JP2608828B2 - Non-invasive oximeter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION This invention relates to non-invasive oximeters.

[0002]

2. Description of the Related Art As a conventional oximeter of this type, there is a pulse oximeter. Conventional pulse oximeters have measured oxygen saturation using the pulsation of biological tissues including blood.

[0003]

However, the measurement using the pulsation is difficult when the pulsation is low and an accurate result cannot be obtained, and since a slight change in the signal is used, it is affected by the body movement of the subject. Cheap.

[0004] The present invention has been made in view of such conventional drawbacks, and its object is to provide a non-observable measurement which can perform measurement independent of the level of pulsation of a subject and which is hardly affected by body movement. It is to provide a blood oximeter.

[0005]

According to a first aspect of the present invention, there is provided irradiation means for irradiating a tissue containing blood with light of three wavelengths, and photoelectric conversion means for receiving the light transmitted through the tissue and converting it into an electric signal. A storage unit for storing the irradiation light intensity of the three wavelengths of light, or a ratio of irradiation light intensity of each other, based on the contents stored by the storage unit and the transmitted light intensity obtained from the photoelectric conversion unit, The difference between the logarithm of the irradiation light intensity ratio and the logarithm of the transmitted light intensity ratio of a certain combination of two wavelengths is obtained, and the logarithm of the irradiation light intensity ratio and the transmission light intensity ratio of the other two wavelength combinations are obtained. An oxygen saturation calculation means for calculating the oxygen saturation by a relational expression between the difference between the two and the logarithm and the extinction degree by blood hemoglobin is provided.

According to a second aspect of the present invention, in the configuration of the first aspect, the irradiation light intensity ratio is obtained by transmitting light generated from the irradiation means through a light attenuation plate having a known light attenuation characteristic. The characteristic of the light attenuating plate is taken into consideration in the calculation of the light intensity ratio.

[0007]

According to the construction of the first aspect, the oxygen saturation calculating means combines a certain two wavelengths based on the contents stored in the storage means and the transmitted light intensity obtained from the photoelectric conversion means.
The difference between the logarithm of the irradiation light intensity ratio and the logarithm of the transmitted light intensity ratio is obtained, and the logarithm of the irradiation light intensity ratio of the other two wavelength combinations,
The difference between the transmitted light intensity ratio and the logarithm is obtained, and the oxygen saturation is calculated from the relational expression between the ratio of the two and the extinction degree of blood hemoglobin.

In the structure of claim 2, the irradiation light intensity of the light of three wavelengths is obtained by measuring the light transmitted through the light attenuating plate whose light attenuation characteristic is known. For this reason, the intensity of light that is weaker than that measured directly is measured. Therefore, the photoelectric conversion means used for the measurement may be the same as the photoelectric conversion means for converting the light transmitted through the tissue into an electric signal, that is, one photoelectric conversion means can measure both the irradiation light intensity and the transmitted light intensity of the tissue. It can be carried out.

[0009]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. First, the principle of the present embodiment will be described.

The three wavelengths of light used in the measurement are λ ₁ = 805 nm λ ₂ = 890 nm λ ₃ = 650 nm, and each has the following characteristics. λ ₁ : Hemoglobin extinction coefficient is independent of oxygen saturation. λ ₂ : The extinction coefficient of hemoglobin has little relation to oxygen saturation. λ ₃ : The extinction coefficient of hemoglobin significantly changes depending on the oxygen saturation. Suffixes 1, 2, and 3 added to symbols used hereinafter indicate light of these wavelengths.

The theory is that the extinction degrees A ₁ , A ₂ , and A ₃ of light of three wavelengths transmitted through living tissues including blood are as follows if incident light is an appropriate scattered light. Theory) and experiments. A ₁ Ξlog (Ie ₁ / It ₁ ) = {Eh ₁ Hb (Eh ₁ Hb + G)} ^1/2 Db + ZtDt (1) A ₂ Ξlog (Ie ₂ / It ₂ ) = {Eh ₂ Hb (Eh ₂ Hb + G)} ^{1 / 2 Db + ZtDt (2)} a 3 Ξ log (Ie 3 / It 3) = {Eh 3 Hb (E h 3 Hb + G)} 1/2 Db + ZtDt (3) where, Ie: irradiation light intensity (energy per unit area ) It: transmitted light intensity (energy per unit area) Eh: extinction coefficient of hemoglobin Hb: concentration of hemoglobin in blood G: scattering constant of blood Db: effective thickness of blood layer Zt: tissue excluding blood (pure) The light extinction ratio (referred to as “tissue”) Ξ The light extinction / thickness is constant at any of the above wavelengths. Dt: thickness of pure structure. Note that Ξ indicates the equal sign of the identity (the same applies below).

The above-mentioned "scattered light suitable as irradiation light" is as follows.

When parallel incident light enters a scatterer, it is scattered as it enters the scatterer, and reaches a degree of scattering peculiar to the scatterer at a deep place above a certain level. Therefore, in the case of direct light irradiation, the extinction ratio differs between the shallow part and the deep part of the scatterer.

For example, if the irradiation light is scattered by a scattering plate having the same scattering property as that of the living tissue, the extinction ratio becomes the same over the whole living tissue.

Therefore, when such a scattering plate is arranged on the surface of the light emitting diode, the above-mentioned "scattering light suitable as irradiation light" is obtained.
Is obtained.

Here, the ratio 減 of the dimming degree between the respective wavelengths is defined as follows. ΨΞ (A ₃ −A ₂ ) / (A ₁ −A ₂ ) (4) Expanding equation (4) from equations (1), (2) and (3) gives the following. ΨΞ (A ₃ −A ₂ ) / (A ₁ −A ₂ ) = log [(Ie ₃ / It ₃ ) / (Ie ₂ / It ₂ )] / log (Ie ₁ / It ₁ ) / (Ie ₂ / It) _{2)] = {log (Ie} 3 / Ie 2) -log (It 3 / It 2)} / {log (Ie 1 / Ie 2) -log (It 1 / It 2)} (4a) formula (4a) Among them, Ie ₃ / Ie ₂ and Ie ₁ / Ie ₂ are irradiation light intensity ratios.

The irradiation light intensity itself is greatly affected by measurement conditions such as the distance between the light source and the light receiving unit, but the irradiation light intensity ratio, which is the light intensity ratio between the three wavelengths, is not affected by the above measurement conditions. .

There are the following methods for obtaining the irradiation light intensity ratio.

In order to directly measure the irradiation light, the illuminance is so high that it may not be possible to measure the transmitted light with a light-receiving unit that receives the transmitted light. Therefore, for example, the irradiation light is transmitted through a light attenuating plate having a uniform dimming degree with respect to the light in the wavelength range to be used, and the irradiation light intensity ratio among the three wavelengths of the transmitted light is calculated.

Thus, the irradiation light intensity ratio Ie ₃ / I
If e ₂ and Ie ₁ / Ie ₂ are obtained, transmitted light intensities It ₁ and Ie
By measuring t ₂ and It ₃ , Ψ can be easily calculated by the equation (4a).

On the other hand, Ψ can also be written as follows based on equations (1), (2) and (3). Ψ = [｛Eh ₃ Hb (Eh ₃ Hb + G)｝ ^1/2 − ｛Eh ₂ Hb (Eh ₂ Hb + G)｝ ^1/2 ] / [｛Eh ₁ Hb (Eh ₁ Hb + G)｝ ^1/2 − ｛Eh ₂ Hb (Eh ₂ Hb + G)} ^1/2 ] (5)

If the hemoglobin concentration is not abnormally high,
That is, when Hb <20 [g / dL], G = FHb
(F: a constant, which will be referred to as a scattering rate).

Thus, equation (5) becomes as follows. Ψ = [{Eh ₃ (Eh ₃ + F)} ^1/2 − {Eh ₂ (Eh ₂ + F)} ^1/2 ] / [{Eh ₁ (Eh ₁ + F)} ^1/2 − {Eh ₂ (Eh ₂ + F)} ^1/2 ] (6)

The following equation is established from the absorption characteristics of hemoglobin with respect to the light wavelength. Eh ₁ = Eo ₁ (7) Eh ₂ = about Eo ₂ (8) Eh ₃ = SEo ₃ + (1-S) Er ₃ = S (Eo ₃ −Er ₃ ) + Er ₃ = Er ₃ −SΔE ₃ (9) S: oxygen saturation Eo: extinction coefficient of oxyhemoglobin Er: extinction coefficient of reduced hemoglobin ΔE ₃ Ξ Er ₃ -Eo ₃

The expression (6) is changed from the expressions (7), (8) and (9) into the following expression. Ψ = [{(Er ₃ −SΔE ₃ ) ((Er ₃ −SΔE ₃ ) + F)} ^1/2 − {Eo ₂ (Eo ₂ + F)} ^1/2 ] / {(Eo ₁ (Eo ₁ + F) ^{1 / 2} )-(Eo ₂ (Eo ₂ + F))｝ ^1/2 (10)

Here, {Eo ₁ (Eo ₁ + F)} ^1/2 {E
b ₁ , {Eo ₂ (Eo ₂ + F)} ^1/2と Eb ₂ , equation (10) gives: Ψ = [{(Er ₃ −SΔE ₃ ) (Er ₃ −SΔE ₃ + F)} ^1/2 − Eb ₂ ] / (Eb ₁ −Eb ₂ ) (11) Equation (11) is solved for the oxygen saturation S to obtain the value. S = ｛− B ± (B ² -4AC) ^1/2 ｝ / 2A (12)

Here, A, B, and C are as follows, respectively. AΞΔE ₃ ² (ΔE ₃ = Er ₃ -Eo ₃ ) BΞ-ΔE ₃ (2Er ₃ + F) CEr ₃ (Er ₃ + F)-[Ψ (Z ₁ -Z ₂ ) + Z ₂ ] ² Thus, the oxygen saturation Can be calculated.

Next, a description will be given based on the apparatus shown in FIG.

The three-wavelength light source 1 has wavelengths of 805 nm, 890 nm,
It has three light emitting diodes that emit light of 650 nm each, and a scattering plate 2 that transmits the light of the light emitting diodes. The scattering plate 2 has a scattering property close to a living tissue to be measured.

The light receiving section 3 is a circuit provided at a predetermined distance from the three-wavelength light source 1 and converts light from the three-wavelength light source 1 into an electric signal. The amplifier 4 is a circuit that amplifies an electric signal output from the light receiving unit 3. The A / D converter 6 is a circuit that converts a signal output from the amplifier 4 into a digital signal. The three-wavelength measurement control circuit 7 has three wavelengths of the three-wavelength light source 1.
It is a circuit that outputs a signal for causing one light emitting diode to sequentially emit light at a predetermined timing. This signal is simultaneously sent to the amplifier 4, the A / D converter 6, the irradiation intensity calculation / storage circuit 9, and the oxygen saturation calculation circuit 10.

The changeover switch 8 is a switch for switching the output of the A / D converter 6 to one of the irradiation light intensity calculation / storage circuit 9 and the oxygen saturation calculation circuit 10 for application. The calibration / measurement switching control circuit 11 is a circuit that controls switching of the changeover switch 8.

The display device 12 and the recorder 13 respectively display and record the calculation result of the oxygen saturation calculation circuit 10.

Here, the irradiation light intensity calculation / storage circuit 9 stores A /
This circuit performs a predetermined operation using the output of the D converter 6 and stores the result. The oxygen saturation calculation circuit 10 has an A /
This is a circuit for performing a predetermined calculation based on the output of the D converter 6 and the content stored in the irradiation light intensity calculation / storage circuit 9 and outputting the result to the display device 12 and the recorder 13.

Next, the operation of the present embodiment will be described.

First, the operator places the light attenuation plate 15 between the three-wavelength light source 1 and the light receiving section 3. Here, the operator operates the calibration / measurement switching control circuit 11 to switch the changeover switch 8.
Is switched so that the A / D converter 6 and the irradiation light intensity calculation / storage circuit 9 are connected. Next, the operator causes the three-wavelength measurement control circuit 7 to start control. Three-wavelength light source 1, irradiation light intensity calculation / storage means 9, oxygen saturation calculation circuit 10 and A
The / D converter 6 is controlled by a control signal from the three-wavelength measurement control circuit 7. That is, the three-wavelength light source 1 has the wavelength λ ₁ (805
nm), λ ₂ (890 nm) and λ ₃ (650 nm) at predetermined intervals. These lights pass through the light attenuating plate 15 and reach the light receiving unit 3, where they are converted into electric signals. The amplifier 4, the A / D converter 6, and the irradiation light intensity calculation / storage circuit 9 operate in synchronization with the lighting timing of the three-wavelength light source. At this time, the signals given to the irradiation light intensity calculation / storage means 9 correspond to Ie ₁ , Ie ₂ , and Ie ₃ in the equation (4a). The irradiation light intensity calculation / storage means 9 stores Ie ₁ , Ie
Memorize e ₂ and Ie ₃ .

Next, the operator takes out the light attenuation plate 15, and
Instead, a living tissue 17 (a finger, an earlobe, or the like) to be measured is disposed between the three-wavelength light source 1 and the light receiving unit 3. Here, the operator operates the calibration / measurement switching control circuit 11 to switch the changeover switch 8 so that the A / D converter 6 and the oxygen saturation calculation circuit 10 are connected. Next, the operator causes the three-wavelength measurement control circuit 7 to start control. The signals corresponding to It ₁ , It ₂ and It ₃ in the equation (4a) are output from the A / D converter 6 in the same manner as the irradiation light intensity measurement using the light attenuating plate 15 described above. Is output to The oxygen saturation calculation circuit 10 also calculates the irradiation light intensity ratios Ie ₃ / Ie ₂ and Ie ₁ / Ie ₂ based on the output of the irradiation light intensity calculation and storage circuit 9. Then, the transmitted light intensity ratio and the irradiation light intensity ratio are substituted into Expression (4a) to obtain （. Next, the oxygen saturation calculation circuit 10 calculates the oxygen saturation S by substituting the obtained Ψ into the equation (12) (Ψ is included in C in the equation (12)). The oxygen saturation S thus obtained is displayed on the display device 12 and recorded by the recorder 13.

According to this embodiment, since the light attenuating plate 15 is used for measuring the intensity of the irradiation light, the measurement can be performed by the same device as the device for measuring the intensity of light transmitted through the living tissue 17. It is desirable that the light attenuating plate 15 has the same degree of scattering for each wavelength of the light source, but if there is a difference in the degree of scattering, the difference is known, and the difference is calculated by the means for calculating the irradiation light intensity ratio. It suffices if a means for correction is included. In this example, the light of three wavelengths is irradiated in order. However, the light of three wavelengths is simultaneously irradiated on the light attenuating plate or the living tissue, and the respective transmitted light is simultaneously received and photoelectrically converted. Immediately Ie ₃ / Ie
₂ , Ie ₂ / Ie ₁ , It ₃ / It ₂ , It ₁ / It ₂
May be obtained, and these may be stored and used in the calculation of the equation (4a).

Further, according to the present embodiment, since the light emitted from the three-wavelength light source 1 is transmitted through the scattering plate, the extinction ratio does not differ between the shallow part and the deep part of the living tissue.

Further, according to the present embodiment, since the ratio of the irradiation light intensities is used, it is less affected by the measurement conditions. Further, according to the present embodiment, the irradiation intensity can be measured and stored in a timely manner, so that it is possible to prevent the light emitting section from being affected by aging or contamination.

In this embodiment, an approximation is used for Eh ₂ as shown in equation (8), but this does not cause an error when the oxygen saturation S = 1 (100%). Although the error increases as the oxygen saturation decreases, the effect is small where the oxygen saturation is low because the allowable error is large.

Equation (8) is an equation in the case where approximation is performed, and this is set as follows without approximation. Eh ₂ = SEo ₂ + (1-S) Er ₂ = S (Eo ₂ −Er ₂ ) + Er ₂ = Er ₂ −SΔE ₂ (8a)

By substituting equation (8a) and equations (7) and (9) into equation (6), the following equation is obtained. Ψ = [(Er ₃ −SΔE ₃ ) (Er ₃ −SΔE ₃ + F)] ^1/2 − [｛(Er ₂ −SΔE ₂ ) · (Er ₂ −SΔE ₂ + F)] ^1/2 / ｛Eo ₁ ( Eo ₁ + F)｝ ^1/2 -[(Er ₂ −SΔE ₂ ) (Er ₂ −SΔE ₂ + F)] ^1/2 (13)

Therefore, instead of the oxygen saturation calculation circuit 10, a numerical value that gradually decreases in increments of, for example, 0.01 from 1 is substituted for S on the right side of the equation (13), and the calculated value on the right side of each is compared with Ψ on the left side. , Ψ, a circuit for outputting the value of S at that time to the display device 12 and the recorder 13 may be provided.

In the above embodiments, the circuits for performing calculations and controls are independent circuits, but these calculations and controls may be performed by a computer. 1, the irradiation light intensity calculation / storage circuit 9, the oxygen saturation calculation circuit 10,
An apparatus in which the portion 16 including the calibration / measurement switching control circuit 11, the three-wavelength measurement control circuit 7 and the changeover switch 8 is replaced with a microcomputer will be described.

In this case, the microcomputer is shown in FIG.
It has the program of the flowchart shown in FIG.
The operation will be described below based on this flowchart.

First, the microcomputer waits until it enters the calibration mode (FIG. 2, step 101). The operator inserts the light attenuating plate 15 between the three-wavelength light source 1 and the light receiving section 3, and switches to the calibration mode by the input means. The microcomputer has a three-wavelength light source 1, an amplifier 4 and an A / D converter 6
To obtain Ie ₁ ′, Ie ₂ ′, Ie ₃ ′ (transmitted light It ₁ , It ₂ , It ₃ ) of the light attenuating plate, and store them in the memory (step 102). Next, the microcomputer converts Ie ₁ ′, Ie ₂ ′, Ie ₃ ′ to Ie ₃ ′ / Ie ₂ ′.
(= Ie ₃ / Ie ₂ ), Ie ₁ ′ / Ie ₂ ′ (= Ie ₁
/ Ie ₂ ) is calculated and stored in the memory (step 103). Next, the operator takes out the light attenuating plate 15 from between the three-wavelength light source 1 and the light receiving unit 3, inserts the living tissue 17 to be measured instead, and switches to the measurement mode by the input means. At this time, the microcomputer is waiting for switching to the measurement mode (FIG. 3, step 104), and when it is in the measurement mode, it controls the three-wavelength light source 1, the amplifier 4 and the A / D converter 6 to make It ₁ , It ₂ , It ₃ and store them in the memory (step 105). Next, the microcomputer stores the Ie ₃ / Ie ₂ , Ie already stored in the memory.
Substituting e ₁ / Ie ₂ , It ₁ , It ₂ , It ₃ into equation (4a) to obtain Ψ (step 106), substituting this Ψ into equation (12) to obtain S (step 107), This S is displayed on the display device 12
To be recorded on the recording device 13 (step
108).

[0047]

According to the present invention, it is not necessary to measure the pulsation of transmitted light due to the pulsation of arterial blood in living tissue. Therefore, even a subject with low pulsation can obtain accurate measurement results. Also, since a large signal can be used, it is possible to perform a measurement that is not easily affected by body movement. Further, according to the present invention, it is possible to measure the overall oxygen saturation including not only arterial blood but also venous blood. By comparing this value with the value of the oxygen saturation of arterial blood, it also serves as an index indicating the lack of oxygen supply to the tissue. This is highly valuable as an index showing the special supply and demand of oxygen in the brain.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an overall configuration of an embodiment of the present invention.

FIG. 2 is a flowchart for explaining the operation of another embodiment of the present invention.

FIG. 3 is a flowchart for explaining the operation of another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 3 wavelength light source 3 Light receiving part 6 A / D conversion circuit 9 Irradiation light intensity calculation / storage circuit 10 Oxygen saturation calculation circuit 11 Calibration / measurement switching control circuit 7 3 wavelength measurement control circuit 12 Display device 13 Recorder 15 Optical attenuation plate

Claims (2)

(57) [Claims] 1. An irradiation unit for irradiating a tissue containing blood with light of three wavelengths, a photoelectric conversion unit for receiving light transmitted through the tissue and converting it into an electric signal, and an irradiation light intensity of the light of three wavelengths. Or a storage unit for storing the irradiation light intensity ratio between them, and an irradiation light intensity of a combination of two wavelengths based on the contents stored in the storage unit and the transmitted light intensity obtained from the photoelectric conversion unit. The difference between the logarithm of the ratio and the logarithm of the transmitted light intensity ratio is calculated, and the difference between the logarithm of the irradiation light intensity ratio and the logarithm of the transmitted light intensity ratio of the other two wavelength combinations is calculated, and the ratio of the two is calculated. And an oxygen saturation calculating means for calculating oxygen saturation based on a relational expression between the degree of extinction due to blood hemoglobin and the degree of extinction due to blood hemoglobin. 2. An irradiation light intensity ratio is calculated based on the light intensity obtained by transmitting light generated from the irradiation means through a light attenuation plate having a known light attenuation characteristic and the characteristics of the light attenuation plate. 2. The non-invasive oximeter according to claim 1, further comprising irradiation light intensity ratio calculating means for storing the light intensity ratio in the storage means.