Science has progressed through many gradual states. It is a
long time since Archimedes are his Greek
contemporaries started down the
path of scientific discoveries, but a
technological historian could easily trace the trends through
the centuries. Engineering has emerged out of the roots
of science, and since the Industrial Revolution
the profession has grown rapidly Again, there are definite
stages that can be traced. The field of medical instrumentation is by no means new. Many instruments were developed as early as the nineteenth country
for example, the electrocardiograph, first used by Einthoven
at the end of that century. Progress was rather slow
until after World War II, when a surplus
of electronic equipment, such as amplifiers and recorders, becomes
available. At that time many technicians
and engineers, both within industry and on their own, started to
experiment with and modify existing equipment for medical use,
this process occurred primarily during the 1950s and
the result were often disappointing, for
the experimenters soon learned that
physiological parameters are not measured in the same
way as physical parameters. They also encountered
a severe communication problem in the medical profession.
Some forms of biomedical instrumentation are unique
to the field of medicine but many are adaptation of widely
used physical measurements. A thermistor, for example, changes its
electrical resistance with temperature, regardless of whether the
temperature is that of an engine or the human body. The
principle are the same only the shape and size of the device might
be different. Another example is the stain gage, which is
commonly used toe measure the stress
in structural components. It
operates on the principle that electrical
resistance is changes by the stretching of
a source of constant voltage,
an electrical output can be obtained that
is proportional to the amount of the stain. Since pressure can
be translated into stains by various means, blood pressure
can be measured by an adaptation of this device.
When the transducer is connected
into a typical circuit, such as
abridge configuration, and this circuit is
excited from source of constant input voltage,
the changes in resistance are reflected
in the output as voltage changes. For
a thermistor, the temperature
is indicated on a voltmeter calibrated
in degrees Celsius of Fahrenheit. In the design or specification of medical
instrumentation systems, each of the following
factors should be considered.
The branch of science that includes the
measurement of physiological variables
and parameters is known as
biometrics. Biomedical instrumentation provides the tools by
which these measurements can be achieved.
Range
:
The range of an instrument is generally considered to include
all the levels of input amplitude and frequency over
which the device is expected to operate. The objective
should be to provide an instrument that will give a usable
reading from the smallest expected value of the variable or
parameter being measured to the largest.
Sensitivity
:
The sensitivity of the instrument determines how
small a variation of the variable
or parameter cab be reliable
measured. This factor differs from the instrument's range in
that sensitivity is not concerned with the absolute levels of
the parameter but rather with the minute changes that cab be
detected. The sensitivity directly determines the resolution of
the device, which is the minimum variation
that can accurately be read. Too high a sensitivity often
results in non-linearities or instability. Thus, the optimum
sensitivity must be determined for any given
type of measurement. Indications of sensitivity are frequently
expressed in terms
of scale length per quantity to be measured-
for example, inches per microampere in a galvanometer
coil or inches per millimeter of mercury. These
units are sometimes expressed reciprocally. A sensitivity
of 0.025 centimeter per millimeter of mercury could be expressed
as 40 millimeters of mercury per centimeter.
Linearity
:
The degree of which variations on the output of an instrument
follows input variations is referred to as the linearity of
the device. IN a linear system the sensitivity would be the
same for all absolute levels of input, whether in the high,
middle, or low portion of the range. In some instruments
a certain form of non-linearity is purposely introduced to create
a desired effect, whereas in others it is desirable to have
linear scales as much as possible over the entire range
of measurement. Linearity should be obtained
over the most important segments, even
id it is impossible to achieve it over the entire range.
Hysteresis
:
Hysteresis is a characteristic of some instrument where by
a given value of the measured variable results in a different
reading when reached in as ascending direction
from that obtained when it is reached in
a descending direction. Mechanical friction in a meter,
for example, can cause the movement of the indicating needle
to lag behind corresponding changes in
the measured variable, thus resulting in
a Hysteresis error in the reading.
Frequency Response
:
The frequency response of an instrument is its variation in sensitivity
over the frequency rage of the measurement. It is important of display
a wave shape that is a faithful reproduction of the original
physiological signal an instrument system should be
able to respond rapidly enough to reproduce all
frequency components of the waveform with equal sensitivity.
This condition is referred to a s a "flat response"
over a given range of frequencies.
Accuracy
:
Accuracy is a measure of systemic error. Errors can occur in
amplitude of ways. Although
not always present simultaneously,
the following errors should be considered:
· Errors due to tolerances of electronic components.
· Mechanical errors in meter movements.
· Component errors due toe drift or temperature variation.
· Errors due to poor frequency response.
· In certain types of instruments, Errors due to change
in atmospheric pressure or temperature.
· Reading errors due to parallax inadequate illumination, or
excessively wide ink trace on a pen recording.
Two additional sources of error should not be overlooked. The first concerns correct instrument zeroing. In most measurements, a zero or a baseline, is necessary. It is often achieved by balancing the wheatstone bridge or a similar device. It is very important that, where needed, balancing or zeroing is done prior to each set of measurements. Another source of error is the effect of the instrument on the parameters to be measured, and vice versa. This is especially true in measurements in living organism.
Signal-to-Noise Ratio
:
It is important that the signal to noise ratio be as high as
possible. In the hospital environment, power-line-frequency
noise or interference is common and is usually picked
up in long leads. Also, interference
due to electromagnetic electrostatic,
or diathermy equipment is possible. Poor
grounding is often a cause of this kind of noise problem. Such "interference
noise" however, which is due to coupling from other
energy sources, should be differentiated from thermal
and shot noise, which originate within the elements
of the circuit itself because of the discontinuous nature
of matter and electrical current. Although thermal
noise is often the limiting factor in the detection of signals
in the fields of electronics, interference noise is often the limiting
factor in the detection of signals in other fields of electronics,
interference noise is usually more of a problem
in biomedical system. It is also important to know and control the signal-to-
noise ratio in the actual environment
in which the measurement
are to be made.
Stability
:
In control engineering, stability is the ability of a system
to resume a steady state condition following a disturbance at the
input rather that be driven into
uncontrollable oscillation. This is a factor that varies with the amount
of amplification, feedback, and other features of the
system. The overall system must
be sufficiently stable over the useful range.
Baseline stability is the maintenance of a constant baseline
value without drift.
Isolation
:
Often measurement muse be made on patients or experimental
animals in such a way that the instrument does not produce a direct
electric connection between the subject ad ground. This requirement
is often necessary for reasons of electrical
safety to a avoid interference be achieved by using magnetic
or optical coupling techniques, or radio telemetry. Telemetry is
also used where movement of the person or animal to be
measured is essential, and thus the encumbrance of connecting leads should
be avoided.
Simplicity
:
All systems and instruments should be as simple as possible
to eliminate the chance of component or human error. Most instrumentation
system require calibration before they are actually used. Each
component of a measurement system is usually calibrated individually at
the factory against a standard. When a medical
system is assembled. it should be calibrated as a whole. This
step can be done external to the living organism or in situ ( connected
to or within the body ) Calibration should always be done by using
error-free devices of the simplest kind for references. An example
would be that of a complicated, remote
blood-pressure monitoring system, which
is calibrated against a simple mercury manometer.
A block diagram of the man instrument system
is shown in figure. The basic components of this system
are essentially the same as in any instrumentation system. The only
real difference is in having a living human being as
the subject. The system components are given below.
The Subject
:
The subject is the human being on whom the measurements
are made. Since it is the subject who makes this system different from
other instrumentation systems, the major physiological systems
that constitute the human body are treated in much greater
detail.
Stimulus
:
In many measurements, the response to some form of external
stimulus is required. The instrumentation used to generate
and present this stimulus to the subject is a vital part
of the man-instrument system when even responses are
measured. The stimulus may be visual auditory
tactile or direct electrical stimulation of some part
of the nervous system.
The Transducer
In general, a transducer in defined as a device
capable of converting one form of energy or signal
to another. In the man-instrument system, each transducer is
used to produce an electric signal that is an analog
of the phenomenon being measured. The transducer
may measure temperature, pressure, flow, or any of the other variables
that cab be found in the body, but
its output is always an electric signal.
As indicated one or more transducers may be used simultaneously to obtain
relative variation between phenomena.
Single Conditioning Equipment :
The part of the
instrumentation system that
amplifies, modifies, or in any other way changes the
electric output of the transducer is called signal-conditioning
( or sometime signal-processing) equipment. Signal conditioning
equipment is also used to combine or relate the output of two
or more transducers. Thus for each item of signal conditioning equipment
both the input & output are electric
signals although the output signal is often greatly modified
with respect to the input. In essence then the
purpose of the signal conditioning equipment is to process
the transducers in order to satisfy the functions
of the system and to prepare
signal suitable for operating the display
or recording equipment that follows.
Display Equipment
:
To be meaningful, the electrical output to of the
signal-conditioning equipment must be converted into a form that can be
perceived by one of man's senses and that can convey the information
obtained by the measurement in a meaningful way. The
input to the display device is the modified electric
signal from the signal-conditioning equipment. Its output is
some form of visual audible, or possibly tactile information. In
the man-instrumentation system the display equipment may include
a graphic pen recorder that produces a permanent
record of the data.
Recording Data-Processing, and
Transmission Equipment :
It is often necessary, or at least desirable, to record
the measured information for possible later use or to transmit it from
one location to another, whether across the hall of the hospital
or halfway around the world. Equipment for these functions
if often a vital part of the man-instrument
system. Also, where automatic storage or processing of data
is required, or where computer control is employed, an
on-line analog or digital computer
may be part of the instrumentation
system. It should be noted that the term
recorder is used in two different contexts in
biomedical instrumentation. A graphic pen recorder is actually a
display device used to produce a paper record
of analog waveform as, whereas the recording equipment referred to
in this paragraph includes devices by which data can be recorded
for future playback, as in a magnetic tape recorder.
Control Devices
:
Where it is necessary or desirable to have automatic control
of the stimulus, transducers, or any other part of the man-instrument
system, a control system is incorporated. This
system usually consists of a feedback loop in which part of
the output from the signal-conditioning or display equipment is used
to control the operation of the system in some way.
The Cardiovascular System :
To an engineer, the cardiovascular system can be viewed as
a complex, closed hydraulic system with a four-chamber
pump connected to flexible and sometimes elastic tubing. In
some parts of the system, the tubing changes
its diameter to control pressure. Reservoirs in the system
change their volume and characteristic to satisfy certain control
requirements and a system of gates and variable
hydraulic resistance continuously alters the pattern
of fluid flow. The four chamber pump acts
as two synchronized but functionally isolated
two-stage pumps collects fluid is pumped into the system immediately
after it has been received from the first stage. One
of the two stage pumps collects fluid from the main hydraulic
system and pump it through an oxygenation system.
The other pump received fluid from the oxygenation system
and pumps it into the main hydraulic system. The fluid which flows
in a laminar fashion acts as a communication
and supply network for all parts of
the system. Carriers of fuel supplies and waste
materials are transported to predetermined destinations by the fluid.
The fluid also contains mechanisms for repairing
small system punctures and for rejecting
foreign foreign element from the system. Sensors provided to detect
changes in the need for supplies, the buildup of waste materials,
and out-of-tolerance pressure in the system are known as chemoreceptors,
Pco2 sensors, and baroreceptors, respectively.
These and other mechanisms control the pump's speed and
efficiency, the fluid flow pattern through the system,
tubing diameters, and other factors. Because part of the system is required
to work against gravity at times, special one-way valves are
provided to prevent gravity from pulling fluid against the direction
of flow between pump cycles. The variables of prime importance
in this system are the pump (cardiac) output
and the pressure, flow rate, and volume of the fluid (blood) at various
locations throughout the system.