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5.1 在记录至少30min内灵敏度必须从7μV/mm增加到至少2μV/mm，其中包含相应的定标这无疑是最重要且最易被忽略的参数。人们仅仅意识到当灵敏度为7μV/mm时，而2μV的信号看不到，因为墨水线平均宽度为1/4 mm，也就是我们能看到的信号大小。明显地，当灵敏度为2μV/mm时，2μV的电压标准将会使记录笔偏转1mm。这种信号在灵敏度为2μV/mm时应该是可见的，当灵敏度为1.5或lμV/mm则会更明显。在大多数电脑显示器上单个像素大约高1/3 mm，因而实际灵敏度应该会稍微高点；例如：2μV的信号可以使跟踪信号在屏幕上移动至少1.5 mm。然而，伴有斜坡的非常缓慢的慢活动还是难以看到。现代仪器设备可以以1.5或1μV/mm的灵敏度连续记录。将灵敏度增加50%～100%能对2μV信号的存在和缺失作出更好的评估。
5.2 充分的和适当的校准程序是必要的。信号校准到与已记录的EEG信号相近是一个很好的尝试，因此，对于ECI来说，2μV或5 pV的校准是合适的。 当灵敏度为2μV或1μV/mm时，50μV的校准信号是无效的，因为记录笔阻尼和显示器痕迹可能会重叠在一起。记录系统内在的噪声水平也应引起注意。
众所周知，短时间常数(低频滤波的高值)削弱了慢电位。在接近ECI的情况下，存在有θ和δ范围内的电位，因而应尽一切努力来避免低频的衰减。然而，已经证实1Hz的低频设置已足够用来确定ECI(Jorgensen，1974；Bennett et al，1976)，不必犹豫地使用60 Hz的陷波滤波。
EEG记录的是真正的脑波，其他生理信号和来源于仪器内部或外部的，并具有机械性、电磁性或静电起源的信号则属伪差。当灵敏度增加时，这些伪差将变得更为明显，因而必须将其识别出以便可以准确评估EEG检查是否正在进行中。我们应该强调的是对抗伪差的最佳保证就是稳定的、低阻抗的电极系统。各种不同的伪差图例可见于《Atlas昏迷和脑死亡脑电图》(Bennett et a1，1976)和《现代临床脑电图的实践》(Chatrian et al，2003)。鉴于目前很难获得Atlas图集，Raven出版社已同意我们可以使用下面一些图表。
7.5 仪器的噪声和外部的干扰可以用“虚拟患者”很方便地检查出，即将1个10 kΩ的电阻器连接在一个信道的输入端1(G1)和输入端2(G2)之间。
对强烈的躯体感觉、听觉或视觉刺激不应有EEG的反应性。在联合研究中，ECI患者的常规记录中并无与刺激相关活动的情况(Bennett et al，1976；NINCDS，1980；walker，1981)。任何来自上述刺激或其他(呼吸道痰液的抽吸和其他能形成有效刺激的护理程序)的明显EEG活动都必须从非脑电生理信号和非生理性伪差中仔细区别开来。例如：当有ECI时，视网膜电图仍存在并对光刺激有反应。在没有ECI的患者中也发现刺激可能对记录反应性的程度有帮助。
在脑死亡的联合研究中(Bennett et al，1976；NINCDS，1980；Walker，1981)，假若没有使用过量的镇静剂，EEG提示有ECl的患者中没有能存活很久的，这一发现证实了早期的调查结果，正如导言部分所总结的。因此，很明显单一EEG显示ECl是确定皮层死亡的高度可靠的检查手段之一。(对于指导医师确定脑死亡的其他指南，请查看附件所列的参考文献)
5. Sensitivity Must Be Increased from 7 uV/mm to at Least 2 uV/mm for at Least 30 Minutes of the Recording, with Inclusion of Appropriate Calibrations
5.1 This is undoubtedly the most important and the most often overlooked parameter. One hasonly to realize that at a sensitivity of 7 uV/mm a signal of 2 uV cannot be seen because the average ink line is 1/4 mm in width, i.e., about the size of the signal one desires to see.
Obviously, the criterion voltage of 2 uV will deflect the pen only 1 mm at a sensitivity of 2 uV/mm. Such a signal should be visible at 2 uV/mm, and more certainly so at a sensitivity of 1.5 or 1 uV/mm. On most computer monitors, a single pixel is about 1/3 mm high, so that the effective sensitivity should be slightly higher; e.g., a 2 uV signal should move the signal trace at least 1.5 mm on the screen. However, very slow activity with gradual wave slopes still may be difficult to see. Contemporary equipment permits extended recording at a sensitivity of 1.5 or 1 uV/mm. This 50—100% increase in sensitivity will allow a more confident assessment of the presence, or the absence, of a 2-uV signal.
5.2 Adequate and appropriate calibration procedures are essential. It is good practice to calibrate with a signal near the size or value of the EEG signal that has been recorded; thus, for electrocerebral inactivity, a calibration of 2 or 5 uV is appropriate. A 50-uV calibration signal at a sensitivity of 2 or 1 uV/mm is useless, since the pens block and monitor traces may overlap. The inherent noise level of the recording system also should be noted.
5.3 Self-limited periods of ECI of up to 20 min may occur in low-voltage records (Jorgensen, 1974), and, therefore, a single recording should be at least 30 min long to be certain that intermittent low-voltage cerebral activity is not missed.
6. Filter Settings Should Be Appropriate for the Assessment of ECI
In order to avoid attenuation of low-voltage fast or slow activity, whenever possible, highfrequency filters should not be set below a high-frequency setting of 30 Hz, and low-frequency filters should not be set above a low-frequency setting of 1 Hz. It is well-known that short time constants (high values of the low filter) attenuate slow potentials. In the situation approaching ECI, there may be potentials in the theta and delta ranges, so every effort should be made to avoid attenuation of these low frequencies. However, it has been demonstrated that a low-frequency setting of 1 Hz is adequate for the determination of ECI
(Jorgensen, 1974; Bennett et al., 1976). There need be no hesitation in the use of the 60-Hz notch filter.
7. Additional Monitoring Techniques Should be Employed When Necessary
The EEG record is a composite of true brain waves, other physiologic signals, and artifacts(either internal or external to the machine, and of mechanical, electromagnetic, and/or electrostatic origin). When the sensitivity is increased, such artifacts are accentuated and therefore must be identified in order to accurately assess whether EEG is present. It should be emphasized that the best insurance against many artifacts is a stable, low-impedance electrode system. A wide range of artifacts is illustrated in the Atlas of Electroencephalography in Coma and Cerebral Death (Bennett et al., 1976) and in Current Practice Of Clinical Electroencephalography (Chatrian et al., 2003.) Because the Atlas is now difficult to obtain, Raven Press has kindly granted permission to use some of the figures, which are found below.
7.1 Since one rarely sees an ECI record without varying amounts of EKG artifact, an EKG monitor is essential.
7.2 If respiration artifact cannot be eliminated, the artifact must be documented by specific technologist notation on the record or be monitored by transducer. Briefly disconnecting the respirator will allow definitive identification of the artifact.
7.3 Frequently, an additional monitor is needed for other artifact emanating from the patient or for artifact induced from the surroundings. The most convenient for this purpose is a pair of electrodes on the dorsum of the hand separated by about 6-7 cm.
7.4 It is now clear that some EMG contamination can persist in patients with ECI recordings. If EMG potentials are of such amplitude as to obscure the tracing, it may be necessary to reduce or eliminate them by use of a neuromuscular blocking agent such as pancuronium bromide (Pavulon) or succinylcholine (Anectine). This procedure should be performed under the direction of an anesthesiologist or other physician familiar with the use of the drug.
7.5 Machine noise and external interference may be conveniently checked by a “dummy patient,” i.e., a 10,000-Ohm resistor between input terminal 1 (G1) and input terminal 2 (G2) of one channel.
7.6 Even with good technique, however, an EEG recorded at the increased sensitivities required above can at times leave the electroencephalographer who interprets the recordings in considerable difficulty. An attempt must be made to determine what portion of the record results from noncerebral physiologic signals, or nonphysiologic artifacts, including the ongoing noise level of the complete system in the particular ICU as indicated, for example, by a recording from the hand. An estimate must then be made of whether or not the remaining activity exceeds 2 uV in amplitude. When this cannot be done with confidence, the EEG report must indicate the uncertainty, and the record cannot be classified as demonstrating ECI (see Section 10).
8. There Should Be No EEG Reactivity to Intense Somatosensory, Auditory, or Visual Stimuli
In the collaborative study, there was no instance of stimulus-related activity in routine recordings of patients with ECI (Bennett et al., 1976; NINCDS, 1980; Walker, 1981). Any apparent EEG activity resulting from the above stimuli or any others (airway suctioning and other nursing procedures can be potent stimuli) must be carefully distinguished from noncerebral physiologic signals and from nonphysiologic artifacts. For example, an electroretinogram can still persist in response to photic stimulation when there is ECI. Stimulation may be of help also in documenting the degree of reactivity of records found not to be characterized by ECI.
9. Recordings Should Be Made Only by a Qualified Technologist
Great skill is essential in recording cases of suspected ECI. The recordings are frequently made under difficult circumstances and include many possible sources for artifact. Elimination of most artifact and identification of all others can be accomplished by a qualified technologist.
Qualifications for a competent EEG technologist for ECI recordings include the requirement of supervised instruction in the techniques of recording in ICU settings. Additionally, Registry in EEG Technology (R. EEG T.) is encouraged for technologists performing such studies. The technologist should work under the direction of a qualified electroencephalographer.
10. A Repeat EEG Should Be Performed If There is Doubt About ECI
In the Collaborative Study of Cerebral Death (Bennett et al., 1976; NINCDS, 1980; Walker, 1981), there were no patients who survived for more than a short period after an EEG showed ECI, provided that overdose of depressant drugs was excluded. This finding confirmed the results of the earlier survey, which were summarized in the Introduction. It is evident, therefore, that a single EEG showing ECI is a highly reliable procedure for the determination of cortical death. (For other guidelines to assist physicians in the determination of brain death, see theReferences.)
In the event that technical or other difficulties lead to uncertainty in the evaluation of the question of ECI, the entire procedure should be repeated after an interval, for example, after 6 h (see Section 7).