导言

 

确诊脑死亡的脑电图(EEG)研究不再局限于大的实验室。许多小型医院都已配备有加强监护病房和EEG设备，因此需要增加脑死亡EEG记录的最低技术标准。

 

第一版(1970)疑似脑死亡的EEG记录最低技术标准反映了当时的现状和20世纪60年代末的技术。随后EEG设备得到相当大的改进并用于临床，许多实验室在这方面已积累了多年的经验。同样重要的是，现在有了更多的能胜任EEG工作的技师。1970年开展脑死亡联合研究的EEG结果也已经发表(Bennett et a1.，1976)。

 

美国EEG协会特别委员会于20世纪60年代末对确定脑死亡EEG标准进行的调查显示：在伴推测为“等电位的”EEG的2650例昏迷病例中，仅有3例符合委员会的标准并显示了大脑功能的恢复。而这3例曾服用过量的神经系统镇静剂，其中2例服用了巴比妥类药物，另1例服用了眠尔通。很多报道的“等电位的”记录再次回顾分析要么是低电压，要么是当时的脑电技术不过硬而产生了低电压活动，也就是说，单是不过硬的技术就能描绘出类似“平坦”的波形。然而，需要指出的是该研究没有包括儿童。因此，儿童组的参考数据目前还没有。委员会于1970年推荐非生理性术语如：“等电位的”或“线性的”(“平坦”一词这里不使用)，并重新命名了“脑电静息”状态。“脑电静息(ECI)”一词后来被国际临床神经生理学会(IFCN；Chatrian et al.，1974)制定的词汇表所采用。目前的指南包括了脑电静息标准的更新，这反映了自第一次这些标准出现以来我们所掌握的知识的不断进展(Chatrian et al.，1974；Bennett et al.，1976；Chatrian，1980；NINCDS，1980；Medical Consultants，198l；WaIker，1981)。

 

定义：脑电静息(ECI或ECS)是指脑电活动低于2μV，头皮电极间的距离不应小于lO cm，电极间的阻抗需低于10 kΩ而高于100 Ω。

 

下面对疑似脑死亡EEG记录的10条指南逐条进行原理的阐述，并加以说明注解。

 

1 应用一套完整的头皮电极设备

脑的主要区域必须被电极覆盖以确定电活动的缺乏不是局灶现象。单信道设备用来确定ECI是难以接受的，例如有时用EEG来监测麻醉水平。额、中央、枕和颞区为最低要求覆盖的区域。同时应增加一个接地电极。然而，在加强监护病房中，如果连接其他电设备的地线已接在患者身上，则EEG的接地电极可以不用。

 

由于记录前，我们并不知道是否将能获得ECI记录，因而在初步检测时使用一套完整的头皮电极设备是很有必要的，正如在指南1(临床脑电图操作的最低技术要求)中第2.3节里所定义的一样。有时由于头部外伤或近期手术的缘故，可能难以获得一套完整的、常规的lO-20系统头皮定位。另外，初始研究不应使用少于特定临床实验室的常规覆盖标准。一套完整的电极应该包括中线电极(Fz，Cz，Pz)。这对于监测低电压电生理活动很有用，同时也相对减少了伪差的产生。由于疑似ECI患者的EEG可能更多地发现有EEG异常，使用完整的电极覆盖是非常有必要的。

 

2 电极间阻抗应<10kΩ，>100Ω

2.1 不匹配的电极间阻抗可能会使EEG失真。当其中一个电极相对另一个配对的电极有更高的阻抗时，放大器变得不平衡。并且很容易使外来的信号过度放大。这将会产生60Hz(美国的市电为60Hz，我国为50Hz——编者注)的干扰或其他伪差。尤其对低电压脑电活动和高灵敏度的仪器而言，需要特别慎重地安放电极。

 

2.2 当阻抗低于100欧姆时，具有显著的电压降低，当然在O欧姆时就没有电压降低。这可能是一次ECI错误记录的原因之一。用来确保具有足够数量级的电极间阻抗的测试应该在记录期间执行。当使用固定阵列电极(“电极帽”或相似的设备)时，有必要确保过剩的导电胶没有从一个电极传到另一个，以免产生短路，这样会减弱电信号。稳定的低阻抗电极对所有临床研究来说是绝对必须的。

2.3 尽管不推荐常规使用针电极，但它实际上已有效地应用于疑似ECI的记录中。电极间阻抗越大就越可能被不同电极间相似测试值抵消掉，其结果就是在记录过程中并不增加伪差出现的可能性(也见指南1：临床脑电图操作的最低技术要求中第2.2节)。

 

3 整个记录系统的完整性应该经过测试

普通的仪器校准测试了放大器和描记器单元的运转，但这并不能排除分流的可能性或电极间开放环路、电极板、电缆或仪器的输入端的问题。如果在一个导联上记录到波幅的增高，EEG显示有ECI，应通过使用铅笔尖或棉签轻轻碰触每一导联组合中每一电极这样的方法，从而在记录中造成一伪差电位，以此来测试系统的完整性。这一测试证实了电极板是与机器相连的；当电极板不经意间没有相连时，会记录到有时类似低波幅的脑电活动。该测试要进一步证实导联组合设置与电极的放置相匹配。

 

4 相邻电极间距离不应小于10cm

在国际10-20系统中，成人的平均电极间距离在6～6.5cm之间。采用普通灵敏度的平均电极间距离的记录很可能提示为ECI；然而，如果使用更长的电极间距离来记录，则可见脑电位。因此，在纵向或横向的双极导联组合时，一些二倍距的电极连接方法可被推荐使用(如：Fpl-C3，F3-P3，C3-O1，etc．)。

 

耳电极参考记录几乎总是因为被EKG(ECG)活化而变得无用，但是Cz作为参考电极会令人满意。在一项研究中(Bennett et a1.，1976)，最佳导联组合是：Fp2-C4，C4-02，Fpl-C3，C3-01，T8(T4)-Cz，Cz-T7(T3)，同时应有一个ECG信道，一个描绘EMG信道。然而，在不能移动的患者身上，枕部的电极线更难放置和固定，并且特别容易受到人工呼吸机引起的运动伪差的影响。包括F7-P7(T5)，F8-P8(T6)、F3-P3、F4-P4和Fz-Pz的导联组合可能会产生一份更好的记录。以上的观点并不意味着实验室通常预先选好的导联组合不能使用。

 

英文原文

 

Introduction

 

EEG studies for the determination of cerebral death are no longer confined to major laboratories. Many small hospitals have intensive care units and EEG facilities. The need forminimal standard guidelines has thus increased.

 

The first (1970) edition of Minimum Technical Requirements for EEG Recording in Suspected Cerebral Death reflected the state of the art and the technique of the late 1960s. Substantially improved EEG instrumentation is now available, and many laboratories have had years of experience in this area. Equally important, there is now a much larger number of competent EEG technologists. Finally, the EEG results of a collaborative study of cerebral death that was being planned in 1970 have been published (Bennett et al., 1976).

 

The survey in the later 1960s by the American EEG Society’s Ad Hoc Committee on EEG Criteria for the Determination of Cerebral Death revealed that, of 2,650 cases of coma with presumably “isoelectric” EEGs, only three whose records satisfied the committee’s criteria showed any recovery of cerebral function. These three had suffered from massive overdoses of nervous system depressants, two from barbiturates, and one from meprobamate. Many of the reported “isoelectric” records were, on review, either low-voltage records or obtained with techniques inadequate to bring out low-voltage activity. That is, inadequate technique alone gave the graphs the appearance of being “flat.” It should be pointed out, however, that this study did not include children. Hence, the comparable data on which to base recommendations for this young age group do not exist at present. The 1970 committee recommended dropping nonphysiologic terms such as “isoelectric” or “linear” (the word “flat” should likewise not be used) and renaming the state “electrocerebral silence.” Subsequently, “electrocerebral inactivity” (ECI) was the term recommended in the Glossary of the International Federation of Clinical Neurophysiology (IFCN; Chatrian et al., 1974).

 

The current Guideline includes an updating of the criteria for electrocerebral inactivity, reflecting what has been learned since the first appearance of these standards (Chatrian et al.,1974; Bennett et al., 1976; Chatrian, 1980; NINCDS, 1980; Medical Consultants, 1981; Walker,1981).

 

Definition

 

Electrocerebral inactivity (ECI) or electrocerebral silence (ECS) is defined as no EEG activity over 2 μV when recording from scalp electrode pairs 10 or more cm apart with interelectrode impedances under 10,000 Ohms (10 KOhms), but over 100 Ohms.

 

Ten guidelines for EEG recordings in cases of suspected cerebral death, with the rationale for each, are set forth with explanatory comments.

 

1.A Full Set of Scalp Electrodes Should Be Utilized

 

The major brain area must be covered to be certain that absence of activity is not a focal phenomenon. The use of a single-channel instrument such as is sometimes used for EEG monitoring of anesthetic levels is therefore unacceptable for the purpose of determining ECI. The frontal, central, occipital, and temporal areas are recommended as the minimal required coverage. A grounding electrode should be added. However, for recordings in intensive care units, a ground electrode should not be used if grounding from other electrical equipment is already attached to the patient.

 

Since, prior to the recording, one does not know whether an ECI record will be obtained, it is desirable to use a full set of scalp electrodes on the initial examination, as defined in Guideline One: Minimum Technical Requirements for Performing Clinical Electroencephalography,Section 2.3. At times, the full set of conventional 10-20 scalp locations may not be accessible because of head trauma or recent surgery. Otherwise, the initial study should not use less than the routine coverage standard for the particular clinical laboratory. A full set of electrodes includes midline placements (Fz, Cz, Pz); these are useful for the detection of residual low-voltage physiologic activity and are relatively free from artifact. Since the EEGs of patients with suspected ECI actually may have EEG abnormalities other than ECI, the use of more complete, rather than less complete, electrode coverage is often essential.

 

2.Interelectrode Impedances Should Be Under 10,000 Ohms But Over 100 Ohms

 

2.1 Unmatched electrode impedances may distort the EEG. When one electrode has a relatively high impedance compared to the second electrode of the pair, the amplifier becomes unbalanced and is prone to amplify extraneous signals unduly. This may result in the occurrence of 60-Hz interference or other artifacts. Situations characterized by low-voltage electrocerebral activity and high instrument sensitivity demand especially scrupulous electrode application.

 

2.2 There is a marked dropoff of potentials with impedances below 100 Ohms and, of course,no potential at 0 Ohms. Such an occurrence could be one possible reason for a false ECI record. A test of inter-electrode impedances to assure that they are of adequate magnitude thus should be performed during the recording. When fixed arrays of electrodes (“electrode cap” or similar devices) are utilized, it is essential that excess jelly does not spread from one electrode to another, creating a shunt or short circuit, which would attenuate the signal.

 

Stable, low-impedance electrodes are absolutely essential for all bedside (i.e., away from the laboratory) studies.

 

2.3 Although not recommended for general use, needle electrodes have been used effectively in suspected ECI recordings. The greater impedance they may have is offset by a greater probability of similar values among different electrodes, so that the likelihood that artifact will occur in the record is not increased. (See also Guideline 1: Minimum Technical Requirements for Performing Clinical Electroencephalography, Section 2.2.)

 

3.The Integrity of the Entire Recording System Should Be Tested

 

Ordinary instrumental calibration tests the operation of the amplifiers and writer units, but it does not exclude the possibility of shunting or an open circuit at the electrodes, electrode board (jackbox), cable, or input of the machine. If, after recording on one montage at increased amplification, an EEG suggesting ECI is found, the integrity of the system may be tested by touching each electrode of the montage gently with a pencil point or cotton swab to create an artifact potential on the record. This test verifies that the electrode board is connected to the machine; records made with the electrode board inadvertently not connected can sometimes resemble low-amplitude EEG activity. The test further proves that the montage settings match the electrode placements.

 

4.Interelectrode Distances Should Be at Least 10 Centimeters

 

In the International 10-20 System, the average adult interelectrode distances are between 6 and 6.5 cm. A record taken with average interelectrode distances at ordinary sensitivity may suggest ECI; however, if it were recorded using longer interelectrode distances, cerebral potentials might be seen in the tracing. Hence, with longitudinal or transverse bipolar montages, some double distance electrode linkages are recommended (e.g., Fpl-C3, F3-P3, C3-O1, etc.).

 

Ear reference recording is almost invariably too contaminated by EKG to be useful but a Cz reference may be satisfactory. In one study (Bennett et al., 1976), the best montage was: Fp2-C4, C4-O2, Fpl-C3, C3-Ol, T8 (T4)-Cz, Cz-T7 (T3), with one-channel EKG and one-channel noncephalic (hand). Occipital leads, however, are more difficult to attach in immobilized patients and are particularly susceptible to movement artifact induced by artificial respirators. A montage that includes F7-P7 (T5), F8-P8 (T6), F3-P3, F4-P4, and Fz-Pz may therefore yield a better record.

 

None of the foregoing should imply that the usual preselected laboratory montages could notalso be used.

 

翻译：秦兵

编辑：格格