopulmonary bypass procedures) surgeries.
The recording electrode montage should consist of one type of electrode, with no mismatching of metals
Electrode leads and cables used for the neuromonitoring system should not be bundled with the cables used for any other device. Separating the electrode leads and cables will reduce the chance of electrical coupling between adjacent lines.
The length of the leads to the preamplifier should be minimized and the leads braided in order to reduce electromagnetic contamination.
Orientation of needle insertion should involve a parallel, anteroposterior alignment since misalignment may cause artifactual amplitude asymmetries or distortions.
Filter Settings
will depend on the frequency of the waveforms of importance
should be determined by the “judicious use” which “will allow emphasis on a particular event as it occurs,”
should be used to acquire and save data with a wider bandpass than those used for on-line or off-line digital filtering
HFF
A high frequency setting of 70 Hz or higher is considered optimal (if available), but not lower than 35 Hz since significant distortion and attenuation of spikes or anesthetic-induced, higher frequency activity can occur.
However, exceptional cases where sources of noise cannot be eliminated (e. g., jaw clenching in an awake patient) and rEEG cannot be easily visualized, a lower high-frequency filter settingmay be required
LFF
A low frequency filter of 0.3–1.0Hz is recommended, as this setting allows the display of slow frequency activity without significant baseline variability and loss of sensitivity for the detection of ischemic events or anesthetic-induced, slow, frequency activity.
A greater than 1 Hz setting should be restricted to brief periods when viewing low-voltage beta or spike activity. It should be noted that the setting of the hardware low frequency filter greatly affects the recovery of the amplifier after the application of electrocautery, If this filter is set too low, there may be prolonged recovery and blocking
60 Hz Noise & Filtering
60-Hz notch filter is often necessary to eliminate extraneous, irreducible noise in the operating room (OR) arena, such as line frequency artifact. However, this should only be used if other measures against 60 Hz interference fails, since this notch filter can distort or attenuate spikes, and other faster frequencies.
Great care should be exercised to reduce excessive noise by trouble-shooting which may include removing, replacing, or unplugging any unwanted current source (e. g., OR table, blood and body warmers, microscope, extraneous power supply, etc.) which does not interfere with normal clinical practice or distraction from neuromonitoring.
Intraoperative Computer-Processed or Quantitative EEG (qEEG)
6.1 Background
Clearly, assessment of the frequency and amplitude of the rEEG is crucial for rapid and accurate
interpretation; however, such assessment is quite difficult sometimes using the raw signal and naked eye alone. Over the past several decades, a number of computer-processed algorithms and display techniques have been developed to make easier the recording and interpretation of the rEEG. The primary advantages of qEEG include:
1) enhanced visual graphics for easier on-line interpretation
2) the ability to determine pre-selected baselines in order to evaluate deviations during critical anesthetic and surgical manipulations
3) the ability to quantify and statistically evaluate the rEEG,
4) to develop a parameter(s) that would allow intraoperative monitoring of depth of anesthesia.
Although qEEG has been used to assist in the analysis of the analog signal and to quantify intraoperative ischemia and depth of anesthesia, in the final analysis, the visual inspection of an artifact-free, contemporaneous, raw signal is still deemed by many as clinically the most critical and superior technique for interpretation. In 1987, the AEEGS supported the position that "the clinical application of quantitative EEG analysis is considered to be limited and adjunctive”. Thus, a real-time view of the rEEG must always be available whenever qEEG is used. Since the analog EEG is an alternating voltage which changes over time, some of the first methods for computer-processing or quantification involved a time domain analysis such as zero-crossing (aperiodic analysis) [72, 74, 75]. An alternative approach to statistical examination of the EEG is frequency domain analysis which is based on signal activity as a function of frequency (power spectral analysis based on Fast-Fourier transformation (FFT) [60, 61, 72, 76-78]. This it typically done using the Fourier transform which is most commonly implemented using FFT. Essentially, the Fourier transform decomposes the analog EEG (a complex waveform) into its component sine waves. The power spectrum is then calculated by squaring the amplitudes of the individual frequency components. Thus, the analog EEG signals which were recorded on the time axis are transformed and displayed on the frequency axis as the amount of power or energy in user-defined bandwidths, typically delta, theta, alpha, and beta.
There are a number of properties of the power spectrum that are important to understand.
First, the power in any signal is related to the square of its amplitude of the signal so that doubling the amplitude of the signal quadruples the power. This means that small amplitude components in the EEG can be obscured by higher amplitude components. One means of reducing this problem is by plotting the logarithm of the power, a method that is occasionally used in the display of EEG power spectra.
Second, the highest frequency in the power spectrum is related to the rate at which the analog EEG is sampled according to the Nyquist relation: fmax = 0.5*fsample where fmax is the maximum frequency in the power spectrum and fsample is the rate at which the raw EEG is sampled.Thus, if the raw EEG is sampled at 250Hz, then the highest frequency in the power spectrum is 125 Hz. If the input signal contains frequencies greater than fmax, the power at these frequencies is falsely represented (or aliased) at a frequency in the range from 0 to fmax. Thus, it is important for the sample frequency to exceed twice the frequency of any significant frequency in the input signal.
Third, the smallest difference in frequencies that can be resolved is 1/T where T is the size of the segment of EEG analyzed in seconds. Thus, if power spectral analysis is performed in one second epochs then the resolution is only 1 Hz. If, however, 10 second epochs are used the resolution is 0.1 Hz.
Typically, an epoch length of 2-2.5 seconds is employed. Several different display formats have been developed for computer-enhanced imaging of the power spectral analysis of the rEEG:
1) the compressed-spectral array (CSA; a pseudo-three-dimensional topographic plot)
2) dot-density spectral array (DSA; a gray- or color-scaled two dimensional contour plot) and color-scaled topographic brain mapping.
These computer-enhanced images can literally give the user an impression that individual anesthetic agents and surgical events may produce their own neural "signature or fingerprint". In addition, derived measures such as spectral edge frequency (SEF; e. g., SEF95 is the frequency below which 95% of the total spectral power is contained), mean and median frequency, absolute and relative power frequency bandwidths, coherence, burst-suppression ratio, and asymmetry indices have also been used to simplify, and assist in the display and interpretation of the rEEG. Perhaps the most informative and widely used computer-processed display technique in routine clinical practice are the CSA and DSA formats with derived indices such as SEF, power frequency bandwidths of the traditionally-defined bandwidths, and median power and peak power frequencies.
| DABNM EXAM | IOM STUDY HOME |
8.0 SUMMARY
8.1 rEEG monitoring during CEA surgery using selective shunting is a standard (Class II and III evidence, strong Type A recommendation).
8.2 rEEG monitoring during CEA surgery using routine shunting is a practice option (Class II and III evidence, Type B recommendation).
8.3 qEEG monitoring is a practice option for level of consciousness (Class II and III evidence, Type B recommendation)
8.4 rEEG monitoring for cerebral aneurysms is a practice option (Class II and III evidence, Type C recommendation)
8.5 qEEG monitoring CEA surgery using routine shunting is a practice option (Class III evidence, Type D recommendation).
8.6 qEEG monitoring CEA surgery using selective shunting is a practice option (Class III evidence, Type D recommendation).
8.7 rEEG monitoring is a practice option for depth of anesthesia is (Class II and III evidence, Type D recommendation).
8.8 qEEG monitoring is a practice option for depth of anesthesia is a practice option (Class II and III evidence, Type D recommendation).
8.9 qEEG monitoring for cerebral aneurysms is a practice option (Class III evidence, Type E recommendation)
8.10 rEEG monitoring during cardiac surgery using cardiopulmonary bypass is a practice option ( Class II and III evidence, Type U recommendation).
8.11 qEEG monitoring during cardiac surgery using cardiopulmonary bypass is a a practice option (Class II and III evidence, Type U recommendation).
8.12 Different types of electrodes may be used for EEG recording, but the standard metal, disc (cup), surface electrodes are preferred to subdermal needle electrodes when this is practical (Class III evidence, Type C recommendation).
7.2 Strength-of-recommendation ratings
Type A. Strong positive recommendation, based on Class I evidence, or overwhelming Class II evidence.
Type B. Positive recommendation, based on Class II evidence.
Type C. Positive recommendation, based on strong consensus Class III evidence.
Type D. Negative recommendation, based on inconclusive or conflicting Class II evidence.
Type E. Negative recommendation, based on evidence of ineffectiveness or lack of efficacy.
Type U. No recommendation, based on divided expert opinion or insufficient data.