Irregular patterns of ECG may be caused by either pathological states or at times artefacts. Undesirable presence of artefacts on ECG trace may be due to the following:
- Interference from the power line
- Shifting baseline
- Muscle tremor
Interference from the Power Line
This interference may be caused by the stray effect of the alternating current on the subject under measurement or because of alternating current fields due to loops in the patient cable. Interference may also be due to loose contacts on the patient cable as well as unclean electrodes. When the machine or the subject under measurement is not properly grounded, power line interference may even completely obscure ECG waveform.
A common cause of 50 Hz interference is the disconnected electrode resulting in a very strong disruptive signal that can damage the stylus of an unprotected direct writing recorder, thus requires swift action.
At times, static charges on the synthetic uniform of the operator may result in a random noise on the trace. This noise is very hard to remove except in those machines which have very high common-mode rejection ratio (CMRR). The noise can be reduced by partially shielding the subject by means of the bed springs. Connection of the springs to the instrument case helps to compensate for a poor CMMR.
Electromagnetic interference from the power lines also causes poor quality tracings. Electrical apparatus like X-ray units, air-conditioners and elevators draw heavy power-line current, which induce 50 Hz signals in the input circuits of ECG machines. Because of the unbalanced linkages, common mode rejection circuits are almost ineffective against them. To minimize this problem, is to have in place, a physical separation between the interference causing sources and the subject under measurement.
The electrical power systems also induce very rapid pulses or spikes on the trace, due to the switching action. Employing a transient suppressor in the mains lead of the machines may help fix this issue.
Related article: 3 Sources of Noise in Biomedical Measurement Systems
Shifting of the Baseline
A shifting baseline may be caused the movement of the subject/patient or electrodes. The baseline shift can be removed by ensuring that the patient lies relaxed and the electrodes are properly attached. Baseline shift/Baseline wander is normally observed immediately after the application of the electrodes; this is due to relatively slow establishment of electromechanical equilibrium at the electrode-skin interface. This can be minimized by choosing the proper electrode material, which will reach equilibrium quickly with a good electrode jelly.
Muscle Tremor
Sporadic trembling of the ECG trace, without shifting of the baseline happens when the patient is not relaxed or is cold. Mostly occurs in older patients. For normal ECG recordings, ensure the patient is warm and advised to relax.
You may also read:
- The Instrumentation for Recording ECG Signals
- What is Echocardiography?
- ECG Electrodes
- Commonly used EEG Electrodes
