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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to blood pressure evaluation
procedures and more particularly to non-invasive techniques for
determining certain waveform information associated with blood pressure.
The most reliable ways presently known for obtaining information relating
to an individual's blood pressure require invasive procedures. Such
procedures are not carried out routinely but only under extreme
circumstances, for example during heart surgery. Under less critical
conditions, blood pressure information including specifically an
individual's systolic (maximum) and diastolic (minimum) blood pressures is
obtained non-invasive. There are two well known non-invasive techniques
presently being used today, one is commonly referred to as auscultation
and the other is based on oscillometry. Both of these non-invasive
techniques use the standard arm cuff which most people are familiar with.
However, in the auscultatory method, the systolic and diastolic pressures
are determined by listening to certain sounds (Korotkoff sounds) which
occur as a result of the cuff first being pressurized and then
depressurized whereas oscillometry actually measures changes in pressure
in the cuff as a result of changes in blood pressure as the cuff is first
pressurized and then depressurized.
As will be seen hereinafter, the various embodiments of the present
invention are based on oscillometry. In order to more fully appreciate
these embodiments, reference is made to applicant's own U.S. Pat. No.
3,903,872 (the Link patent) for obtaining blood pressure information
non-invasively. This patent which is incorporated herein by reference
describes, among other things, a way of obtaining the diastolic pressure
of an individual in accordance with a technique which will be discussed in
more detail hereinafter. In U.S. Pat. Nos. 4,009,709 and 4,074,711 (Link
et al) which are also incorporated herein by reference, non-invasive
techniques using oscillometry are disclosed for obtaining the systolic
pressure of an individual. These techniques will also be discussed
hereinafter.
OBJECTS AND SUMMARY OF THE INVENTION
While the various procedures described in the Link and Link et al patents
just recited and other patents held by applicant are satisfactory for
their intended purposes, it is an object of the present invention to
provide additional uncomplicated and yet reliable techniques for obtaining
different types of information relating to an individual's blood pressure.
A more specific object of the present invention is to provide a different
uncomplicated and yet reliable technique for generating non-invasively a
waveform closely approximating an individual's true blood pressure
waveform which, heretofore, has been obtainable by invasive means only.
Another particular object of the present invention is to provide a new way
for measuring and calculating the mean arterial pressure of an individual.
As will be described in more detail hereinafter, the objects just recited
are achieved by means of oscillometry. In accordance with this technique,
a suitably sized cuff, for example one which is 20 inches long and 5
inches wide, is positioned around the upper arm of an individual, a human
being specifically or a mammal in general (hereinfter referred to as the
patient) and initially pressurized to a level which is believed to be
clearly greater than the patient's systolic pressure, for example 180
Torr. It is assumed that this pressure will also cause the patient's
artery within the sleeve to completely collapse. Thereafter, cuff pressure
is gradually reduced toward zero during which time the cuff continuously
changes in pressure in an oscillating fasion due to the combination of (1)
the internal blood pressure changes in the patient's artery and (2)
changes in cuff pressure. The latter at any given time in the procedure is
known and oscillatory changes in cuff pressure can be readily measured,
for example with an oscilloscope. By using these two parameters in
conjunction with information which may be made available from methods
disclosed in the above-recited U.S. patents it is possible to achieve the
foregoing objectives in an uncomplicated and reliable way utilizing the
techniques of the present invention to be described hereinafter.
In this regard, it should be noted at the outset that the typically 5" wide
pressure cuff entirely surrounds a corresponding 5" length of artery. The
tissue of the arm is for the most part incompressible, and therefore any
change in the volume of the artery, caused for example by pulsations of
blood, results in a corresponding change in the volume of air in the air
bladder which is within the cuff and therefore adjacent to the arm. This
change in air volume produces a small but accurately measurable pressure
change in the air. This equivalence of pressure pulsations in the cuff
bladder to volume pulsations of the artery is the essence of oscillometry.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to more fully appreciate the various techniques of the present
invention, the following more detailed background information is provided
in conjunction with FIGS. 1-5 of the drawings where:
FIG. 1 (corresponding to FIG. 6 in U.S. Pat. No. 3,903,872)
diagrammatically illustrates the shapes of successive cuff pressure versus
time pulses (cuff pulses) as the measured cuff pressure changes from 90
Torr to 80 Torr to 70 Torr, assuming the patient has a diastolic pressure
of 80 Torr;
FIG. 1A diagrammatically illustrates a full series of cuff pulses
corresponding to those in FIG. 1 from a cuff pressure of 160 Torr to a
cuff pressure of zero;
FIG. 2 diagrammatically illustrates a curve corresponding to arterial or
cuff volume (V), that is, the volume of the patient's artery within the
cuff (as measured by cuff volume) versus wall pressure (P.sub.w) across
the artery wall within the cuff and, superimposed on this curve, a curve
which is intended to correspond to the actual blood pressure waveform of a
patient, the two curves being provided together in order to illustrate the
principles of oscillometry, as relied upon in the above-recited patents;
FIGS. 3 and 4 diagrammatically illustrate the cuff curve of FIG. 1 in ways
which display techniques for obtaining a given patient's systolic and
diastolic blood pressures in accordance with the Link and Link et al
patents recited above; and
FIG. 5 diagrammatically illustrates a compliance curve for the patient's
artery, that is, a curve which displays the ration .DELTA.V/.DELTA.P
against the arterial wall pressure P.sub.w, where .DELTA.V is the
incremental change in the arterial volume corresponding to a preselected
constant change in blood pressure .DELTA.P. This curve is initially
determined in order to provide the cuff or arterial volume curve (V/P
curve) of FIG. 2 by means of integration, as will be seen;
FIG. 6 diagrammatically illustrates an arterial v/p curve of a given
individual with specific emphasis on the degree of linearity of its
segments;
FIG. 7 diagrammatically illustrates the use of the arterial curve of FIG. 6
in combination with the given individual's cuff pulses at a fixed cuff
pressure to approximate the individual's actual blood pressure curve;
FIG. 8 schematically illustrates an arrangement for providing the
approximated curve just referred to in association with FIG. 7;
FIGS. 9 (a)-(d) diagrammatically illustrate four blood pressure waveforms
having different blood pressure constants K and equivalently, different
mean blood pressures; and
FIGS. 10 and 11, respectively, illustrate a block diagram and flow diagram
of the technique described with respect to FIGS. 6-8.
FURTHER BACKGROUND OF THE INVENTION
Turning first to FIG. 1, this figure diagrammatically illustrates three
successive waveforms 10h, 10i and 10j which correspond to the change in
volume in a pressurized cuff, as described above, at three different cuff
pressures, specifically cuff pressures of 90 Torr, 80 Torr and 70 Torr. In
actual practice, a greater number of waveforms (hereinafter referred to as
cuff pulses) are generated starting at a cuff pressure of 160 Torr and
ending at a cuff pressure of zero, as will be seen in FIG. 1A. By
generating these waveforms at known cuff pressures, both the diastolic and
systolic pressures of a patient can be determined in accordance with the
above-recited patents. While this will be explained in more detail below,
it is important to note initially that each waveform has what may be
referred to as a systolic rise S.sub.r at one end of the waveform, a
diastolic decline D.sub.d at the opposite end and a maximum amplitude A.
While the systolic rise S.sub.r is fairly consistent and distinctive from
one cuff pulse 10 to another, both the diastolic decline D.sub.d and
amplitude A vary from pulse to pulse for reasons to be explained
hereinafter. It is because of these variations that the techniques
disclosed in the Link ad Link et al patents recited above are able to
determine the diastolic and systolic pressures. Specifically, as will be
seen, when the diastolic pressure of a patient is equal to the cuff
pressure, the cuff pulse generated has a diastolic decline which is
greater in slope than the diastolic decline of any of the other cuff
pulses. Thus, assuming that the diastolic decline has a maximum slope at
the cuff pulse 10i illustrated in FIG. 1, the patient providing these
waveforms would have a diastolic pressure of 80 Torr. At the same time,
this patient's systolic pressure can be determined by first finding which
of the cuff pulses displays a maximum amplitude A and then, moving up in
cuff pressure, finding the cuff pulse having half that amplitude. The cuff
pressure responsible for producing this half-amplitude pulse will equal
the patient's systolic blood pressure. In order to more fully understand
these capabilities, reference is made to FIGS. 2-5 in conjunction with the
above-recited Link and Link et al patents.
Turning now to FIG. 2, attention is directed to the curves illustrated
there in order to explain why the cuff pulses of FIG. 1 result from
changes in cuff pressure. The generally S-shaped curve 12 illustrated is
shown within a horizontal/vertical coordinate system where the horizontal
axis represents the wall pressure P.sub.w across the artery wall of a
given patient, within the confines of the applied cuff, and the vertical
axis represents arterial volume V of the artery within the cuff, as
measured by the internal volume of the cuff itself. In order to fully
understand this V/P curve (hereinafter merely referred to as an arterial
or a cuff curve), it is important to keep in mind the definition of
P.sub.w. The wall pressure P.sub.w of the artery of the patient at any
given time is equal to the blood pressure P.sub.b of the patient within
the artery at that time less the applied pressure of the cuff P.sub.c.
Thus:
P.sub.w =P.sub.b -P.sub.c (1)
For purposes of the present discussion, it will be assumed that pressure is
measured in Torr (mmHg) and that the section of the horizontal axis to the
right of the vertical axis represents positive wall pressures while the
section of the axis to the left of the vertical axis represents negative
wall pressures. As a result, when no pressure is applied to the cuff (e.g.
P.sub.c =0), P.sub.w at any given point in time is equal to the blood
pressure of the patient at that time. As the cuff is pressurized, P.sub.w
decreases (moves to the left along the horizontal axis). When the cuff
pressure P.sub.c is equal to the blood pressure P.sub.b at any given point
in time, P.sub.w at that time is equal to zero (e.g. at the vertical
axis). As the cuff pressure is increased beyond the blood pressure at any
point in time, P.sub.w at that time becomes more negative (moves further
to the left on the horizontal axis).
With the definitions of the vertical axis V and the horizontal axis P.sub.w
in mind, attention is now directed to an interpretation of the generally
S-shaped cuff curve 12 within this coordinate system. For the moment, it
is being assumed that this curve is characteristic of the particular
patient being evaluated. That is, it is being assumed that the patient's
artery within the cuff and therefore the cuff itself will change in volume
along the S-shaped curve and only along the curve with changes in P.sub.w.
Hereinafter, with regard to FIG. 3, it will be shown that the arterial
curve 12 of a given patient can be generated from his cuff pulses 10 and
corresponding cuff pressures P.sub.c. Thus, for the time being, it will be
assumed that the arterial curve illustrated in FIG. 2 corresponds to that
of the given patient.
With the foregoing in mind, the arterial curve of FIG. 2 will now be
examined. Let it first be assumed that no pressure is applied to the
patient's cuff so that P.sub.c equals zero. As a result, P.sub.w equals
the blood pressure P.sub.b of the patient. In this regard, it is important
to note that P.sub.b varies with time between the patient's diastolic
blood pressure P.sub.b (D) and his systolic blood pressure P.sub.b (S).
For purposes of this discussion, let it be assumed that these values are
known and that specifically the patient's diastolic blood pressure is 80
Torr and his systolic blood pressure is 120 Torr. Thus, with no pressure
in the cuff, P.sub.w oscillates back and forth with time between P.sub.b
(D) and P.sub.b (S), that is, between 80 Torr and 120 Torr. This 40 Torr
measuring band is illustrated by dotted lines in FIG. 2 at 14 and actually
represents the patient's pulse pressure .DELTA.P which is equal to 40 Torr
in this case.
The patient's actual blood pressure waveform 15 is superimposed on the
V/P.sub.w coordinate system in FIG. 2 within the pulse pressure band 14.
As seen there, this waveform is made up of a series of actual blood
pressure pulses 16, each of which corresponds to a single beat of the
patient's heart. Note that each pulse starts at a minimum pressure (the
diastolic pressure of the patient) and sharply increases along its leading
edge which is the systolic rise S.sub.r until it reaches a maximum (the
patient's systolic blood pressure), at which time it drops back down along
a trailing edge which includes a dichrotic notch and a diastolic decline
D.sub.d to the minimum pressure again.
At those points in time when the patient's blood pressure is at a minimum
(that is, at the diastolic ends of pulses 16), the volume of the patient's
artery and therefore the volume of the cuff is fixed by the arterial curve
at the value indicated at V.sub.1 (P.sub.w =80). On the other hand,
whenever the patient's blood pressure is maximum (at the systolic end of
each blood pressure pulse 16), the arterial curve fixes arterial and
therefore cuff volume at the slightly higher value indicated at V.sub.2
(P.sub.w =120). Therefore, it should be apparent that for each heart beat,
assuming a cuff pressure P.sub.c of zero, the volume V (the cuff volume
moves between the values V.sub.1 and V.sub.2, thereby generating a series
of cuff pulses 10q corresponding to those illustrated in FIG. 1 but at a
cuff pressure P.sub.c =0, as shown in FIG. 1A. Thus, as the patient's
blood pressure rises from a minimum to a maximum, the volume of the artery
rises from V.sub.1 to V.sub.2 in a generally corresponding manner and as
the patient's blood pressure drops back down to a minimum, the arterial
volume falls from V.sub.2 to V.sub.1 in a generally corresponding manner.
Thus, each of the arterial pulses 10 in FIG. 2 has a systolic rise S.sub.r
and a diastolic decline D.sub.d corresponding to the systolic rise and
diastolic decline of each blood pressure pulse 16.
Having shown how the cuff pulses 10q are dependent upon the volume curve at
a cuff pressure of zero, we will now describe how the arterial curve
causes these arterial pulses to change with applied cuff pressure. Let us
assume now a cuff pressure of 50 Torr. Under these conditions, P.sub.w
oscillates back and forth between 30 Torr and 70 Torr. The 30 Torr value
is determined by subtracting the cuff pressure P.sub.c of 50 Torr from the
diastolic blood pressure P.sub.b (D) of 80 Torr and the 70 Torr value is
determined by subtracting the same P.sub.c of 50 Torr from the systolic
blood pressure P.sub.b (S) of 120 Torr. Thus, the entire 40 Torr band has
merely been shifted to the left an amount equal to 50 Torr as indicated by
the band 14'. Under these circumstances, P.sub.w oscillates back and forth
along a steeper segment of the arterial curve so as to cause the volume of
the patient's artery and therefore the volume of the cuff to oscillate
between the values V.sub.3 and V.sub.4. This results in the production of
arterial pulses 101 at a P.sub.c of 50 Torr. Note that the amplitude of
each cuff pulse 101 is greater than the amplitude of each cuff pulse 10q.
This is because the 40 Torr band 14' at a cuff pressure of 50 Torr is on a
steeper part of the volume slope than the band 14 at a cuff pressure of
zero. Indeed, as we increase the cuff pressure P.sub.c (which decreases
P.sub.w) and therefore move the pressure band to the left on the
horizontal axis, we first continue to move along steeper sections of the
arterial curve and thereafter less steep sections. Therefore, the
amplitude A (see FIGS. 1 and 1A) of the corresponding cuff pulses 10q, 101
and so on will first increase to a maximum and then decrease again. At a
cuff pressure P.sub.c of 100, the entire 40 Torr pressure band is shifted
to the left so as to uniformly straddle opposite sides of the vertical
axis, as indicated at 14". This results in a corresponding cuff pulse 10g
having approximately a maximum amplitude (.DELTA.Vmax in FIG. 2).
Moving still further to the left, at for example, a cuff pressure P.sub.c
of 160 Torr, the entire 40 Torr band is moved a substantial distance to
the left of the vertical axis, as indicated at 14''' such that the
resultant change in volume (amplitude of the corresponding cuff pulse 10a)
is quite small. By increasing the cuff pressure to even a greater amount,
the band is moved still further to the left, eventually producing very
small changes in volume V. From a physical standpoint, this represents a
collapsed artery. In other words, sufficient cuff pressure P.sub.c is
being applied over and above the internal blood pressure P.sub.b to cause
the wall of the artery to collapse. At the other extreme, that is, when
the cuff pressure P.sub.c is zero, there are no external constraints
placed on the artery and the latter is free to fluctuate back and forth
based on its internal pressure P.sub.b only. Between these extremes, the
amplitude A of cuff pulse 10 (e.g. .DELTA.V) will increase to a maximum
and then decrease again, as stated. It is this characteristic of the
volume curve which is used to determine the patient's systolic pressure in
accordance with the previously recited Link et al patents, as will be
described with regard to FIGS. 3 and 4.
As previously mentioned, it should be noted that a blood pressure increase
causes an arterial volume increase. This arterial volume increase causes a
cuff bladder air volume decrease which in turn causes a cuff bladder
air-pressure increase. Therefore a blood pressure increase results in a
cuff air pressure increase. This is emphasized as follows:
______________________________________
blood .fwdarw.
arterial .fwdarw.
cuff air
.fwdarw.
cuff air
pressure volume volume pressure
increase increase decrease increase
Thus: blood .fwdarw.
cuff air
pressure pressure
increase increase
______________________________________
Referring to FIG. 3, the same arterial curve 12 illustrated in FIG. 2 is
again shown but with a single superimposed pressure band 14'''' at a cuff
pressure P.sub.c of 120 Torr. Assume again that the diastolic pressure of
the patient is 80 Torr and his systolic pressure is 120 which means that
P.sub.c is equal to the patient's systolic pressure. Under these
circumstances, P.sub.w oscillates back and forth within band 14''''
between wall pressures of -40 Torr and zero, as shown. This results in a
change in arterial volume .DELTA.V (e.g., the amplitude A of a
corresponding cuff pulse) which is approximately equal to one-half of the
maximum change in arterial volume (e.g., max cuff pulse amplitude). It may
be recalled that a maximum change in volume .DELTA.V max (and therefore a
maximum cuff pulse amplitude Amax) results from a cuff pressure P.sub.c of
about 100 Torr (e.g. the pressure band 14" in FIG. 2). Thus, when the cuff
pressure P.sub.c is equal to the patient's systolic blood pressure P.sub.b
(S), the amplitude A of the resultant cuff pulse 10 is having a maximum
amplitude. Therefore, a patient's systolic blood pressure can be
determined by first generating a series of cuff pulses across the cuff
pressure spectrum, as in FIG. 1A. From these pulses, the one having
maximum amplitude Amax is determined and then the cuff pulse having half
that amplitude (at a greater cuff pressure) is found. The cuff pressure
P.sub.c used to generate that pulse corresponds to the patient's systolic
pressure. In other words, by evaluating the amplitudes of the various cuff
pulses, the one corresponding to the band 14'''' illustrated in FIG. 3 can
be found. Once that pulse is found, its associated cuff pressure is
assumed to be equal to the patient's systolic pressure. This is discussed
in more detail in Link et al U.S. Pat. Nos. 4,009,709 and 4,074,711 and
means are provided in these latter patents for electronically making these
evaluations.
Returning to FIG. 2, it should be noted that the actual blood pressure
waveform 15 is shown having a uniform repetition rate, for example 60
pulses/minute, and that each blood pressure pulse 16 making up this
waveform is identical to the next one. Both of these aspects of the
waveform are assumed for purposes herein. Moreover, each pulse has its own
systolic rise S.sub.r and diastolic decline D.sub.d, as mentioned
heretofore. It should also be noted that the arterial curve 12 dictates
the relationship between V and P.sub.w at each and every point on the
waveform 15 of individual blood pressure pulse 16, not merely at the
extreme diastolic and systolic end points of each pulse. Thus, one could
measure the change in volume .DELTA.V at two different cuff pressures
along the diastolic decline only. In this case, the measuring band (e.g.
the pressure difference between the two measuring points) is substantially
narrower than band 14. As best illustrated in FIG. 4, .DELTA.V.sub.1 ' is
determined for a cuff pressure P.sub.c of zero using the pressure band 18
which encompasses a small part of the diastolic decline of each blood
pressure pulse 16. .DELTA.V.sub.2 ' is determined for a cuff pressure of
P.sub.c of 50 Torr by shifting the band to 18' and, .DELTA.V.sub.3 ' is
determined for a cuff pressure P.sub.c of 80 Torr (e.g. the patient's
diastolic blood pressure) by shifting the band to 18". Note that .DELTA.V
is maximum when the cuff pressure P.sub.c is equal to the patient's
diastolic blood pressure. Therefore, by determining the change in volume
.DELTA.V at the end of the diastolic slope of the patient's actual blood
pressure waveform for each and every cuff pressure, the one cuff pressure
producing a maximum change will correspond to the patient's diastolic
blood pressure. The lowest pressure part of the diastolic decline D.sub.d
forming part of each pulse 16 is particularly suitable for this purpose
since it can be readily located during each cycle of the waveform. This is
because it immediately precedes the systolic rise S.sub.r which is readily
distinguishable each time it appears. This procedure is described in more
detail in the previously recited Link U.S. Pat. No. 3,903,872 along with
means for carrying out this procedure electronically.
The foregoing discussions for obtaining a given patient's systolic and
diastolic blood pressures have assumed that the patient's arterial curve
corresponded to the one illustrated in FIGS. 2, 3 and 4. While this
assumption is reasonably valid, it is possible to determine the patient's
own volume curve using the using the narrower bands 18, 18' and so on as
measuring bands, the change in volume .DELTA.V (e.g., the change in cuff
volume) resulting from different cuff pressures P.sub.c is plotted, as
shown in FIG. 5. Thus at a cuff pressure P.sub.c of zero, there is a
relatively small change in volume .DELTA.V, as evidenced by the small
.DELTA.V.sub.1 ' in FIG. 4. As the cuff pressure P.sub.c increases, the
change in volume .DELTA.V continues to increase to a maximum
(.DELTA.B.sub.3 ' in FIG. 4) and then decreases. In mathematical terms,
this curve represents incremental changes in volume with incremental
changes in pressure or dV/dP (FIG. 5). By integrating this curve we obtain
the cuff curve or the V/P curve of FIGS. 2-4.
DETAILED DESCRIPTION
Having discussed FIGS. 1-5 in regards to the prior art techniques for
obtaining diastolic and systolic blood pressures for a given patient in
accordance with the techniques described in the above-recited Link and
Link et al patents, attention is now directed to the various aspects of
the present invention, as discussed briefly above, in conjunction with
FIGS. 6-11 recited above.
Turning to FIGS. 6-9, a technique is provided for generating a waveform
which closely approximates the actual blood pressure waveform of a
patient. In order to more fully appreciate this technique, reference is
again made to FIG. 2. It may be recalled that a particular patient's cuff
pulses at any given cuff pressure is dictated by the S-shaped cuff curve
12 in FIG. 2. For example, assuming a systolic pressure of 120 Torr and a
diastolic pressure of 80 Torr, the resultant measuring (pulse pressure)
band may be moved along any section of the S-shaped curve by selecting a
particular cuff pressure. Thus, with a cuff pressure of zero, the band is
located to the far right, as viewed in FIG. 2 and by providing a cuff
pressure of 160, the band is located to the far left. It is known that the
most linear sections of the arterial curve provide cuff pulses which most
approximate the actual blood pressure waveforms. To illustrate arbitrarily
this known art the S-shaped cuff curve of FIG. 2 is shown in FIG. 6
divided into three sections, sections 2 and 3 being the least linear while
section 1, is the most linear. Thus, if the pulse pressure band of FIG. 6
has its center along section 2 for example, that is, at a fixed cuff
pressure of around 50 Torr, the resultant cuff pulses are not close
approximations of the patient's actual blood pressure waveform. By
operating in section 3, there is practically no gain at the diastolic end
of the waveform, that is, this section of the curve is practically
horizontal, resulting in very bad waveform distortions.
The most ideal section of the curve to operate on in order to produce fixed
cuff pulses which most approximate the actual waveform is section 1 which
is more linear and which displays moderate to low gain, that is, a gradual
slope. This can be achieved by operating at a fixed cuff pressure of
anywhere from zero to approximately 80 Torr. Once the cuff pressure is
selected, corresponding cuff pulses of the given patient are continuously
produced at the selected pressure. These cuff pulses are shown at 10m' in
FIG. 7 and correspond to a cuff pressure of, for example, 40 Torr (see
FIG. 1A). At the same time, the patient's systolic and diastolic pressures
and arterial curve are used in combination with the cuff pulses to provide
ultimately an approximation of the patient's blood pressure waveform, as
will be seen below. The patient's arterial curve is reproduced in FIG. 7
at 12'. Both the systolic and diastolic pressures of the patient and curve
12' can be readily provided.
With the continuous pulses 10m' and curve 12' shown in FIG. 7, a waveform
16' can be generated between fixed wall pressures (P.sub.w) which are
dictated by the patients systolic and diastolic pressures and the cuff
pressure selected. In the example above where the cuff pressure P.sub.c is
40 Torr, the patients systolic pressure P.sub.s is 125 and his diastolic
pressure P.sub.d is 85, the operating P.sub.w band B is between 45 Torr
and 85 torr, as illustrated in FIG. 7. These wall pressures dictate the
section of curve 12' which produces waveform 16'. To generate this
waveform from continuous pulses 10m', a first point P.sub.1 at the
beginning of pulse 10m' (at time t.sub.1) is found and a corresponding
point P.sub.1 in band B is plotted. This is easily done since both of
these points represent the diastolic pressure of the patient and the
beginning of the pulse and waveform. A second point P.sub.2 at time
t.sub.2 (as referenced from time t.sub.1) can be found and so on until a
series of points are found, as shown. From these points, the waveform 16
can be generated. The shape of waveform 16' correctly represents the true
blood pressure waveform whereas the shape of waveform 10m' from which 16'
is derived may be highly deformed by the arterial V/P curve.
In accordance with the present invention, suitable cuff means generally
indicated at 30 in FIG. 8 is positioned around the arm of a patient in the
normal operating manner and maintained at one of these preferably low
pressures, for example, a cuff pressure of 40 Torr by pump means 32.
However, the present invention is not limited to this cuff pressure range.
Thus, for example, a cuff pressure of 100 Torr could be selected but
higher cuff pressures of this type might be uncomfortable for the patient.
The resultant cuff pulses are continuously monitored by transducer 34.
Suitable and readily providable electronic circuitry 36 is also provided
with the patients arterial curve and his systolic and diastolic pressures
and uses the information to generate the waveform 16'. This waveform can
be placed on an oscilloscope or monitor 38 or read out permanently as an
approximation of the patient's actual blood pressure waveform, as shown in
FIG. 1A. Moreover, in its displayed or readout state, the waveform can be
appropriately labeled with its systolic and diastolic points in order to
more aptly represent the patient's true blood pressure waveform.
In yet another application of the present invention, any single one or many
of the cuff pulses obtained when the cuff pressure is ramped slowly down
or up in pressure can be transformed by the apparatus described above into
a waveform 16' which accurately represents the shape of the true blood
pressure waveform. Thus during a normal oscillometric measurement of blood
pressure as described elsewhere above, a single or many cuff pulses can be
transformed into accurate representations of the blood pressure waveform
and suitably presented on a monitor for a doctors examination.
The foregoing has been a discussion of how a particular patient's actual
blood pressure waveform can be closely approximated without an invasive
device. This may be an important diagnostic tool to a doctor, especially
if it turns out that his patient has an unusual waveform. This is best
exemplified in FIGS. 9a-d which diagrammatically illustrate a number of
waveforms having different mean values. The mean pressure P.sub.b (m) of a
blood pressure waveform is equal to the diastolic blood pressure P.sub.b
(D) plus a particular fraction K of the pulse pressure which is the
difference between the patient's systolic blood pressure P.sub.b (s) and
his diastolic blood pressure. Equation 2A shows this and equation 2B shows
the same thing in a convenient short hand notation and equation 2C solves
equation 2B for K.
P.sub.b (m)=P.sub.b (D)+K(P.sub.b (s)-P.sub.b (D) (2A)
M=D+K(S-D) (2B)
K=M-D/S-D. (2C)
Noting that the mean pressure M can be calculated by integrating the
waveform (its pressure amplitude P) over time T (the duration of the
waveform) so that:
##EQU1##
With the above equations in mind, the FIG. 9a waveform can be shown to have
a K value (which is commonly referred to as the blood pressure constant)
of about 0.50. The FIG. 9b waveform approximates a K value of 0.6 while
the FIG. 9c waveform approximates a K value of 0.2. Finally, the FIG. 9d
waveform approximates a K value of 0.33. This latter waveform m | | |
