JPH0698881A - Pulse oximeter - Google Patents

Abstract

(57) [Abstract] [Purpose] Even when extraneous light including frequency components is mixed in the photodiode of the probe, the effect of this optical noise is effectively removed, and accurate oxygen saturation can be measured. [Structure] Light-emitting diodes 3 and 5 for respectively irradiating a living tissue 6 including arterial blood flow with red light and infrared light, and irradiation of infrared light after intermittent irradiation of red light at a predetermined frequency n times The light emitting diode 3, so that two lights are irradiated to the living tissue 6 at the timing of intermittently n times at the same frequency.
5, a control circuit 1 for controlling 5, a photodiode 7 for detecting outputs of red light and infrared light after being absorbed by the living tissue 6, an amplifier 8 for amplifying a light reception signal of the photodiode 7, and this amplifier A low-frequency removing circuit 9 for removing low-frequency components from the output signal of 8 at a predetermined low-frequency cutoff frequency;
The ratio Φ of the pulsating components of the absorbance due to the arterial blood flow for the two wavelengths is calculated from the signal obtained by detecting the output signal of the low-frequency elimination circuit 9, and the oxygen saturation S of the arterial blood is calculated from the obtained ratio Φ of the absorbance. And an arithmetic circuit 1 for calculating

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention utilizes non-invasive continuous measurement of oxygen saturation of arterial blood of a subject by utilizing the difference in absorption characteristics of two different wavelengths of red light and infrared light in a living body. The present invention relates to a pulse oximeter, and more particularly to a pulse oximeter capable of effectively removing the influence of noise caused by external light.

[0002]

2. Description of the Related Art A pulse oximeter has been conventionally used for non-invasive continuous measurement of oxygen saturation of arterial blood. In this pulse oximeter, a probe is attached to a fingertip or an earlobe of a subject, and light of different wavelengths of red and infrared is radiated from a probe to a living body in a time-sharing manner to obtain transmitted or reflected light of two different wavelengths Ratio of pulsating component of absorbance Φ
Is to measure the oxygen saturation S. A reference wavelength of, for example, 660 nm is used for red light, and a wavelength of, for example, 940 nm is used for infrared light. Two light emitting diodes emitting these wavelengths and one photodiode for receiving light are built in the probe. Has been done. Now, assuming that the pulsation component of the absorbance of the red light wavelength is ΔA1 and the pulsation component of the absorbance of the infrared light wavelength is ΔA2, different 2
The wavelength absorption ratio Φ is given by the following equation. Φ = ΔA1 / ΔA2 The oxygen saturation S can be calculated as a function f of this absorbance ratio Φ. S = f (Φ)

By the way, in such a pulse oximeter, when external light from indoor lighting or the like enters the photodiode of the probe during measurement, a noise component due to optical noise is mixed into the measurement signal, and oxygen saturation is accurately achieved. The degree S cannot be measured. Therefore, various attempts have conventionally been made to remove the influence of noise due to such external light. For example, in the pulse oximeter shown in FIG.
The voltage shift circuit 42 is used to reduce the influence of noise due to this external light. The configuration and operation of this pulse oximeter will be described below. In this figure,
The light emitting diode 3 built in the probe is a diode for emitting red light, and the light emitting diode 5 is a diode for emitting infrared light. A DC power supply Vc is connected to the light emitting diodes 3 and 5, and driving transistors 2 and 4 are connected in series. A control signal K1 for red light emission shown in FIG. 5A is output to the base of the transistor 2 from the arithmetic / control circuit 41 composed of a microcomputer and the like, and infrared light emission shown in FIG. A control signal K2 for is output to the base of the transistor 4. The control signals K1 and K2 are, for example, 250H
has a frequency of z and these control signals K1, K2
Thus, the transistors 2 and 4 are alternately turned on and off, so that the light emitting diodes 3 and 4 emit light alternately in a cycle of 2 msec. The red light and infrared light emitted from this probe enter the living tissue 6 of the subject including arterial blood flow, and the transmitted light or reflected light after being absorbed by this living tissue 6 is provided in the probe. It is detected by the photodiode 7 of the light receiving element.

The received light output of the photodiode 7 is amplified by the differential amplifier 8 to obtain an amplified signal K4 shown in FIG. 5 (d). In the figure, R is a red light signal, and IR is an infrared light signal. This signal K4 is input to the voltage shift circuit 42 at the next stage. The voltage shift circuit 42 includes a capacitor 43 connected in series to the output terminal of the differential amplifier 8.
And a semiconductor switch 44 connected between the output terminal of the capacitor 43 and the ground. The semiconductor switch 44 has an on / off control signal K from the control circuit 41 only when both the transistors 2 and 4 are off, that is, while the light emitting diodes 3 and 5 are both off.
3 (FIG. 5 (c)), the light emitting diode 3 or 5 is turned off while the light emitting diode 3 or 5 is emitting light. Therefore, the light emitting diode 3,
When both 5 are turned off and the switch 44 is turned on and short-circuited to the ground, the output of the voltage shift circuit 42 is 0V.
And when the light emitting diode 3 or 5 emits light,
Due to the action of the voltage shifting capacitor 43, it is possible to extract the received light voltage with the output of the differential amplifier 8 at the moment when the semiconductor switch 44 is switched from ON to OFF as a reference level. By this operation, the noise component due to the external light can be removed.

An output terminal of the voltage shift circuit 42 is connected to an input terminal of an amplifier 29 for detecting a red light reception output or an amplifier 32 for detecting an infrared light reception output through a signal changeover switch 26 composed of an analog switch or the like. . The signal changeover switch 26 is changed over so that the amplifier 29 is selected by the changeover control signal K4 from the arithmetic / control circuit 41 when the red light emitting diode 3 is emitting light, and the infrared light emitting diode 5 emits light. If so, the amplifier 32 is switched by the switching control signal K4. It is released when the light emitting diodes 3 and 5 are off. Capacitors 45 and 46 for holding the voltage when the signal changeover switch 26 does not select the amplifier 29 or 32 (at the time of release) are connected to the input terminals of these amplifiers 29 and 32, respectively. The detection output signal of each amplifier 29, 32 becomes a voltage signal indicating the absorbance of the living tissue 6 of red light or infrared light,
This signal is input to the A / D converters 34 and 36 via the low pass filters (LPF) 33 and 35, respectively. Each detection signal converted into a digital signal is taken into the arithmetic / control circuit 41, and the oxygen saturation S is calculated.

[0006]

In the above-described conventional pulse oximeter, when a constant external light such as natural light enters the photodiode 7 in the probe as optical noise, it is as shown in FIG. 6 (a). The noise component EN superimposed on the output of the differential amplifier 8 becomes DC, and the voltage shift circuit 42
By shifting the output voltage of the amplifier 8 to the ground side (0 V side) by the noise amount EN, noise can be easily removed. FIG. 6B shows the output waveform of the voltage shift circuit 42 after removing the DC noise component EN, which is superimposed on the output voltage component ER for red light and the output voltage component EIR for infrared light. It can be seen that the noise components have been removed cleanly.

When the external light is not natural light but is emitted from indoor lighting such as a fluorescent lamp, such illuminating light contains various frequency components having a commercial AC frequency as a fundamental wave. Therefore, when the optical noise includes such a frequency component, the voltage shift circuit 42 cannot easily remove the noise. FIG. 7A shows an output voltage waveform of the differential amplifier 8 on which the noise EAC of such an AC component is superimposed. Since the voltage shift circuit 42 performs voltage shift with reference to the output voltage of the amplifier 8 at the moment when the semiconductor switch 44 is switched from ON to OFF,
In this case, the voltage shift circuit 42 outputs a voltage waveform as shown in FIG. 7B which differs depending on the gradient of the noise component EAC. As shown in this output waveform, when the optical noise has an upward slope, an error is mixed in such a manner that the output voltage component R due to the red light and the output voltage component IR due to the infrared light are added with the noise component N1. Therefore, when the optical noise has a downward gradient, an error is mixed in by subtracting the noise component N2. When the external light contains a frequency component as described above, the voltage shift circuit 42 alone cannot sufficiently remove noise, and an error that differs depending on the amplitude state of noise is mixed in the measurement signal. S
Was difficult to measure.

Therefore, it is conceivable to shorten the light emission cycle of red light and infrared light emitted alternately as much as possible. By doing so, the voltage change of the optical noise in one cycle included in the output signal of the amplifier 8 becomes small and approaches DC, and the error after the noise component is removed by the voltage shift circuit 42 is sufficiently small. it can. However, since pulsed noise is also emitted from the fluorescent lamp, if such noise components P1 and P2 jump into the boundary time at which light is switched off to light emission as shown in FIG. As shown in FIG. 7D, the received light output signal output from 42 is accompanied by a very large error amount, and the accurate oxygen saturation S cannot be measured. N3 and N4 indicate signal voltages buried as errors.

As described above, in the voltage shift circuit 42, it is difficult to remove the noise component due to the external light including the frequency component, but by setting the emission frequency to a frequency that is less likely to be affected by the noise due to the external light, the noise is reduced. A method of removing it can be considered. The frequency component of the noise light emitted from the fluorescent lamp contains harmonics that are an integral multiple of the commercial AC frequency, and if the commercial frequency is 50 Hz, an integer multiple of the noise frequency exists. Therefore, assuming that the red and infrared emission frequencies for alternately emitting light are set to fHz, the frequency of noise carried on the signal after detection is (f−n · 50) Hz. Now, when the light emission frequency is set to f = 575 Hz, the minimum frequency of optical noise is 25 Hz, and this noise frequency exceeds the maximum frequency of the pulsation due to the arterial blood flow to be detected, and the detected signal is passed through the low-pass filter 3
This noise component can be removed by passing it through 3, 35. However, this commercial AC frequency is nominally 50 Hz and 60 Hz in each country, but in many countries this commercial frequency has an error, so even if it is set to the emission frequency calculated from the nominal commercial frequency, It is difficult to effectively remove the noise component due to optical noise. If the pulse oximeter is a battery-operated device, the commercial AC power supply is not drawn into the device, so the commercial frequency used cannot be detected, and the pulse oximeter is affected by noise. You will not be able to set a light emission frequency that does not exist. Therefore, when such a noise removing method is adopted, the optical noise cannot be removed by the battery-powered pulse oximeter unless the commercial frequency used in advance is known.

The present invention has been proposed in order to solve the problems of the conventional technique, and even when external light including a frequency component is mixed in the photodiode of the probe, this optical noise is removed. An object of the present invention is to provide a pulse oximeter capable of effectively performing the above.

[0011]

By the way, in the conventional pulse oximeter, after a low-pass removing circuit is connected to the next stage of the differential amplifier 8 and noise from external light is removed by this low-pass removing circuit. Another possible method is to input a signal to the voltage shift circuit 42. By this method, the noise from the external light can be removed to some extent, and by increasing the cutoff frequency of the low-frequency removing circuit, the noise is directed to the removal direction. However, regardless of the received light output signal (see FIG. 8A) extracted from the differential amplifier 8, when the cutoff frequency is increased, the infrared light side signal becomes a red light side signal with a small voltage. 8 (c) because the output waveform of the low frequency removing circuit (see FIG. 8 (b)) is greatly distorted because it is pulled in.
As shown in, the separation of the voltage signal R reflecting red light and the voltage signal IR reflecting infrared light is not successful. This is due to the following reasons. In the conventional pulse oximeter, the light emitting diodes 3 and 5 for red light and infrared light are alternately emitted at a cycle of 2 msec, and the differential amplifier 8 outputs 50 as shown in FIG.
A signal K4 equivalent to that obtained by AM-modulating the carrier of 0 Hz with the signal A2 of 250 Hz is extracted. The power spectrum distribution of the output signal of the differential amplifier 8 at this time is shown in FIG. As is clear from this figure, the information contained in the received light output signals of red light and infrared light after being absorbed by the living tissue 6 is distributed in the occupied band width W2 of 250 Hz to 750 Hz. Therefore, in order to obtain a sufficient degree of separation between the voltage signal reflecting the red light and the voltage signal reflecting the infrared light, the cut-off frequency of the low-frequency elimination circuit cannot be increased to 250 Hz or higher, and the conventional pulse oxidizer cannot be used. The meter cannot sufficiently remove the noise.

Therefore, in the pulse oximeter according to the present invention, first and second light sources for respectively irradiating living tissue including arterial blood flow with two wavelengths of red light and infrared light, and a living body of red light. Irradiation and blocking of tissue at a predetermined frequency
Red light and red light are continuously generated by repeating the first timing for continuously performing the irradiation once and the second timing for continuously irradiating and blocking the infrared light to the biological tissue n times at the predetermined frequency. Control means for controlling the first and second light sources so that the external light is irradiated to the living tissue, and the living tissue for the two wavelengths of red light and infrared light emitted from the first and second light sources. A light receiving element that detects the light output after being absorbed, an amplifier that amplifies the light receiving signal obtained from this light receiving element, and a low band removal that removes low frequency components from the output signal of this amplifier at a predetermined low frequency cutoff frequency. Circuit and the signal after detecting the output signal of this low frequency elimination circuit, the ratio of the pulsating component of the absorbance due to arterial blood flow for two wavelengths is calculated, and the oxygen saturation of arterial blood is calculated from the obtained ratio of the absorbance. Performance It is constituted to have a means. Here, the first and second light sources can be composed of a red light emitting diode and an infrared light emitting diode, and a control means for controlling these light sources and a calculation means for calculating the oxygen saturation are calculation / control circuits including a microcomputer or the like. Can be configured by

[0013]

According to the above-mentioned configuration, the red light is turned on and off n times at the predetermined frequency fo, and then the infrared light is turned on and off n times at the predetermined frequency fo, and the two wavelengths of the biological tissue are emitted. Since light can be emitted, a signal equivalent to that obtained by AM-modulating the carrier having the frequency fo at the frequency fo / n including the information about the absorbance for two wavelengths can be extracted from the light receiving element. As a result, it is possible to narrow a significant frequency band (occupied band width) including the information on the absorbance centering on the carrier frequency fo, and as a result, the cut-off frequency of the low-frequency elimination circuit can be increased, and It is possible to effectively remove the noise of external light having a mixed frequency component.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the pulse oximeter according to the present invention will be described below in detail with reference to the drawings. An example of this pulse oximeter is shown in the block diagram of FIG. In the description, the same parts as those in the conventional example are designated by the same reference numerals, and part of the description of the overlapping parts will be omitted. In this figure, the red light emitting diode 3 and the infrared light emitting diode 5 are shown.
Is controlled to turn on and off based on the control signal S1 for emitting red light shown in FIG. 2A and the control signal S2 for emitting infrared light shown in FIG. 2B from the arithmetic / control circuit 1. Red light and infrared light from the light emitting diodes 3 and 5 are applied to the living tissue 6 including arterial blood flow. Here, after the red light emitting diode 3 repeatedly emits light n times (n is an integer of 2 or more) and turns off during the period T1 (first timing period), the infrared light emitting diode 5 next period T2 ( In the second timing period), light emission and light extinction are repeated n times, and these light emitting diodes 3 and 5 continuously emit light by repeating these timings. Note that T1 = T2 is set. The red light and the infrared light emitted toward the living tissue 6 are oxygen saturation S of the arterial blood flow contained in the living tissue 6.
After being attenuated by the light absorption characteristic peculiar to the wavelength, the transmitted light or the reflected light is received by the photodiode 7. In this embodiment, transmitted light of red light and infrared light is detected.

The received light output of the photodiode 7 is amplified by the differential amplifier 8 and then input to the low frequency band removing circuit 9. FIG. 2C shows the output signal S3 of the differential amplifier 8. This low frequency elimination circuit 9 includes a capacitor 10 connected in series to the output terminal of the amplifier 8, a resistor 11 connected between the output terminal of the capacitor 10 and the ground, and a buffer amplifier connected in series to the output side. It is composed of 12 and. The low-frequency removing circuit 9 removes the low-frequency component in the received light signal by the low-frequency cutoff frequency determined by this circuit, and the output signal S4 (signal waveform of FIG. 2 (d)) is detected by the AM detection circuit 13. Sent to.

To explain the structure of the AM detection circuit 13, the output terminal of the buffer amplifier 12 is connected to the inverting input terminal of the differential amplifier 14 via the resistor 16, and the non-inverting input terminal of the differential amplifier 14 is grounded. Has been done. The output terminal of the differential amplifier 14 is connected to the anode of the diode 17 and the cathode of the diode 18. The cathode of the diode 17 is connected to the inverting input terminal of the differential amplifier 14 via the resistor 19, and is connected to the inverting input terminal of the differential amplifier 15 at the next stage via the resistor 21. Further, the cathode of the diode 17 is grounded via the resistor 21. The anode of the diode 18 is connected to the non-inverting input terminal of the differential amplifier 14 via the resistor 22, and is connected to the non-inverting input terminal of the differential amplifier 15 of the next stage via the resistor 24. Also,
The connection point between the anode and the resistors 22 and 24 is grounded via the resistor 23. The non-inverting input terminal of the differential amplifier 15 is grounded via the resistor 25A. The output terminal of the differential amplifier 15 is connected to the inverting input terminal via the resistor 25B and also to the input terminal of the signal switching circuit 26 including an analog switch. The resistors having the same resistance value are used.

The detection signal S5 of the received light signals of the red light and the infrared light shown in FIG. 2 (e), which are both detected by the AM detection circuit 13, are output from the arithmetic / control circuit 1.
By switching the signal switching circuit 26 by the switching control signal S6 shown in (f), the signals are distributed to the amplifiers 29 and 32 through the smoothing filters. The signal S7 shown in FIG. 2 (g) is the detection signal on the red light side after being distributed, and the signal S8 shown in FIG. 2 (h) is the detection signal on the infrared light side. The smoothing filter includes resistors 27 and 30 inserted in series to the input terminals of the amplifiers 29 and 32, and capacitors 28 and 31 connected between the input terminal of the amplifier 32 and the ground.

The red light detection signal and the infrared light detection signal that have passed through the amplifiers 29 and 32 are voltage signals reflecting the information on the absorbance after each light has passed through the living tissue 6. These detected signals are input to the A / D converters 34 and 36 through the low pass filters 33 and 35, converted into digital signals, and then taken into the arithmetic / control circuit 1. The calculation / control circuit 1 calculates the ratio Φ of the pulsating components of the absorbance of red light and infrared light, and calculates the oxygen saturation S from the ratio Φ of the absorbance.

Thus, in this pulse oximeter,
After the red light emitting diode 3 repeatedly emits and turns off the light n times based on the light emission control signals S1 and S2, the infrared light emitting diode 5 repeats the light emitting and extinction n times during the rest period. Since 3 and 5 are emitting light, the output voltage waveform as shown in FIG. Now, the light emission frequency f0 of each light emitting diode 3, 5 in the light emission period is set to 500
If the frequency is set to Hz, the output signal S3 of the amplifier 8 becomes a signal equivalent to that obtained by AM-modulating the carrier of 500 Hz with the signal A1 of 250 / nHz. Output signal S of amplifier 8 at this time
The power spectrum of No. 3 is shown in FIG. As is clear from this figure, the output signal of the amplifier 8 is 500+ (250 / n) Hz from the lower sideband of the frequency f1 of 500- (250 / n) Hz centered on the carrier frequency f0 of 500 Hz.
Is distributed in the range of the upper sideband of the frequency f2, and the occupied band width W1 of the spectrum having information is 500 / nHz. Therefore, the occupied band width W1 is reduced to 1 / n as compared with the conventional one. Note that fa and fb indicate the conventional lower sideband and upper sideband. As a result, the low frequency removing circuit 9
Even if the low cutoff frequency of is increased, useful signal components are not removed, and the low frequency removal circuit 9 can sufficiently remove the noise of external light including frequency components of fluorescent lamps and the like. Becomes

Next, the case where the light emitting frequency f0 of each of the light emitting diodes 3 and 5 is increased from 500 Hz to several kHz will be described. If this light emission frequency is set to, for example, 10 kHz and the number of consecutive light emission times n is, for example, 20 times,
The occupied band width W1 is 500 Hz, and the frequency f of the lower sideband is
1 becomes 9.75 kHz. As a result, even if external light such as a fluorescent lamp contains a noise component of several kHz, the cutoff frequency of the low-frequency elimination circuit 9 can be set to 9 kHz or higher, and the noise of the external light including the frequency component can be reduced. Can be completely removed.

In the pulse oximeter shown in FIG. 1, the output signal S4 of the low frequency band removing circuit 9 is detected by the AM detecting circuit 13.
The signal S4 after the low frequency band is removed
Is also possible through the voltage shift circuit 42 shown in FIG. 4 and then input to the signal switching circuit 26.

[0022]

As described above, according to the present invention, when red light and infrared light having two wavelengths are incident on living tissue in a time division manner to obtain information on the absorbance of different wavelengths, red light is predetermined. Since the infrared light is intermittently incident at the frequency f0 of n times and then the infrared light is intermittently incident at the same frequency f0 n times, the biological tissue is irradiated with light of two wavelengths. From the element, it is possible to take out a received light signal in which the occupied bandwidth for the frequency including the information on the absorbance associated with the center frequency f0 is narrowed, and as a result, the cut-off frequency of the low frequency elimination circuit can be increased. As a result, the noise of the external light including the frequency component mixed in the light receiving element can be effectively removed by this low-frequency removing circuit, and the oxygen saturation S can be measured with higher accuracy than before.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a pulse oximeter according to the present invention.

FIG. 2 is a waveform diagram for explaining the operation of the pulse oximeter of FIG.

FIG. 3 is a diagram showing a power spectrum of a received light output signal.

FIG. 4 is a circuit diagram showing a conventional pulse oximeter.

FIG. 5 is a waveform diagram for explaining the operation of a conventional pulse oximeter.

FIG. 6 is a waveform diagram for explaining the operation of the voltage shift circuit when optical noise is a DC component.

FIG. 7 is a waveform diagram for explaining the operation of the voltage shift circuit when the optical noise includes a frequency component and a pulse component.

FIG. 8 is a waveform diagram for explaining a problem when a low-frequency removing circuit is used in a conventional pulse oximeter.

FIG. 9 is a diagram showing a power spectrum appearing in a light reception output by a conventional pulse oximeter.

[Explanation of symbols]

 1 Calculation / Control Amplification Circuit 3 Red Light Emitting Diode 5 Infrared Light Emitting Diode 6 Living Tissue 7 Photodiode 8 Differential Amplifier 9 Low Frequency Removal Circuit 13 AM Detection Circuit 26 Signal Switching Circuit 29, 32 Amplifier 33, 35 Low Pass Filter 34, 35 A / D converter

Claims (1)

[Claims] 1. A first and a second light source for respectively irradiating living tissue including arterial blood flow with light of two different wavelengths, red light and infrared light, and predetermined irradiation and blocking of the living tissue with red light. By repeating the first timing for continuously performing n times at the frequency and the second timing for continuously performing irradiation and blocking of the infrared light to the biological tissue n times at the predetermined frequency, the red color is continuously obtained. 1st and 2nd so that living tissue is irradiated with light and infrared light
Means for controlling the light source, a light receiving element for detecting the light output after being absorbed by the biological tissue for the two wavelengths of red light and infrared light emitted from the first and second light sources, and An amplifier that amplifies the received light signal obtained from the device, a low-frequency removing circuit that removes low-frequency components from the output signal of this amplifier at a predetermined low-frequency cutoff frequency, and an output signal of this low-frequency removing circuit after detection. A pulse oximeter, comprising: a calculation means for calculating the ratio of the pulsating component of the absorbance due to arterial blood flow for two wavelengths from the signal, and the oxygen saturation of arterial blood from the obtained ratio of the absorbance.