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3.8 When instrument settings (sensitivities, filters, paper speed, montage) are changed during the recording, the settings should be clearly identified on the record at the time of the change. If technologically feasible, the final calibration(s) should include each sensitivity and filter settings used in the recording, and should include calibration voltages appropriate to the sensitivities actually used. It is especially important to record calibration signals at very high sensitivities when these settings have been used. With some digital systems, a gain factor display may be the only available assessment for function of the analog system. More comprehensive calibrations, such as those described above, are encouraged.
3.9 The baseline record should contain at least 20 min of technically satisfactory recording. Longer recordings are often more informative. Although the possibility of reformatting digital EEG allows the entire recording to be performed in a single montage, this is not acceptable practice. Observing only one montage may prevent recognition of poor connections in electrodes that happen not to be included, and also prevent appreciation of subtle abnormalities that require special technical maneuvers (such as placement of additional electrodes.)
The EEG is a short sample in time from the patient’s life. Within reasonable limits, the longer the recording, the better the chance of recording an abnormality or abnormalities demonstrating the variability of these. Experience in many centers shows that a very minimum of 20 min of artifact-free recording is necessary to assess baseline waking EEG activity. The addition of photic stimulation, hyperventilation, and especially sleep—which should be recorded whenever possible—often requires an increase of recording time.
3.10 The recordings should include periods when the eyes are open and when they are closed.
Proper EEG recordings requires examining the effect of stimuli upon the EEG. A comparison between the eyes-open and eyes-closed condition constitutes one important means for assessment. Some rhythms can be masked by the alpha activity and are visible only when the alpha rhythm has been attenuated by eye-opening. Certain forms of eye movement may appear to be frontal delta or theta activity but eye-opening and closing helps in differentiation. Finally, paroxysmal activity may appear only when the eyes are opened or only when the eyes are closed or at the times these conditions change. Thus, failure to record with eye-opening and closing as a routine procedure can reduce chances of obtaining potentially important information. This procedure is so simple that it is unjustifiable not to request eye-opening and closure whenever patient cooperation per-mits, or to manually open and close the eyes when it does not.
3.11 Hyperventilation should be used routinely unless medical or other justifiable reasons (e.g., a recent intracranial hemorrhage, significant cardiopulmonary disease, sickle cell disease or trait, or patient inability or unwillingness to cooperate) contraindi-cate it. It should be performed for a minimum of 3 min with continued recording for at least 1 min after cessation of overbreathing. At times, hyperventilation must be performed for a longer period in order to obtain adequate activation of the EEG. To evaluate the effects of this activation technique, at least 1 min of recording with the same montage should be obtained before overbreathing begins. The record should contain an assessment of the quality of patient effort during hyperventilation. It is often helpful to record electrocardiographic (ECG) activity directly on one EEG channel during this and other parts of the recording, particularly if spikes and sharp waves, or pulse or ECG artifact, are in question. With an additional (e.g., 17th) channel, the ECG can be monitored continuously.
3.12 Sleep recordings should be taken whenever possible but not to the exclusion of the waking record.
It is increasingly evident that considerable additional information can be obtained by recording during drowsiness and sleep. Some laboratories use sleep recording routinely. Sleep recording is usually essential for patients with suspected or known convulsive disorders.
3.13 The patient’s level of consciousness (awake, drowsy, sleeping, or comatose), and any change thereof, should be noted by the technologist on the EEG recording. Any commands or signals to the patient, and any movement or clinical seizure activity or absence thereof, should also be noted on the recording. Careful observation of the patient with frequent notations is often essential, particularly when unusual waveforms are observed in the tracing. Abbreviations used should be standardized, with their definitions readily available to the reader.
In stuporous or comatose patients and those showing invariant EEG patterns of any kind, visual, auditory, and somatosensory stimuli should be applied systematically during recording. The stimuli and the patient’s responses or failure to respond should be noted on the recording paper as near as possible to their point of the occurrence.
It is the responsibility of the electroencephalographer to recognize the patterns usually associated with different states of consciousness. However, observations by the technologist about the patient’s clinical status can be of considerable interpretative value, particularly when discrepancies or unusual correlations occur.
To facilitate assessing awake background activity, it is important for the technologist to ascertain that the patient is maximally alert for at least a portion of the record.
3.14 Special procedures that are of some risk to the patient should be carried out only in the presence of a qualified physician, only in an environment with adequate resuscitating equipment, and with the informed consent of the patient or responsible relative or legal guardian.
3.15 EEGs for the evaluation of cessation of cerebral function (“cerebral death”) require special procedures and extraordinary precautions (see Guideline 3: Minimum Technical Standards for EEG Recording in Suspected Cerebral Death).