108 Algorithms and measuring principles
a reference electrode is the most common method for effecting an
essentially greater suppression of interference.
The adhesive quality and the composition of the gel applied to the elec-
trodes have an additional important effect on ECG measurements. The
best results are obtained when the electrodes adhere well. Electrodes
that have dried out or adhere badly are unsuitable.
Irrespectively of all interference signals, the heart rate can be measured
only when the amplitude of the measured ECG signal itself is large
enough to be detected by the monitor. Normally the amplitude of the
ECG signal detected on the body surface cannot be influenced and
varies from person to person.
The monitor measures the potential difference between the two sens-
ing electrodes. Depending on where the electrodes are attached, the
measured signal is strong, weak, positive, or negative. It is even poss-
ible that a signal cannot be seen at all. This is the case when the elec-
trodes are inadvertently placed on the same potential line. The pro-
pagating signal from a heartbeat can be pictured as contour lines around
a hill on a map. When two persons are standing on the same line, the
difference in height between these persons is zero. Accordingly, no
ECG signal can be measured when the sensing electrodes are placed
on such a line on the body surface. To prevent too weak ECG signals
from triggering false alarms, it is therefore important that the optimal
electrode positions are determined before the start of monitoring.
10.3 Measuring principle for the apnea monitor
Unlike the heartbeat, respiration is not accompanied by its own
electric signal. A different principle must therefore be adopted for
measuring respiration. The most common method is the so-called
impedance pneumography. Fluctuations in impedance are caused by
both respiratory movements of the thorax and other movements. For
this reason, an apnea monitor can function correctly only when the
patient is lying still. Here too, the measured signals are very weak and
must be amplified. To prevent false alarms wherever possible, it is
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