Biomedical Engineering Reference
In-Depth Information
Table 2.3-1 Energy forms and associated information-carrying
variables
Energy
Variables
(specific fluctuation)
Common
measurements
Figure 2.3-1 A classic systems view of a physiological system
that receives an external input, or stimulus, that evokes an output,
or response.
Chemical
Chemical activity
and/or concentration
Blood ion, oxygen,
carbon dioxide,
pH, hormonal
concentrations,
and other chemistry
concentrations of other blood components; and sounds
generated by the heart and its valves.
Often it is desirable to send signals into a biological
system for purposes of experimentation or therapy. In
a general sense, all drugs introduced into the body can be
considered biosignals. We often use the term
stimulus
for
signals directed into some physiological process, and if an
output signal is evoked by these inputs we term it a
re-
sponse.
(Terms shown in
italics
are an important part of
a bioengineer's vocabulary.) In this scenario, the bi-
ological system is acting like an input-output system,
a classic construct or model used in systems analysis
(
Figure 2.3-1
).
Classical examples include the knee-jerk reflex, where
the input is a mechanical force and the output is me-
chanical motion, and the pupillary light reflex, where the
input is light and the output is a mechanical change in the
iris muscles. Drug treatments can be included in this
input-output description, where the input is the mo-
lecular configuration of the drug and the output is the
therapeutic benefit (if any). Such representations are
further explored in the sections on systems and analog
modeling (Sections 2.3.3.2 and 2.3.3.3).
Systems that produce an output without the need for
an input stimulus, for example the electrical activity of
the heart, can be considered biosignal
sources.
(Although
the electrical activity of the heart can be moderated by
several different stimuli, exercise for example, the basic
signal does not require a specific stimulus.) Input-only
systems are not usually studied, because the purpose of
any input signal is to produce some sort of response: even
a placebo, which is designed to produce no physiological
response, often produces substantive results.
Because all of our interactions with physiological sys-
tems are through biosignals, the characteristics of these
signals are of great importance. Indeed, much of modern
medical technology is devoted to extracting new physi-
ological signals from the body or gaining more in-
formation from existing biosignals. The next section
discusses some of the basic aspects of these signals.
Mechanical
Position
Force, torque, or pressure
Muscle movement,
cardiovascular
pressures, muscle
contractility
Valve and other
cardiac sounds
Electrical
Voltage (potential
energy of charge
carriers)
Current (charge
carrier flow)
EEG, ECG, EMG,
EOG, ERG, EGG, GSR
Thermal
Temperature
Body temperature,
thermography
ECG, electrocardiogram; EEG, electroencephalogram; EGG, electrogastrogram;
EMG, electromyogram; EOG, electrooculogram; ERG, electroretinogram; GSR,
galvinic skin response.
that carries the information (the specific energy fluctua-
tion) depends upon the type of energy involved.
Table 2.3-1
summarizes the different energy types that can be used
to carry information, and the associated variables that
encode this information.
Ta b l e 2 . 3 - 1
also shows the
physiological measurements that involve these energy
forms as discussed later in the chapter.
Biological signals are usually encoded into variations of
electrical, chemical, or mechanical energy, although oc-
casionally variations in thermal energy are of interest. For
communication within the body, signals are primarily
encoded as variations in electrical or chemical energy.
When chemical energy is used, the encoding is usually
done by varying the concentration of the chemical within
a
physiological compartment,
for example, the concen-
tration of a hormone in the blood. Bioelectric signals use
the flow or concentration of ions, the primary charge
carriers within the body, to transmit information. Speech,
the primary form of communication between humans,
encodes information as variations in air pressure.
Outside the body, information is commonly trans-
mitted and processed as variations in electrical energy,
although mechanical energy was used in the seventeenth
and early eighteenth centuries to send messages. The
semaphore telegraph used the position of one or more
large arms placed on a tower or high point to encode
letters of the alphabet. These arm positions could be
2.3.2 Biosignals
Much of the activity in biomedical engineering, be it
clinical or research, involves the measurement, processing,
analysis, display, and/or generation of signals. Signals are
variations in energy that carry information. The variable
