JPH0386152A - Oxymeter - Google Patents

Abstract

PURPOSE:To heighten measuring accuracy by finding the change quantity of a pulse wave signal from a signal in which only a fluctuation component is taken out and amplified from the pulse wave signal. CONSTITUTION:The measurement of the saturation of oxygen in an arterial blood is performed by irradiating a measured part with light of two wavelength R and IR, and measuring the light transmitting or reflected on the measured part at a time phi1 and a time phi2 a little later, and finding the change quantity between the time of a measured value at every wavelength, and computing the saturation of the oxygen from the change quantity or the measured value at the time phi1. Therefore, the change quantity per hour of the pulse wave signal can be found from the signal from which a DC signal component is eliminated, and since only the required component of the signal can be amplified by eliminating a DC component even without increasing the dynamic range of a circuit unnecessarily, amplification can be performed sufficiently, and the S/N of the circuit to find a difference can be improved, and also, by finding the change quantity of the pulse wave signal from that of an amplified AC component, the number of digits of the change quantity used in computation can be expanded, which improves the S/N of the measured value as the whole of computation.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a device or oximeter for measuring the oxygen saturation of blood.

[Conventional technology]

An oximeter is a device that shines light onto a specific location on the human body, such as a fingertip, and measures the transmitted or reflected light from that location, and calculates and records the oxygen saturation in arterial blood from the measured value. To measure oxygen saturation using such an absorbance method,
Normally, logarithmic operations are required, and using a logarithmic conversion element,
The applicant of the present invention proposed a method that does not require logarithmic calculation because measurement accuracy cannot be obtained. First, the principle of the conventional method will be explained. Broadly speaking, the measuring parts of the human body can be divided into layer A, which is a collection of tissues other than blood such as bones and skin, layer B, which is venous blood, and layer 0, which is arterial blood. The 0 layer is further divided into a B layer of deoxyhemoglobin and a Ho layer of oxyhemoglobin. The thickness of each layer is Xa, Xb, X
If c=Xh+Xho, the oxygen saturation S is: S=Xho/Xc=Xho/(Xh+Xho) (1
) is required. Now, let the intensity of the light irradiated to the measurement site be Io, the intensity of the transmitted light be I, and the absorption coefficient of each layer be A.
If a, Ab, Ah, Aho, I = 1
. , e-(Aa-J', L+4-λ'h+Arλ
``1+Ata・A) ,,---(2.The absorption coefficients Aa, Ab and thicknesses Xa, Xb of layers A and B do not change over time, so the exponent part of Aa・Xa+Ab −Xb is summarized. If we replace it with the constant K, the above equation (2) becomes I=K, 1.., -(At・
If we set Xh+Xho=d in equation (1), then Xho=S -d, Xh=
d (1-3), therefore, equation (3) is 1 = 1o, eAt0-';) + Ate#5) (1
,------. Since d is a component that changes over time, d( is expressed as n, the transmitted light intensity at time XIJ to is I, and time t
If the transmitted light intensity from o to At r& is Io, then 1'
=x+(dI/dt)At, Bi = ■ r
AA(/-8”)+Ata-3jpJt], Mut---
--('51, therefore, (I-I)/1 = [Ah-(1-3)+Aho-3 d' (t
)×Δt ・・・・・・・・・・・・(
6). Since the two numbers S and d' (t) in equation (6) are unknown, S can be determined by performing measurement using two types of light having different wavelengths. For example, as two types of light to be used, one is a wavelength λ1 where the absorption coefficients Ah and Aho of both oxidized and reduced hemoglobin are equal, and the other is a wavelength λ2 where both absorption coefficients are different. For example, when measuring at the wavelength λ1, Equation (6) is as follows. [(I-xo)/Ilλ1 =Ah” ・d' (t) ・At・・・・・・・・・
...(7)
What does the value mean? ) By transforming the above equation (7), d' (t) At can be obtained. Substituting this into equation (6), [(I-I')
/I λ 2/[(I -I')/I
] λ 1= [Ah ・ (1-3) 10
Aho -S ko /Ah' Therefore, [(I-1')/I]λ2/[(I -I')/I
] If λ1=P, 5= (Ah' -P-Ah)/ (Aho-Ah)-43
), Ah', Aho, and Ah are known coefficients, so it is possible to find S using P. That is, the wavelength λ1. By using two types of light of λ2 and measuring the time variation difference (r-I') of the transmitted light and the transmitted light intensity (2), the oxygen saturation level can be determined. In the above explanation of the principle, the oxygen saturation S is expressed in the form of a linear function of the variable P, but in actual living blood, the scattering effect of blood cells can be calculated by
IQlffi nt=I=K, ■o, e
-A4' t "4a]de - (31' may also be expressed. Therefore, in this case, the oxygen saturation S will be expressed in the form of a quadratic function of the variable P. However, as described above Even when using an equation that takes into account the scattering effect of The configuration diagram of the device is shown.The oximeter uses two types of light sources: a red light source 1 that has a high absorption rate for oxyhemoglobin, and an infrared light source that has the same absorption rate for oxyhemoglobin and deoxyhemoglobin. A light source is used to measure the light from light sources 1 and 2 that has passed through the living body, and the pulse wave signals at the time points formed by light sources 1 and 2 are collected in a sample hold section (hereinafter referred to as S-H section) 27° 28. At the same time, memorize (hereinafter referred to as hold),
The difference (amount of change) between the pulse wave signal held in the S-8 section 28 and the pulse wave signal for the light source 2 after the above-mentioned sample and hold is determined by the differential circuit 21, and this is determined by the comparison circuit 23 to be output from the reference voltage circuit 24. Difference (amount of change) between the pulse wave signal for light source 1 and the pulse wave signal for light source 1 previously held in the S-8 section 27 at the time when the signals match.
is obtained by the differential circuit 20, the output signal is sampled and held in the S-8 section 26, and the sampled and held values are A-D converted in the S-8 section 26, 27, and 28, and the digitized values are used. Then, the calculation/control circuit 32 calculates the above formula (8
) to determine the oxygen saturation level S in arterial blood. However, in the above method, it is necessary to find the amount of change in the pulse wave signal at two points in time, and the amount of change is found directly from the pulse wave signal, but the pulse wave signal is a signal in which a small change component is superimposed on a large DC component. Since the amount of change per hour is a very small value with respect to the pulse wave signal strength, the number of effective digits of the amount of change is very small compared to the number of effective digits of the pulse wave signal measurement value. However, there was a problem that the S/N ratio was poor overall. When the amplification factor of the pulse wave signal is increased in order to improve the S/N ratio, the DC component is also amplified, so there is a problem in that the dynamic range of the circuit must be increased.

[Question H that the invention attempts to solve]

An object of the present invention is to expand the number of effective digits of the amount of change in the pulse wave signal that is required, and to improve the S/N ratio of the calculated oxygen saturation value.

[Means for Solving the Problems] A pulse wave oximeter includes at least two light emitting means that emit light with different wavelength distributions, and a light emitting means that irradiates a biological measurement site from the light emitting means and transmits or reflects light from the biological measurement site. A light receiving means that receives light and converts the received light into an electrical signal, a sample hold means A that samples and holds the output signal of the light receiving means for each light source, and a pulse wave alternating current component for each light source from the output signal of the light receiving means. sample and hold means B for each light source that samples and holds each amplified alternating current component; and a difference circuit that calculates the difference between the signal held by the means B and the subsequent alternating current component for each light source; A digital calculation means for calculating the oxygen saturation level from the output of each of the difference circuits and the output signal of the sample and hold means A was provided. In order to further narrow the dynamic range, a detection means is provided which is connected to one of the difference circuits for each light source and detects that its output matches a predetermined value, and the digital calculation means Depending on the timing of coincidence detected by the circuit, the output of the other difference circuit and the output of the sample-hold means A are AD-converted to calculate oxygen saturation. Alternatively, the sample and hold means A may be deleted, and a calculation means may be provided for calculating the oxygen saturation level from the output of the other difference circuit and the output of the light receiving means for each light source in accordance with the timing of coincidence detected by the detection circuit. Ta.

[For use]

To measure oxygen saturation in arterial blood, light of at least two wavelengths is irradiated onto the measurement site, and the light reflected or transmitted through the measurement site is measured at the same time and at a slightly later time, and the measured value is The present invention calculates the amount of change in each wavelength for each wavelength, and calculates the oxygen saturation level from the amount of change and the measured value at the time of the above formula.
The amount of change per time in the pulse wave signal is determined by the signal excluding the DC signal component.If the DC signal is removed, only the necessary components of the signal can be amplified without unnecessarily increasing the dynamic range of the circuit. As a result, sufficient amplification can be performed, improving the S/N ratio of the circuit that calculates the difference. Also, by determining the amount of change in the pulse wave signal from the amount of change in the amplified AC component, it is possible to perform calculations. By increasing the number of significant figures of the amount of change used, the S/N ratio of the measured value is improved as a whole in the calculation.

[Implementation r! A]

FIG. 1 shows an embodiment of the present invention.
is a light source that emits red light. This light source 1 performs calculations and
It lights up when the clock Φ1 (FIG. 2a) output from the control section 32 is ON. 2 is a light source that emits infrared light. This light source 2 lights up when the clock Φ2 (FIG. 2 b) output from the calculation/control unit 32 is ON.
Φ3 shown in is a clock for dark current sampling of the photometry means, and the clock Φ1. Φ2. As shown in FIG. 2 a, b, and c, Φ3 is transmitted by the arithmetic/control unit 32 at 1/3 duty (or lower duty), and Φ1 transmits the light from the light source 1, and Φ2 transmits the light from the light source 1. The light from light source 2,
Φ3 is used as a timing signal when both light source 1 and light source 2 are turned off to measure the influence of external light other than the light source (dark light), that is, the background. 3 is a light amount adjustment section, and a calculation/control section 32, the light source 2
In this embodiment, the infrared light source 2
However, the purpose of adjusting the light amount is to irradiate the measurement site with the light emitted from light source 1 and light source 2, and to adjust the amount of transmitted light or light that is transmitted or reflected from the measurement site. The objective is to ensure that the difference in intensity between two reflected lights falls within a predetermined range, thereby simplifying the configuration and operation of a subsequent processing circuit. Therefore, the light quantity control may be performed using either one or both of the two types of light sources. The light is received and output as an electrical signal. Reference numeral 5 denotes an IV converting section which converts the output current of the light receiving section 4 into a voltage signal and outputs a signal as shown in FIG. 2d. Note that the I-V converter 5
It is better to install it as close as possible to the light receiving part 4 in order to improve the resistance to external noise. 6 is a DC amplification part,
−■ A signal obtained by subtracting the output of the dark light sample and hold (S-H) section 7 (FIG. 2 e) that samples the signal using clock Φ3 from the output of the converter 5 (FIG. 2 d) (FIG. 2 f) The dark light S-H section 7 holds the output of the DC amplification section 6 during the period when both the light source 1 and the light source 2 are off.The timing of this sample and hold is determined by the arithmetic/control section. 32
8 is a DC amplification gain adjustment section which adjusts the gain of the DC amplification section 7 using a DC gain adjustment signal from the arithmetic/control section 32. Adjustment is made so that the output signal strength of the DC amplifying section 7 falls within a certain range. The signal strength of the pulse wave signal varies greatly depending on the measurement site or person.
It is necessary to adjust the signal strength so that it falls within an appropriate range, and the above adjustment is performed by adjusting the gain of the DC amplification section 7. 9 is LPF (low pass filter), D
High frequency noise is removed from the output signal of the C amplification section 6. The output signal of the DC amplifier section 7 from which high frequency noise has been removed is sent to the DC-R channel S-8 section 27 and the DC-IR3-8
The detection signals of red light and infrared light are respectively clocked Φ1. Sampling is performed when Φ2 is ON, and held when Φ2 is OFF. That is, in synchronization with the lighting timing of light source 1, the DCR channel S-8 unit 27 samples red light, and in synchronization with the lighting timing of light source 2,
The DC-IR channel S-8 section 28 performs sampling of infrared light.
Figure 3 shows the output signal of the DC-IR channel S-8 section 28. 29 is a DC-R channel LPF, 30 is a DC-IR channel LPF, and The output signals of the channel S-8 section 27 and the IR channel S/8 section 28 are subjected to low-pass filtering processing to obtain a pulse wave signal used for calculating the amount of change in the pulse wave signal. This signal is expressed by the above formula (61, (71, etc.),
Signal [I]λ1 used for denominator. [I] becomes λ2. IO is an HPF (bypass filter) that cuts low frequency noise and extracts the alternating current component of the pulse wave signal, and is the main part of the present invention. An example of an HPFIO circuit is shown in FIG. Note that the cutoff frequency switching portion is omitted in this circuit diagram. Analog switch SW1. SW2 is
1 during each clock. ON, O by clock Φ2
It will be FF. When the clock Φl is ON, that is, when the light source l is on, the operational amplifier and C
1, R1, R2, and R3 constitute a bypass filter. Similarly, when the clock Φ2 is ON, the light source 2 is in a lighting state, and in FIG.
, R2, and R3 constitute a bypass filter. That is, bypass filtering processing is performed in a time-division manner. The output signal of the HPF 10 at this time is as shown in FIG. Reference numeral 11 denotes a frequency adjustment section, which adjusts the limit frequency to be cut by the HPF 10 under the control of the calculation/control section 32. 12 is an AC amplification section, which amplifies the pulse wave signal (Fig. 4) output from ) , adjusts the 12 gains on the ACC amplification under the control of the arithmetic/control unit 32. 14 is the R (red light) channel S-H section, and 15 is the IR (
Infrared light) channel S-H section, ACC amplification upper 12
The output signals (FIG. 4) are clocked Φ1. Φ2 is ON
By sampling when OFF and holding when OFF, the R channel pulse wave signal (Figure 5a) and the IR channel pulse wave signal (Figure 5b) are separated and sampled and held.16 17 is the R channel LPF, and 17 is the IRR channel AC.
Channel S-8 section 14 and IR channel S-8 section 1
The output signal of step 5 is subjected to low-pass filtering processing to obtain the AC component of the pulse wave signal used for calculating the amount of change in the pulse wave signal. Figures 5a and 5b show the R channel LPF 16. I
An example of an output signal of the R channel LPF 17 is shown. Next, R channel LPF16. IR channel LPF
The amount of change per time is calculated from the output signal of No. 17, and in this embodiment, the amount of change in the R channel is measured when the amount of change in the IR channel reaches a predetermined value. This is because P in equation (8) or equation (9) described below has a change in infrared light in the denominator, so the calculation is simplified by keeping the amount of change in the IR channel constant. First, the operation of the timing circuit 25 that controls sampling and holding of the amount of change will be explained. The output signal (FIG. 5 d) of the IR channel differential circuit 21 is converted into an absolute value by the absolute value circuit 22, and the absolute value is compared with the output signal (reference voltage) of the reference voltage section 24 by the comparison circuit section 23, When the output of the absolute value circuit 22 and the reference voltage match, a match signal is transmitted from the comparison circuit 23.The timing circuit 25 transmits the -
Sample and hold signals tl and t2 (
Disseminate Figure 5. When a coincidence signal is issued, the sample and hold signal t1 becomes an ON (sampling) signal at first, and the R channel AC-8-8 section 18. Sent to IRR channel AC8-8 part 19, R channel AC-3
-8 part 18, IRR channel AC5-8 part 19 is L
The output signals of PF16 and 17 are sampled. The sample hold signal t1 immediately becomes an OFF (hold) signal, and the sampled signal is transferred to the comparison circuit section 23.
It is held until a match signal is output from . The sample hold signal t2 becomes 0FF (hold) due to the coincidence signal.
signal, R channel S-8 section 40. IRR channel AC8/H section 41. AC difference S-8 section 26, R channel S-8 section 40. IR channel D-C-
5.H part 41. The AC difference S-8 section 26 includes an LPF 29.
The output signals of 30 and the differential circuit 20 are A-D converted by an A-D converter 31 and sampled and held until they are sent to an arithmetic/control section 32 . FIGS. 5c and 5d show the R channel differential circuit 20. A signal output from the rR channel differential circuit 21 is shown. The calculation/control unit 32 converts the output of the AC-3/8 unit 26 into A-
D conversion result (AC-R) and R channel D
The result of A-D conversion of the output of the CS-8 section 40 (DC
-R) and the result of A-D conversion of the output of the IR channel DC-3-8 section 41 (DC-I R), calculate P using the following formula, and use the value of P to calculate the above-mentioned Arterial blood oxygen saturation is determined by equation (8) or the like. P= [(AC-R)/(DC-R)]/[K/
(DC-I R) Co......Formula (9) Here, K is the pre-calculated A of the reference voltage section 24.
−D conversion value. The calculation/control unit 32 controls each part as described above, and also calculates arterial blood oxygen saturation (SaO2).
Furthermore, the results are displayed on the display section 34 and the data output section 3
5, the data is output to an external device and stored in the data memory section 36. Reference numeral 33 denotes an operation input section for setting various setting values, such as the alarm limit value of 5a02. In the above embodiment, the DC-R channel LP
F27. The outputs of the DC-IR channel LPF 28 are sent to the R channel DC-3-8 section 40 . IR channel DC-3-8 part 4
However, the sample and hold signal t2 immediately changes the outputs of the DC-R channel LPF section 29 and the DC-I R channel LPF 30 to A-1.
If controlled to perform D conversion, the above R channel DC
-3-8 part 40. It is also possible to omit the IR channel DC-8/8 section 41. In addition, the DC-R channel LPF section 29 and the DC-IR channel LPF section 29
The output signal of 30 is actually in the form of a slight alternating current component (pulse wave signal) superimposed on a direct current component, so the output signal of DC-R channel LPF 29 and DC-IR channel LPF 30 at any given time D-conversion value or DC for a predetermined time
-R channel LPF29. DC-IR channel LP
Using the average value of the A-D conversion values of F30, the above formula (
1) may be used as DC-R or DC-IR. Next, a second embodiment of the A-D converter 31 and its surrounding parts will be described. Generally, the strength of the pulse wave signal of a living body varies greatly depending on the individual or the part to be measured. However, in the first embodiment described above, the reference voltage section 24
There is only one type of reference voltage output from the voltage source, and if the strength of the change per unit time of the pulse wave signal of the living body is extremely large or small, problems may occur. When the amount of change in the pulse wave signal per unit time is large, the comparison circuit 23 frequently outputs a coincidence signal. On the other hand, if the amount of change in the pulse wave signal per unit time is small, it will be difficult for the comparison circuit section 23 to match them, and in the worst case, they may not match at all. This problem can be solved by providing multiple reference voltages. FIG. 7 shows the configuration of the second embodiment. Note that parts unnecessary for the explanation of the second sickle example are omitted. The reference voltage unit 24 is capable of generating a plurality of reference voltages, and switching between these voltages is performed by the calculation/control unit 32. In this embodiment, four types of reference voltages are prepared. Furthermore, a new R channel AC amplifier section is added between the R channel differential circuit 2o and the AC difference S-8 section 26. This R channel AC amplification section 38 is
Four types of amplification factors can be switched corresponding to four types of reference voltages of the reference voltage section 24. That is, the AC component of the IRR channel side pulse wave signal is
If the voltage is small, the output voltage of the reference voltage unit 24 is selected to be small. At the same time, the amplification factor of the R channel AC amplifying section 38 is selected to be large. This is because, as described above, when the AC component of the pulse wave signal on the IRR channel side is small, the AC component of the pulse wave signal on the R channel side is also small. Conversely, when the AC component of the IRR channel side pulse wave signal is large, the output voltage of the reference voltage section 24 is selected to be large, and the amplification factor of the R channel AC amplifier section 38 is selected to be small. Note that in order to adjust the ratio of the DC component of the pulse wave signal of the R channel and the DC component of the pulse wave signal of the IR channel to a ratio within a predetermined range by the light amount control unit 3, the AC component of the pulse wave signal of the R channel side is The ratio of the AC component of the pulse wave signal on the IR channel side and the AC component of the pulse wave signal on the IR channel side falls within a certain range.

【effect】

According to the present invention, the difference (amount of change) in the pulse wave signal is obtained from a signal obtained by extracting only the fluctuation component from the pulse wave signal and amplifying it, so that the number of effective digits of the amount of change in the pulse wave signal can be increased. The size was appropriate compared to the number of effective digits of the wave signal, the S/N ratio was improved, and the accuracy of oxygen saturation measurement was increased.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention, and FIG. 2 is a diagram of the above-mentioned embodiment.
FIG. 3 is an output signal diagram of the DC-R (IR>channel S/H section) of the above embodiment. FIG. 4 is an output signal diagram of the HPF of the above embodiment. 5 is an output signal diagram of the R (IR) channel LPF of the above embodiment, an output signal diagram of the R (IR) channel differential circuit, and a sample hold signal diagram, and FIG. 6 is an example circuit diagram of the HPF of the above embodiment, Fig. 7 is a block diagram of a second embodiment of the A-D converter 31 and its surrounding parts, and Fig. 8 is a block diagram of a conventional example. 1... Red light source, 2... Infrared light source. Light source, 3... Light amount adjustment section, 4... Light receiving section, 5... I-V conversion section, 6
...DC amplifier section, 7...Dark light S-H section, 8...
・DC amplification gain adjustment section, 9...LPF, 1o...
HPF, 11... Frequency adjustment section, 12... AC amplification section, 13... AC amplification gain adjustment section, 14... R (
Red light) channel S/H section, 15...IR (infrared light) channel S-H section, 16...R channel LPF
, 17...IR channel LPF, 1B/R channel Ac-5-H section, 19-IR channel AC-
S-H section, 2°...R channel differential circuit, 21.
...IR channel differential circuit, 22...absolute value circuit,
23... Comparison circuit section, 24... Reference voltage section, 25.
...Timing circuit, 26...AC differential S-H section, 2
7...DC-R channel s-H section, 28-DC-I
R channel 7 channel S-H section, 29/DC-R channel LP
F, 30 ・DC-IR channel LPF, 31...
A-D conversion section, 32... calculation/control section 32, 33...
- Operation input section, 34... Display section, 35... Data output section, 36... Data memory section.

Claims (3)

[Claims] (1) At least two light emitting means that emit light with different wavelength distributions, receiving light that is irradiated from the light emitting means onto a biological measurement site, transmitted or reflected by the biological measurement site, and converting the received light into an electrical signal. A light receiving means, a sample holding means A for sampling and holding the output signal of the light receiving means for each light source, a means for extracting and amplifying only the pulse wave AC component for each light source from the output signal of the light receiving means, and a means for extracting and amplifying each pulse wave AC component for each light source from the output signal of the light receiving means; Sample and hold means B for each light source to sample and hold; a difference circuit that calculates the difference between the signal held in the means B and the subsequent alternating current component for each light source; An oximeter characterized by comprising an output signal and digital calculation means for calculating oxygen saturation. (2) The digital calculation means further includes a detection means that is connected to one of the difference circuits for each light source and detects that the output thereof matches a predetermined value.
The oxygen saturation level is calculated by AD converting the output of the other difference circuit and the output of the sample hold means A according to the timing of coincidence detected by the detection circuit. oximeter. (3) at least two light emitting means that emit light with different wavelength distributions; receiving light that is irradiated from the light emitting means onto a biological measurement site, transmitted or reflected by the biological measurement site; and converting the received light into an electrical signal; a light receiving means, a means for extracting and amplifying only the pulse wave alternating current component for each light source from the output signal of the light receiving means, a sample holding means for each light source for sampling and holding each amplified alternating current component, and a pulse wave AC component held by the sample holding means. a difference circuit that calculates the difference between the received signal and the subsequent alternating current component for each light source; a detection means that is connected to one of the difference circuits and detects that the output matches a predetermined value; An oximeter comprising a calculation means for calculating oxygen saturation based on the output signal of the light receiving means for each light source and the output signal of the other differential circuit according to the timing of the oximeter.