1) Lack of reference electrode. This will not cause inaccuracy of the analysis but will make it far more susceptible to artefact, particularly electrical interference.
2) Radio-frequency interference from a variety of sources, radio-telemetry, paging systems, lighting and diathermy. The interference may be picked up by the patient and electrode leads acting as an aerial. The CFAM3 does have radio frequency interference rejection filtering in its input but, if the interference is large enough it may be rectified and added to the signal and could cause amplifier overload. If possible try to locate the source by switching off possible sources for about 10 seconds or by observation. Diathermy, often being pulsatile in use may cause corresponding pulsatile deflections in the amplitude trace. It should also be visible in the EEG trace. The trace should be marked when this occurs to prevent subsequent misinterpretation. Electromagnetic pulses are also caused by lightning with corresponding deflections seen in the EEG trace.
3) Intermittent voltages, typically from an external cardiac pacemaker, may affect the tracing and may also be indicated by deflections in the impedance and muscle channels.
4) Electrical interference on the mains supply. Most such interference is filtered out, but high voltage spikes, particularly such as occur in industrial areas may cause deflections in the trace.
5) Faults in connection to mains supply, earthing or other apparatus. The CFAM may give warning of potentially dangerous electrical faults in other apparatus by unusual patterns of artefact, amplifier overloads, spurious impedance readings, muscle and 50/60Hz trace changes. These may be associated with movement of that apparatus, operating it and switching it on, plugging it into the mains supply. Identify by switching off / disconnecting possible sources for about 10 seconds. If there is any suspicion of this call a service engineer to investigate the problem immediately. Possible sources could be electrical items attached to the bed frame and incorrectly connected electronic monitors.
6) Electromagnetic interference. Changing magnetic fields passing through the patient, the electrode leads or the isolation amplifier will induce electrical currents in them. The most likely sources are electric motors, transformers, mains cables, and headphones used to generate acoustic evoked potentials or for listening to radio programmes or tape recorders. Identify by switching off possible sources for about 10 seconds.
7) Electrostatic interference. Persons having accumulated an electrostatic charge and then touching a patient can cause deflections. Electrostatically charged items such as clothing moving in the vicinity of the patient and the electrode leads can induce changes of potential at the input to the isolation amplifier. It may cause artefactual deflections and amplifier overloads. The cause is usually easy to detect by observation of movements and the CFAM trace, taking into account that the trace has an approximately 2 second delay in the EEG and a 4 second delay in the amplitude-frequency plots.
8) Stimulus related artefact. Stimuli given for averaged evoked potential computation or for stimulating efferent motor potentials or for assessment of neuromuscular block may generate electrical potentials across the recording electrodes. The occurrence of these would normally be observed on the EEG trace. If computing an averaged evoked response on the CFAM, these potentials would occur at the time of the stimulus rather than at the time of brain response.
