JP2010233908A - Pulse oximeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately measure an SpO<SB>2</SB>value also on a subject of low perfusion with low pulsation without causing an adverse effect by an increase of light emission energy in a pulse oximeter. <P>SOLUTION: The pulse oximeter measures the SpO<SB>2</SB>value noninvasively utilizing the difference of absorption characteristics of two wavelengths of red light R and infrared light IR (the ratio of pulse wave amplitude components) in a living body. When AC components R<SB>AC</SB>, IR<SB>AC</SB>and DC component R<SB>DC</SB>, IR<SB>DC</SB>of transmitted light are periodically measured in sequence to nullify the influence due to the difference of the light emission levels of light emitting elements that generate light of two wavelengths respectively, the detection periods R-DC, IR-DC of the DC components R<SB>DC</SB>, IR<SB>DC</SB>are shortened by a time α, while the detection periods R-AC, IR-AC of the AC components R<SB>AC</SB>, IR<SB>AC</SB>are extended by the time α as shown in (b) with respect to a conventional detection period T1 shown in (a). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention irradiates a living tissue with blood flow such as an artery or a vein with red light and infrared light, and utilizes transmitted or reflected light to saturate blood oxygen from the difference in absorption characteristics of two wavelengths. The present invention relates to a pulse oximeter that measures the degree of blood noninvasively, and particularly relates to an improvement in accuracy in the case of low perfusion with small pulsation and small alternating current component of a signal.

  The pulse oximeter as described above was obtained by alternately irradiating the subject's finger with the red light and infrared light, and photoelectrically converting the transmitted light or reflected light with a light receiving element such as a silicon photodiode. A current output corresponding to the incident light intensity is extracted as a voltage signal using a current-voltage conversion circuit using an operational amplifier. Thereafter, the voltage signal is further amplified by an amplification circuit, converted into a digital signal by an analog / digital conversion circuit and input to a computing device, and the blood oxygen saturation is calculated from the difference between the absorption characteristics of the two wavelengths. The

Specifically, the size of the red light and infrared light data changes according to the pulsation of the blood vessel. Within a certain period of time, the AC component (pulse wave signal) in which the red light is changed is R _AC , the AC component (pulse wave signal) in which the infrared light is changed is IR _AC , and the DC component in which the red light is not changed is R _{If DC} and the direct current component in which infrared light is not changed are IR _DC , the ratio P of the change ratio between red light and infrared light is expressed by the following equation.

P = (R _AC / R _DC ) / (IR _AC / IR _DC ) (1)
This ratio P is obtained from the pulse wave amplitude ratio of transmitted light or reflected light of red light and infrared light. When the light emission amounts of the light sources (LEDs) that generate the red light and the infrared light are different from each other, Since the wave amplitude is affected by the absolute level of the transmitted light or reflected light, the AC components R _AC and IR _AC are normalized by the DC components R _DC and IR _DC in order to eliminate this effect.

Then, the ratio P is, depends on the characteristics such as the wavelength and the half value width of the light source in use, of its ratio P, premeasured the blood oxygen saturation by blood type oximeter and (SpO ₂ value) by using a table representing the relationship, the moment of SpO ₂ value can be obtained, the SpO ₂ value of the instantaneous, by performing processing such as moving averages for example, SpO ₂ values displayed by the pulse oximeter Can be obtained.

  At this time, in Patent Document 1, the A / D count value at the time of non-light emission measured before and after the A / D count value at the time of light emission of each light source of red light and infrared light is subtracted, and this is 1 It is the data at the time of light emission. In this way, by subtracting the count value at the time of non-light emission, the externally incident external light component and the offset component of the circuit are removed, and only a pure signal component by light emission is taken out.

JP 2000-325330 A

In such a pulse oximeter, a signal obtained by irradiating a living body with light from a light source and transmitting or reflecting the light fluctuates depending on the pulsation of a blood vessel in the living tissue as described above. However, the ratio of the fluctuation component, that is, the alternating current components R _AC and IR _AC to the non-variation component, that is, the direct current components R _DC and IR _DC , is a blood vessel due to individual differences in the state of peripheral blood vessels and the surrounding cold and warm conditions. It varies depending on the difference in the contraction state, etc., and when it is large, it is close to 40%, but when it is small, it may be 0.5% or less.

In the above-mentioned Patent Document 1, the external light component and the offset component of the circuit can be removed as described above, but there is a problem that such minute AC components R _AC and IR _AC cannot be measured with a good S / N. . That is, in addition to the components, noise propagating through the space or the substrate enters the components R _AC , IR _AC ; R _DC , IR _DC . This noise is generated regularly and randomly at a substantially constant level, and enters the DC components R _DC and IR _DC having a large amplitude level and the AC components R _AC and IR _AC having a small amplitude level at substantially the same level. . Therefore, in the AC components R _AC and IR _{AC having} a small amplitude level, the amplitude level largely varies (S / N greatly decreases) due to the noise, and as a result, the calculated SpO ₂ value varies.

Therefore, in order to suppress such variations in the _AC components R _AC and IR _AC , it can be considered that the influence of the random noise is suppressed by increasing the sampling period and obtaining an average value therebetween. However, when this happens, the pulse emission period in the pulse oximeter becomes longer, and the living body is continuously irradiated with a light having a large amount of light that can be seen through the blood vessels. Therefore, adverse effects due to heat may occur, and the advantage of the pulse oximeter that is non-invasive, that is, non-invasive is greatly impaired. In addition, if the sampling period is lengthened, the power consumption of the light source increases, and in particular, in a portable type pulse oximeter that can measure the state at night or while moving, the operable time is shortened and the battery is increased in size. There is also a problem.

  An object of the present invention is to provide a pulse oximeter capable of improving the S / N against random noise and accurately measuring without increasing the light emission energy of the light source.

  The pulse oximeter of the present invention irradiates biological tissue with blood flow with light of the first and second wavelengths, and saturates blood oxygen based on the ratio of the amount of received light transmitted or reflected by the biological tissue. In the pulse oximeter that calculates the degree, first and second light sources that respectively generate light of the first and second wavelengths, and a light emission control unit that drives the first and second light sources, A calculation process for calculating the blood oxygen saturation based on a ratio of a light receiving element that receives or reflects light of the first and second wavelengths transmitted or reflected through the living tissue and a ratio of the amount of light received by the light receiving element A measurement value obtained by sequentially measuring the fluctuation component (AC) and non-fluctuation component (DC) of the transmitted or reflected light as the amount of received light at the first and second wavelengths. The light emission control unit uses the ratio of The fluctuation component (AC) detection period and non-fluctuation component (DC) detection period at the first wavelength, and the fluctuation component (AC) detection period and non-fluctuation component (DC) at the second wavelength. The detection period is sequentially repeated every predetermined period, and the emission period per predetermined period is larger in the detection period of the fluctuation component (AC) than in the detection period of the non-fluctuation component (DC). And

  According to said structure, blood oxygen saturation is measured noninvasively using the difference (ratio of a pulse-wave amplitude component) of the light absorption characteristic of two wavelengths of red light and infrared light in a biological body. In the pulse oximeter, in order to cancel the influence of the difference between the light emission levels of the first and second light sources that respectively generate the two wavelengths of light, the blood oxygen is based on the ratio of the amount of light received by the light receiving element. A calculation processing unit that calculates a saturation degree uses a variation component (pulse wave amplitude component (AC)) and non-variation component (DC) of the transmitted or reflected light as the amount of received light at the first and second wavelengths. The fluctuation component (AC) is normalized with the non-fluctuation component (DC) using the ratio of the measured values measured sequentially. For this reason, the light emission control unit that drives the first and second light sources includes a detection period of a fluctuation component (AC) and a detection period of a non-fluctuation component (DC) at the first wavelength, and the second light source. The fluctuation component (AC) detection period and the non-fluctuation component (DC) detection period at the wavelength are sequentially repeated at predetermined intervals. In this case, in the present invention, the light emission control unit increases the light emission energy per the predetermined period in the detection period of the fluctuation component (AC) as compared with the detection period of the non-fluctuation component (DC).

  Therefore, even if noise that propagates through space or the substrate and is generated randomly at a constant and constant level enters the fluctuation component (AC) and the non-fluctuation component (DC) at the same level, A relatively large amplitude is obtained, and for a non-variable component (DC) having an originally good S / N, the S / N is reduced by a decrease in the emission energy per predetermined period, but only a relatively small amplitude is obtained. However, for the fluctuation component (AC) originally having a poor S / N, the S / N can be remarkably improved by the increase in the emission energy. As a result, the total luminescence energy remains constant, and even if the perfusion subject has low pulsation and the pulsation amplitude component (AC) is low and the measurement conditions are low, without causing adverse effects due to the increase in luminescence energy, The blood oxygen saturation can be measured well.

  In the pulse oximeter according to the present invention, the light emission control unit makes the detection period of the fluctuation component (AC) longer than the detection period of the non-fluctuation component (DC), so The emission energy is increased to improve the S / N of the fluctuation component.

  According to the above configuration, the number of samples in the detection period of the fluctuation component (AC) is increased or the integration time is lengthened without increasing the time average current associated with light emission, and the fluctuation component (AC) is increased. S / N can be improved.

  Preferably, values obtained by subtracting the response time constants of the light receiving element and the amplifier for converting the photoelectric conversion current obtained by the light receiving element from the detection periods of the fluctuation component (AC) and the non-fluctuation component (DC), respectively. The ratio is approximately 10 times.

  According to said structure, S / N of a fluctuation | variation component (AC) can be raised, suppressing deterioration of S / N of the said non-fluctuation component (DC).

  In the pulse oximeter of the present invention, the light emission control unit increases the light emission amount in the detection period of the fluctuation component (AC) as compared with the light emission quantity in the detection period of the non-fluctuation component (DC). The light emission energy per predetermined period is increased to improve the S / N of the fluctuation component.

  According to the above configuration, it is possible to increase the amount of emitted light during the detection period of the fluctuation component (AC) without increasing the time average current associated with light emission, thereby improving the S / N of the fluctuation component (AC) itself. it can.

  Furthermore, in the pulse oximeter of the present invention, the light emission control unit makes the detection period of the fluctuation component longer than the detection period of the non-fluctuation component, and the amount of emitted light in the detection period of the fluctuation component. By increasing the amount of emitted light in comparison with the non-variable component detection period, the light emission energy per predetermined period is increased, and the S / N of the varying component is improved.

  According to the above configuration, the number of samples in the detection period of the fluctuation component (AC) is increased or the integration time is increased without increasing the time average current associated with light emission, and the amount of emitted light is further increased. , The S / N of the fluctuation component (AC) can be improved.

  As described above, the pulse oximeter of the present invention uses the difference between the absorption characteristics of two wavelengths of red light and infrared light in the living body (ratio of pulse wave amplitude components) to reduce blood oxygen saturation. In a pulse oximeter for invasively measuring, in order to counteract the influence of the difference in the light emission levels of the first and second light sources that respectively generate the light of the two wavelengths, When the fluctuation component (pulse wave amplitude component (AC)) and the non-fluctuation component (DC) are sequentially measured and the fluctuation component (AC) is normalized by the non-fluctuation component (DC), the fluctuation component (AC ) In the detection period is larger than the detection period of the non-variable component (DC).

  Therefore, the total light emission energy remains constant, and the S / N of the fluctuation component (AC) is improved without causing adverse effects due to the increase of the light emission energy, the pulsation is small, and the pulse wave amplitude component (AC) is reduced. Even for low-perfusion subjects with small measurement conditions, blood oxygen saturation can be accurately measured.

1 is a block diagram showing an electrical configuration of a pulse oximeter according to an embodiment of the present invention. It is a wave form diagram for demonstrating operation | movement of the pulse oximeter of one Embodiment of this invention. It is a block diagram which shows the electrical structure of a part of pulse oximeter which concerns on other forms of implementation of this invention. It is a wave form diagram for demonstrating the drive operation of the light emitting element of the pulse oximeter of one Embodiment of this invention. It is a flowchart for explaining the operation of measuring SpO ₂ values. 6 is a flowchart for explaining in detail the measurement operation in FIG. It is a wave form diagram for demonstrating the drive operation | movement of the light emitting element of the pulse oximeter of the other form of implementation of this invention. It is a wave form diagram for demonstrating the drive operation of the light emitting element of the pulse oximeter of the further another form of implementation of this invention. It is a wave form diagram for demonstrating the drive operation | movement of the light emitting element of the pulse oximeter of the other form of implementation of this invention.

(Embodiment 1)
FIG. 1 is a block diagram showing an electrical configuration of a pulse oximeter 10 according to an embodiment of the present invention. In the pulse oximeter 10, the drive circuit 1 has a light emitting element 2a that is a first light source that emits red light R having a first wavelength, and a second light source that emits infrared light IR having a second wavelength. The light emitting elements 2b are alternately turned on, and the light R and IR emitted from them are incident on the living body 3 such as the fingertip, and after a part of the light is absorbed, the remaining transmitted light is detected by the light receiving element 4. The The light receiving element 4 made of a silicon photodiode or the like photoelectrically converts the transmitted light and inputs it to the current / voltage conversion circuit 5 as a current output. In the present embodiment, the two light emitting elements 2a and 2b are alternately lit and the light receiving element 4 is shared in a time division manner. However, when individual light receiving elements are provided for the respective light emitting elements 2a and 2b, Such time division use may not be performed.

  The current-voltage conversion circuit 5 includes a feedback resistor R1 in the amplifier 51. The current-voltage conversion circuit 5 converts the current generated in the light receiving element 4 into a voltage and amplifies the voltage with reference to a predetermined reference voltage Vref1 (GND level in FIG. 1). Then, it is given to the amplifier circuit 6 at the next stage. The amplifier circuit 6 includes an input resistor R2 and a feedback resistor R3 in the amplifier 61, amplifies the voltage output from the current-voltage conversion circuit 5 with reference to a predetermined reference voltage Vref2 (GND level in FIG. 1), The data is supplied to the analog / digital conversion circuit 71 of the arithmetic processing unit 7 configured to include a CPU and its peripheral circuits.

In the arithmetic processing unit 7, the analog / digital conversion circuit 71 converts the analog signal amplified by the amplifier circuit 6 into a digital signal at a predetermined sampling period, and supplies the digital signal to the arithmetic unit 72. The arithmetic unit 72, and integrated by the integrating unit 721 the input sampled data, by dividing the integrated value by the number of samples in the average value calculating unit 722, the AC component _R AC, _{IR AC} and DC component _{R DC,} An average value to be IR _DC is obtained, and from the average value, the SpO ₂ calculation unit 723 obtains the ratio P according to the equation 1, and refers to a table stored in advance from the ratio P, and the arterial oxygen saturation Read the degree (SpO ₂ value). The read SpO ₂ value is output to a display device (not shown) or an external data recording device.

  On the other hand, in the arithmetic processing unit 7, the timing control unit 73 defines the sampling timing of the analog / digital conversion circuit 71 and controls the drive circuit 1 through the digital / analog conversion circuit 74. Therefore, the timing control unit 73, the digital / analog conversion circuit 74, and the drive circuit 1 function as a light emission control unit that drives the light emitting elements 2a and 2b that are the first and second light sources. The drive circuit 1 is composed of four FET bridges and the like, and when no voltage is applied between the terminals of the light emitting elements 2a and 2b connected in reverse parallel to each other, the light emitting elements 2a and 2b are turned off, When a voltage is applied, only the light emitting element 2a is lit with a luminance corresponding to the applied voltage, and when a voltage of the other polarity is applied, only the light emitting element 2b is lit with a luminance corresponding to the applied voltage. For this reason, the digital / analog conversion circuit 74 outputs an analog voltage and supplies it to the gate of the FET. Note that, at the time of initial setting or the like, the light intensity adjustment of the light emitting elements 2a and 2b is performed by adjusting the analog voltage corresponding to the thickness of the finger of the subject.

In the pulse oximeter 10 described above, during the lighting period of the light emitting elements 2a and 2b as shown in FIG. 2A, the output of the amplifier circuit 6 is output by the analog / digital conversion circuit 71 as shown in FIG. Discrete sampling is performed, the sampling values are integrated by the integrating unit 721, and the integrated value is divided by the number of samples in the average value calculating unit 722, whereby the AC component R _AC , IR _AC and the DC component R _DC , IR _The average value for _DC is obtained. On the other hand, the configuration of the arithmetic processing unit 7 ′ can be simplified by using a circuit as shown in FIG.

  FIG. 3 is a block diagram showing an electrical configuration of a pulse oximeter 20 according to another embodiment of the present invention. The pulse oximeter 20 is similar to the pulse oximeter 10 described above, and corresponding portions are denoted by the same reference numerals and description thereof is omitted. In FIG. 3, the configuration upstream of the amplifier circuit 6 is omitted. It should be noted that in this pulse oximeter 20, a double integration circuit 8 is provided between the amplification circuit 6 and the arithmetic processing unit 7 '.

In the double integration circuit 8, the output from the amplification circuit 6 is input from the switch SW1 to the amplifier 81 via the input resistor R4. An integrating capacitor C1 is provided between the input and output terminals of the amplifier 81, the output of which is monitored by a comparator 82, and the output of the comparator 82 is input as a stop signal to the counter 75 of the arithmetic processing unit 7 ′. The accumulated charge of the integrating capacitor C1 can be discharged from the discharge resistor R5 via the switch SW2. On the other hand, in the arithmetic processing unit 7 ′, when the timing control unit 73 lights the light emitting elements 2a and 2b, the lighting period is made to conduct the switch SW1, and as shown in FIG. The integrating capacitor C1 is charged with the output from the amplifier circuit 6. When the lighting period ends, the timing control unit 73 cuts off the switch SW1 and turns on the switch SW2 instead, and inputs a start signal to the counter 75, and the integration capacitor as shown in FIG. The discharge time of the accumulated charge of C1 is counted. When the accumulated charge becomes 0, the output of the comparator 82 is inverted, and the stop signal is input to the counter 75 as described above, and the counting operation is stopped. Thus, the average value of the output of the amplifier circuit 6 can be obtained from the charge accumulated in the integrating capacitor C1. For this reason, the output of the counter 75 is directly input to the SpO ₂ calculation unit 723 serving as the calculation unit 72 ′.

FIG. 4 is a waveform diagram for explaining the operation of the pulse oximeters 10 and 20 configured as described above. FIG. 4A shows the conventional operation, and FIG. 4B shows the operation of the present embodiment. In obtaining the AC component R _AC , IR _AC and DC component R _DC , IR _DC , the conventional light emission control unit detects the DC component R _DC detection period of the red light R, for example, as shown in FIG. R-DC, detection period R-AC of AC component _RAC of red light R, detection period IR-DC of DC component IR _DC of infrared light IR, and detection period of DC component IR _DC of infrared light IR IR-DC is sequentially performed every predetermined period T, and within the period T, the light emitting elements 2a and 2b are pulse-lit only for a certain period T1. Note that the above detection order is an example, and other orders may be used as long as the detection order is periodically repeated at the same frequency.

On the other hand, it should be noted that, in the light emission control unit of the present embodiment, detection is performed in the same order at the same period T as shown in FIG. 4B, but FIG. , The detection periods R-DC and IR-DC of the direct current components R _DC and IR _DC are shortened by the time α. Instead, the detection periods R-AC and IR− of the _AC components R _AC and IR _AC are reduced. AC is extended by the time α. Therefore, if the current during lighting of the light emitting elements 2a and 2b is the same A, the conventional average light emitting current I1 is
I1 = A × T1 / T (2)
The average light emission current I2 of the present embodiment is
I2 = (A × (T1−α) + A × (T1 + α)) / 2T = A × T1 / T (3)
And
I2 = I1 (4)
It is.

Thereby, in the pulse oximeter 10 shown in FIG. 1, the number of times of sampling shown in FIG. 2 (b) increases, and in the pulse oximeter 10 shown in FIG. 3, the integration time shown in FIG. 2 (c) increases. . For example, the period T is 6.6 msec, the period T1 is 400 μsec, and the time α is 250 μsec (in FIG. 2, for the sake of easy understanding, the ratio of the period T1 is shown to be considerably larger than actual). ing.). Accordingly, the DC component _R DC, the detection period R-DC of _{IR DC,} IR-DC is 150μsec, and the alternating current component _R AC, _{IR AC} detection period R-AC, IR-AC becomes 650Myusec.

Here, the current-voltage conversion circuit 5 generates a response time constant for a pulsed signal. Further, when the direct current components R _DC , IR _DC and the alternating current components R _AC , IR _AC are very small, the gain in the amplifier circuit 6 is set high and the response speed decreases, so as shown in FIG. The rise is slow, and it takes about T2 = 100 μsec, for example, as the time to start sampling. Therefore, the substantial detection period T1 ′ is 50 μsec for the DC components R _DC and IR _DC and 550 μsec for the AC components R _AC and IR _AC . Within the detection period T1 ′ of 550 μsec, for example, the analog / digital conversion circuit 71 performs sampling every 6 μsec, and therefore, sampling can be performed about 90 times. On the other hand, in the case of the conventional T1 = 400 μsec, the substantial detection period T1 ′ is 300 μsec, and the number of samplings is about 50 times.

Therefore, the S / N in the detection periods R-AC and IR-AC of the _AC components R _AC and IR _AC is compared to the conventional case.
S / N = {(T1 + α-T2) / (T1-T2)} ^1/2 (5)
Doubled. For example, setting the α to 0.5 times the (T1-T2), the alternating current component _R AC, _{IR AC} detection period R-AC, IR-AC becomes 1.5 times, S / N is about 1.2 Double (= √1.5) improvement. On the other hand, the direct current component _R DC, although the S / N ratio of the _{IR DC} decreases, the variation in SpO ₂ measurement is normally problem, the alternating current component _R AC, _{IR AC} DC component _R DC, 1% of _{IR DC} a case in the following, the alternating current component _R AC, is quite large contribution of _{IR AC} of S / N improvement can contribute to improve the accuracy of SpO ₂ measurement.

FIG. 5 is a flowchart for explaining the measurement operation of the SpO ₂ value. In step S 0, pulse emission periods T and T 1 are set in the timing control unit 73. In step S1, the red LED flag is first set to H and the DC component flag is set to H from the timing control unit 73 to the digital / analog conversion circuit 74 and the analog / digital conversion circuit 71. In step S2, the red LED is turned on and red. The measurement operation described later with the light R is performed, and the measurement result is stored in the SpO ₂ calculation unit 723 as the direct current component R _DC of the red light R in step S3. Thereafter, in step S4, a non-emission (non-detection) period is entered for the period T-T1, both the red LED flag and the DC component flag are reset (undefined), and the red LED is turned off.

Subsequently, in steps S11 to S14, as in steps S1 to S4, the red LED flag is continuously set to H and the DC component flag is switched to L in step S11, and the red LED is similarly turned on and red in step S12. It is performed measurement operation by the optical R, the measurement result in step S13 is stored in the SpO ₂ calculation section 723 as an AC component _{R AC} of the red light R. Thereafter, in step S14, a non-emission (non-detection) period is set for the period T-T1, both the red LED flag and the DC component flag are reset (undefined), and the red LED is turned off.

Instead, in steps S21 to S24, as in steps S1 to S4, the red LED flag is switched to L in step S21, the DC component flag is also set to H, and the infrared LED is turned on in step S22. A measurement operation using the light IR is performed, and the measurement result is stored in the SpO ₂ calculation unit 723 as a direct current component IR _DC of the infrared light IR in step S33. Thereafter, in step S24, a non-emission (non-detection) period is entered for the period T-T1, both the red LED flag and the DC component flag are reset (undefined), and the infrared LED is turned off.

Subsequently, in steps S31 to S34, as in steps S11 to 14, the red LED flag continues to be L and the DC component flag is switched to L in step S31, and the infrared LED is similarly turned on in step S32. It is performed measurement operation by infrared light IR, the measurement result in the step S33 is stored in the SpO ₂ calculation section 723 as an AC component _{R AC} of the infrared light IR. Thereafter, in step S34, a non-emission (non-detection) period is entered for the period T-T1, both the red LED flag and the DC component flag are reset (undefined), and the red LED is turned off.

Along with the start of the reset operation in step S34, in step S41, the SpO ₂ calculation unit 723 calculates the ratio P from the _AC components R _AC , IR _AC and DC components R _DC , IR _{DC according} to the equation 1. In step S42, the corresponding SpO ₂ value is read from the table.

FIG. 6 is a flowchart for explaining in detail the measurement operation in steps S2, S12, S22, and S32. This operation corresponds to the discrete sampling pulse oximeter 10 shown in FIG. In step S51, the light emission times of the light emitting elements 2a and 2b that become the detection periods R-AC and IR-AC and the detection periods R-DC and IR-DC are changed to AC components R _AC , IR _AC and DC components R _DC , for each IR _DC, the conjunction is set to the T1 + alpha and T1-alpha, number of samples in the analog / digital converter circuit 71, is set in accordance with the sampling time of the T1-T2 + alpha and T1-T2-alpha, further The light emission luminance of the light emitting elements 2a and 2b output through the digital / analog conversion circuit 74, that is, the drive current value is also set.

  In step S52, either the red LED or the infrared LED is turned on according to the red LED flag and the DC component flag, and the counting operation of the light emission time is started. Subsequently, in step S <b> 53, sampling of light transmitted through the living body 3 by the light receiving element 4 is performed by the analog / digital conversion circuit 71 and stored in the integrating unit 721. In step S54, 1 is added to the variable K representing the number of samples to be updated, and in step S55, it is determined whether or not the variable K has reached the value set in step S51. If not, the process returns to step S53 to repeat sampling, and when a set number of samples are obtained, the process proceeds to step S56.

In step S56, the process waits until the count value of the light emission time counter started in step S52 reaches the value set in step S51, and when that value is reached, the LED is turned off in step S57. In step S58, all the obtained sampling values are integrated by the integrating unit 721, and the integrated value is divided by the number of samples in the average value calculating unit 722, whereby the AC components R _AC , IR _AC, and DC components are divided. An average value of R _DC and IR _DC is obtained, and the ratio P can be calculated in step S41.

As described above, the pulse oximeters 10 and 20 according to the present embodiment use the difference between the absorption characteristics of the two wavelengths of the red light R and the infrared light IR in the living body 3 (the ratio of the pulse wave amplitude component) to SpO. _In a pulse oximeter that non-invasively measures binary values, the AC components R _AC and IR of transmitted light are used to cancel the influence of the difference in the light emission levels of the light emitting elements 2a and 2b that generate light of the two wavelengths, respectively. _{When the AC} and DC components R _DC and IR _DC are sequentially measured sequentially, the detection periods R-DC and IR-DC of the DC components R _DC and IR _DC are compared with the standard detection period T1 by the time α. while shorter, alternating current component _R AC, _{IR AC} detection period R-AC, extending the IR-AC.

Therefore, the noise that propagates through the space and the substrate and is randomly generated at a constant and constant level enters the AC components R _AC and IR _AC and the DC components R _DC and IR _DC at the same level. However, with respect to the direct current components R _DC and IR _DC , a large amplitude is obtained with respect to noise, so the S / N is very good, whereas even if the emission energy is reduced, the S / N is slightly reduced. The AC components R _AC and IR _AC have smaller amplitudes than the DC components R _DC and IR _DC depending on biological characteristics, and may be 1/100 or less depending on the conditions, and S / N becomes a problem. On the other hand, S / N can be improved by increasing the light emission energy as described above. As a result, the total light emission energy (the average light emission current I2) remains constant (equal to the conventional average light emission current I1), and the pulsation is small without causing an adverse effect due to the increase in the light emission energy, and the AC component R _AC can be performed on even the subject of poor hypoperfusion of measurement conditions IR _AC is small, the measurement of high accuracy SpO ₂ value.

Further, the response time constant T2 of the light receiving element 4 and the amplifier 51 for converting the photoelectric conversion current obtained by the light receiving element 4 into current-voltage is set as the detection period of the _AC components R _AC , IR _AC and DC components R _DC , IR _DC . By setting the ratio {(T1-T2 + α) / (T1-T2-α)} of the values T1 ′ subtracted from T1 to about 10 times as described above, the DC component R _DC , while suppressing the deterioration of the S / N of the IR _DC, AC component _R AC, I am possible to increase the _{IR AC} of S / N, the AC component _R AC, DC component _R DC and _{IR AC,} the amplitude ratio of the _{IR DC} R Measurement performance can be improved even if _AC / R _DC and IR _AC / IR _D are low signals such as about 1/100 or less due to biological characteristics.

  Furthermore, in the case of the discrete sampling shown in FIG. 1 and FIG. 2B, the sampling cycle is constant and the total number of samplings is also constant, so that the signal processing of the CPU such as the analog / digital conversion circuit 71 is performed. There is no change in speed, and the power consumption of the CPU does not increase. In addition, since the processing only needs to increase / decrease the number of averaging of discrete sampling or only increase / decrease the integration time of continuous sampling, the S / N can be improved without complicating the program.

(Embodiment 2)
FIG. 7 is a waveform diagram for explaining an operation in a pulse oximeter according to another embodiment of the present invention. The pulse oximeter of the present embodiment can use the configuration of the pulse oximeters 10 and 20 described above, and only the driving method of the light emitting elements 2a and 2b by the driving circuit 1 from the digital / analog conversion circuit 74 is different. is there. It should be noted that in the present embodiment, the detection periods R-DC and IR-DC of the direct current components R _DC and IR _{DC and} the detection periods of the AC components R _AC and IR _AC are the same as in FIG. R-AC, whereas the IR-AC is equal in the standard period T1, the light emitting element 2a, 2b of the DC component _R DC, the detection period of the _{IR DC R-DC,} the driving current in IR-DC A1 Is reduced by ΔA from the standard value A of FIGS. 4A and 4B, and instead, the drive current A2 in the detection periods R-AC and IR-AC of the _AC components R _AC and IR _AC is changed to the standard value. The value A is increased by ΔA. The standard value A is appropriately set in consideration of current consumption of the circuit, influence on the living body 3, and the like.

Thus the alternating current component _R AC, _{IR AC} detection period R-AC, when increasing the emission energy per unit cycle T in IR-AC, by an increase of the light emission amount, the AC component _R AC, _{IR AC} itself S / N can be improved.

Here, when the noise is sufficiently large, such as about 1000 times, such as the DC component R _DC , IR _DC is 1 V, for example, with respect to 1 mVrms, the S / N improvement rate is a decrease in signal of the DC component R _DC , IR _DC . The influence by is negligible, and the S / N change in the _AC components R _AC and IR _AC becomes dominant. For example, if ΔA = A / 2 in FIG. 7, the amount of light simply becomes 1.5 times, so the S / N is also improved by 1.5 times.

If the light emission current is the same in the detection periods R-AC and R-DC and the light emission current is the same in the detection periods IR-AC and IR-DC, Equation 6 is obtained, and the AC components R _AC and IR at the ratio P are obtained. There is no problem in the method of normalizing _AC with the direct current components R _DC and IR _DC . However, when the emission currents are different in the detection periods R-AC, R-DC, IR-AC, and IR-DC as shown in FIG. In addition, it is necessary to apply correction when calculating the ratio P as shown in Equation 9.

R-AC emission amount / R-DC emission amount = IR-AC emission amount / IR-DC emission amount (6)
Light emission amount at current amplitude A2 of K1 = R / Light emission amount at current amplitude A1 of R (7)
K2 = light emission amount at IR current amplitude A2 / light emission amount at IR current amplitude A1 (8)
P = {(R _AC / R _DC ) / (IR _AC / IR _DC )} × {K2 / K1} (9)

(Embodiment 3)
FIG. 8 is a waveform diagram for explaining an operation in a pulse oximeter according to still another embodiment of the present invention. As in the second embodiment, the pulse oximeter of the present embodiment can use the configuration of the pulse oximeters 10 and 20 described above, and the light emitting elements 2a and 2b by the drive circuit 1 from the digital / analog conversion circuit 74 can be used. Only the driving method is different. It should be noted that in this embodiment, the detection periods R-DC and IR-DC are timed in the first embodiment in view of the small decrease in S / N due to the reduction in the light emission amount of the direct current components R _DC and IR _DC. Instead of shortening by α, the light emission current in the detection periods R-DC and IR-DC is set to A ′ reduced by ΔA ′. Regarding the light emission energy, the average light emission current in the detection periods R-DC, IR-DC, R-AC, and IR-AC is made equal to the conventional average light emission current (Formula 2).

That is,
ΔA ′ = (energy increment in detection period R-AC, IR-AC) / T1
= A × α / T1 (10)
And therefore
A ′ = A−ΔA ′ = A (1−α / T1) (11)
It becomes.

The improvement rate of the S / N is the same as that described in the second embodiment, and the influence of the signal drop of the direct current components R _DC and IR _DC can be ignored, and the change of the S / N in the alternating current components R _AC and IR _AC Become dominant. That is, the S / N improvement rate is equal to Equation 5 in comparison with the prior art.

(Embodiment 4)
FIG. 9 is a waveform diagram for explaining an operation in a pulse oximeter according to another embodiment of the present invention. The pulse oximeter of this embodiment can also use the configuration of the pulse oximeters 10 and 20 described above, and only the driving method of the light emitting elements 2a and 2b by the driving circuit 1 from the digital / analog conversion circuit 74 is different. is there. It should be noted that in the present embodiment, in order to increase the emission energy per unit period T in the detection periods R-AC and IR-AC of the AC components R _AC and IR _AC , the above-described FIG. The adjustment of the indicated pulse width and the increase in the amount of emitted light shown in FIG. 7 are used in combination. Therefore, the decrement −ΔA ′ of the direct current components R _DC and IR _DC and the increment ΔA of the alternating current components R _AC and IR _AC can be reduced, and the burden on the drive circuit 1 can be reduced.

That is, as in FIG. 4B, the detection periods R-AC and IR-AC of the AC components R _AC and IR _AC are increased by α, respectively, and the detection periods R− of the DC components R _DC and IR _DC are increased. It is assumed that DC and IR-DC are decreased by α, respectively. Furthermore, when the decrement of the direct current components R _DC and IR _DC is ΔA ″ and the increment of the alternating current components R _AC and IR _AC is ΔA ′ ″ with respect to the standard value A, the detection period R-DC or IR-DC The light emission current A '' at
A ″ = (T1−α) / T × (A−ΔA ″) (12)
The light emission current A ′ ″ in the detection period R-AC or IR-AC is
A ′ ″ = (T1 + α) / T × (A + ΔA ′ ″) (13)
It becomes.

With respect to the light emission energy, the average light emission current in the detection periods R-DC, IR-DC, R-AC, and IR-AC is made equal to Equation 2 which is the conventional average light emission current. That means
{(T1-α) / T × (A−ΔA ″) + (T1 + α) / T × (A + ΔA ′ ″)} / 2
= A × T1 / T (14)
If you organize,
ΔA ″ −ΔA ′ ″ = (ΔA ″ + ΔA ′ ″) × α / T1 (15)
Since (ΔA ″ + ΔA ′ ″) × α / T1> 0, it is necessary to set the amplitude to satisfy ΔA ″> ΔA ″ ′.

Regarding S / N, from Equation 5 of improvement by pulse width α and increased amplitude ratio = (A + ΔA ″) / A,
S / N = {(A + ΔA '') / A} × {(T1 + α-T2) / (T1-T2)} 1/2
... (16)
It becomes.

DESCRIPTION OF SYMBOLS 10,20 Pulse oximeter 1 Drive circuit 2a Red light emitting element 2b Infrared light emitting element 3 Living body 4 Light receiving element 5 Current voltage conversion circuit 51, 61, 81 Amplifier 6 Amplifying circuit 7, 7 'Arithmetic processing part 71 Analog / digital Conversion circuit 72, 72 'Calculation unit 721 Integration unit 722 Average value calculation unit 723 SpO ₂ calculation unit 73 Timing control unit 74 Digital / analog conversion circuit 75 Counter 8 Double integration circuit 82 Comparator C1 Integration capacitors R1, R3 Feedback resistor R2, R4 Input resistance R5 Discharge resistance SW1, SW2 switch

Claims (4)

A pulse that irradiates a living tissue with blood flow with light of the first and second wavelengths and calculates blood oxygen saturation based on the ratio of the amount of received light transmitted or reflected by the living tissue. In the oximeter,
First and second light sources that respectively generate light of the first and second wavelengths;
A light emission controller for driving the first and second light sources;
A light receiving element that receives light of the first and second wavelengths transmitted or reflected by the biological tissue;
An arithmetic processing unit that calculates the blood oxygen saturation based on the ratio of the amount of received light at the light receiving element;
The arithmetic processing unit uses a ratio of measured values obtained by sequentially measuring the fluctuation component and the non-fluctuation component of the transmitted or reflected light as the received light amount at the first and second wavelengths,
The light emission control unit has a predetermined period for detecting a fluctuation component and a non-fluctuation component in a first wavelength, and for detecting a fluctuation component and a non-fluctuation component in a second wavelength. The pulse oximeter is repeatedly repeated each time, and the emission energy per predetermined period is increased in the detection period of the fluctuation component in comparison with the detection period of the non-fluctuation component.   The light emission control unit increases the emission energy per the predetermined cycle and improves the S / N of the fluctuation component by making the detection period of the fluctuation component longer than the detection period of the non-fluctuation component. The pulse oximeter according to claim 1, wherein:   The light emission control unit increases the light emission energy per the predetermined period by increasing the light emission amount in the fluctuation component detection period compared to the light emission quantity in the non-fluctuation component detection period. 2. The pulse oximeter according to claim 1, wherein the S / N ratio of the gas is improved.   The light emission control unit makes the detection period of the fluctuation component longer than the detection period of the non-fluctuation component, and sets the light emission quantity in the detection period of the fluctuation component to the light emission quantity in the detection period of the non-fluctuation component 2. The pulse oximeter according to claim 1, wherein the emission energy per predetermined period is increased by improving the S / N ratio of the fluctuation component by increasing the emission energy.

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