JPH03170866A - Method and apparatus for measuring degree of saturation of oxygen - Google Patents

Abstract

PURPOSE:To enhance the S/N ratio of the intensity of reflected light and to accurately detect the degree of saturation of oxygen by narrowing the frequency range of a signal showing the intensity of the reflected light from blood. CONSTITUTION:AC signals f1, f2 having different frequencies outputted from oscillators 11, 12 are amplified by amplifying/driving circuits 15, 16 and lights flashing at respective frequencies are emitted from light emitting diodes 17, 18 reacted in synchronous relation to the circuits 15, 16. These two light signals are allowed to be incident to blood and the intensities of reflection thereof are detected by a photodiode 19 to be amplified by an amplifier 20. A noise component is cut by high-pass and low-pass filters 21, 22 and reflected lights are separated at every frequencies by band-pass filters 23, 24. Subsequently, the reflected lights are synchronously detected at every signal components of frequencies by synchronous demodulators 25, 26 and signal components of other frequencies are removed by low-pass filters 27, 28. The reflected light intensity signals of lights outputted from the diodes 17, 18 are converted by an A/D converter 29 and the degree of saturation of oxygen of hemoglobin in blood is calculated by an operation part 30.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a method for measuring the oxygen saturation of hemoglobin, which measures the oxygen saturation of hemoglobin using the light absorption characteristics (reflected light characteristics) of hemoglobin in blood. and its equipment.

[Prior Art] It is known that the light absorption (reflection) characteristics of blood generally change due to absorption and scattering by pigments and particles in the blood, and in particular, vary greatly depending on the bonding state of hemoglobin with oxygen and the wavelength of irradiated light. It is being Therefore, a device for measuring the oxygen saturation level of hemoglobin in blood using such characteristics was proposed by the applicant in Japanese Patent Application No. 62-186.
No. 135 and Japanese Patent Application No. 186136/1982.

FIG. 4 is a diagram showing a schematic configuration of a conventional apparatus for measuring oxygen saturation of hemoglobin. In FIG. 4, a pulse generator 201 generates pulses with predetermined intervals and a predetermined time width that do not overlap in time, and passes them through an LED drive circuit 202 to light emitting diodes 203 (wavelength 660 nm), 20
4 (length 800 nm) are emitted alternately. Light from these light emitting diodes 203, 204 is irradiated into the blood through optical fibers 20, 5, 206, respectively. The reflected light from the blood is detected by a photodiode 207 and converted into a voltage signal by a detection amplifier 208.

In the analog switch 209, switches SWI and SW2 are turned on and off by a signal from the pulse generator 20l.
For example, when the light emitting diode 203 lights up, only the switch SWI is turned on, and the voltage signal from the detection amplifier 208 is applied to the capacitor 210.
An average signal voltage (reflected light intensity of light with a wavelength of 660 nm) is generated at both ends of 0. This average signal voltage is further applied to amplifier 2.
After being amplified by l2, it is converted into a digital signal by an A/D converter 29. In addition, a light emitting diode 20
Similarly, the reflected light intensity by 4 (reflected light intensity of light with a wavelength of 800 nm) is inputted and converted into a digital signal by turning on the switch SW2. Then, the calculation section 30
The oxygen saturation level of hemoglobin in the blood is calculated based on the ratio (I./I.) of these reflected light intensities, and the output section 3
1 is displayed and output.

[Problem to be solved by the invention] However, in the conventional circuit that inputs the reflected light intensity,
The light emitting diodes 203 and 204 are both driven by a pulse signal, and the reflected light intensity signal detected by the photodiode 207 is also pulsed, and the frequency components contained in this signal are in the frequency range including the high frequency range rather than the low frequency range. It becomes a wide signal. This signal needs to be accurately amplified and input to a subsequent A/D converter or the like.

Therefore, an amplifier 208, 212 that amplifies this signal
213 is required to have good frequency characteristics, but amplifiers with good frequency characteristics generally have low amplification factors and S/N ratios and are expensive.

Furthermore, since the amplification factor of the amplifier becomes low, it becomes difficult to detect weak signals, so the output of the light emitting diode must be increased. For this reason, there have been problems such as an increase in the current flowing through the light emitting diode, shortening the life of the light emitting diode, and increasing heat generation and changing the emission wavelength. In addition, there is a risk that the dark current of the photodiode and the bias current of the amplifier will be superimposed on the signal component, causing an error, so a photodiode with a small dark current and an amplifier with a small bias current are required. The use of diodes and amplifiers also increased the cost of the device.

The present invention has been made in view of the above-mentioned conventional example, and by narrowing the frequency range of the signal indicating the reflected light intensity of light reflected in blood, the S/N' ratio of the reflected light intensity is increased. It is an object of the present invention to provide a method and apparatus for measuring oxygen saturation, which can detect oxygen saturation with high accuracy.

[Means for Solving the Problems] In order to achieve the above object, the method for measuring oxygen saturation of the present invention has the following configuration. That is, the method includes light emitting sources that output light of different wavelengths, irradiates the blood with light from the light emitting sources, and measures the oxygen saturation level of hemoglobin in the blood based on the intensity of the reflected light. driving each of the light emitting sources with first and second alternating current signals having at least two different frequencies, and converting the reflected light intensity of the light reflected in the blood out of the light from each light emitting source into an electrical signal. a step of separating the electrical signal into signals containing frequency components of the first and second AC signals to extract first and second reflected light intensity signals; and calculating the oxygen saturation level of hemoglobin in the blood based on the reflected light intensity signal.

Further, in order to achieve the above object, an oxygen saturation measuring device according to another invention has the following configuration. That is, the oxygen saturation method includes light emitting sources that output light of different wavelengths, irradiates the blood with light from the light emitting sources, and measures the oxygen saturation of hemoglobin in the blood based on the intensity of the reflected light. The measuring device includes a light emitting means for driving each of the light emitting sources to emit light using first and second alternating current signals having at least two different frequencies; a conversion means for converting reflected light intensity into an electrical signal; and separating the electrical signal into signals containing frequency components of the first and second alternating current signals, and outputting first and second reflected light intensity signals. output means for
and calculation means for calculating the oxygen saturation level of hemoglobin in the blood based on the second reflected light intensity signal.

[Function] In the above configuration, each of the light emitting sources that output light of a different wavelength is driven by at least two first and second alternating current signals having different frequencies, and blood from the light from each light emitting source is driven. The intensity of the light reflected inside is converted into an electrical signal. The electric signal is separated into signals containing frequency components of the first and second AC signals, the first and second reflected light intensity signals are extracted, and the first and second reflected light intensity signals are separated. It operates to calculate the oxygen saturation level of hemoglobin in the blood.

[Embodiments] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Description of the oxygen saturation measuring device (Fig. 1)] Fig. 1 is a block diagram showing the schematic configuration of the oxygen saturation measuring device of the embodiment, and the same parts as the conventional configuration shown in Fig. 4 are designated by the same symbols. It shows.

In the figure, 11 and 12 are both oscillators, and AC (sine waveform) signals (t'+, t
's) is output. 15.16 is the amplification drive circuit,
Each input AC signal (a sine wave converted into a rectangular wave) is amplified, and the corresponding light emitting diode (17, 18) is driven in synchronization with the AC (rectangular wave) signal to emit light. Therefore, the light emitting diode 17 emits light that blinks at a frequency f1 with a wavelength of 660 nm, for example, and the light emitting diode 18 emits light that blinks at a frequency f2 and a wavelength of 800 nm, for example.

These two optical signals are irradiated into blood (not shown), and the intensity of the reflected light is detected by the photodiode 19. A detection amplifier 20 converts the current output from the photodiode 19 into a voltage signal 4l. Therefore, this voltage signal 41 is a voltage signal in which a reflected light intensity signal of light blinking at frequency f1 and a reflected light intensity signal of light blinking at frequency f2 are multiplexed. 21 is a high pass filter (HPF), 22 is a low pass filter (LPF), and noise components contained in the voltage signal 4l are cut by these filters.

23 and 24 are bandpass filters (BPF), and the bandpass filter 23 passes signals in a frequency band approximately centered on the frequency f1, and the bandpass filter 24 passes signals in a frequency band approximately centered around the frequency f2. In this way, each of the signals separated by these bandpass filters 23, 24 is an electrical signal modulated at frequency f,
This becomes an electrical signal modulated at frequency f2. 25 and 26 are both synchronous demodulators, the synchronous demodulator 25 synchronously detects the signal component of frequency f1, and the synchronous demodulator 26 synchronously detects the signal component of frequency 2.

As a result, for example, the reflected light intensity signal of the light emitted from the light emitting diode 17 is extracted by the synchronous demodulator 25, and the signal component of the frequency f is removed by the low pass filter (LPF) 27. A reflected light intensity signal of the light is output. Similarly, the reflected light intensity signal of the light emitted from the light emitting diode 18 is extracted by the synchronization detector 26, and further filtered by a low-pass filter (LPF).
28 removes the signal component of the frequency f2, and outputs a reflected light intensity signal of the light from the light emitting diode l8.

The reflected light intensity signal from each of these light emitting diodes is
The signal is input to the A/D converter 29 and converted into a digital signal. Then, the calculation unit 30 calculates the ratio N of these reflected light intensities. /r2), the oxygen saturation of hemoglobin in the blood is calculated and displayed/outputted by the output unit 31.

Regarding the calculation in this calculation section 30,
No. 186135 and Japanese Patent Application No. 62-186136, the details are not explained here.

[Explanation of a specific example of the circuit (Figs. 2 and 3)] Figs. 2 and 3 are circuit diagrams showing a specific example of the configuration of Fig. 1, and for the sake of explanation, the common parts with Fig. 1 are Indicated by the same symbol.

Here, the light emitting diode 17 is driven to blink by a rectangular wave signal having a frequency of 320 Hz, and the light emitting diode 18 is driven to blink by a rectangular wave signal having a frequency of 740 Hz. Also,
The wavelength of the light emitted by the light emitting diode l7 is 660 nm,
The wavelength of the light emitting diode 18 is 800 nm. The phase shifter l3 incorporated in the drive circuit of the light emitting diode 17 outputs a reference signal REF1 based on the sine waveform signal, and this reference signal REF1 is input to the synchronous demodulator 25 to output a detected signal. is adjusted so that it is the maximum. Furthermore, the phase shifter 1 incorporated in the drive circuit of the light emitting diode 18
3 similarly outputs REF2, and the detection signal output from the synchronous demodulator 26 is adjusted so as to be at the maximum level.

In FIG. 3, reflected light from the sample blood is detected by photodiode 19 and converted to a voltage signal 41 by sense amplifier 20. In FIG. This voltage signal 41 is passed through a 50Hz low-pass filter 21 and a 5KHz high-pass filter 2.
2 to remove noise components, the signal is amplified by an amplifier 34 to a predetermined signal level.

The electrical signal amplified in this way is passed through the subsequent bandpass filter 2.
3 and 24. Bandpass filter 23 is 250Hz
It is composed of a high-pass filter 23a of 400 Hz and a low-pass filter 23b of 400 Hz.
It is a third-order Butterworth-type filter with a passband characteristic of Hz. Therefore, this bandpass filter 23 is
Electrical signals modulated at 320Hz pass through easily, but
The other signal component modulated at 740 Hz is difficult to pass through. Similarly, the bandpass filter 24 includes a 600Hz high-pass filter 24a and a 920Hz low-pass filter 2.
4b, and is a third-order Butterworth-type filter having a pass band characteristic of 600 Hz to 920 Hz. Therefore, this bandpass filter 24 has 740H
The electrical signal modulated at 320 Hz is easily passed through, but the other signal component modulated at 320 Hz is difficult to pass through. In this way, the electrical signals are separated into two different frequencies.

Next, the synchronous demodulator 25 inputs the phase-adjusted reference signal REF1 from the phase shifter 13, synchronously detects the electric signal from the bandpass filter 23, and generates a reflected light intensity signal of the light from the light emitting diode 17. is being detected. And furthermore, 5
The 320 Hz signal component is removed by the Hz low-pass filter 27 to obtain a DC voltage signal proportional to the amplitude of the output signal of the band pass filter 23, that is, a reflected light intensity signal of the light from the light emitting diode 17. Similarly, the synchronous demodulator 26 detects the reflected light intensity signal of the light from the light emitting diode l8, and the 5Hz low-pass filter 28 detects the reflected light intensity signal.
The DC voltage signal is extracted by removing the 0 Hz frequency component.

As explained above, according to this embodiment, the reflected light intensity signal is detected without using a signal with a wide frequency range, so the reflected light intensity signal can be amplified with an amplifier that amplifies a signal with a narrow frequency range. Can be done. This results in
Since the amplification factor of the detection signal can be increased, the reflected light intensity can be detected with a good S/N ratio.

In addition, because it does not require a high-speed response detection amplifier or an amplifier circuit with a wide frequency range, it is possible to keep circuit costs low. It also reduces the physical burden on the light source, which reduces manufacturing costs. , which has the effect of extending the product life.

Note that in this example, the number of light emitting diodes is 32 each.
I set it to drive with 0Hz and 720Hz square waves, but
The present invention is not limited to these frequencies, and any frequency that can be separated from each other may be used. Preferably 100
It is between Hz and 100 KHz, and it is sufficient that the two frequencies have a difference of one octave or more.

Further, the circuit configuration shown in this embodiment is shown as an example of the present invention, and it goes without saying that it does not specify the present invention.

[Effects of the Invention] As explained above, according to the present invention, the S/N ratio of the reflected light intensity is increased by narrowing the frequency range of the signal indicating the reflected light intensity of the light reflected in the blood. This has the effect of allowing accurate detection.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the schematic configuration of the oxygen saturation measuring device of this embodiment, Fig. 2 is a circuit diagram showing a specific example of the light emitting diode drive circuit of this embodiment, and Fig. 3 is a photodiode of the embodiment. FIG. 4 is a block diagram showing a specific example of a signal detection circuit, and FIG. 4 is a block diagram showing the configuration of a conventional light emitting diode drive circuit and a detection circuit for the intensity of reflected light. In the figure, 11.12... Oscillator, 13... Phase shifter, 1
5,16...Amplification drive circuit, 17.18...Light emitting diode, 19...Photodiode, 20...Detection amplifier, 21.23a, 24a...High bass filter (HPF), 22, 27.28...Low-pass filter (LPF), 23.24...Band filter (BP)
F), 25.26...synchronous demodulator, 29...A/
D converter, 30... calculation section, 31... output section.

Claims (3)

[Claims] (1) A method that includes light emitting sources that output light of different wavelengths, irradiates the blood with light from the light emitting sources, and measures the oxygen saturation of hemoglobin in the blood based on the intensity of the reflected light. Each of the light emitting sources is driven by first and second alternating current signals having at least two different frequencies, and the reflected light intensity of the light reflected in the blood among the light from each light emitting source is converted into an electric signal. converting the electrical signal; separating the electric signal into signals containing frequency components of the first and second alternating current signals to extract first and second reflected light intensity signals; and extracting first and second reflected light intensity signals; 2. A method for measuring oxygen saturation, comprising: calculating the oxygen saturation of hemoglobin in the blood based on the reflected light intensity signal of step 2. (2) Oxygen saturation, which is equipped with light emitting sources that output light of different wavelengths, irradiates the blood with light from the light emitting sources, and measures the oxygen saturation of hemoglobin in the blood based on the intensity of the reflected light. a light emitting means for driving each of the light emitting sources to emit light using first and second alternating current signals having at least two different frequencies; and light reflected in the blood out of the light from the light emitting sources. converting means for converting reflected light intensity into an electrical signal; and converting the electrical signal into signals containing frequency components of the first and second alternating current signals to generate first and second reflected light intensity signals. An apparatus for measuring oxygen saturation, comprising: an output means for outputting, and a calculation means for calculating the oxygen saturation of hemoglobin in the blood based on the first and second reflected light intensity signals. . (3) The output means includes first and second synchronous demodulation means that demodulate frequency components of the first and second AC signals in synchronization with each other, and outputs of the first and second synchronous demodulation means. 3. The oxygen saturation measuring device according to claim 2, further comprising a low-pass filter means for extracting only low frequency components of the oxygen saturation level.