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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pulse spectrometer, and particularly to
a pulse spectrometer which irradiates the measurement region of an in vivo
tissue with light and conducts spectroscopic analysis for noninvasively
determining the concentration of blood constituents.
2. Description of the Prior Art
In conventional noninvasive measurement of in vivo tissue blood
constituents by near infrared spectrometry the measured value tends to
fluctuate with the amplitude of the pulsation at the measurement region.
The measurement therefore has to be conducted over a long period for
averaging out the effects of the pulsation.
When in vivo tissue is irradiated with light, the transmission of the light
through the tissue is accompanied by light absorption by the blood and
light absorption by the tissue. In noninvasive measurement of blood
component concentration, the measurement accuracy is degraded by the
tissue light absorption.
Water has a particularly large effect on measurement accuracy because it
makes up 50-60% of body tissue and has a major absorption peak at
wavelengths in the vicinity of 980 nm, which is the wavelength region in
which the absorption peaks of most of the blood constituents to be
measured fall.
These circumstances make it necessary to eliminate the effect of the tissue
so as to increase the relative amount of light absorbed by the blood
constituents. For eliminating the effect of the tissue, it is advantageous
to extract the amount of change in light absorbance of the blood caused by
pulsation. Moreover, the fact that concentration measurement requires an
absolute quantity of blood makes it essential to measure the amount of
absorbance.
The pulse oximeter was developed on the basis of this knowledge to
positively utilize blood pulsation for obtaining spectroscopic information
from blood in an in vivo tissue. The pulse oximeter measures the oxygen
saturation degree of blood hemoglobin by using the change in absorbance
caused by pulsation as a signal source and measuring the amount of change
at each of two wavelengths during each segment of a subdivided pulse
period. Since this method can determine the oxygen saturation degree by a
comparison between two frequencies, the calculation uses only the change
in the amount of absorption by the blood.
As this signal processing method ignores the total amount of absorption,
however, it cannot measure the hemoglobin concentration of the blood and
other such concentrations that relate to the absolute value of the light
absorbance. It is therefore not appropriate for analyzing blood component
concentration.
Japanese Patent Laid-open Publication No. Hei 4(1982)-60650 teaches another
device that utilizes pulsation by using a blood pressure and blood oxygen
saturation degree measurement technique based on what is known as the
volumetric vibration method. This device applies cuff pressure to the
measurement region for reading the light absorbances of the tissue, vein
and artery layers in various combinations and then calculates the oxygen
saturation degrees of the arterial blood and veinal blood by determining
the difference in light absorbance of the combinations at two wavelengths,
and further measures the blood pressure from the cuff pressure at the
change points of the pulse amplitude.
Regarding the application of cuff pressure, Japanese Patent Laid-open
Publication No. Hei 5(1993)-503856 teaches a method of determining
arterial blood oxygen saturation degree and arterial blood pressure in
which the light absorbance of the arterial blood is measured under a cuff
pressure approximately equal to the arterial pressure so as ascertain the
change in light absorbance with pulsation at maximum amplitude.
The prior art devices for analyzing in vivo tissue blood constituents by
near infrared spectrometry utilize only measured change in the arterial
signal and difference in light absorbance. As a result, they are capable
of determining only the blood oxygen saturation degree and do not employ
any means or method for utilizing the light absorbance value or other
blood component concentration data.
SUMMARY OF THE INVENTION
An object of this invention is to overcome the foregoing drawbacks of the
prior art by providing a pulse spectrometer capable of accurately
measuring light absorbance data related to blood constituents and using
the measured data for analyzing blood component concentration.
In accordance with the invention, the above object is achieved by a pulse
spectrometer which noninvasively determines the concentration of blood
constituents in in vivo tissue by irradiating a measurement region of the
in vivo tissue with light and spectroscopically analyzing light from the
in vivo tissue. The pulse spectrometer comprises a light source for
irradiating an in vivo tissue measurement region with light, fixing means
for fixing the measurement region by applying pressure thereon,
spectroscopic means for separating light from the measurement region into
its spectral components, a first light-receiving element for receiving the
spectrally separated light, a second light-receiving element for receiving
scattered light from the vicinity of the measurement region, means for
optimally controlling the pressure applied by the fixing means based on a
signal obtained from the second light-receiving element, and signal
component separation means for, based on the signal from the second
light-receiving element, separating out only the blood flow variation
component from among the components of the signal from the first
light-receiving element.
In this arrangement, the separation of only the blood flow variation
component from among the components of the signal from the first
light-receiving element based on the signal obtained from the second
light-receiving element makes it possible to measure signals corresponding
to the systole and diastole that occur synchronously with the
pulsation-induced variation in blood quantity, extract only data relating
to the blood from the in vivo tissue data, and quantitatively determine
various blood component quantities from the shape of the light spectrum.
Moreover, by determining the difference in spectrum between the systole
and diastole it becomes possible to directly calculate the light
absorbance spectrum of the blood without need for a reference light for
light source spectrum correction.
BRIEF DESCRIPTION OF THE DRAWINGS
The purposes and features of the present invention will become more
apparent from a consideration of the following detailed description taken
in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram for illustrating the arrangement of a pulse
spectrometer of the invention;
FIG. 2 is a block diagram showing the arrangement of a pulse detection
means of the pulse spectrometer of FIG. 1;
FIG. 3 is a graph showing the time-course fluctuation of the
logarithmically converted value of a signal obtained from light
transmitted through in vivo tissue;
FIG. 4 is a graph showing a blood component absorption spectrum measured by
an embodiment of the invention; and
FIG. 5 is a graph showing how pulse amplitude varies with cuff pressure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention will now be described in detail on the basis of the preferred
embodiment illustrated in the drawings. The embodiment described in the
following is an application of the invention to the measurement of a light
spectrum carrying information regarding the component concentration of
blood in a human finger.
A block diagram of the pulse spectrometer according to the invention is
shown in FIG. 1. The pulse spectrometer has a light source 1 comprising a
halogen lamp, a power supply for driving the light source 1, a mirror for
selectively reflecting near infrared light of a wavelength of 600-1100 nm,
and a condenser lens for converging light on a measurement region. The
mirror provided in the light source 1 is for protecting the measurement
region from being burned by heat waves emitted by the halogen lamp and for
removing stray light in the spectroscope. It is possible to replace the
mirror with one or more filters arranged to selectively transmit the same
wavelength band. (The wavelength band is not limited to that mentioned
above and can be changed as desired in accordance with the absorption
wavelengths of the blood constituents to be measured.)
The subject to be tested inserts his or her finger f upward (as seen in
FIG. 1) into a probe 2 having windows for light passage on the left, right
and bottom. The finger tip (the measurement region) ft is positioned at
the windows. The fixing of the finger f by the probe and the detection of
pulses will be explained later.
Light emitted by the light source 1 and entering the probe 2 from the right
side in FIG. 1 irradiates the measurement region ft, passes through the
finger tissue, exits the left side of the probe 2 and enters a
spectroscope 6.
The spectroscope 6, which is a monochromator comprising a diffraction
grating and a convex mirror, spectrally separates the light passing
through the measurement region into individual wavelengths in the 600-1100
nm wavelength range. The light spectrum is detected by a photodiode array
7 and, under the control of a CPU (central processing unit) 8, is sampled
by a sampling circuit 14, is converted to a digital signal by an A/D
converter 10, and is stored in a buffer memory 11. The CPU 8 issues
commands for sequentially reading the data stored in the buffer memory 11.
The structure of the probe 2 and the arrangement of the pulse detection
means are shown in the block diagram of Figure 2. Light scattered within
the tissue and passing through a window 2c at the bottom of the probe 2
(under the finger tip ft) is detected by a photodiode 21 for use in pulse
monitoring.
The photoelectric current output by the photodiode 21 is converted to
voltage by a photoelectric current-voltage converter 21a, amplified by an
amplifier 22, passed through a band-pass filter 23 for removing the signal
dc component and high-frequency noise, amplified by an amplifier 25,
converted to a digital signal by an A/D converter 24, and input to the CPU
8. The band-pass filter 23 is used for removing the dc component of the
signal so as to enable pulse extraction and for removing power source
noise and other high-frequency noise components.
As it monitors the pulses detected via the photodiode 21, the CPU 8
determines the maximum and minimum values during each pulse cycle and
synchronously with the occurrence of each of the values sends a signal to
the sampling circuit 14 for sampling a signal from the photodiode array 7.
Furthermore, as it monitors the pulses, the CPU 8 prevents the pulse
amplitude from falling to zero by controlling a pump 26 and a solenoid
valve 27 through a drive circuit 3 so as to regulate the pressure in a
balloon 2b. The regulated pressure value is maintained throughout the
measurement for fixing the finger f.
The pressure of the balloon 2b is set to the optimum value for stabilizing
the finger without hindering the flow of arterial or veinal blood at the
measurement region and for stabilizing the pulse amplitude over time.
Although the present embodiment assumes the pressure value to be 20 mm Hg,
this is not limiting and the pressure is regulated to the optimum value
for each measurement.
As will be understood from the foregoing, in the illustrated embodiment,
the measurement region is fixed by the balloon during signal integration
and the balloon pressure is controlled for obtaining constant pulse
amplitude.
Although the finger tissue transmits light of wavelengths in the near
infrared band to some degree, a large part of the light is scattered
during passage. As a result, the light directed onto the measurement
region disperses equally in all directions from the point of incidence, so
that light leaves the finger similarly in all directions. In addition, the
amount of light leaving the tissue varies as the amount of light absorbed
by the blood changes with the variation in the amount of blood present in
the tissue at the measurement region that is caused by the pulsation of
the heart.
As the light transmitting through the measurement region is being picked up
by the spectroscope 6, the photodiode 21 simultaneously picks up light
from another direction for measuring the change in blood quantity due to
heart pulsation and the signal is processed synchronously with the
pulsation. Specifically, as will be explained in more detail later, the
maximum and minimum values of the intensity of the light received by the
spectroscope 6 during a single pulse cycle are detected and the light
spectrum is produced at the time of each detected value.
For enhancing the accuracy of the measurement by this method, it is
important for the pulses to be detected at a steady and strong amplitude.
Although the earlier mentioned Japanese Patent Laid-open Publication No.
Hei 5(1993)-503856 teaches that pulse strength can be effectively
increased by applying a cuff pressure substantially equal to the arterial
pressure, for realizing the purpose of the present invention it is even
more important for the pulses to be steady, the measurement region to be
fixed and the length of the optical path to be constant.
In this invention, therefore, the approach adopted for fixing the
measurement region and stabilizing the pulse amplitude is to facilitate
pulse amplitude control not by use of a cuff pressure equal to the
arterial pressure but by use of a cuff pressure at a point that is
linearly correlated with the pulse amplitude. Thus, it was found that
measurement is preferably conducted while, as shown in FIG. 5, the cuff
pressure is controlled not at a point at which the pulse amplitude becomes
maximum, but at a region at which the linearity between the cuff pressure
and the pulse amplitude is maintained, that is, at which the amplitude
becomes about 2/3 of the maximum amplitude.
FIG. 3 shows the pulse component measured in this state as expressed in
terms of light absorbance. The absorption by the in vivo measurement
subject is attributable to that by the tissue, that by arterial blood and
that by veinal blood. Since the absorptions by the tissue and the veinal
blood are affected only slightly by the pulsation, they are steady over
time. On the other hand, the absorption by the arterial blood varies
greatly with pulsation and, as can be seen in FIG. 3, fluctuates
periodically. What is required is the concentration of the blood
constituents, particularly the concentration of the physiologically
important arterial blood constituents.
It is therefore necessary to ascertain the absorption by the arterial blood
only, independently of the effects of the tissue and veinal blood. The
method used for this purpose by the illustrated embodiment is to determine
the difference between the maximum and minimum values of the light
absorbance spectrum synchronously with the pulses detected via the
photodiode 21. Since the change in light absorbance with pulsation
reflects the rise and fall in the amount of arterial blood, the method of
determining the difference between the maximum and minimum values
synchronously with the pulsation is an effective way of extracting
information relating solely to the arterial blood.
Specifically, the analysis is conducted by subjecting the minimum and
maximum value signals to divisional operation and logarithmic conversion
at each wavelength. If required, moreover, the accuracy is enhanced by
integrating the division-subjected signals over multiple periods.
When, as in this invention, a method for determining the difference
spectrum is adopted, the measurement can be conducted without need for the
reference light required by ordinary spectroscopes. More specifically,
defining the light source spectrum as I0 (.lambda.) and the spectrum at
the maximum pulse value as Imax (.lambda.), the light absorbance Abs max
can be expressed as
Abs max=-log (Imax (.lambda.)/IO (.lambda.)) (1)
Similarly, defining the spectrum at the minimum pulse value as Imin
(.lambda.), it follows that
Abs min=-log (Imin (.lambda.)/IO (.lambda.)) (2)
Therefore, since the difference spectrum (1)-(2) is
Abs max-Abs min=-log (Imax (.lambda.)/Imin (.lambda.)) (3)
the light absorbance can be obtained without measurement of a reference
light.
FIG. 4 shows light absorbance spectral data measured by the invention. The
spectral data of this figure was obtained by sampling the outputs of the
photodiode array 7 at a sampling interval of 100 ms, dividing the spectrum
at the maximum value by the spectrum at the minimum value in one cycle of
pulse, integrating the divisional values over multiple cycles, and
subjecting them to logarithmic conversion in the CPU 8, thus defining the
light absorbance. In the illustrated embodiment data consistency is
enhanced by integration over 100 cycles. The number of integration cycles
does not necessarily have to be 100, however, but can be selected as
desired in view of the light quantity and the required measurement
accuracy.
This invention does not particularly specify the measurement region. While
the illustrated embodiment uses a finger as the measurement region, it is
alternatively possible to use the ear lobe or any other portion of the
body which permits detection of light passing therethrough.
Being equipped with a measurement region fixing means and a pulse detection
means, the embodiment described in the foregoing detects a pulse signal
from the measurement region, detects the maximum and minimum values of the
pulse signal during each cycle thereof, and based on the so-obtained
information determines the difference between the spectra of the light
passing through the measurement region. Owing to this arrangement, the
pulse spectrometer is able to eliminate information related to the tissue
of the measurement region, extract only information related to the blood
at the measurement region, and quantitatively determine various blood
component quantities from the shape of the light spectrum. Moreover, by
determining the difference spectrum between the systole and diastole it
becomes possible to directly calculate the light absorbance spectrum of
the blood without need for a reference light for light source spectrum
correction.
In addition, since fluctuation in the transmitted light quantity is
suppressed by regulating the fixing pressure for the measurement region
synchronously with the detected pulsation, the absorption spectrum
produced by the blood constituents can be independently extracted. This is
particularly advantageous in the case of measuring glucose and other blood
constituents that produce minute signals since it enables the accuracy of
the measurement to be enhanced by minimizing the effect of water. The
measured spectral data can thus be used for noninvasively determining not
only the oxygen saturation degree but also various components contained in
the blood.
As set out in the foregoing, the invention provides a high-performance
pulse spectrometer which noninvasively determines the concentration of
blood constituents in in vivo tissue by irradiating a measurement region
of the in vivo tissue with light and spectroscopically analyzing light
from the in vivo tissue. For this, the pulse spectrometer according to the
invention comprises a light source for irradiating an in vivo tissue
measurement region with light, fixing means for fixing the measurement
region by applying pressure thereon, spectroscopic means for separating
light from the measurement region into its spectral components, a first
light-receiving element for detecting the spectrally separated light, a
second light-receiving element for detecting scattered light from the
vicinity of the measurement region, means for optimally controlling the
pressure applied by the fixing means based on a signal obtained from the
second light-receiving element, and signal component separation means for,
based on the signal from the second light-receiving element, separating
out only the blood flow variation component from among the components of
the signal from the first light-receiving element.
Thus, since the pulse spectrometer is able to separate the blood flow
variation component from among the components of the signal from the first
light-receiving element based on the signal obtained from the second
light-receiving element, it is able to measure signals corresponding to
the systole and diastole occurring synchronously with the
pulsation-induced variation in blood quantity, independently extract only
data relating to the blood from the in vivo tissue data, and
quantitatively determine various blood component quantities from the shape
of the light spectrum. Moreover, by determining the difference spectrum
between the systole and diastole, the pulse spectrometer is able to
directly calculate the light absorbance spectrum of the blood without need
for a reference light for light source spectrum correction.
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