Biomedical Engineering Reference
In-Depth Information
cases, and are pathological. Obesity does not change the
ECG waveforms; the
R
amplitude may be somewhat
reduced but not into the ''low voltage'' region.
Indifferent electrode
With an ideal current dipole in an infinite volume, zero
potential will exist in all directions if the distance to the
dipole is large enough. Unfortunately an indifferent
(neutral) ECG electrode does not exist because the
human body is not large enough. If we go out in one di-
rection along a limb, the limb proper is isoelectric with
respect to the heart activity but not with respect to other
sources (e.g. respiration). If we go out along a second
limb that too will be isoelectric, however the potential
will not be equal to that of the first limb. Therefore none
of them represents a true indifferent electrode.
Since this is an unsolvable problem, the ideal unipolar
lead does not exist. Non-ideal solutions are to use one
electrode at some remote point (e.g. at a limb or an ear-
lobe). Another solution is to add the voltages picked up by
more than one electrode, such as two limb electrodes
(augmented leads) or three limb electrodes (Wilson cen-
tral terminal).
Frequency spectrum
Routine bipolar diagnostic ECG is performed with a pass
band of, for example, 0.05-60 Hz. The American Heart
Association (AHA) recommends 150 Hz as minimum
bandwidth and 500 Hz as minimum sampling rate for
recording pediatric ECG. DC voltage is thus filtered out
because it is related to the electrodes and not to elec-
trophysiology. In monitoring situations the low frequency
cut-off frequency may be increased to 0.5 Hz or more
because the patient induced motion artefacts are re-
duced. With skin surface electrodes only the QRS com-
plex has frequency components above 60 Hz, however in
the QRS there are components up to and above 1000 Hz
(Franke et al., 1962) which are not used clinically. With
electrodes in direct connection with the heart the useful
frequency spectrum extends to 500 Hz and more. With
a pacemaker assisted heart the pacemaker pulses may
have a width of
<
1 ms, and the frequency spectrum
extends far above 1 kHz.
Ground, reference, indifferent electrode
In Einthoven's time without amplifiers there was no need
for a ''ground'' electrode, the galvanometer is a floating
input device. But mains supplied amplifiers needed
a ''ground'' electrode to reduce noise, and it was the RL
which was chosen for that purpose. Later safety philos-
ophy advocated that the patient should be electrically
floating with respect to ground (cf. Section 4.1.17.2).
This implied that ''ground'' was not ground in the
meaning the ground wire of the mains. Instead of
''ground'' this is the
reference
electrode, meaning that the
amplifier and the patient are connected together so that
they roughly are at the same potential. But then we also
had the ECG tradition that in unipolar leads the refer-
ence may not only be one electrode, but for example,
a Wilson terminal, used as ''zero'' or indifferent refer-
ence. Here
ground
means the protective ground electri-
cally connected to the building and the room, supplied by
the mains and having the symbol . The purpose of
a
reference
electrode is to obtain potential equalization
between the patient and the electronic circuitry, symbol
, or . The reference electrode is usually an
electrode on the RL. A
neutral
or
indifferent
electrode is
a part of a unipolar electrode system, and is usually
obtained by summing the potentials from more than one
electrode (cf. the Wilson terminal).
Zero line and isoelectric level
Because the ECG amplifier is AC coupled no true zero
voltage is recorded. Instead the zero is a line defined by
the low-pass filter of the ECG amplifier so that the
positive and negative areas of the total ECG waveform
are equal. The zero line can be determined by simply
shorting the two input wires of a lead (no signal).
Noise
True physiological signals
The limb leads presuppose that the patient is lying down
so that EMG signals from the limb and body muscles do
not intervene. Slow baseline respiration waves are seen if
the characteristic frequency of the low-pass filter is 0.05
Hz or lower.
Exogenous noise
50/60 Hz mains noise is electric or magnetic field
coupled from power line wires to patient wires or
patient body.
Patient movement generates triboelectricity
which may severely disturb the ECG waveform
(electrostatic noise).
Solutions are shielding, active op-amp circuit
clamping the body to ground, increased distance to the
noise source, increased common mode rejection
circuitry, averaging over several heart beats, wireless
telemetry of electrode pick-up signals.
4.1.2 Impedance plethysmography
Plethysmography is the measurement of volume. Dy-
namic plethysmography is usually associated with volume
changes due to the heartbeats, but may also be related to,
for example, respiration or peristaltic movements of the
