This set-up will also regularly print unprocessed EEG samples on the chart. However remember that a full EEG may indicate that asymmetry or other phenomena are better shown with a different electrode placement.
If, on the CFAM EEG display, you see unusual waveforms or seizure waveforms, place the highlight on the sample button and press the down arrow. This will print the waveforms on the chart recorder.
Further, if the patient appears to be having seizures and these are not visible on the EEG in two-channel recording, then go to four-channel recording. This should enable recognition of the majority of focal seizures, although for proper localisation a full EEG should also be undertaken if possible. In general it is always advisable to obtain a conventional EEG recording as limited channels, as on brain monitors, my miss something of significance. .
Ideally you should always try to use four-channel monitoring..
Why more than one channel of recording?
When you record EEG you are recording the difference in voltage between the two recording electrodes. In Fig1, A represents the voltage on an electrode in the left hemisphere and B the voltage from the right hemisphere. The voltage recorded by the EEG recorder is A minus B. If A were to be equal to B then the monitor would record zero volts. Similarly, if B were to be reduced to zero, then the monitor would record only voltage A. I.E. In the presence of a serious reduction of amplitude on one channel, the voltage actually recorded could increase.
In general of course, in EEG, A and B are not the same but are usually positively correlated to some extent and this means that it is possible for the voltage under one electrode to become heavily attenuated, for example as a result of a haemorrhage, without significant indication of this asymmetric reduction being observable on a bi-parietal CFM trace. Hemispheric asymmetries are a common feature of Intensive Care recordings and particularly so in Neonatal Intensive Care. Sometimes they will develop during the course of recording. For this reason RDM Consultants do not supply single channel monitors and encourage customers never to use single channel inter-hemispheric recordings in Intensive Care.
It is suggested that, to comply with Toby requirements, you might consider undertaking a two-channel left-right comparison recording prior to recording on a single channel in order to show whether or not an asymmetry exists. Use the CFAM format trace at 30cm.hr for this as it is much better for the purpose. Similarly, whenever you are not recording for the purposes of comparison with the Toby study, you should revert to the two channel recording as asymmetries can develop over time. These EEG asymmetries, typically from haemorrhages, may be visible in the EEG before their origin is detectable by ultra-sound imaging.
In Fig 2. A represents the voltage recorded between two electrodes in the left hemisphere, and B from two electrodes on the right. If , because of a haemorrhage on the right, the voltage under both electrodes reduces by roughly the same proportion, then the voltage difference between them (B) will also reduce, so the two channel trace correctly indicates a right sided problem.
The two channel electrode placements preferably should be C3-P3 and C4-P4 if you are looking for asymmetries related to perfusion, and F3-P3, F4-C4 if you are looking mainly for seizures. You can then use P3 and P4 for the single channel CFM recording. Normally CFAMs are supplied with the two channel recording positions ready set under the heading 2CHANNEL, or you can set another name.
