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FIELD OF THE INVENTION
This invention relates to a sphygmomanometer. More particularly, the
invention relates to a non-invasive manometer having a bladder which is
mainly attached to an appendage such as a finger, an earlobe, a leg, a
tail or the like. Also included is a means for applying pressure to the
bladder and, hence to the blood. An optical means senses the varying
diameter (volume) of the blood vessel depending upon the applied pressure
and the internal pressure with the blood vessel in order to measure the
blood pressure.
BACKGROUND OF THE INVENTION
Conventionally, a sphygmomanometer is applied indirectly to an appendage
such as a finger or an earlobe because it is convenient. For example, U.S.
Pat. No. 4,406,289 discloses the use of a servo balance technique. Another
example is U.S. Pat. No. 4,597,393 which discloses a method of calculating
and estimating the lowest blood pressure by observing the linearity of the
blood vessel and the associated peripheral organization. Further examples
are U.S. Pat. Nos. 3,104,661, 3,920,004 and 4,437,470 which disclose the
use of a plurality of cuffs or sensors. Also, a sphygmomanometer may
include an optical means which senses a change in the vessel diameter or
volume due to the applied pressure and the internal blood vessel pressure.
The optical means converts the change in blood pressure to an electric
signal. Using the optical means requires the use of a cover to eliminate
the influence of external light, solar light, or the like and in addition
requires a place where little external light is present and/or the
external light is stable. If these requirements are not considered, an
error will result from the sphygmomanometry. For example, when the average
blood pressure is to be measured using a signal processor shown in FIG. 1,
a light emitting element drive circuit Dr activates a light emitting
element Ea to emit intermittent light. The output signal from a
photosensitive element Da is amplified by an amplifier A4. The
intermittent components contained in the amplified signal are eliminated
by a filter F1 in order to obtain an output signal having a continuous
waveform. The photosensitive element Da receives a quantity of varying
light which depends on a change in diameter of the blood vessel. The
output signal from the photosensitive element Da is an electric signal
which corresponding to the change in the diameter of the blood vessel. As
shown in FIG. 2(a) if the pressure PO of a bladder B (FIG. 1) is
temporarily increased higher than the highest vessel pressure and then
decreased gradually, the output signal from the signal processor changes
as shown in FIG. 2(b). In this case, the blood pressure corresponding to
the maximum amplitude of the output signal is handled as an average blood
pressure. Reference characters F and A in FIG. 1 denote a finger and a
blood vessel, respectively.
In the U.S. Pat. No. 4,406,289, the device used is large in size, and
requires a long time to attach the device and to adjust it, so that the
device is not suitable for an application in which the measurement of
blood pressure is instantaneously desired. U.S. Pat. No. 4,597,393 has
several problems such as (a) producing an error due to an assumption that
the elasticity of the blood vessel and its associated peripheral
organization is linear, (b) producing an error due to an increase in the
quantity of data due to the calculation of a small waveform area, and (c)
producing an error due to reducing the quantity of data, etc. In U.S. Pat.
Nos. 3,104,661, 3,920,004, and 4,437,470, it is hard, to attach two or
more device elements. Pressurization cannot be allowed because it may
cause an error in the measurement by a sensor on the distal side. In
addition, it is impossible to determine the lowest blood pressure or a
method for the determination is unclear. In the sphygmomanometer which
uses an optical means, the filter cannot eliminate external light similar
in frequency to the pulsating wave. Thus, all the prior art, in which
light emitting elements are disposed at opposing positions on a portion of
a human body to sense a change in the vessel diameter as a change in the
received quantity of light which is then changed to an electric signal
which is in turn processed to measure the blood pressure, will cause an
error in the sphygmomanometry.
SUMMARY OF THE INVENTION
Therefore, it is an object of this invention to provide a sphygmomanometer
which eliminates the above problems involving the measurement accuracy and
device complexity that the prior art has, and performs sphygmomanometry
with high accuracy and with a simple structure.
The means to solve the above problems according to this invention will be
now described.
A first embodiment is a non-invasive manometer similar to that shown in
FIG. 3 and includes (a) a pressurization means B and C attached to a
finger F which applies a pressure to a finger or the like, (b) a means S
for changing the pressure applied to the finger, and (c) an optical means
Ea, Eb and Da, Db for sensing a change in the diameter (volume) of the
blood vessel due to the applied pressure and the internal pressure of the
blood vessel A. As shown in FIG. 4, either light emitting unit of the
optical means may include a plurality of elements (Ea, Eb) or the
photosensitive unit may include a plurality of elements (Da, Db). One of
the light emitting elements (or photosensitive elements), Ea (or Da), is
disposed closer to the heart than other light emitting elements (or
photosensitive) elements, Eb (or Db). The plurality of elements of the
optical means are operated in a time-divisional manner. The elements of
the optical means are shielded from external light by shield SH. As shown
in FIG. 4, comparing means Cp is provided which compares the outputs of
two or more elements (Ea, Da), (Eb, Db) of the optical means positioned on
the heart side and the peripheral side. The highest blood pressure is
determined when the output Ss of the optical means (Eb, Db) positioned on
the peripheral side appears while the lowest blood pressure is determined
when the output Sd of the comparing means Cp disappears.
In a second embodiment, the sensing unit has almost the same structure as
that of the first invention except that it has no shield, so that the
photosensitive elements Da, Db receive external light. As shown in FIG. 8,
either light emitting unit of the optical means may include a plurality of
elements Ea, Eb or the photosensitive unit may include a plurality of
elements Da, Db. One of the light emitting (or photosensitive) elements,
Ea (or Da) is disposed closer to the heart than the other Eb (or Db). As
shown in FIG. 7, the plurality of elements of the optical means are
operated in a time-divisional manner and have a phase where the respective
light emissions are zero. The optical signals received during the zero
intervals are stored, by capacitors Ca, Cb, etc. A subtraction means
(differential amplifiers A31, A32) subtracts the stored signals from the
signal obtained during the light emission. Comparing means Cp compares the
outputs from two or more optical means (Ea, Da), (Eb, Db) positioned on
the heart side and on the peripheral side. The highest blood pressure is
determined when the output Ss from the optical means (Eb, Db) positioned
on the peripheral side appears while the lowest blood pressure is
determined at a time when the output Sd from the comparing means Cp
disappears.
A third embodiment is an additional embodiment to the first and second
embodiments. Usually, the waveforms obtained from the two different
positions do not coincide because of factors due to the human body as well
as due to the device used. For example, the sensitivities of the light
emitting diodes (as the light emitting elements) and the phototransistors
(as the photosensitive elements) vary. The third embodiment is intended to
attain a better coincidence of the waveform outputs from the two positions
in order to determine the lowest blood pressure. As shown in FIGS. 9 and
10, one of the signals Sa', Sb' of FIGS. 4 and 8 is applied to a
variable-gain amplifier A4. First, when a pressure value, for example, of
30 mm Hg, which is obviously lower than the lowest blood pressure is
attained, a close-loop control circuit is temporarily operated to control
the gain of the amplifier such that both the signals Sa', Sb' are equal in
order to permit a highly accurate measurement of the lowest blood
pressure.
A fourth embodiment is also a non-invasive manometer. As shown in FIG. 12,
this device is attached to a finger F or the like. At least one of the
light emitting units Ea and the photosensitive unit Da of the optical
means includes at least one element. The optical means operates in a
time-divisional manner. All the light emitting units have a rest or zero
phase. The output from the photosensitive element Da during the rest
interval (or the zero interval) of the light emitting element Ea is
stored. The difference between the stored output and the output signal
from the photosensitive element Da during a light emitting interval is
obtained in order to eliminate the influence of external light.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a conventional signal processor used for
measurement of an average blood pressure;
FIG. 2 illustrates sphygmomanometry;
FIG. 3 is a cross-sectional view of a part of a sensor of the
sphygmomanometer according to this invention;
FIG. 4 is a circuit diagram of a signal processor used in the method shown
in FIG. 3;
FIGS. 5 and 6 illustrate a blood vessel and blood flow for determining the
highest and lowest blood pressures;
FIG. 7 illustrates the phase relationship of light emission and extinction
by the light emitting elements;
FIG. 8 is a circuit diagram of a signal processor showing another
embodiment of this invention;
FIG. 9 is a circuit diagram of an additional circuit for improvement to
this invention;
FIG. 10 illustrates the operation of the circuit of FIG. 9;
FIG. 11 is a circuit diagram of a signal processor showing another
embodiment of this invention;
FIG. 12 is a circuit diagram showing another embodiment of the
sphygmomanometer according to this invention; and
FIG. 13 illustrates the measurement of respective signals in FIG. 12.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 3 is a partially cross-sectional view showing the structure of a
sensor in a first and second embodiment of the invention. Reference
character F denotes a finger, the left half of which is shown in a
cross-sectional view. Reference character A denotes an artery within the
finger. A bladder B applies air pressure to the finger. A cuff C limits
the expansion of the bladder. A pressure changing device S controls the
air pressure within the bladder. Light emitting elements Ea, Eb emit, for
example, infrared rays. Photosensitive elements Da, Db are, for example, a
phototransistor. In FIG. 3, light emitting element Ea is disposed opposite
to the photosensitive element Da so that light from element Ea reaches the
photosensitive element through the finger (including the artery). For
example, if the bladder B is opaque, they are disposed between the finger
and the bladder. If the bladder is transparent, they may be disposed on
the inner surface of the bladder or between the bladder and the cuff C.
The light emitting element Eb and the photosensitive element Db are
similarly disposed opposite to each other. It is to be noted that the
elements Ea and Da are disposed closer to the heart rather than at the
center of the bladder B, while the elements, Eb and Db are disposed on the
peripheral side rather than at the center of the bladder. The quantity of
light transmitted from the light emitting element Ea to the photosensitive
element Da is influenced by a change in the volume of the artery (blood
vessel) A due to a change in the internal blood pressure. Similarly, the
photosensitive element Db outputs a signal representing the change in the
volume of the artery due to the light Lb from the light emitting element
Eb. In this case, if the light emitting elements and the photosensitive
elements receive external light components, an error may be involved in
the measurement. In order to avoid this, the first invention protects the
optical means from external light using a shielding means SH. For example,
a pair of gloves may be used. The second invention electrically eliminates
external light without using the shielding means SH.
First, the first embodiment will be described as follows:
Light emitting elements Ea, Eb emit light alternately using a two-phase
clock so that their emission intervals do not overlap. The photosensitive
elements Da, Db are turned on synchronously with the light emission clock
to receive the light La, Lb from the corresponding opposing light emitting
elements Ea, Eb. A clock frequency of 60 or as high as the maximum
expected pulsation rate is used as the light emitting on-off frequency in
order to obtain a correct pulsation waveform.
The operation of the first embodiment will now be described using the
circuit of FIG. 4.
First, the light emitting element Ea emits light at a phase P1 via a switch
SW11. The photosensitive element Da is shielded from external light, so
that it receives only light La from the light emitting element Ea. The
light La is changed by the photosensitive element Da into a voltage signal
which is then amplified by an amplifier A1 into a signal Sa.
Similarly, at a phase P2, light emitting element Eb emits light Lb which is
received by the photosensitive element Db and changed into a voltage
signal Sb. These signal voltages Sa, Sb are detected by detectors D1, D2,
which eliminate the clock components as the carriers and which output
signals Sa', Sb' corresponding to changes in the volume of the blood
vessel. Comparator Cp compares signals Sa', Sb' to provide a difference
signal Sd. When the signals Sa', Sb' coincide in waveform and amplitude,
the comparator produces a zero output.
The determination of the highest and lowest blood pressures SYSTOLIC and
DIASTOLIC will be described with respect to FIGS. 5 and 6.
First, the bladder B is pressurized temporarily higher than the highest
blood pressure by pressure changing means S. The bladder is then
depressurized gradually by pressure changing means S. At this time, the
volume of the blood vessel changes are shown in FIG. 5(a), 5(b), 5(c) and
the signals Sa', Sb' change, as shown in FIG. 6. Namely, when the bladder
pressure is higher than the highest blood pressure, the blood vessel
remains closed and no blood flows (see FIG. 5(a)). Therefore, the volume
of the blood vessel on the peripheral side does not change and no change
due to light Lb occurs, while the volume of the blood vessel on the heart
side does change and hence light La changes. Namely, no signal Sb' is
produced but signal Sa' is produced (see the left portion of FIG. 6).
When the bladder pressure decreases slightly below the highest blood
pressure, the blood vessel opens only during the interval in which the
internal pressure is higher than the external pressure to produce a flow
of blood (see FIG. 5(b)). This causes a quantity of light Lb to change to
thereby produce a signal Sb' (see SYSTOLIC in FIG. 6).
When the bladder pressure decreases below the lowest blood pressure, there
is no interval in which the blood vessel closes (see FIG. 5(c)), so that
changes for the light La and Lb are equal and hence signals Sa', Sb' are
equal. Comparator Cp compares signals Sa', Sb' to output the difference
Sd, so that the difference becomes zero at the lowest blood pressure (see
DIASTOLIC at the lower part of FIG. 6).
In this way, the highest blood pressure can be determined at a point where
signal Sb' appears while the lowest blood pressure can be determined from
a point where the difference signal Sd between signals Sa' and Sb'
disappears.
Now, the second embodiment of the invention will be described as follows:
The sensor of FIG. 3 has quite the same structure as that of the first
embodiment but is not externally shielded by shield means SH. Therefore,
the photosensitive elements Da, Db receive external light. The light
emitting elements Ea, Eb emit light by a multi-phase clock so that their
respective light emitting intervals do not overlap. The photosensitive
elements Da, Db are switched ON or OFF simultaneously with the light
emission clock. As shown in FIG. 7, light emitting element Ea emits light
at phase P1, light emitting elements Eb emit light at phase P2 and none of
the elements Ea, Eb emit light at phase P3. A clock frequency of sixty or
as high as the highest expected pulsation rate is used as the light
emitting on-off frequency in order to reproduce a correct pulsation
waveform. At phase P3, where neither element Ea or Eb emit light, the
photosensitive elements Da, Db receive external light Lc. The peripheral
light (lamp light or solar light) illuminates the finger F and is
scattered at each end of the cuff C by finger F and arrives at the
photosensitive elements Da, Db. This light is designated Lc.
The operation of the second embodiment will now be described with reference
to FIG. 8. The signal obtained by photosensitive element Da is amplified
by amplifier Al and delivered via switch SW12 to a differential amplifier
A31. The operation of switches SW11, SW12 and differential amplifier A31
will be described. At phase P1, light emitting element Ea emits light via
switch SW11. Photosensitive element Da receives light La from light
emitting element Ea as well as external light Lc. Namely, it receives
light La+Lc at phase P1. This light is changed into a voltage signal which
is amplified by amplifier A1 to a signal Sa+Sc.
At phase P2, similarly, light emitting element Eb emits light and
photosensitive element Db receives light Lb+Lc which is changed into a
voltage signal Sb+Sc. At phase P3, element Db receives only light Lc and
the signal Sc is connected via switches SW31, SW32 to non-inverting inputs
of the two amplifiers A31, A32. The signal voltages are held on the
non-inverting input sides even after completion of phase P3 by capacitors
Ca, Cb which are connected at the non-inverting inputs. At phase P1, the
signal Sa +Sc is connected to non-inverting input of amplifier A31 and the
signal Sc is connected via switch SW31 to the non-inverting input of
amplifier A31. Therefore, the differential amplifier A31 outputs the
difference between the two inputs, namely, Sc-(Sa+Sc)=-Sa. Similarly, at
phase P2, the differential amplifier A32 outputs a signal Sc-(Sb+Sc)=-Sb.
As described above, the differential amplifiers A31, A32 output signal
components free from signal Sc due to external light. The signal voltages
Sa, Sb are detected by detectors D1, D2. Namely, the carrier, including a
clock component, is eliminated from the signal voltages Sa, Sb to provide
signals Sa', Sb' corresponding to the changes in the volume of the blood
vessel. Comparator Cp, which is similar to that in the first embodiment,
signals Sa' and Sb' in order to output the difference signal Sd. When both
signals Sa' and Sb' coincide in waveform and amplitude, the output of the
comparator Cp is zero.
In the above arrangement, the determination of the highest blood pressure
SYSTOLIC and the lowest blood pressure DIASTOLIC is the same as in the
first embodiment and a description therefore will be omitted.
According to the above method, the lowest blood pressure can be highly
accurately measured. However, this method may be influenced by variations
in the elements used or caused by a living body whose lowest blood
pressure is to be measured. A third embodiment will be described in which,
the waveform outputs from two positions coincide in order to determine the
lowest blood pressure by elimination the influence caused by a living
body, etc.
FIG. 9 shows the circuit diagram of the third embodiment. FIG. 10
illustrates the operation of the circuit of FIG. 9. In FIGS. 9 and 10,
signals Sa', Sb' correspond to a change in the volume of the blood vessel
in FIG. 1. Either one of the signals Sa', Sb' is input via a variable-gain
amplifier A4 to comparator Cp of FIG. 4. The gain of amplifier A4 is
adjusted so that both signals are equal in amplitude. Namely, a
closed-loop control circuit is temporarily operated in which the output
from comparator Cp is delivered via amplifier A5 to the variable-gain
amplifier A4.
The operation of the closed-loop control is performed when a pressure value
Pc (for example, 30 mmHg) is attained which is obviously lower than the
lowest blood pressure at the beginning of pressurization and
depressurization of the sphygmomanometer, as shown in FIG. 10.
Pressurization is first stopped for a time t to maintain a constant
pressure.
The reason for maintaining a constant pressure is that the signals Sa', Sb'
must be first equalized below the lowest pressure. Namely, the output from
comparator Cp must be zero. However, if signals Sa', Sb' are not equal,
the output from comparator Cp will not be zero, in which case the output
from comparator is applied to the variable-gain amplifier A4 and the gain
of the amplifier is controlled so that the output from the comparator will
be zero. This controlled value is held until the measurement ends by
storing the value, for example, in a capacitor Cc.
As soon as the controlled value is stored, pressurization is again
performed to a value higher than the highest blood pressure and then a
measurement is taken as mentioned above. Instead of the variable-gain
amplifier, for example, an amplifier may be used which has a plurality of
resistors and a switch to select one of the resistors to determine a
desired gain. The switch may be controlled by a digital signal. This
system can absorb any disruptive factors caused by the living body or by
the device, for example, variations in the sensitivity of the LEDs and
phototransistors.
The concept of equalizing outputs from two sets of optical systems may be
realized by other means, one example of which will be described below.
As shown in FIG. 11, part of an analog means may be replaced with a digital
means. The optical means used is the same as those described with
reference to FIGS. 4 and 8, but without the differential amplifiers A31,
A32; detector circuits D1, D2 and comparator Cp. Instead, the functions of
these circuits are performed digitally.
More particularly, the respective outputs of the two sets of optical means
are converted by an analog to digital (A/D) converter Ct and are stored as
a digital variable in a memory M. Of course, the outputs from the optical
means are stored separately at a phase P where no light is emitted from
the light emitting elements. Differential amplifiers A31, A32 function to
eliminate the influence of external light by subtracting a signal at phase
P3 from a signal at phase Pl. This similarly occurs at phase P2. The
respective signals are stored as a digital quantity. A central processing
unit CPU determines the difference by performing a digital subtraction.
Detectors D1,D2 function to extract frequency components of the pulsation
wave by eliminating the clock component (the carrier). The A/D convertor
is operated synchronously with a clock signal, and the clock components
are automatically eliminated. Also, comparator Cp may be implemented as a
digital device.
This system saves several analog parts but requires many memory elements.
A fourth embodiment will now be described with reference to FIG. 12 which
is an example of a circuit used where an average blood pressure is
measured using a set of optical means. The manometer used includes a
sensor T which measures the blood pressure in a non-invasive manner. The
sensor is attached to an appendage such as a finger F, an earlobe or the
like. A pressure changing unit S changes the pressure applied to the
finger, the earlobe or the like. An optical means senses a change in the
diameter or volume of the blood vessel due to the applied pressure and the
internal pressure in the blood vessel A. The optical means includes a
light emitting element Ea and photosensitive element Da. Use of one pair
of light emitting elements Ea and photosensitive element Da of the optical
means is not important. At least one of the light emitting unit Ea and the
photosensitive unit Da may include a plurality of elements. The optical
means is driven by a two-phase clock generator P in a time-divisional
manner and has a phase where all the light emitting elements stop
emission. Reference character La denotes light from light emitting diode
Ea through the finger to the corresponding photosensitive element Da.
Reference numeral Lb denotes external light entering the finger and
arriving at photosensitive element Da. Reference characters S1, S2, S3
denote electric switches in which switches S1, S2 are controlled by a
clock .phi.1 from the two-phase clock generator P while switch S3 is
controlled by a clock .phi.2. Clocks .phi.1, .phi.2 are alternating
outputs from the two-phase clock generator P. Reference character C
denotes a capacitor which holds a voltage whether switch S3 is OFF or ON.
Differential amplifier A2 outputs a signal corresponding to the difference
between the two input signals.
When switches S1, S2 are turned on, a power source E lights the light
emitting element Ea. At this time, photosensitive element Da receives the
sum of light La from light emitting element Ea and external light Lb to
output a signal which is amplified by amplifier Al and output via a filter
F1 as a signal Sa+Sb to one terminal of differential amplifier A2. When
switch S3 is then turned ON, photosensitive element Da receives only
external light Lb and outputs a signal Sb to the other terminal of
differential amplifier A2. Thus, differential amplifier A2 outputs the
difference between the two inputs, namely, SO=(Sa+Sb)-Sb=Sa which includes
a pulsation wave component free from external light Lb component. FIG. 13
shows the waveform of these signals in which the clock output .phi.1 is
roughly depicted. The clock frequency is in fact, higher than the
pulsation frequency. For example, the clock frequency may be 100 times
higher than the pulsation frequency so that the original waveform can be
reproduced by filter F1.
In the conventional manometer which uses an optical system with one set of
elements, it is difficult to obtain the value of the lowest blood
pressure. Therefore, the conventional manometer calculated the highest
blood pressure as well as the average blood pressure. In order to obtain
the average blood pressure with higher accuracy, the optical means must be
disposed near the center of the bladder. In the present invention, this
means obtaining an intermediate signal between signals Sa' and Sb'
corresponding to a change in the volume of the blood vessel, so that
obviously the highest blood pressure will be measured as a value higher
than the actual highest pressure. Since the distance between the two sets
of optical means is short, there are few differences in physical
characteristics between the blood vessels and between the skin portions
and also few differences in the phase between the signals. Since the same
pressure is applied to both of the optical systems, the outputs from the
optical systems highly accurately coincide at the lowest blood pressure.
The time-divisional light emission serves to decrease power consumption.
Providing a phase P3 where no light emitting elements emit light and
prolonging the interval of phase P3 serve to further decrease the power
consumption.
As described above in detail, a manometer according to this invention is
capable of measuring the highest blood pressure with higher accuracy than
the prior art in which a set of optical means is provided at the center
portion where pressure is applied. In the present invention, distance
between two sets of optical means is short, which permits measurement of
the lowest blood pressure with higher accuracy than the method in which
sensors are disposed at distant positions. Turning on the light emitting
elements in a time-divisional manner eliminates the entrance of external
light and eliminates mutual interference between the two sets of optical
means, and reduces power consumption.
From the foregoing description of the preferred embodiments of the
invention, it will be apparent that many modifications may be made
therein. It should be understood that these embodiments are intended as
one example of the invention only, and that the invention is not limited,
thereto. Therefore, it should be understood that the appended claims are
intended to cover all modifications that fall within the true spirit and
scope of the invention.
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