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The oldest and most widely used technique for measuring the blood pressure
of a patient is to completely occlude an artery by a pressurized cuff
whose pressure is then allowed to bleed down. A mercury manometer is used
to determine the pressure in the cuff, and a stethoscope is utilized to
listen for Korotkoff sounds. The cuff pressures when particular types of
sounds are heard are indications of systolic and diastolic pressures. The
various methods based on listening for Korotkoff sounds are inherently
inaccurate, especially when measuring diastolic pressure since what is
required is a determination of the disappearance of sound as it gradually
fades out. Even systolic pressure determinations are inaccurate because
what is often thought to be a first pressure pulse, and sometimes even a
second pressure pulse, are nothing more than artifacts which do not
represent a flow of blood through the still fully occluded artery. A mean
error of .+-.8 mm Hg can be expected in readings of systolic and diastolic
pressures based on Korotkoff sounds. (Best and Taylor, Physiological Basis
of Medical Practice, 9th Edition, Chapter 7, page 3-151.) Nor are
present-day automatic instruments based on Korotkoff sounds any more
reliable. Not only is it difficult to monitor electronically a fading
sound, but the methodologies employed do not provide consistent, reliable
results.
Other measurement approaches have met with equally little success. With
respect to oscillometric methods, it is very difficult to determine
diastolic pressure because one has to look for changes in oscillations of
a mercury column, and they are barely noticeable with narrow-width
pressure pulses. Hot-wire anemometer-type transducers offer somewhat
better accuracy, but they require the use of two cuffs (occluding and
sensing, in which the pressure in the sensing cuff is maintained
constant). One of the shortcomings of these and other prior art devices is
that two transducers and associated electronics are required.
Representative prior art, in addition to the Best and Taylor text referred
to above, are the following:
______________________________________
Patent
No. Date Inventor Title
______________________________________
2,827,040
March 18, 1958
S. R. Gilford
Automatic
Sphygmomano-
meter
3,224,435
Dec. 21, 1965
M. Traite Method of
Measuring
Blood Pressure
3,229,685
Jan. 18, 1966
D. L. Ringkamp
Blood Pressure
et al Measuring
3,480,005
Nov. 25, 1969
W. C. Edwards
Apparatus for
Measuring Blood
Pressure With
Plural Brake
Controlled
Indicators
3,581,734
June 1, 1971
M. E. Croslin
Sphygmomano-
et al meter
3,742,937
July 3, 1973
B. Manuel et al
Cardiac Monitor
3,742,938
July 3, 1973
T. J. Stern Cardiac Pacer
and Heart Pulse
Monitor
3,814,083
June 4, 1974
J. C. Fletcher
Apparatus and
et al Method For Pro-
cessing Korotkov
Sounds
3,841,314
Oct. 15, 1974
R. E. Page Pulse Activity
Indicator
3,885,551
May 27, 1975
H. L. Massie Artifact Rejection
For Blood Pres-
sure Monitoring
3,894,533
July 15, 1975
R. L. Cannon Vital Sign Trend
Intuitive Display
System
3,903,872
Sept. 9, 1975
W. T. Link Apparatus and
Process For
Producing
Sphygmometric
Information
3,978,848
Sept. 7, 1976
D. H. Yen et al
Monitoring Appa-
ratus And Method
For Blood Pres-
sure and Heart
Rate
4,009,709
March 1, 1977
W. T. Link et al
Apparatus and
Process For
Determining
Systolic Pressure
4,074,711
Feb. 21, 1978
W. T. Link et al
Apparatus And
Process For
Determining
Systolic Pressure
______________________________________
Other Publications:
1. L. A. Geddes et al--"The Meaning of the Point of Maximum Oscillations in
Cuff Pressure in the Indirect Measurement of Blood Pressure, Part I",
Cardiovascular Research Center Bulletin, July-Sept., 1969, pages 15-25.
2. Physiological Basis of Medical Practice, Ninth Edition, John R. Brobeck:
Chapter 7, Section 3-"Measurement of Blood Pressure and Flow", pages
148-163; Chapter 8, Section 3--"Control Mechanisms of the Circulatory
System", pages 164-188; Chapter 9, Section 3-"Regulation of Systemic and
Pulmonary Circulation", pages 189-210.
3. George E. Burch--"Sphygmomanometric Cuff Size and Blood Pressure
Recordings", JAMA, Sept. 3, 1973, Vol. 225, No. 10, pages 1215-1218.
4. Electronic Design, Vol. 24, No. 19, September 13, 1976, page 28, "Semis
invade medical transducers; microprocessors monitor EKG and blood
pressure".
5. "Computer Automation of Blood-Pressure Measurements", Proceedings of the
IEEE, Vol. 63, No. 10, October 1975, pages 1399-1403.
The basic problem with most prior art automated blood pressure measuring
instruments is that they look for "gross" indications, e.g., the presence
of a pulse based upon a sound level or some other parameter reaching a
detectable level. From a theoretical standpoint, the most accurate
measurement determinations could be made were the pressure waveform in the
cuff actually traced out on paper during the course of a measurement
cycle, much as is done in the case of ECG waveform analysis. The pressure
waveform would show a decreasing occluding cuff pressure, on which blood
pressure pulses are superimposed. Such a paper trace would provide to the
physician the maximum amount of information from which systolic and
diastolic pressures could be determined. If a trace is not to be made and
an instrument is to perform the analysis, then ideally the processing
section of the instrument should be provided with the exact waveform of
the pressure in the cuff. It is possible to do this by sampling the cuff
pressure at a sufficiently high rate and to then process the samples. If
the sampling rate is so high that numerous samples are taken during the
occurrence of each pulse, then from the standpoint of information theory
the processing section of the instrument will have available sufficient
data from which the complete waveform may be reconstructed.
However, while this general principle may have been recognized by prior art
researchers, they have not employed effective methodologies in analyzing
the sampled data. One problem in this regard is that the analysis must be
done "on the fly". In the illustrative embodiment of the invention, a
sample is taken approximately every 2.5 milliseconds; thus 400 samples are
taken each second, and an 8k memory would be required to store the data
for a measurement cycle of 20 seconds--if all of the data is to be stored
prior to the actual processing which determines the final measurement
values. A cost-effective instrument must therefore perform the processing
as samples are taken without storing a complete history of the pressure
waveform. The methodologies employed in the prior art for performing this
type of "on-the-fly" processing have not provided accurate or consistent
results.
For example, consider the methodology for systolic pressure determination
disclosed in Link et al Pat. No. 4,009,709. Link et al theorize that the
DC pressure in the cuff (the value of the slowly changing occluding
pressure) when there is detected a blood pressure pulse whose amplitude is
one-half of a maximum amplitude value represents the systolic pressure,
where the maximum amplitude value is the maximum average amplitude over
four successive pulses. In the Link et al instrument, a "sliding average"
of the pulse amplitude over four successive pulses is taken, and a
threshold level is constantly up-dated to equal the maximum sliding
average. Link et al pump up the pressure continuously. As the occluding
pressure increases, the pulse amplitudes rise and then fall. By using an
increasing pressure during the measurement cycle, maximum pulse amplitudes
are detected before the occluding pressure reaches the relatively high
value which represents systolic pressure. It is in this way that the
threshold level is determined before a pulse is actually detected whose
amplitude is less than one-half of the threshold level. The Link et al
technique requires a smooth pump-up of the cuff pressure and thus does not
allow a cheap, conventional-type manually-operated bulb pump to be used.
On the other hand, it is possible to use a bulb to pump up the pressure to
a value which completely occludes the artery, and then to allow the
pressure to bleed down smoothly as in conventional instruments. But in
such a case, the systolic pressure is reached before the threshold level
can even be determined. This, in turn, requires that a considerable amount
of data be stored since "on-the-fly" processing is possible to only a
limited extent.
But quite apart from the difficulties in implementing such a technique, the
Link et al methodology has not proven to provide consistently correct
systolic pressure measurements. The basic premise of Link et al is that
the systolic pressure is the DC cuff pressure when a particular pulse is
detected, and that particular pulse is the first one in a decreasing
amplitude sequence whose amplitude corresponds to one-half of the maximum
amplitude (or, more accurately, the maximum average amplitude over four
successive pulses). This criterion has not been established, but even were
it valid the Link et al system does not take into account the existence of
artifacts. For example, if a patient moves his arm during the course of a
measurement cycle and in the process squeezes the cuff, there will be a
very large pressure rise which may control the maximum average pulse
amplitude which is used as the threshold value--the threshold value and
therefore the systolic pressure determination being completely erroneous
in such a case.
What is important in an automated blood pressure measurement instrument is
not only the selection of the proper criteria for determining systolic and
diastolic pressures, but also validation of the results. Throughout the
following detailed description of the invention, it will be noted that
considerable attention is paid to validating the measurement cycle. One
such example is the analysis of each individual pulse; a pulse is not
considered to be valid if its amplitude is too large. Another example
relates to the determination of systolic pressure. The sequence of pulse
amplitudes in the region of systolic pressure must be one of plurality of
predetermined valid sequences. It is this kind of constant concern for
validating the measurement results (both intermediate and final) which
contributes to reliable instrumentation.
In the illustrative embodiment of the invention a display is provided for
guiding the operator--physician or patient--through the measurement cycle.
As the bulb is used to pump up the cuff pressure, the operator is informed
not only of the instantaneous cuff pressure, but also of the particular
actions which are required. This, in and of itself, is an important
feature of the invention. Furthermore, for a measurement cycle to provide
accurate results, it is essential that the artery be completely occluded
before the cuff pressure is monitored for the presence of pulses. What the
system of the invention does is to check that no pulses have been detected
for about 2.5 seconds before it assumes that the artery has been
completely occluded and the cuff pressure should be allowed to continue
bleeding down. If full occlusion for 2.5 seconds is not ascertained, the
display informs the operator to pump up the cuff pressure.
The basic systolic pressure methodology involves an analysis of the
amplitudes of four successive pulses, when pulses first appear as the
occluding pressure bleeds down. (As will be described below, artifacts are
rejected and it is not necessarily the first four pulse amplitudes which
are operated upon.) Systolic pressure is taken to be the cuff pressure at
the onset of a particular one of the four pulses, but only if the pulse
amplitudes have a sequence which is one of a plurality of known valid
sequences, e.g., four successive pulses exhibit increasing amplitudes,
except for the third which may have the largest amplitude. There are quite
a few valid sequences, some of which will be described in detail below.
The diastolic pressure methodology is actually similar to the Link et al
methodology for determining systolic pressure. (There is no apparent
reason why the same type of methodology should be effective to determine
both systolic and diastolic pressures; in fact, it is not effective for
systolic pressure determinations as taught by Link et al, but it is
effective for diastolic pressure determinations.) A theshold value is
determined based upon maximum pulse amplitude information; in the
illustrative embodiment of the invention, the threshold level partially
depends upon the maximum average pulse amplitude over four pulses. But the
threshold value is not based solely upon the maximum amplitude
information; it is also a function of a constant value. Moreover, instead
of comparing the amplitude of a single pulse with the threshold value in
order to determine diastolic pressure, the comparison involves the average
pulse amplitude over four pulses in the vicinity of diastolic pressure.
The methodology of the invention does not lend itself to a more detailed
general description. Suffice it to say that the method of the invention
allows "on-the-fly" analysis of samples taken at a sufficiently high rate
such that they allow the complete cuff pressure waveform to be reproduced.
The criteria for determining systolic and diastolic pressures have proved
to be accurate and reliable. Throughout the processing, validation checks
are performed. Any indication of erroneous measurements having been taken
results in an appropriate error message. Accurate measurements of pulse
rates are also provided. In connection with a pulse rate measurement,
while it is not particularly difficult to count detected pulses (as is
known in the prior art), it is the rejection of a measurement cycle due to
the presence of artifacts that gives rise to the high accuracy of my
method and apparatus.
Further objects, features and advantages of my invention will become
apparent upon consideration of the following detailed description in
conjunction with the drawing, in which:
FIG. 1 is a perspective view of the instrument of my invention;
FIG. 2 depicts a portion of the circuit board within the housing of the
instrument, and several of the components mounted on the board;
FIGS. 3-6 are a schematic of the circuit of the instrument, with the
figures being arranged as shown in FIG. 7;
FIG. 8 depicts two resistor networks utilized in the circuit of FIGS. 3-6;
FIG. 9 depicts the seven segments of each display element, together with
the fifteen characters which can be formed by energizing appropriate ones
of the segments (the 16th character is a blank, obtained by energizing
none of the segments);
FIGS. 10-13, 15-18, 20, 23 and 24 are flow charts depicting most of the
method of my invention, and should be read in conjunction with the
complete source listing which is reproduced below and which will be
described;
FIG. 14, which is not drawn to scale, depicts the cuff pressure throughout
a measurement cycle;
FIG. 19 depicts, in enlarged scale, the cuff pressure in the vicinity of a
single blood pressure pulse;
FIG. 21 depicts the envelope of the pulse amplitudes--not the cuff
pressure, but just the amplitudes of individual pulses such as that shown
in FIG. 19--throughout a measurement cycle; and
FIG. 22 depicts several illustrative pulse sequences which will be
discussed in conjunction with the systolic pressure measurement
methodology.
Hardware
FIG. 1 depicts the instrument of my invention. It includes a conventional
cuff 40, with tubing 42 connecting the cuff to pump-up bulb 44. As the
bulb is pumped, the pressure in the cuff rises. There is a bleed valve 45
in the bulb which allows air in the cuff to bleed out at a rate of several
mm Hg per second, the actual bleed rate depending upon the cuff pressure.
Tubing 47 connects the cuff to a manifold within the instrument housing.
The overall cuff arrangement is standard except that the take-off tubing
47 is extended to the instrument rather than to a mercury column as in
conventional blood pressure measuring instruments.
The instrument itself includes three switches and a twelve-character
display DP1 (under a red translucent strip 43). Switch S1 (on the top) is
the main on/off switch which, when operated, connects the internal
batteries to the circuit. (The unit also includes a jack 49 for insertion
of the plug of a charging circuit when it is necessary to recharge the
batteries.) Switch S2 is the reset/exhaust switch which is spring-loaded
to an open position. When it is momentarily closed, as will be described
below, the instrument resets and initiates a new cycle of operation.
Switch S3, another normally-open, spring-loaded push-button, is the
recall/cuff control. When it is operated, one of two different sequences
takes place depending upon the state of the instrument at the time the
button is operated. Toward the beginning of the overall cycle, operation
of switch S3 closes take-off tubing 47 as will be described shortly, so
that the pressure in the cuff can be pumped up by repeatedly squeezing
bulb 44. At the end of a measurement cycle, the final values are displayed
for only ten seconds, also as will be described below, and the display is
then blanked to conserve power. Operation of switch S3 causes the
previously determined values to be displayed once again, for another ten
seconds.
The display itself consists of 12 character positions, each of which has
seven light-emitting diode segments as shown at the left of FIG. 9.
Depending upon which of the segments are energized, any one of 15
characters can be displayed at each position, the 15 characters also being
shown in FIG. 9 and it being obvious which of the seven segments are used
to form each of the fifteen characters. A blank may be displayed simply by
energizing no segments. The display elements are also used to form
numerals as is well known in the art.
The instrument also includes a light-emitting diode LD1 (under a red
translucent area 51 on the case) which, when illuminated, represents one
of two things. First, the light is on whenever the system is in the
process of detecting a blood pressure pulse (a rise in the occluding cuff
pressure). Second, after the final display has been blanked in order to
conserve power, the light is turned on to indicate to the operator that
the display can be recalled if switch S3 is momentarily operated. Lastly,
the positions on the display of the final measurement values are printed
on the case.
FIG. 2 depicts just one part of the circuit board 18 on which the circuit
components are mounted within the housing. Switch S3 can be seen in the
drawing. In addition, a manifold 20 is mounted on the board, and spaced
from it by spacer 22. The manifold provides open communication between
input pipe 24 (on which take-off tubing 47 of FIG. 1 is placed), a pipe
segment 26, and a valve V1. The valve is normally open, but when its two
leads (not shown) have a potential applied across them, the valve closes.
A pressure transducer T1 is mounted on the other side of the board--the
side on which all of the chips used in the circuit are mounted--and the
input port of the transducer is connected to pipe segment 26. It is
apparent that since pipe 24 is connected via take-off tubing 47 to the
cuff, transducer T1 has as its "input" the cuff pressure. Valve V1 is used
to open the cuff to the atmosphere, within and through the housing, so
that the cuff pressure can rapidly decrease at the end of a measurement
cycle. The valve is closed automatically by the circuit after switch S3 is
operated so that the pump-up procedure can commence. It is important to
note that transducer T1 is located within the instrument housing and is
not positioned in the cuff (although it could be). Thus there are no
circuit elements which are in contact with the patient.
The schematic of the circuit is shown in FIGS. 3-6. Many of the chips are
identified on the schematic, and the omitted chip identifications, as well
as the component values, are as follows (many of the resistors are
contained in four resistor networks, identified by the symbols RA1-RA4,
which will be discussed below):
______________________________________
C1 22uf R18* 15K (RA1)
C2 .01uf R19 10K
C3 6.8uf R20 2K
C4 6.8uf R21* 20K (RA1)
C5 .01uf R22 1K
C6 .47uf R23* 5K (RA2)
C7 .01uf R24 100
C8 .01uf R25* 10K (RA1)
C9 22uf R26 100
C10 .01uf R27* 7.5K (RA1)
C11 .47uf R28* 402K (RA1)
C12 .01uf R29* 10K (RA1)
C13 .47uf R30* 330 (RA4)
C14 .01uf R31* 10K (RA1)
C15 10f R32* 330 (RA4)
C16 22uf R33* 330 (RA4)
C17 .47f R34* 330 (RA4)
C19 1uf R35* 162K (RA1)
C20 1uf R36* 40.2K (RA1)
C21 .01uf R37* 4.7K (RA3)
C22 270pf R38* 4.7K (RA3)
C23 68pf R39* 4.7K (RA3)
C24 20pf R40* 4.7K (RA3)
C25 20pf R41 47 (1/2W)
C26 .1uf RA1 Custom 16-Pin Dip,1
C27 .1uf RA2 Custom 16-Pin Dip,1
R1* 18.7K (RA2) RA3 Bourns 4310R-102-47
R2* 19.6K (RA2) (10-Pin Sip), 1%
R3* 1M (RA2) RA4 Bourns 4310R-102-33
R4* 10K (RA2) (10-Pin Sip), 1%
R5* 18.7K (RA2) D1 IN4001
R6* 1M (RA2) D2 IN4001
R7* 10K (RA2) D6 IN4001
R8* 21.5K (RA2) Z1 IN5523
R9 1K IC3 LM324
R10* 20K (RA2) IC10 LM393N
R11* 100K (RA2) IC11 DS88L12N
R12* 20K (RA2) LD1 RL209-2
R13* 5.6K (RA1) V1 Angar Scientific
R14* 100K (RA2) Controls, Model
R15* 330 (RA4) 336073 (East Hanover,
R16* 20K (RA1) New Jersey)
R17* 4.7K (RA3) DP1 NSA7120
______________________________________
In FIG. 3, the numeral 12 depicts eight 1.4-volt batteries. Although each
battery has a nominal voltage of 1.4 volts, the system is designed to
operate even if the overall voltage falls as low as 9.4 volts. Terminals
10 simply depict the points at which a charging circuit may be connected
to the instrument to recharge the batteries. When switch S1 is closed,
power is furnished to the circuit. A potential of 9.6 volts is shown to
the right of switch S1, since this is a typical actual potential in normal
use. Chip IC1 is a voltage regulator which derives a 5-volt regulated
potential at its output pin 2. The circuitry directly below switch S1 and
chip IC1 is a standard circuit for deriving a -5.1-volt potential at the
junction of Zener diode Z1 and resistor R26. This negative potential is
required for proper operation of chip IC4. Chip IC2 is arranged as a
10-kHz oscillator. The configuration is standard, and five of the six
inverters on the chip are connected in parallel to lower the output
impedance so that charge can be dumped faster into capacitor C16. The
circuit is shown on page 1-50 of the "Data Conversion Design Manual"
published by Teledyne Semiconductor, 1979.
Transducer T1 on FIG. 3 is a National Semiconductor chip --a pressure
transducer utilizing a piezoresistive circuit which derives an output
voltage across pins 3 and 4 which is proportional to applied pressure. It
is the pressure port of the transducer (not shown in FIG. 3) which is
coupled to pipe segment 26 in FIG. 2. Amplifier D of chip IC3 on FIG. 3,
and the associated components, are used to develop a -3.75-volt reference
voltage which is applied through resistor R27 (FIG. 4) to the positive
input of amplifier A of chip IC3.
At this point, two things should be noted. First, many of the resistors are
marked in the schematic with asterisks. These asterisks identify the
resistors as being included in one of four resistor networks, as will be
described below in connection with FIG. 8. The second point to note is
that no invention is claimed in the various sub-systems per se of the
overall circuit. Thus, with reference to FIG. 3, the derivation of the +5
and -5.1-volt power supplies, as well as the -3.75-volt potential, and the
connections to the pressure transducer T1, are all known in the art. The
invention resides in the manner in which the sub-systems are
interconnected to allow the system to sequence in the manner to be
described below.
The ambient output of transducer T1 may range between +50-mv and -50-mv.
The analog-to-digital converter chip IC4 (FIG. 5) works on positive inputs
only, and thus an offset is introduced by amplifier A of chip IC3 (FIG.
4). The amplifier itself is used in a unity gain configuration, and the
coarse and fine potentiometer controls R19 and R22 are used to provide an
ambient potential difference across pins 12 and 13 of differential
amplifier B of chip IC3 which is in the 30-mv to 50-mv range. The output
at pin 14 of amplifier B of chip IC3 is extended to the positive input of
amplifier C of the same chip. This is the gain amplifier which is provided
with coarse and fine potentiometer controls R20 and R24. The potential at
pin 1 of chip IC3 is extended to the analog input at pin 14 of chip IC4,
the analog-to-digital converter. It is this chip, on FIG. 5, which derives
samples of the instantaneous cuff pressure, as reflected by the analog
output at pin 1 of chip IC3.
The ambient output when the cuff pressure is open to the atmosphere need
not be precise. In fact, it varies with temperature and atmospheric
pressure. The system self-calibrates itself by deriving a reference
pressure at the output of the analog-to-digital converter when the cuff is
at atmospheric pressure. Thus at the start of any measurement cycle, the
analog signal furnished to the converter is non-zero, but this is of no
moment because the system subtracts the reference pressure from each
actual sample taken. Thus all sample values which are processed by the
apparatus are pressures which are relative to atmospheric pressure.
In the factory, however, the offset and gain potentiometers are adjusted to
provide accurate readings. Tubing 42 in FIG. 1 is connected to a pump-up
bulb without a bleed hole and to an accurate mercury manometer. If the
cuff is initially at atmospheric pressure, the instrument should read a
pressure of zero, since each sample, less the reference atmospheric
pressure, should provide a value of zero. During the factory-calibrate
mode, the instrument actually displays the cuff pressure as will be
described below. The operator manipulates the two offset potentiometers
until a pressure reading of zero is obtained. Thereafter, the bulb is
pumped up. Since a bleed hole is not provided in the bulb, the pressure in
the cuff remains constant at the pumped-up value. The instrument may
display a pressure value which is different from the actual value as
represented on the manometer. The two gain potentiometers are adjusted
until the pressure reading (relative to the reference pressure) displayed
by the instrument is correct. By thus manipulating both pairs of
potentiometers, the instrument can be calibrated in the factory.
Thereafter, it is the use of the reference pressure subtraction technique
which insures that all displayed pressures are pressures which are
relative to atmospheric pressure, so that temperature and altitude
considerations are of little importance.
Comparators A and B of chip IC10 on FIG. 4 serve to develop two test
signals. The output of comparator A is high whenever the battery
potential, connected to the positive input, is greater than 9.8 volts. The
output of comparator B is high whenever the battery potential exceeds 9.4
volts. The two signals at the outputs of the comparators are used in two
different ways.
During normal processing, as will be described below, the "test" signals at
the outputs of the two comparators are used to inform the system of the
state of the battery. If both test signals are high, indicating a battery
potential greater than 9.8 volts, the system provides no
"state-of-the-battery" message to the operator. But if the output of
amplifier B is high and the output of amplifier A is low, it is an
indication that the battery potential exceeds 9.4 volts but does exceed
9.8 volts. In such a case, the instrument is capable of performing up to
25 more measurements so it continues to function. However, the operator is
provided with a message indicating that the batteries should be recharged.
If both test signals are low, the system will not allow measurements to be
taken, and a message is displayed which informs the operator that the
batteries must be recharged before the instrument can be used.
It will be noted that pin 5 of chip IC10 is connected through resistor R8
(21.5k) to ground. Resistor R9 (1k) is in parallel with it, but this
resistor is left floating. In the factory, a test clip, symbolized by the
numeral 14, can be used to ground the lower end of resistor R9. By so
doing, the output of comparator B is forced low. A factory technician does
this when the unit is to be calibrated.
The system includes a microprocessor and firmware for controlling its
cycling. (The Intel 8048 chip which is used includes the firmware together
with the microprocessor on the same chip, although other microprocessors
with separate ROM chips can be employed.) The firmware includes
instructions for cycling the system in the factory-calibrate mode; these
instructions are not actually accessed during normal use of the
instrument, and control cycling of the machine only in the
factory-calibrate mode. During the factory-calibrate procedure, all the
system does is to measure cuff pressure and to display it so that the
operator may manipulate the potentiometer controls. The instructions for
cycling in the factory-calibrate mode are included in the firmware which
is shipped in the unit despite the fact that, after factory calibration,
this part of the firmware is not used (unless re-calibration is ever
required, in which case the unit may be thought of as being calibrated in
the "factory"). The system therefore must have a way of knowing whether it
is to cycle in the normal mode or in the factory-calibrate mode. It is
jumper 14 which does this.
When a unit is being calibrated in the factory, fresh batteries are in it
and thus the output of comparator A of chip IC10 is high, indicating that
the battery potential is above 9.8 volts. But when resistor R9 is
connected to ground by the jumper, the output of comparator B of chip IC10
is forced low, indicating that the battery potential is below 9.4 volts.
The two test conditions are thus inconsistent with each other, since they
indicate battery potentials which are both above and below an intermediate
level. When the system detects these inconsistent test conditions, it
branches to the factory-calibrate mode of operation.
The advantage of this technique is that it allows a branch to be controlled
in the firmware without the need for another test input to the
microprocessor. As will become apparent below, all of the pins of the 8048
microprocessor are utilized, and there is no available pin which can be
used as a separate test input. Were such a pin available, it would be
relatively simple to apply an appropriate potential to it in the factory
which would cause a branch to the factory-calibrate mode of operation. But
in the absence of an available pin, it would appear that there is no way
for the microprocessor to test whether it should branch to the
factory-calibrate mode of operation. But since two battery test signals
are required anyway, an effective state test can be controlled by forcing
the two battery test signals to represent inconsistent conditions. Such
inconsistent signals never arise during the normal mode of operation,
since the battery potential can never be both above and below an
intermediate level.
On FIG. 5, chip IC4 is a Teledyne 8704 analog-to-digital converter,
arranged in a standard configuration. The chip is interfaced directly to
chip IC5, an Intel 8048 microprocessor with on-board ROM and RAM. The
analog signal which is to be converted to a digital sample appears at pin
14 of chip IC4. The converter generates a 10-bit sample at pins 3-12. The
data bus of the microprocessor has only 8 lines, D80-D87, and consequently
only the eight least significant bits of each sample are connected to the
data bus inputs of the microprocessor. The two most significant bits, 8
and 9, are extended to the bit 0 and bit 1 inputs of port 2 of the
microprocessor, pins 21 and 22. The microprocessor reads in one sample at
the same time that it initiates the formation of a new one, i.e., at the
same time that it initiates a new conversion cycle. During normal
processing, the microprocessor is so fast that it | | |
