Two channels are normally used as a left right hemisphere comparison. This is the preferred initial recording derivation. For example this may be used to monitor unilateral head injury, comparing the damaged side with the undamaged side as a reference, or as a warning indicator of unilateral failure of perfusion such as may occur during carotid artery surgery.
Electrode placement is normally frontal to parietal F3-P3, and F4-P4,if you are trying to detect seizures, or C3-P3 and C4-P4 if you are trying to detect failure of cerebral perfusion such as in carotid artery surgery, so that both left and right have electrodes over the boundary zones of the cerebral arteries. The reference electrode may be placed at the vertex.
Five EEG electrode leads are used. These are connected to Channel 1 red (+ ) and black (-), Channel 2 red and black, and reference inputs. The EEG signal is recorded between red and black inputs. A positive voltage on the red input deflects the EEG trace downwards if the EEG is plotted negative upwards (the polarity is shown on the EEG plot).
The independent reference electrode reduces the amount of electrical interference that may be induced into the input amplifier. It is essential for minimal artefact recordings to use this electrode.
Whenever possible avoid placing an electrode close to the forehead or low down by one of the ears. In these positions you will get muscle artefact which may persist even with the levels of muscle relaxant used in surgery.
This would normally be used where an indication of localisation of seizure activity is required. It may be necessary to move the electrodes to various locations over a suitable period of time in order to establish an effective recording position. However, the CFAM3 is not an EEG replacement. Its function in this case would normally be to monitor the effectiveness of anti-convulsant therapy. Suitable starting locations may be F3-C3, F4-C4, P3-O1, P4-O2.
Note, if you want input leads on two channels to share an electrode location, suitable adapter plugs can sometimes be obtained from electronic component distributors.
