ELECTROENCEPHALOGRAPH (EEG)

Typical Number in Hospital: 3

Cost Bands: 5,6

References: 3,9

Electrical signals from the brain can be picked up by electrodes attached to the scalp. Differential signals between electrodes over different parts of the brain, and monopolar signals between each electrode and a reference, typically give signal amplitudes of between 10 and 300 [u]V, with frequency content between 0.5 and 40 Hz. The apparatus normally produces a wide paper strip with eight or sixteen simultaneous tracings showing the patterns derived from electrodes placed over different parts of the brain, and the relationship between these. Computer analysis of electroencephalograms is possible, but in most everyday use the paper strip is read directly by a technician or neurologist.

These machines are found in the neurology department or in a special sub-section of this called the EEG department. Patients are referred for investigation of suspected abnormalities of the brain which may arise from disease, injury, or may be congenital in origin (e.g. epilepsy). There are standard electrode configurations and standard procedures for the test, since the EEG may change considerably even with the mood of the patient. In some tests a stimulus is applied such as a flashing light. Evoked response averaging of the EEG is sometimes applied to identify which part of the EEG waveform relates to a particular stimulus, but is more often performed on dedicated machinery in the departments which use the results (e.g. electric response audiometry in the audiology department).

A typical EEG machine would consist of a head harness to hold the electrodes, a set of electrodes and leads, an input selector box with switches to group the electrodes into the standard configurations, preamplifiers with variable settings and time-constant switch to define the low-frequency cut-off, and filters for high-frequency cut-off. There would also be a calibrating facility. Following the preamplifiers are the pen- drive amplifiers, with shift and ink controls. The actual writing mechanism may be pen and ink, or it may be an ink spray type, in which ink is sprayed on to the paper from a jet mounted on a galvanometer a few centimetres from the paper. This type has the advantage of a very high frequency response. Photographic type recorders are possible, but unusual.

A one or two channel version is sometimes used as a 'cerebral function monitor' in intensive care or in the operating theatre. This produces a slow-moving paper chart record of the EEG on which the amplitude is taken to indicate the state of the patient. The waveform of the EEG is completely ignored. Since clinical death is commonly defined from the absence of an EEG, these devices have a use which is quite distinct from the diagnostic uses in the EEG department.