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BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates generally to auscultatory blood pressure and pulse
monitoring method and apparatus. More specifically, it concerns an
ambulatory device for the automatic or demand recording and
trans-telephonic communication, for later diagnosis, of information
including blood pressure, heart rate, time of day, event code and figure
of merit. The improved method includes systolic blood pressure-adaptive
cuff pressurization and figure of merit computation to ensure that
information comfortably, reliably and accurately is monitored and
reported.
Typically, ambulatory blood pressure monitoring and recording equipment
continuously measures a patient's systolic and diastolic blood pressure
for a given period of time. Recent advances in monitoring equipment
include the ability of the patient trans-telephonically to communicate
information regarding blood pressure to a remote site for permanent
storage and either simultaneous or later diagnosis, rather than requiring
the patient to return the equipment or a magnetic tape cassette to the
physician after each recording session. A serious shortcoming of
state-of-the-art equipment is the fact that the pressure cuff which
locates the microphone used to pick up auscultatory signals indicative of
blood pressure may be less than optimally positioned on the patient's arm,
resulting in inadequate signal strength and incomplete or misleading blood
pressure data.
Frequently, it is desirable to record blood pressure on demand by the
patient, based upon predetermined, physician-selected criteria, e g. upon
awakening, while eating or after exercising, rather than continuously or
at given times of day that may bear no relation to the individual
patient's activity or idiosyncratic behavior. Further, it is desirable
continuously to alter the maximum (occlusive) cuff pressure to adapt to
the ambulatory patient's level of activity, thereby to ensure that blood
flow in the limb is fully occluded but to ensure that the patient's limb
is not unnecessarily, and often uncomfortably or even painfully,
constricted. Finally, it is desirable to monitor and report to the
prescribing physician not only blood pressure and heart rate data, but
also information indicative of the quality of, or the figure of merit that
may be accorded, such data.
It is a primary object of this invention to provide an ambulatory blood
pressure monitoring and recording device capable of indicating to a
prescribing physician the quality, or figure of merit, of a
contemporaneous or historic blood pressure measurement.
Another object of the invention is to provide such a device that is
individual patient physiology- and activity-adaptive, wherein the maximum
cuff pressure tracks the rise and fall of the patient's systolic blood
pressure, thereby to provide only marginally higher pressure than the
amount needed to occlude blood flow in the limb around which the cuff is
placed.
These and other objects of the invention will be understood in reference to
the following detailed description of the preferred embodiment of the
invention, and by reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified, schematic block diagram of the apparatus of the
invention, made in accordance with its preferred embodiment.
FIG. 2A-2E is a flowchart of the figure of merit software routine as it is
implemented in accordance with the preferred method of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, a simplified, schematic block diagram of the
apparatus of the present invention, indicated generally at 10, is
illustrative of its preferred embodiment. In the interest of brevity and
clarity, detail intentionally is omitted from FIG. 1, e g. latches,
drivers and other detailed, device-level circuitry are not shown It will
be understood by those skilled in the art that, depending upon the
particular devices chosen to perform the required functions, such
circuitry also may be required.
Apparatus 10 is a lightweight, ambulatory, battery-powered device that is
capable of measuring, recording and transmitting blood pressure and heart
rate data, as well as supplemental information found to have particular
utility to a prescribing physician It is self-contained, requiring neither
external power nor pump, and conveniently may be carried upon the person
of the patient A blood pressure and heart rate reading takes approximately
one minute, during which cycle the patient's systolic and diastolic blood
pressure are measured by inflating the cuff to a programmed maximum
pressure and monitoring the pressure and K-sound inputs as the cuff slowly
and steadily deflates In the preferred embodiment of the invention,
apparatus 10 is capable of recording at programmed intervals between ten
and ninety minutes or on demand for a period of twenty-four hours. As will
be seen, apparatus 10 provides the patient with the ability
trans-telephonically to transmit data over standard phone lines, thus
obviating a visit to the physician each time a recording session is
complete.
Apparatus 10 includes computer means, or a microprocessor or
microcontroller 12; program storage means, or a read-only memory (ROM) 14;
and data storage or recording means, or a read and write memory (RAM) 16.
In a manner that will be described below in reference to FIG. 2, a program
that is stored in ROM 14 is executed in microcontroller 12 upon
application of power via a power switch (not shown), and RAM 16 thereafter
is used to store blocks of data pertaining to the patient's blood
pressure, heart rate, etc.
Korotkoff sounds (K-sounds) and static (DC) and dynamic (AC) blood pressure
data are acquired and are discriminated from noise and other artifacts
(particularly motion artifacts), as by any of a variety of known means and
methods that form no part of the present invention. The K-sounds are
sensed by a microphone (not shown) which is taped on the patient's arm
underneath a pressurizable cuff 18 that surrounds the arm. In the
auscultatory method, K-sounds are detected acoustically to produce a
signal representative thereof. In the preferred embodiment of the
invention, K-sound detecting means include the microphone and its signal
conditioner, K-sound amplifier 20, the output of which may be selected as
an output from a multiplexer (not shown) associated with an analog to
digital converter (ADC) 22, which produces an eight-bit digital value
approximately once every 100 microseconds. This byte then is inputted to
microcontroller 12 via one of its data ports. Cuff 18 is pressurized, and
static pressure (the slowly, steadily declining cuff pressure) and dynamic
pressure (the pulse waveform blood pressure) are monitored by pressure
sense and control circuit 24, which includes a pressure transducer, a pump
and a cutoff valve, all of which are integral to the housing (not shown)
in which the diagrammed circuitry is enclosed Static and dynamic pressure
readings are provided in eight-bit digital format to microcontroller 12
via the multiplexer associated with ADC 22.
Referring still to FIG. 1, the remaining parts of the block diagram of
apparatus 10 now briefly will be described. A programmable time of day
clock 26 provides means for recording, with the blood pressure and heart
rate data, the time of day at which, and the date on which, the reading
was taken. As they must retain their otherwise volatile memories in the
event of battery discharge or failure, RAM 16 and clock 26 are powered by
a battery backup circuit 28. Trans-telephonic means for communicating
blood pressure and other data to a remote site for diagnosis includes a
voltage controlled oscillator (VCO) and speaker driver circuit 30 enable
frequency shift keyed (FSK) tone generation via speaker S, as by a
telephone transmitter being placed adjacent a small hole in the enclosure.
The tone frequencies used in the preferred embodiment of the invention are
nominally 1900 and 2500 Hz. A modified RS-232 serial interface 32 is
provided for data communication from apparatus 10 to a parallel printer
(shown schematically to the left in FIG. 1) or to and from data terminal
equipment (DTE), e.g. a computer (shown schematically to the right in FIG.
1), which complies with the RS-232 telecommunications standard. Finally, a
four-digit, seven-segment liquid crystal display (LCD) 34 and a
two-pushbutton (four-position) keyboard 36 enable the physician, the
patient or the service technician to view certain parameters stored in ROM
14 or RAM 16 and, if desired, to change them.
Turning now to FIG. 2, a flowchart of the FIGURE OF MERIT software routine
implemented according to the preferred method of the invention, is shown
It will be appreciated that diagramming conventions have been adopted,
including a generally top-to-bottom and left-to-right directional flow.
The ellipses indicate entry points to routines or subroutines, exit points
from routines or subroutines, or off-sheet connectors to other parts of
FIGS. 2A through 2E, which span five sheets (e.g. entry/exit
points/off-sheet connectors designated 100, 102, 104, 106, 108, 110, 112,
114, 116, 118, 120). The rectangles indicate task or action blocks (e.g.
actions blocks designated 122, 124, 126, 128, 130, 132, 134, 136, 138,
140, 142, 144, 146, 148, 150, 152, 154, 156, 158, 160, 162, 164, 166, 168,
170, 172, 174). The rhombuses indicate decision blocks (e.g. decision
blocks 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200,
202, 204, 206, 208, 210, 212, 214, 216, 218, 220, 222, 224, 226, 228),
with the answer to the enclosed question determining the decisional path
next to be taken. The flowcharts of FIGS. 2A through 2E are quite
self-explanatory to those of ordinary skill in computer science and allied
arts, and will not, therefore, be discussed in detail. It is noted that,
unless otherwise indicated in the specification, numeric values are
decimal and alphanumerics within quotes, e.g. "A1", are American Standard
Code for Information Interchange (ASCII) characters.
Referring first to FIG. 2A, which contains the entry point to the FIGURE OF
MERIT routine, it is noted that K-COUNTER is a variable that represents
the number of acceptable K-SOUNDs that are encountered. (K-sound
acceptability may be based upon any of a wide range of known criteria
including, for example, an acceptably low rate of change in the indicated
heart rate, as may be derived from AC pressure, as well as corresponding
K-sound, waveform timing.) It is initialized to zero. Upon entry to the
FIGURE OF MERIT routine, the variable SAMPLE COUNTER contains the number
of K-SOUNDs that have been sampled over the sampling interval (which may
be a test interval which number or count is referred to herein as K-sound
frequency or a more normal, automatic or demand recording session or
event). Finally, upon entry to the routine, variable RAM BYTE POINTER
contains the address of the first of consecutive bytes in RAM 16 which
contain K-SOUND amplitude values. Thus, RAM BYTE POINTER is used as a
pointer into the RAM buffer segment of memory.
The software flowcharted in FIG. 2A counts the number of acceptable K-SOUND
samples over the interval and assigns a NUMBER CODE between 1 and 4
(inclusive), depending upon whether K-COUNTER is less than twelve; greater
than or equal to twelve but less than eighteen; greater than or equal to
eighteen but less than twenty-four; or greater than or equal to
twenty-four The NUMBER CODEs assigned for each of the above categories
are, respectively, "4"; "3"; "2"; and "1." Once a NUMBER CODE is assigned
based upon K-sound frequency, control is transferred to an AMPLITUDE INDEX
ANALYSIS routine.
FIGS. 2B and 2C flowchart the software routine that analyzes the peak
amplitudes of the K-sounds, categorizes them as falling within one of four
K-sound ranges and encodes them into a corresponding one of four, symbolic
ALPHA CODEs. These ALPHA CODEs, when considered in light of the
distinctive, symbolic NUMBER CODEs assigned in the routine described
immediately above, have been found to give valuable information to the
physician regarding the quality, or figure of merit, of the data which was
simultaneously recorded therewith.
The software flowcharted in FIG. 2B initializes three range limit
variables, HIGH, MID and LOW, to the preferred values 192, 128 and 64,
respectively (the maximum value of the output of eight-bit ADC 22 is 255,
which may be made to represent the maximum K-sound amplitude of interest
by conventional null and gain adjustment of amplifier 20 and ADC 22);
initializes a variable POINTER to the address of the RAM buffer;
initializes a variable LOOP COUNTER to the number of K-sound samples to be
analyzed and clears three variables, HIGH COUNTER, MID COUNTER and LOW
COUNTER. Peak amplitude values representing acceptable K-sounds are
compared with these variables and HIGH COUNTER, MID COUNTER and LOW
COUNTER selectively are incremented to indicate the frequencies with which
the sampled peak amplitudes are within the three ranges. When LOOP COUNTER
is decremented to zero (when the samples are exhausted), control is passed
to the AMPLITUDE INDEX ENCODING routine.
FIG. 2C illustrates the process by which the results of the amplitude index
analysis are encoded into the ALPHA CODE alluded to above Each of the
HIGH, MID and LOW COUNTERs are compared to a predetermined value which, in
the preferred method of the invention, is 4. If more than four HIGH
amplitude K-sounds were counted, then the variable ALPHA CODE is set to
"A." If more than four MID amplitude K-sounds were counted, then ALPHA
CODE is set to "B." If more than four LOW amplitude K-sounds were counted,
then ALPHA CODE is set to "C." If none of the above is true, then ALPHA
CODE is set to "D." The two-digit, alphanumeric code which is formed by
concatenating a NUMBER CODE and an ALPHA CODE representing, respectively,
the frequency and amplitude of the K-sounds, is referred to herein as a
figure of merit. It may be thought of as representing a two-dimensional
matrix in which, for example, a blood pressure measurement having
associated with it a figure of merit of "A1" would earn a high confidence
rating, whereas a blood pressure measurement having associated with it a
figure of merit of D4 would be suspect. Preferably, the alphanumeric code
generated by the routine is stored in a data record, within RAM 16, for
example, containing the blood pressure and heart rate data recorded during
the (demand) event or (automatic) session, thereby to associate a figure
of merit with the blood pressure data the quality of which it
characterizes.
Operatively coupling the software illustrated in FIG. 2 with computer means
provides means integral with more conventional blood pressure monitoring
and recording apparatus for qualitatively characterizing such blood
pressure data. This characterizing means includes software means for
evaluating the data, based upon predetermined criteria e.g. based upon the
frequency and amplitude of the accompanying K-sounds. The result of such
evaluation is quality indicia, such as the figure of merit or the quality
index produced by the illustrated software which executes in
microcontroller 12. Finally, in the illustrated embodiment, these quality
indicia are encoded, by the software flowcharted in FIGS. 2C through 2E,
to produce symbolic data interpretable as representing the K-sound-based
figure of merit or quality index.
It will be appreciated that, by executing the software described above
concurrently with conventional K-sound and AC blood pressure sampling and
accepting software (e g. the AMPLITUDE INDEX ANALYSIS routine may be
defined as a task which is begun whenever the K-sound RAM buffer fills), a
highly significant, stored result obtains. It is now possible, during
analysis and diagnosis of blood pressure history, to evaluate the `cold`
blood pressure data in the light of objective data recorded simultaneously
therewith Such a figure of merit also may be viewed in real time by the
patient or physician on LCD 34, thereby giving immediate feedback on the
merit of any recorded measurements. If indicated, appropriate corrective
action may be taken immediately, e.g one may better locate cuff 18 or its
integral microphone, relative to the limb.
In a proposed modification to the preferred method of the invention, the
alphanumeric figure of merit is converted to a single one of four numeric
digits, referred to herein as a K-sound quality index, by the mapping
illustrated in Table 1.
TABLE 1
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1 2 3 4
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A 1 1 2 3
B 1 2 2 3
C 2 2 3 4
D 3 3 4 4
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It may be seen from Table 1 that a number between one and four has been
assigned to each of sixteen matrix positions at the intersection of a
NUMBER CODEd column and an ALPHA CODEd row. It will be appreciated that
other assignments, or mappings, may be employed within the spirit of the
invention. For example, if it is thought that K-sound frequency is a
relatively better indicator of the quality of a blood pressure measurement
than is K-sound amplitude, the mapping may be biased (weighted), or made
to be asymmetric with respect to its top-to-bottom, left-to-right,
diagonal axis.
Turning next to FIGS. 2D and 2E, a flowchart illustrating this MATRIX TO 1
OF 4 CONVERSION routine is described It will be appreciated how
straightforwardly the mapping illustrated in Table 1 may be performed. It
will also be appreciated that the mapping may, within the spirit of the
invention, be done in hardware rather than software. The software
flowcharted in FIG. 2D first determines which ALPHA CODE is contained in
the figure of merit. It then determines, for whichever of the four cases,
which NUMBER CODE is contained in the figure of merit. The input paths to
the SET 1 OF 4 CODE TO "1" (indicating relatively high quality) action
block include any of the following figures of merit "A1", "A2" or "B1. "
The input criteria to the SET 1 OF 4 CODE TO "2" action block include
"A3", "B2", "B3", "C1" or "C2. " If none of these figures of merit is
present in the figure of merit buffer segment of RAM 16, then control is
transferred to the appropriate continuation ROW. In FIG. 2E, it may be
seen that "A4", "B4", "C3", "D1" or "D2" figures of merit result in the
setting of the 1 OF 4 CODE to "3." Finally, "C4", "D3" or "D4" as a figure
of merit maps into a "4" K-sound quality index (indicating relatively low
quality).
It is important that cuff 18 be pressurized to a level above that of the
patient's systolic blood pressure. In the preferred embodiment of the
present invention, cuff 18 is pressurized to a default (maximum) value,
stored as variable MAXP, under the control of microcontroller 12, and
systolic blood pressure readings then are taken. If the measured systolic
blood pressure value, stored as variable SYS, is within a predetermined
range of, or exceeds, the default value MAXP (determined, for example, by
too early a systolic blood pressure, or C, peak in the pressure pulse
waveform), then the default value is increased by a predetermined amount.
The improved method thus ensures that reliable readings are taken of the
patient's systolic blood pressure, e.g that cuff 18 is pressurized
sufficiently to occlude the flow of blood in the patient's arm.
It is possible, but not desirable, to overinflate a blood pressure cuff
This is especially a risk with prior art blood pressure monitoring
equipment having automatic, cuff pressurizing pumps As there is no trained
physician manually inflating the cuff and determining when arterial
occlusion has occurred, there is a risk of discomfort, or worse, pain or
injury, in using automatic blood pressure monitoring equipment.
Conventional, ambulatory blood pressure monitoring equipment is capable
only of establishing a maximum cuff pressure based upon physiological
norms, i.e. a pressure is established that is believed to be sufficient to
occlude blood flow, but insufficient to cause discomfort.
Conventional ambulatory blood pressure monitoring equipment fails to
accommodate differences among individual patients. The fact is, different
patients require different occlusion pressures. Further, an individual's
systolic pressure fluctuates widely over time. For example, a person's
systolic pressure generally is lower when the person is at rest than when
the person is exercising. For this reason, it is believed to be important
that the cuff pressurization dynamically track an individual's systolic
pressure as it rises and falls throughout the day. The improved method of
the present invention accomplishes this dynamic tracking by measuring the
systolic blood pressure, comparing it to the cuff pressurization maximum
and adding to or subtracting from the maximum a predetermined, incremental
value. The novel systolic pressure-adaptive cuff pressurization method of
the present invention is especially useful for diagnosis and treatment of
patients who are sensitive to cuff tightness.
The novel, systolic pressure-adaptive cuff pressurization feature of the
present invention may be better understood by reference to the following
pseudo-code listing:
if SYS>MAXP then
MAXP :=(MAXP+20);
else if MAXP<(SYS+25) then
if (SYS+15)<MAXP then
MAXP :=(MAXP+10);
else MAXP :=(MAXP+20);
else if MAXP>(SYS+45) then
if MAXP>(SYS+50) then
MAXP :=(MAXP-10);
else MAXP :=(MAXP-5);
else no adjustment.
It is believed that the method of adjusting the maximum cuff pressure to
accommodate the rise and fall of the patient's systolic pressure may be
understood from the pseudo-code. Briefly summarized, the method involves
replacement of the default maximum cuff pressure value with a value that
is equal to the default value plus or minus an incremental value which has
been determined by trial and error to produce the desired response to a
wide range of systolic pressure dynamics. By comparing the measured
systolic blood pressure with a first predefined maximum pressure and,
based upon such comparison, either adding a predefined amount or
subtracting a predefined amount from the first predefined maximum to
produce a second maximum cuff pressure value, over a period of time
including one or more recording sessions in which the MAXP adjustments
described above are made, the maximum cuff pressure in successive
pressurization and measurement cycles will seek a level that is within a
predetermined range above the patient's systolic blood pressure. In this
way, the maximum cuff pressure tracks the rise and fall of the patient's
systolic blood pressure.
It may be seen from the cuff pressurization method that the desired
`window` for MAXP is 25-45 mm Hg above the patient's systolic blood
pressure In the preferred embodiment of the invention, the maximum
increment, or upward adjustment, of MAXP is 30 mm Hg above the
physician-selected maximum value. Not indicated is that, in the preferred
embodiment, MAXP is not permitted to fall below 140 mm Hg. This is to
avoid unduly long `warm-up` periods, during which the cuff pressure is
below the systolic blood pressure and blood pressure and heart rate
readings are assumed to be inaccurate. Of course, the particular values
used or the steps taken to adjust the maximum cuff pressure dynamically to
adapt to the patient may differ from the specifics of the preceding
discussion, without departing from the spirit of the invention.
The software described in reference to FIG. 2, when operatively coupled
with K-sound detecting means described in reference to FIG. 1, provides
means for qualitatively characterizing a patient's blood pressure data
proximate in time to the monitoring and recording of the blood pressure
data. The important advantage of this improvement to ambulatory patient
blood pressure monitoring apparatus is that the physician is provided not
only with blood pressure data, but also with supplemental information that
may be interpreted as more broadly indicative of the context in which the
recording was made. For example, when the Figure of Merit is analyzed, it
may be found that the K-sound frequency was too low to accord any weight
to the blood pressure readings taken during the previous twenty-four hour
period, and repositioning of the microphone and/or the pressure cuff may
be indicated. It may also be discovered that the amplitude of the K-sounds
was too low, indicating that an individual patient's physiology requires a
more sensitive, and perhaps an invasive, diagnostic monitoring technique.
Finally, particular readings may be determined to be suspect, and may thus
be disregarded by the physician, when, despite the general high quality
index of most of the readings, one or two are indicated as having been
taken during a time when low frequency or low amplitude K-sounds were
detected. This might indicate excessive motion on the part of the patient
during certain blood pressure readings, for example, rather than an
aberrational blood pressure or pulse, which might otherwise indicate a
dysfunctional cardiology.
The improved method of ambulatory patient blood pressure monitoring and
recording then includes the steps of (1) monitoring patient blood pressure
data over a predetermined period of time, (2) monitoring Korotkoff sound
amplitude and or frequency for the predetermined period of time, and (3)
generating a code which characterizes the blood pressure data based upon
an evaluation of the K-sound data. Specifically, generating a code
involves assigning one of a first set of plural symbols to represent the
Korotkoff sound frequency, assigning one of a second set of symbols
distinctive from said first set of symbols to represent the Korotkoff
sound amplitudes, and concatenating said one of said first set of symbols
with said one of said second set of symbols. In accordance with the
preferred method of the invention, one set of symbols is alphabetic and
the other is numeric, as in "A1", and D4. As described above, computer
means 12 cooperate, in the preferred embodiment of the invention, with
pressure sense means, Korotkoff sound detecting means and memory means to
monitor the blood pressure and Korotkoff sounds, and to generate the code,
which by one method is a two-digit figure of merit, and by another is a
single-digit quality index.
By another improved method of the invention, systolic pressure-adaptive
means for pressurizing a cuff are capable of adapting to the patient's
systolic blood pressure, thereby to ensure that the maximum cuff pressure
is within a predetermined pressure range above the patient's systolic
blood pressure during such blood pressure data monitoring and recording.
Cuff pressure is adjusted by elevating the cuff pressure (which may be
thought of as counter-pressure) to a predetermined maximum value,
measuring the patient's systolic blood pressure as the static cuff
pressure declines, and comparing the measured systolic pressure with the
maximum pressure value. An adjustment then is made to the maximum pressure
value (variable MAXP in memory), based upon the comparison result, which
adds to or subtracts from that value an amount calculated to maintain the
cuff pressure within a predetermined range above the patient's systolic
blood pressure.
Accordingly, while a preferred method for practicing the invention, and a
preferred embodiment of the apparatus of the invention and a proposed
modification thereto have been described herein, it is appreciated that
further modifications are possible that come within the scope of the
invention.
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