续上期：疑似死亡脑电图记录最低技术标准（一）

 

5 灵敏度与定标

 

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。

 

5.3 时长达20min的ECI自限性时期可能会发生在低压记录中(Jorgensen，1974)，因此，单次记录应至少长达30min以确保没有错过间歇性低电压脑电活动。

 

6 滤波设置应适合于ECI评估

 

为了避免低电压快活动或慢活动的衰减，无论何时高频滤波都不应设置低于30Hz，低频滤波则不应高于lHz。

 

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。

 

7 如有需要，应使用额外的监测技术

 

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。

 

7.1 如果没有ECG伪差的话，我们就很少能看到ECI记录，因而ECG监测是必需的。

 

7.2 如果呼吸伪差不能被消除的话，则必须有专业技师在记录上标注出或用传感器监测这些伪差。只要简单的断开呼吸器就能肯定地鉴定出这些伪差。

 

7.3 通常情况下，对于来源于患者或周围环境的伪差来说，额外的电极监测是必要的。最简单的做法是在手背间隔6～7cm加上一对电极。

 

7.4 现在我们已很清楚地知道了一些肌电信号混入EEG中并见于ECI记录的患者。如果肌电电位的波幅影响了记录的图形，则有必要使用神经肌肉阻断剂来减少或消除之，如溴化双哌雄双酯(巴夫龙)或琥珀酰胆碱。该过程应该在麻醉师或其他熟悉这类药物的医师的指导下进行。

 

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。

 

7.6 然而，即使有很好的技术，一份有高灵敏度的EEG记录有时也会让EEG医师很难解释。我们必须努力确定哪部分记录源于非脑电生理信号或非生理性伪差，其中包括特定ICU中全部系统的噪音水平。然后必须估计余下电活动的波幅是否超过2μV。当不能作出肯定的答复时，EEG报告必须注明为不确定性结果且记录到的结果不能归类于ECI。

 

8 感觉刺激

 

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。

 

9 记录只能由合格的技师执行

 

专业的技术对于记录疑似ECI病例来说是必要的。记录通常是在很困难的环境下进行，同时包括很多可能的伪差来源。消除大多数伪差和识别所有的伪差都需要一名专业的技师来完成。ECI记录的EEG技师需要相应的资质要求，其中包括ICU设备记录技术的监督指导要求。此外，也鼓励技师用EEG技术注册表(R．EEG T．)执行此类研究。技师应该在合格的EEG医师的指导下开展工作。

 

10 如对ECI有怀疑则应重复检测

 

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)

 

如果出现因技术或其他困难而导致ECI评估的不确定性，整个程序应该在间隔一段时间后例如6h后，重复一次(见第7节)。

 

英文原文

 

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).

翻译：秦兵

编辑：格格