Biomedical Engineering Reference
In-Depth Information
4.1
Chapter 4.1
Clinical applications
of bioelectricity
Sverre Grimnes and
B
rjan G. Martinsen
Bioimpedance research and development have been
going on for a very long time, and the number of
clinical
applications does regularly increase but at a low rate.
Bioimpedance is a general transducing mechanism for
many physiological events, and the instrumentation is
low cost. Basically the situation may have the aspect of
a technology seeking a problem, but this must be
reverted by taking another approach: go to the clinics and
start with their problems and needs and see whether
bioimpedance technology has a solution. Many of the
transducing mechanisms can indeed offer solutions, but
they must compete with the other solutions already in
clinical use. Ultrasound imaging instrumentation for ex-
ample can measure many of the parameters that bioim-
pedance can also measure. As the medical doctors
already have the ultrasound probe in their hands as
a multiparameter measuring device the bioimpedance
technology must offer some definite advantages. The job
is to find those clinical application areas.
Another job is to standardize the methods used by
different models and companies. A problem related to
standardization is when different models use non-scientific
methods based on non-published algorithms. Another
problem is that it is difficult to describe the clinical ap-
plications in detail because the more important they are
the more patent rights and commercial interest complicate
the picture.
units. Also ECG control is extremely important during
resuscitation and defibrillation.
During a routine ECG examination four electrodes
are connected to the limbs. Three of these are for ECG
signal pick up and one is a reference electrode (right leg
(RL)) for noise reduction. Six electrodes are connected
in the thorax region at well-defined positions near the
heart.
4.1.1.1 Three limb electrodes
(six limb leads)
The most basic ECG examination is with three skin
surface electrodes on the limbs: one at the left arm (LA),
one at the right arm (RA) and one at the left leg (LL).
Figure 4.1-1
illustrates how six limb leads are derived
from these three electrodes.
It is unusual that pick-up electrodes are placed so far
away from the source organ; the main reasons in ECG are
standardization and reproducibility. In our language, the
limbs are salt bridges to the thorax. By this, the coupling
to the thorax is well defined and electrocardiograms can
be compared even when recorded in different hospitals
and at long intervals. The position of the electrodes on
each limb is uncritical because the distal part of each limb
is isoelectric (with respect to ECG, not with respect to,
for example, electromyography (EMG) sources of the
armmuscles). Such is the reproducibility that the bipolar
leads form the basis for determining the axis of the
electric heart vector.
4.1.1 Electrocardiography
Electrocardiography (ECG) is an important clinical ex-
amination routinely performed with 12 leads, 9 skin
surface electrodes and a reference electrode. ECG is also
important for long term monitoring in intensive care
Three bipolar leads: I, II, III
Channel I is the voltage difference RA-LA; channel II:
RA-LL; channel
III: LA-LL. Einthoven found the
