自适应LMS算法的无创动脉血氧饱和度.pdf

978-1-4244-6497-5/10/$26.00 2010 IEEE 13762010 3rd International Conference on Biomedical Engineering and Informatics(BMEI 2010)A Design of Noninvasive Tester of Human Arterial Oxygen Saturation Wang Yongqing，Zhang Zhipeng，Wang Hongrui，Liu Xiuling,Sun Rongxia(College of Electronic&Informational Engineering,Hebei University,Hebei Baoding,071002)Abstract-The testing circuit of body noninvasive arterial oxygen saturation is designed according to the characteristics that the light absorption of arterial blood varies with pulse.The driving circuit of testing source and anti-Rossi circuit of compound signal are controlled by DSP sequence signal,and the sending of the testing signal is synchronous with the separation-receiving signal.The high frequency interference of the stray light is attenuated adopting second-order low pass filter of voltage control source.The constant segment and pulsating quantity is extracted respectively from the compound signals by the anti-Rossi circuit of compound signal,second-order high pass filter and secondary low-pass filter.The reverse phase circuit and AC signal amplifying circuit is used respectively for benefiting AD sampling and raising the testing precision.A way to employ moving average method to eliminate noise of the oxyhemoglobin AC signal is proposed,which has raised the testing accuracy.Keywords-arterial oxygen saturation;noninvasive;testing;oxygen saturation compound signal;moving average method I.INTRODUCTION The arterial oxygen saturation is a important index which estimates whether human body circulatory system and the respiratory system appears handicap 1.The blood flows through the lungs in the periodic diastole and systole.A certain amount of reduced hemoglobin in blood is combined with oxygen in the alveolus,thus becoming hematoglobulin.The oxygen is carried into capillary vessel,and then the oxygen is released for providing metabolism of the cell tissue with energy.At present,the measurement method of arterial oxygen saturation is divided into invasive measure and noninvasive measure.Because in the invasive measure the testing result lacks timeliness,causing suffering to patient easily,and the operation method is too complicated.So the noninvasive measure,which characterized by continuity or real-time or safety,gradually replaces the invasive measure.II.Measuring Principle of Arterial Oxygen Saturation The noninvasive tester of arterial oxygen saturation is designed according to the characteristics that the light absorption of arterial blood varies with pulse.Because the arterial pulses was present periodically.The light absorption spectrum of arterial blood is a pulsating change,which is called pulse component(namely AC signal I).When the systole is presented,the most blood volume is presented,and the minimum measuring light intensity is shown.When the diastole is presented,the minimum blood volume is presented,and the most measuring light intensity is shown.But the light absorption intensity is regarded as a constant component(namely DC signal I)by other human tissue,such as venous blood or muscle or cutis.As shown in figure 1.Figure1.Absorption Curve of the Ray by Human Tissue The arterial oxygen saturation means the percent of the content of the hematoglobulin in the content of all incorporable oxygen in the blood 2,namely:%10022+=HbHbOHbOCCCS （1）When the light of special wavelengths irradiates the testing body part,according to the Lambert-Beer law,and the light absorption intensity is mainly related to the content of haemoglobin in the blood 3,the intensity of the transmitted themeasuringlightintensityI DCImaxIminIAC 1377light through the testing body part is LCLCLCDCHbHbHbOHbOeeeII=22000 (2)Among which,The incident light intensity is I0.The total absorption coefficients of human tissue and venous blood is 0.The material concentration of human tissue and venous blood is C0.The path length of the transmitted light is L.The absorption coefficients of hematoglobulin in arterial is HbO2,and the material concentration of that is CHbO2.The absorption coefficients of reduced hemoglobin is Hb,and the material concentration of that is CHb.The light path length L varies with the diastole and systole.If the variation of the light path length is L,the variation of the light intensity is IAC,then)(02200LLCCLCACDCHbHbHbOHbOeeIII+=(3)If the testing body part is irradiated by two kinds of ray at wavelength of1 and2,it deduces that)()1(211211HbHbOHbHbODCACCCLSSII+=(4)()1(222222HbHbOHbHbODCACCCLSSII+=(5)If the incident light at wavelength of2 is selected,let HbO22=Hb2.And making 2211/DCACDCIIIIRAC=.The arterial oxygen saturation S is 11211122HbHbOHbHbHbOHbRS=(6)According to the red and infrared absorbency spectrum curve of hemoglobin,it is known that there is a great difference in the absorption coefficients between two kinds of haemoglobin at about 650nm,and the absorption coefficients of hematoglobulin is coincident approximatively with the reduced hemoglobin at 850nm950nm,as shown in figure 24.So the two kinds of light source at wavelength of 660nm and 940nm are adopted in the tester.Figure2.The Absorbency Spectrum Curve of Hemoglobin III.Circuit Design and Working principle The circuit block diagram of arterial oxygen saturation monitor is shown in figure 3.The driving circuit of testing source and anti-Rossi circuit of compound signal are controlled by DSP sequence signal,so the sending of the testing signal is synchronous with the separation-receiving signal.The high frequency interference of the stray light is attenuated adopting second-order low pass filter.The constant segment and pulsating quantity is extracted respectively by second-order high pass filter and secondary low pass filter.The reverse circuit and AC signal amplifying circuit are designed to facilitate the testing data acquisition and raise the testing precision.Figure3.The Circuit Block Diagram of Arterial Oxygen Saturation Tester A.Driving Circuit of Light Source The driving circuit of light source of the tester is shown in figure 4(a).Among them,the DSP sequence control signal ends are RLED-CTRL1 or RLED-CTRL2 or IRLED-CTRL1 and IRLED-CTRL2.The DSP sequence control signal with a drivingcircuit of testing sourcelight sourcetesting result display photocellI/V,amplifying circuitlow pass filter,reverse circuithigh pass filterAC signal amplifying circuit low pass filteranti-Rossi circuit of compound signal AD conversionsignal processing sequence signal DSP finger1.51.20.90.60.30Wavelength/nm 600 700 800 900 1000Absorption Coefficients Hematoglobulin Reduced Hemoglobin 1378frequency of 200Hz(as shown in figure 4(b)drives red LED D1 and infrared LED D2 alternant irradiance.At T1,Q1 and Q4 is in conducting state,so the red LED D1 is radiant.At T3,Q2 and Q3 is in conducting state,so the infrared LED D2 is radiant.At T2 and T4,D1 and D2 are all not radiant.It is realized that the testing source works periodically in the following order:red light,extinction,infrared light,extinction.(a)(b)Figure 4(a).The Driving Circuit of Testing Source Figure 4(b).DSP Sequence Control Signal B.Photoelectric and I/V Conversion and Amplifying Circuit The tester uses the silicon photocell as photoelectric conversion element.The spectral response range of the silicon photocell is 400nm1100nm 5,and its peak wavelength is about 850nm.So the silicon photocells matche the two the light sources with the peak wavelength at 850nm and 940nm.The silicon photocell and I/V convertor which of the input impedance is equivalent to 0 is connected to keep the silicon photocell in a quasi short-circuit state,as shown in figure 5.In order for that the change is linear between the short circuit current and illumination intensity,the tester adopts reversed input type of I/V conversion circuit to convert the current signal which has been measured by the photocell into voltage signal.It is known that the pulse oxygen saturation signal is a anti-phase signal according to figure 1.The anti-phase amplifying circuit can convert the oxygen saturation signal into in-phase signal.As shown in figure 5,among which 71RiUBTO=(7)Figure 5.Photoelectric and I/V Conversion and Amplifying Circuit C.Primary Second-order Low Pass Filter Various kinds of stray light interference signal is interwoven with the red light and infrared light signal which is received by the photocell.The main frequency range of human pulse signal is 0.1Hz10Hz 6.So the power interference and other high frequency interference signal,which is above 50Hz,is attenuated farthest by low pass filter with the cutoff frequency of 20Hz,and the oxygen saturation compound signal(UDC+UAC)is extracted.Because the active filter is characterized by those following advantages,the open loop gain is high,the frequency adjustment range is wide,the frequency stability and its precision is well.Consequently,the tester adopts the second order voltage-controlled voltage source low pass filter,as shown in figure 6.Figure 6.Second-order Low Pass Filter Among them,the cutoff frequency HzRCfc2021=(8)Get C1=C2=C=0.1,then R10=R11=67.8 K.Because the amplitude-frequency characteristic is the best of all when Q0.707.So Aup=31/Q1.586.And because 586.1189=+=RRAup (9)According to the principle of maintaining a basic symmetry between the external connection resistance of non-inverting input end and that of inverted input end in the second order high pass filter circuit,it deduces that 989810RRRRR+=(10)Solving the simultaneous equations which is comprised of the equation(9)and(10),it gets that R8=5.7K and R9=3.3K.D.Anti-Rossi Circuit of Red and Infrared Compound Signal The output oxygen saturation signal is red and infrared UO1iBTrled-ctrl1rled-ctrl2irled-ctrl1irled-ctrl2T1 T2 T3T4UO1UO2 1379compound signal after the second-order high-pass filter.It is known that according equation(6)the calculate of the oxygen saturation requires red light DC and AC signal(UDC1 and UAC1),infrared light DC and AC signal(UDC2 and UAC2).So the anti-Rossi circuit red light and infrared light signal is designed in the tester,as shown in figure 7.Figure 7.The anti-Rossi circuit of red and infrared Compound signal The tester uses four bidirectional analog switch CD4066(U4)as signal separate elements.When the CNTL which is the control end of CD4066 is high level,the switch is in conducting state,so the output signal is equal to the input signal.When CNTL is low level,the switch is in cut-off state.The four gating ends are controlled by DSP sequence signal(Figure4(b).At T1,the switch U4B is in conducting state,the red light signal is exported.At T3,the switch U4C is in conducting state,the infrared light signal is exported.At T2 and T4 the output signal is 0.The separation of the red light signal from infrared light signal is realized.And the red light and infrared light sequence signal after the anti-Rossi circuit corresponds with the drive sequence signal of the light source.The voltage follower U5,which has the function of impedance conversion,is designed after the anti-Rossi circuit in order to reduce the attenuation of weak oxygen saturation signal in the back circuit.E.Second-order high pass filter It is known that according equation(6)the calculation of the oxygen saturation requires the pulse signal UDC and the constant signal UAC.So the tester use the second-order voltage-controlled voltage source high-pass filter to eliminate the constant component,and its cutoff frequency is 0.1 Hz,as shown in Figure 8.The output signal of high pass filter is the pulse component of the pulse oxygen saturation signal UAC1 and UAC2.Because the cutoff frequency fc=1/2RC=0.1Hz,let C3=C4=1F，so R14=R15=R=1.6M.In the same way with second order low pass filter,the amplitude-frequency characteristic is the best of all when that Q0.707.The calculation shows that R12=4.3M,R13=2.5M.Figure 8.Second-order High Pass Filter F.Amplifying Circuit of Ac Signals The oxygen saturation pulse component is far less than the constant component after the high pass filter.The red and infrared pulse component UAC1 and UAC2 magnifies certain multiple in order to facilitate data processing,and raise the calculation precision.Only when UAC1 and UAC2 magnifies the same multiple do the calculation of oxygen saturation can not be affected.So the two-path amplifying circuit is designed to magnify respectively the red and infrared pulse signal UAC1 and UAC2,as shown in Figure9 Figure 9.Amplifying Circuit of AC Signals G.Secondary low pass filter and reverse circuit The pulse component is weaker than the constant component in oxygen saturation signal.In order to facilitate AD collection,simplify the software program,and raise testing date precision,the tester extracts the constant component UDC from the compound signal(UAC+UDC)by the low pass filter with the cutoff frequency of 0.1Hz,as shown in figure10.In addition,according to the equation(7)the output signal of the anti-Rossi circuit of the compound signal UDC+UAC0.And because that UACUDC,so that UDC0.In order to facilitate data processing in the AD conversion,the tester adopts reverse circuit which makes that UDC*0.UO5(UO5)UO4(UO4UO4(UO4)UO3(UO3)UO3UO3UO2 1380 Figure10.Secondary Low Pass Filter and Reverse Circuit IV.Data Processing by Moving Average Method The moving average method is a analysis method of the time series which of eliminating the anomalies interference in the testing date by the function of average and move.So the tester uses the method to eliminate low frequency interference signal which exists in the pulse component of the oxygen saturation signal.IAC which is in the oxygen saturation formula(equation 6)is the maximum value of the collected oxygen saturation AC signal UACmax.The moving average of the maximum value of several cardiac cycles of red light signal and infrared light signal are collected by the moving average method.Then the oxygen saturation is calculated with the equation(6)which the moving average is put in.First using the specified threshold value K to find the UACmax1 by bilateral threshold detection method.And UACmax1 is regarded as the signal base point.Then the second maximum value UACmax2 is found.That UACmax3，UACmax4，UACmax5，UACmaxnis found in turn at regular intervals of time length between UACmax1 and UACmax2.The moving average of the maximum value is:nUUniiACAC=1maxmax*(11)V.CONCLUSION The testing circuit of body noninvasive arterial oxygen saturation is designed according to the law that the light absorption of arterial blood varies with pulse.The second-order voltage-controlled voltage source high-pass filter is adopted to reduce the effect of the stray light on the testing to a certain extent.The reverse circuit is designed,thus facilitating data acquisition and AD conversion.The two-path in-phase proportion amplifying circuit is used to magnify the weak oxygen saturation AC signal equally,thus raising the testing precision.The moving average method is adopted to eliminate interference signal,thus r