Sources of, & the remedial action to combat the following sources of artifact: Environmental AC fields; EMG interference; Electrode-skin interface problems

1. Noise originating from sources outside the patient

a) Electrostatic sources

i) Patient acts as one plate of a capacitor

ii) Seen when a charged body is brought up close to an uncharged one, an equal & opposite charge develops on the uncharged body

iii) Eg, if an unearthed body is close to any cable or lamp element that is connected to mains, he will develop a surface charge of equal & opposite potential even though no current is flowing between the two bodies

iv) As the mains potential has a frequency of 50 Hz, the induced potential will also have this frequency

v) Other sources of electrostatic charge include the operating table, other persons, electronic equipment

b) Electromagnetic induction

i) An interference that occurs in the vicinity of wires carrying AC currents

ii) Results in 50 Hz interference

iii) Due to the generation of a magnetic field by the flow of a current: all conductors carrying mains currents are surrounded by electromagnetic fields

iv) The 50 Hz interference is a difference in potential, relative to the ground, that is impressed upon any subject in proximity to the wire carrying alternating current; the subject takes on a potential that is neither that of ground, nor of the power line, but somewhere in between. Since the utility current is fluctuating, the voltage of the subject is also fluctuating

v) Effect is minimised by the fact that the electromagnetic field generated by the live wire is to a greater degree cancelled out by the neutral cable flowing adjacent to the live cable but flowing in the opposite direction

a) However, if leakage of current occurs, the two currents are no longer equal & self-cancelling thereby generating an e.m.f.

(1) The effect is multiplied if the wires are coiled

(2) Leads connecting patient electrodes to sensitive amplifiers are most frequently affected

c) Radiofrequency Interference [> 100 kHz]

i) May enter via

a) Mains distribution system mixed up with 50 Hz current; sources include diathermy, electric motors

b) Radiopropagation whereby activated diathermy probe held in air acts as radiotransmitter aerial while the patient ECG lead acts as a receiving aerial

d) Remedial action

i) Differential analyser

ii) High CMRR

iii) Shielded patient circuitry with copper or aluminium enclosure

iv) ‘Floating” RL ECG

v) Surrounding each lead with a braided copper screen to minimise electrostatic induction

vi) Keep leads as short as possible

vii) Mitigate effects of electromagnetic fields by ensuring all of patient’s leads are of the same length, are closely bound together or even twisted together until close to the electrodes thereby ensuring that the induced signals are identical & therefor susceptible to CMRR

viii) Eliminate source of unwanted electromagnetic radiation (only real cure)

2. Noise originating from the patient

a) EMG [electromyogram]

i) Frequency (Hz) overlap those of EEG & ECG

ii) Signal can be much larger (increased mV) than either EEG or EGG

iii) Muscular activity (especially shivering) can lead to gross interference

iv) ‘Muscle artifact’

b) Remedial action

i) Minimise patent movement ie relax

ii) Minimise shivering

iii) Avoid muscle area to place electrodes; use bony prominences

3. Noise originating from patient-electrode contact

a) Unfortunately, recording electrodes do not act as a passive conductor

i) The placement of a metal next to an electrolyte solution as seen on the surface of skin produces an electrochemical half-cell resulting in the generation of an electromagnetic force

ii) If a differential amplifier is connected to a pair of such electrodes, their output potentials are compared

a) If the cells are identical, the outputs will be self cancelling yielding zero output

b) If the cells are not identical, the difference in potential between the two cells will be amplified

(1) Additionally, the small current produced by the offset potential may result in polarisation

(2) A polarised electrode will distort any signal

iii) Mechanical movement of recording electrodes results in changes in potential

a) Due to alteration in the physical dimensions of the electrode-skin half cell thus modifying cell potential and skin-electrode impedance

b) Remedial action

i) Abrade skin, remove hair, alcohol to ensure adhesion of electrode

ii) Use of Ag:AgCl electrode which does not polarise