ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 52

2. Power Line interference (PLI): These are picked up on the lead wires of
neighboring power line cables, due to capacitive coupling with ECG lead wires.
So, a 50/60 ± 0.2 Hz current flows through the lead wires to ground through
the patient body. Sometimes, the equivalent voltage drop that appears as a
common-mode signal to the input of the ECG amplifier can be as high as
20 mV, which is four times greater than the maximum ECG amplitude itself. A
detailed discussion on PLI is given in [ 4 ]. A measurement technique of power
line interference technique is described in [ 5 ]. In another approach [ 6 ], line
interference reduction is described where the line reference signal is simulta-
neously measured and a scaled version is subtracted from the ECG by signal
averaging.
3. Electrode pop or contact noise: Sometimes loss of contact between the patient
skin and electrode may cause a temporary saturation of the amplifier output for
certain period of time.
4. Baseline wander: This is due to respiration of the patient during ECG. The
lung volume change during the respiration process changes the impedance
between the heart muscle and electrode. This causes baseline (isoelectric level
segments) of the ECG to oscillate at a very-low-frequency drifting between
0.15 and 0.3 Hz.
5. Motion Artifacts: Due to improper 'preparation' of the skin, or patient
movement, a slow movement of the electrodes may occur in long-term
recording using wearable sensors. Motion artifact has a significant overlap with
ECG signal spectrum in the range 1-10 Hz. This results in an abrupt baseline
jump or complete saturation of amplifier output for 0.5 s.
6. Electrosurgical noise: This is the noise generated by neighboring medical
equipments in the clinical setup at frequencies between 100 kHz and 1 MHz.
7. Amplifier noise: Noise and drift are two unwanted signals that are generated
within the amplifier that contaminate a biopotential signal under measurement.
'Noise' generally refers to undesirable signals with spectral components above
0.1 Hz, while 'drift' generally refers to slow changes in the baseline at fre-
quencies below 0.1 Hz. The noise and drift are measured either in microvolts
peak to peak (lV p-p ) or in microvolts root mean square (RMS) (lV RMS ) and
appear as if they were applied as differential input voltage.
8. Quantization noise: The AD converter, having a finite bit width, samples the
amplified ECG to generate a sequence of discrete data, which are digitized
representations approximated to the nearest bit. Quantization noise thus refers
to this truncation of the analog samples, which are of infinite resolution, to a
binary number of finite width and equal to ±1 LSB of the AD converter.
A detailed discussion on ECG artifacts can be found in [ 7 - 8 ]. These artifacts
can be minimized by suitable designs and clinical setup; however, it is impossible
to completely eliminate those altogether using hardware designs. Nowadays, many
soft computational techniques are available which are used for denoising digitized
ECG.