CN102579053A

CN102579053A - Reflective pulse blood oxygen detecting method based on diffusion theory

Info

Publication number: CN102579053A
Application number: CN201210053496XA
Authority: CN
Inventors: 赵会娟; 孟伟; 刘明; 高峰
Original assignee: Tianjin University
Current assignee: Tianjin University
Priority date: 2012-03-02
Filing date: 2012-03-02
Publication date: 2012-07-18

Abstract

The invention belongs to the technical field of human body physiological parameter measurement, and relates to a reflective pulse blood oxygen detecting method based on diffusion theory, which includes the steps: (1) establishing the relationship between organizer absorption coefficient and SaO2 (oxygen saturation); (2) establishing the relationship between reflection light intensity and organizer absorption coefficient; (3) establishing the relationship between an R value and the reflection light intensity; (4) acquiring pulse wave signals of a red light wavelength and a near-infrared light wavelength; (5) detecting features of pulse wave signals of the infrared light wavelength to extract peak values which are the maximum value and the minimal value of the reflection light intensity; (6) calculating blood oxygen saturation coefficient R; and (7) plugging the R value into a calibration curve to obtain needed blood oxygen saturation coefficient in real time. The reflective pulse blood oxygen detecting method based on the diffusion theory can be used for realizing non-invasive real-time measurement, and is higher in precision and wider in application range.

Description

Reflection type pulse blood oxygen detection method based on diffusion theory

Technical Field

The invention belongs to the technical field of human body physiological parameter measurement, and relates to a pulse blood oxygen detection method.

Background

The physiological activities of the human body require internal supply of energy and nutrients, and the activities performed are different, requiring different energies. Blood transports oxygen and nutrients to organs and plays an important role in human metabolism. Hypoxia affects the normal operation of a series of functions of the human body, causes hypoxia symptoms in a light case, and endangers life in a severe case. Therefore, the monitoring of the oxygen transport and combination process in human tissues and the oxygen concentration in human internal tissues and blood have far-reaching significance for clinical diagnosis and treatment work.

The detection of the blood oxygen saturation provides effective clinical basis for doctors in time in the clinical anesthesia operation and intensive care patient monitoring application, and is an indispensable important index for clinical monitoring. Has important significance in the fields of newborn infant, premature infant monitoring, brain surgery, cardiovascular surgery and the like. The application of the pulse oximeter is very important in places where the blood oxygen saturation values of patients are very unstable, such as hospital intensive care units, operating rooms and emergency departments. It is also commonly used for the need and efficiency of positive pressure ventilation used in the treatment of sleep apnea.

The advantage of non-invasive measurement over invasive measurement is that it does not cause pain to the subject, while ensuring real-time, rapid, and accurate measurement. Realizes the functions of monitoring, adjuvant therapy, pathological research and the like. The detection is convenient and the application range is wider.

The non-invasive blood oxygen measurement is based on the principle that the total amount of blood changes along with the pulsation of the artery, so that the absorption of the arterial blood to light changes, and the essence of the non-invasive blood oxygen measurement is the change of an absorption coefficient, and the size of the absorption coefficient is influenced by the blood oxygen saturation value. The blood oxygen saturation in the blood at the moment can be calculated by measuring the absorption of the light by the human tissue.

Although the pulse oximetry is widely used clinically, in actual practice, there are still some problems with the pulse oximetry. The oximeter taking the original Lambert beer law as a theoretical model takes the change of blood volume and arterial blood absorption coefficient as an investigation object, and ignores the scattering of light by tissues so as to cause the deviation of a calculation result. The blood oxygen value is generally measured with the accuracy of 2% -6% in the range of 75% -100%, and the error of the measurement value is larger under the condition of lower blood oxygen saturation.

According to the diffusion transport theory of light, Marble and Burns et al propose a new pulse oximetry transmission model in 1994, which model is based on the three-dimensional photon diffusion theory and assists the existing system to a certain extent. According to the theory, the influence of the multiple scattering effect on the calibration curve is analyzed, and the reason that the measurement is inaccurate under the condition of low oxygen saturation is proved to be mainly due to the existence of the multiple scattering effect. This provides a new direction for the research of oximetry.

Disclosure of Invention

The invention aims to improve the traditional arterial blood oxygen saturation measuring method, and provides a pulse blood oxygen detecting method which can realize non-invasive and real-time measurement and has higher precision and wider application range. Therefore, the invention adopts the following technical scheme:

a reflection type pulse blood oxygen detection method based on diffusion theory comprises the following steps:

(1) establishing tissue body absorption coefficient mu_aAnd S_aO₂The relationship between:

defining the tissue volume absorption coefficient mu_aIs composed of

Wherein V_aAnd V_vThe volume fractions of arterial and venous blood respectively,in order to remove the absorption coefficient of other tissues than blood,

is the absorption coefficient of the arterial blood and is,

the venous blood absorption coefficient and has:

wherein H is hematocrit, μ_a(HbO₂) Is the absorption coefficient of oxygenated hemoglobin in blood cells, mu_a(Hb) is blood fineAbsorption coefficient of reduced hemoglobin in cells, SaO2 is arterial oxygen saturation, SvO2 is venous oxygen saturation;

let the arterial vessel volume fraction V_aGet the formula of minimum absorption coefficient as 0

Setting arterial vasodilatation time V_aThe maximum absorption coefficient is obtained as a formula of 0.1

(2) Establishing the reflection light intensity R (rho) and the tissue body absorption coefficient mu_αThe relationship between:

the expression of the theoretical value of the emergent light intensity of the tissue body is organized by a diffusion equation model and is as follows:

where D is the diffusion coefficient, ρ is the radial distance between the light source and the detector, μ_aAnd mu'_sAbsorption coefficient and reduced scattering coefficient for the skin tissue as a whole;

when mu is_aTake the minimum value mu_aminThe reflected light intensity R (rho) is taken to the maximum value R_max(ρ); when mu is_aTaking the maximum value mu_amaxThe reflected light intensity R (rho) is taken to the minimum value R_min(ρ)；

(3) Establishing a relationship between R value and R (rho)

The absorption coefficient R value of the tissue body is controlled by the maximum reflection light intensity R_max(p) and the minimum reflected intensity R_min(ρ) represents, S is established_aO₂Making a calibration curve reflecting the relation between the R and the R according to the one-to-one corresponding relation of the R and the R;

(4) collecting pulse wave signals under red light wavelength and near infrared light wavelength;

(5) and (3) carrying out characteristic detection on the pulse wave signal waveform under the infrared light wavelength, wherein the extracted peak value is the maximum value and the minimum value of the reflected light intensity:

(6) calculating the blood oxygen saturation coefficient

Wherein,

is the maximum reflected light intensity under the irradiation of red light,

the minimum reflected light intensity under the irradiation of red light;

the maximum reflected light intensity under the irradiation of near infrared light,

the minimum reflected light intensity under the irradiation of near infrared light;

(7) and (4) bringing the R value obtained by actual measurement in the step (6) into the calibration curve to obtain the required blood oxygen saturation value in real time.

The reflection type pulse blood oxygen measuring method based on the diffusion theory has the advantages that: (1) the measurement accuracy of the calculation model taking the diffusion equation as the blood oxygen saturation degree is improved to a certain extent compared with the Lambert beer law, and the measurement range is expanded to a certain extent. (2) The reflection type measuring method is used for replacing the transmission type measuring method, so that the application range is wider, and the use is more convenient. (3) The real-time performance of the difference is guaranteed by adopting the circular queue, and the individual difference set by the threshold is eliminated by adopting a self-learning method, so that the real-time performance, the effectiveness and the stability of the detected signal are guaranteed. (4) The noninvasive and real-time pulse blood oxygen saturation detection is realized.

Drawings

FIG. 1 is a system block diagram of a pulse oximetry platform;

FIG. 2 is a flow chart of an embodiment of a pulse oximetry method;

FIG. 3 is a schematic diagram of an analytical solution of a diffusion equation in a planar semi-infinite space using an extrapolation boundary method;

FIG. 4 is a graph showing the effect of detecting characteristic points of volume pulse waves.

Detailed Description

The principle of the hardware platform on which the detection method of the invention is based is shown in figure 2, the pulse blood oxygen saturation measurement platform comprises a voltage conversion circuit for providing stable and proper voltage for the whole hardware platform, a light source driving circuit for supplying power to a dual-wavelength LED, a switch circuit for generating two columns of orthogonal pulse sequences to respectively modulate light with two wavelengths and control the LED to be lightened, a D-LED type dual-wavelength light emitting Diode, a Photosensitive Diode (PD) for converting the light intensity of the received light into current I, an I-U conversion circuit for converting a current signal which is not easy to measure into a voltage signal which is easy to measure, a sampling and holding circuit for filtering ambient light, a differential amplification circuit and a correction circuit, a DSP application template produced by TI corporation for further processing of the digital signal obtained after the conversion by the ADC, and a PC for displaying the analysis detection result in real time.

The present invention will be described in more detail with reference to the following embodiments and the accompanying drawings.

As shown in FIG. 1, the flow chart of the reflection type pulse oximetry method based on diffusion theory of the present invention includes known optical parameters of tissue under corresponding wavelengths, the calculated absorption coefficients of arteriovenous vessels are derived from the known quantity and the correlation formula, and on the basis, the maximum and minimum reflected light intensities obtained by the diffusion equation and the R value further calculated by definition are used. Thereby establishing S_aO₂Corresponding relation with R and drawing corresponding calibration curve as real-time solving S by using R value_aO₂The basis of (1). In the actual measurement, the pulse oxyhemoglobin saturation measuring platform shown in figure two is used for detecting the waveform of the pulse wave and displaying the waveform on a computer in real time, the peak value of the output light intensity after being reflected by the tissue body is extracted by a self-adaptive differential threshold method, the real-time R value is further obtained, the R value is brought into the calibration curve, and the real-time S can be obtained by reading the graph_aO₂。

As shown in fig. 3, the actual boundary conditions are processed by using an extrapolated boundary method to exploit Dirichlet boundary conditions on the virtual boundaries. The original text is changed into z ═ z₀And is located at z ═ z₀The negative mirror image point source corresponding to the point source at moves to z ═ z₀+2z_b) To (3). The extrapolation boundary method is more in line with the actual situation.

As shown in fig. 4, the waveform is a pulse wave signal at the wavelength of near infrared light, and the red points are the maximum value point and the minimum value point of all waveforms. And calculating the R value in real time according to the characteristic points.

The invention relates to a reflection type pulse blood oxygen detection method based on diffusion theory, which utilizes a diffusion theory equation, selects a semi-infinite tissue layer model, adopts a reflection type measurement method, analyzes tissue components, lists an expression for calculating optical parameters of the tissue components, and brings the expression into the diffusion equation to calculate the blood oxygen saturation. Is determined by S_aO₂A forward model of the R value is derived. The blood oxygen saturation of the tissue body is calculated according to the reconstructed value of the optical parameter, and noninvasive and real-time detection of the pulse blood oxygen saturation can be realized.

In the embodiment of the invention, the selected LED is a D-LED type, namely a dual-wavelength light emitting diode, and respectively emits light with the wavelength of 660nm and 880 nm. Two lines of light waves are modulated by orthogonal pulse sequences, so that the double-wavelength LED red light and the infrared light emitting diode are alternately lightened, and the two lines of light waves are respectively detected. The light intensity of the received light is converted into a current I by a Photodiode (PD), and the current I is converted into a voltage U ═ V by a resistor with a resistance of 100K_PDAnd (6) outputting. The current signal which is not easy to measure is converted into the voltage signal which is easy to measure, so that the subsequent extraction and processing are convenient. And a sample hold chip LF398 is adopted to detect the ambient light and the dark current, and a subsequent differential amplification circuit is utilized to filter the ambient light and the dark current. And D/A conversion is carried out on the signal by using an F2812 chip which can realize integration of other functional modules such as a DSP and an A/D. And uploading the processed data to a computer in real time.

Tissue volume absorption coefficient mu by intermediate variable_aEstablishing the blood oxygen saturation S_aO₂One-to-one correspondence relationship with R for obtaining S in real time by calculating R value_aO₂。

(1)μ_aAnd S_aO₂The relationship between them. Mu.s_aFrom the hematocrit H, the absorption coefficient μ of oxygenated hemoglobin in the blood cells_a(HbO₂) In blood cellsAbsorption coefficient mu of reduced hemoglobin_a(Hb) and arterial oxygen saturation level S_aO₂Venous oxygen saturation S_vO₂Joint determination, S_aO₂When the concentration is changed between 20% and 100%, S_vO₂＝S_aO₂-10％。

(2) R value and mu_aThe relationship between them. The intensity of the reflected light is measured by mu_aDetermine, for mu_aTaking different values, the reflected light intensity changes accordingly. When the artery vessel is contracted, the artery vessel volume fraction can be approximately regarded as 0, and then the minimum absorption coefficient mu of the tissue body can be obtained_aminThe corresponding reflected light intensity is maximum; when the artery vasodilation is carried out, the absorption coefficient is the sum of the absorption coefficients of the arterial blood, the venous blood and the bloodless tissue, and the maximum absorption coefficient mu of the tissue body can be obtained_amaxThe corresponding reflected light intensity is minimal. The maximum value and the minimum value of the reflected light intensity can be used for representing the R value.

To this point R value and S_aO₂The one-to-one correspondence between the two can be determined, and accordingly, a calibration curve showing the relationship between the two is drawn.

The pulse wave signal waveforms under the red light wavelength and the near infrared light wavelength which are displayed on a computer in real time are subjected to feature extraction through a self-adaptive differential threshold method, and the peak value of the waveform is obtained and used for calculating the value of the blood oxygen saturation coefficient R. And substituting the R value measured in real time into the calibration curve to obtain the required blood oxygen saturation value. The specific implementation steps are as follows:

1. tissue absorption coefficient mu by means of intermediate variables_aEstablishing S by diffusion equation_aO₂And a one-to-one correspondence with R. The specific process is as follows:

1) establishing tissue body absorption coefficient mu_aAnd S_aO₂The relationship between them.

The total absorption coefficient of the tissue volume can be written as a weighted sum of the absorption coefficients of the tissues of the layers. Is formulated as:

wherein V_aAnd V_vThe volume fractions of arterial and venous blood respectively,

in order to remove the absorption coefficient of other tissues than blood,

is the absorption coefficient of the arterial blood and is,

the venous blood absorption coefficient and has:

wherein H is hematocrit, μ_a(HbO₂) Is the absorption coefficient of oxygenated hemoglobin in blood cells, mu_a(Hb) is the absorption coefficient of reduced hemoglobin in blood cells, SaO2 is the arterial oxygen saturation, and SvO2 is the venous oxygen saturation.

When the artery vessel is contracted, the artery vessel volume fraction V can be approximately regarded as_aSubstituting equation (1) with 0 results in a minimum absorption coefficient

When the artery vasodilation is performed, the absorption coefficient is the sum of the absorption coefficients of the arterial blood, the venous blood and the bloodless tissue, and V at the moment is set_aThe maximum absorption coefficient obtained by the formula (1) is 0.1

2) Establishing the reflection light intensity R (rho) and the tissue body absorption coefficient mu_aThe relationship between them.

The invention adopts a semi-infinite tissue layer model to simulate fingers. The light source is simply regarded as a monochromatic point light source, and the wavelength of the monochromatic point light source is respectively in the red light region and the near infrared light region of the spectrum. In the reflective case, the detector is in line with the light source at a radial distance ρ. The detector is a reflection type detector, and the detection mode is that the source is detected on the same side of the finger. And applying a diffusion equation on the half-limit tissue layer, and solving by using a full-space solution and mirror image principle and an extrapolation boundary method.

where D is the diffusion coefficient, ρ is the radial distance between the light source and the detector, μ_aAnd mu'_sAbsorption coefficient and reduced scattering coefficient for the skin tissue as a whole, z_x，mTo accommodate diffusionP-dipoles of equation boundary conditions, for the same m, z_3，mAnd z_4，mAnd m can be a pair of dipoles without any limit, and only three groups, namely m is 0 and +/-1, are simply taken for calculation on the premise of ensuring the accuracy. This establishes the reflected light intensity R (ρ) and the tissue volume absorption coefficient μ_aThe relationship between them.

When mu is_aTake the minimum value mu_aminThe reflected light intensity R (rho) may then be taken to a maximum value R_max(ρ); when mu is_aTaking the maximum value mu_amaxThe reflected light intensity R (rho) can be taken to the minimum value R_min(ρ)；

3) And establishing a relation between the R value and R (rho).

The value of the tissue absorption coefficient R can be determined by the maximum reflected light intensity R_max(p) and the minimum reflected intensity R_min(ρ) to establish S_aO₂And making a calibration curve reflecting the relation between the R and the R according to the one-to-one corresponding relation of the R and the R.

2. Collecting pulse wave signals under red light wavelength and near infrared light wavelength;

3. and (3) carrying out characteristic detection on the pulse wave signal waveform under the wavelength of near infrared light, wherein the extracted peak values are the maximum value and the minimum value of the reflected light intensity.

4. Calculating the blood oxygen saturation coefficient

R = \frac{(R_{\max}^{r} - R_{\min}^{r}) / R_{\max}^{r}}{(R_{\max}^{ir} - R_{\min}^{ir}) / R_{\max}^{ir}} - - - (7)

Wherein

Is the maximum reflected light intensity under the irradiation of red light,

the minimum reflected light intensity under the irradiation of red light;

the minimum reflected light intensity under the irradiation of near infrared light.

5. And (3) substituting the R value obtained by actual measurement and calculation into the calibration curve established in the step (1), so as to obtain the required blood oxygen saturation value in real time.

Claims

1. A reflection type pulse blood oxygen detection method based on diffusion theory comprises the following steps:

defining the tissue volume absorption coefficient mu_aIs composed ofWherein V_aAnd V_vThe volume fractions of arterial and venous blood respectively,

in order to remove the absorption coefficient of other tissues than blood,

is the absorption coefficient of the arterial blood and is,

the venous blood absorption coefficient and has:

wherein H is hematocrit, μ_a(HbO₂) Is the absorption coefficient of oxygenated hemoglobin in blood cells, mu_a(Hb) is the absorption coefficient of reduced hemoglobin in blood cells, SaO2 is the arterial oxygen saturation, SvO2 is the venous oxygen saturation;

(2) Establishing the reflection light intensity R (rho) and the tissue body absorption coefficient mu_aThe relationship between:

(3) Establishing a relationship between R value and R (rho)

(5) detecting the characteristics of the pulse wave signal waveform under the infrared light wavelength, wherein the extracted peak values are the maximum value and the minimum value of the reflected light intensity;

(6) calculating the blood oxygen saturation coefficientWherein,

is the maximum reflected light intensity under the irradiation of red light,

the minimum reflected light intensity under the irradiation of red light;