SINGLE CHANNEL RECORDING
Conventionally for single channel CFM recording the electrode positions were P3 to P4, with the reference electrode being placed at the vertex.
The three EEG electrode leads are connected to Channel 1 red (+), black (-) and reference (green) inputs.
The EEG signal is recorded between red and black inputs. A positive voltage on the red input deflects the EEG trace on the screen downwards if the EEG is plotted negative upwards (the polarity is shown on the EEG plot).
The 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.
Reasons for single channel electrode positions.
As an indicator of adequate cerebral perfusion, the recording electrodes are over the joint boundary zone area of the distributions of the posterior, middle and anterior cerebral arteries. This placement is normally sensitive to, for example, sudden falls of blood pressure that exceed the tolerance of auto-regulation, when little or no indication may be obtained from electrodes placed elsewhere, for example frontally.
It is also a location least likely to be affected by scalp muscle activity, and generally has the highest amplitude (and hence best signal to noise ratio) of EEG. Similarly eye movement artefact is minimised in this position.
The placement minimises interference to and from nursing procedures.
Localised seizure activity
When a single channel is to be used to monitor localised seizure activity it is recommended that a conventional EEG recording be obtained first to determine the best electrode placement. This would normally be a balance between amplitude of the seizure activity relative to background and minimising possible associated scalp muscle activity.
Why you should avoid single channel recording
Because left-right asymmetries are commonplace and this derivation cannot indicate them. Typical examples are haemorrhages as a result of birth trauma and carotid artery surgery.
This is dealt with in detail separately. See entry on Single Channel CFM Studies.
