CN112089418A

CN112089418A - Detection method of thoracic electrical impedance based on frequency conversion and amplitude modulation method of human tissue conductivity

Info

Publication number: CN112089418A
Application number: CN202011027046.4A
Authority: CN
Inventors: 汪金刚; 闫阳天; 张一鸣
Original assignee: Chongqing University
Current assignee: Chongqing University
Priority date: 2020-09-25
Filing date: 2020-09-25
Publication date: 2020-12-18
Anticipated expiration: 2040-09-25
Also published as: CN112089418B

Abstract

The invention relates to a method for detecting electrical impedance of a thoracic cavity based on a method of frequency conversion and amplitude modulation of electrical conductivity of human tissue, and belongs to the technical field of medical detection. The method includes: S1: constructing a thoracic cavity electrical impedance measurement system, and using the system to output a specific detection amplitude and frequency; S2: determining the measurement current frequency required for thoracic cavity electrical impedance detection according to the relationship between the electrical conductivity of the biological tissue and the detection frequency; S3: According to the response of the human body to the current excitation at different amplitudes, determine the measurement current amplitude required for the detection of thoracic electrical impedance; S4: After setting a specific measurement frequency, use the cross four-electrode method to carry out detection experiments respectively, and obtain the same measurement frequency The mean value of measurements at different amplitudes was used to complete a set of tests; the fitting relationship between the measured thoracic electrical impedance and pulmonary expiratory air volume at different detection frequencies was determined through multiple sets of experiments. The invention realizes the continuous online rapid non-invasive detection of lung breathing ability, and improves the security, comfort and compliance of detection.

Description

Thoracic cavity electrical impedance detection method based on human tissue conductivity frequency conversion amplitude modulation method

Technical Field

The invention belongs to the technical field of medical detection, and relates to a thoracic electrical impedance detection method based on a human tissue conductivity frequency conversion amplitude modulation method.

Background

In the prior art, there are two main ways for detecting common lung function: the first is volume measurement and shaping, and the instrument realizes the detection of the change rule of the gas volume of the lung by measuring the gas volume change of a buoy or a piston cavity connected with the respiratory tract of a person to be detected; the second is flow measurement, the instrument measures the gas flow with a certain flow cross section area, and then integrates the time to obtain the volume of the respiratory gas, thereby realizing the detection of the change of the gas volume in the lung.

Both of the above-mentioned detection methods have some considerable problems.

1) When the volume measurement sizing detector is used for detection, the inertia of the buoy and the friction force generated when the piston moves can cause serious distortion of the measurement result. Moreover, the gas storage cavities of the buoy and the piston and the breathing pipeline can be reused in use, and the risk of cross infection is extremely high.

2) The flow measuring type detector has higher accuracy than a capacity measuring type detector, and the price and the material consumption of the detector are expensive; part of the airway can still be reused in the test process, and certain cross infection risks still exist.

Since both of these two common lung function detectors require the respiratory airway to be connected to the measurement airway of the device, the potential for cross-infection inevitably exists. Patients also experience discomfort during the measurement, resulting in poor compliance at the time of testing. And real-time online monitoring cannot be carried out, and the requirements of accuracy, safety and convenience cannot be met.

The invention provides a thoracic cavity electrical impedance rapid measurement method based on a variable frequency amplitude modulation method, aiming at the problems of high detection cost, high cross infection risk, single detection mode of a bioelectrical impedance measurement system and the like of the existing lung function detection technology.

Disclosure of Invention

In view of the above, the present invention aims to provide a thoracic electrical impedance non-invasive rapid measurement method based on a human tissue conductivity frequency conversion amplitude modulation method, which utilizes a combined design of hardware and software to establish a thoracic electrical impedance parameter acquisition system based on the frequency conversion amplitude modulation method, so as to realize real-time detection of thoracic electrical impedance. The invention can realize the rapid detection of non-respiratory tract contact and meet the requirements of a lung function detection device on safety, accuracy and convenience.

In order to achieve the purpose, the invention provides the following technical scheme:

a thoracic electrical impedance detection method based on a human tissue conductivity frequency conversion amplitude modulation method is characterized in that a thoracic electrical impedance measurement system is constructed, output of specific measurement current frequency and measurement current amplitude is achieved according to the relationship between human tissue conductivity and measurement frequency, 7 measurement frequencies and 3 measurement amplitudes are set in the detection process, and measurement of different measurement current amplitudes under the same measurement current frequency is achieved by using a cross four-electrode method, and the method comprises the following specific steps:

s1: constructing a thoracic electrical impedance measurement system, outputting a specific detection amplitude and frequency by using the system according to a signal generation principle, increasing a detection mode and improving measurement precision;

s2: according to the change relation between the biological tissue conductivity and the detection frequency, determining the measurement current frequency required by the thoracic electrical impedance detection for comprehensively reflecting the electrical characteristics of the thoracic part, and setting the measurement current frequency band to be 64KHz-1 MHz; in order to realize operability and accuracy of the detection process, firstly, the minimum measurement current frequency is determined to be 64KHz, the maximum measurement current frequency is determined to be 1MHz, and the frequencies of the left end and the right end of a frequency point with a sudden change of slope are selected according to a conductivity-measurement current frequency curve of a measurement current frequency section by the rest measurement current frequencies, wherein the selected frequency is +/-35 percent of the frequency of the sudden change point

S3: determining the measuring current amplitude required by the thoracic cavity electrical impedance detection according to the reaction phenomenon of the human body to current excitation under different amplitudes, and selecting the measuring current amplitude section to be 500 muA-1.5 mA; in order to realize the operability and the rapidity of the detection process, experimental operations show that: when the current amplitude is lower than 500 muA, the electric signal measured by the electrode is very weak, and the subsequent analysis is difficult to realize, and when the current amplitude is higher than 1.5mA, the compliance and comfort of part of detected personnel in the detection process can be greatly reduced, so that the minimum detection amplitude is 500 muA, and the maximum detection amplitude is 1.5 mA; in order to increase the detection mode and improve the measurement precision, 1mA is selected as the detection amplitude in equidistant sampling, and 3 detection amplitudes are determined: 500 μ A, 1mA and 1.5 mA;

s4: respectively carrying out detection experiments with different amplitudes on each frequency selected in the step S2 by using a cross four-electrode method, and calculating the measurement mean values of different measurement current amplitudes under the same measurement current frequency, namely completing a group of detections; and finishing all the selected measurement experiments for measuring the current frequency, and determining the fitting relation between the chest electrical impedance measurement mean value and the lung expiratory air volume under different detection frequencies through the measurement experiments.

Further, in step S1, constructing a thoracic electrical impedance measurement system including an FPGA having a DDS function and a peripheral circuit; the peripheral circuit comprises a single chip microcomputer, a signal generation module, a signal conversion module, a signal processing module, a switch array module and a signal acquisition and demodulation module;

the signal generation module is used for outputting a digital current output signal with specific frequency; the frequency of the output signal is directly controlled by the singlechip, and the amplitude of the output signal is realized by changing the reference voltage by the singlechip through the signal conversion module; then the current required by detection is output through the signal processing module, the switch array module is used for controlling the current to be injected into the thoracic cavity, the signal acquisition and demodulation module is used for realizing signal demodulation, filtering amplification and analog-to-digital conversion to obtain a measured impedance value, the single chip microcomputer is used for controlling the peripheral circuit and the FPGA to realize the setting of the amplitude and the frequency of the detection signal and transmit the measured impedance value to the upper computer system.

Further, the signal generation module generates a waveform digital signal by using an FPGA (field programmable gate array) and utilizing a DDS (direct digital synthesizer) technology according to a frequency control word and a waveform control word provided by the STM32F103 singlechip, and the waveform digital signal is composed of a phase accumulator, a phase register and a waveform lookup table;

the synthesis frequency of the DDS is:

the lowest frequency of output is:

wherein ,f₁Is a reference clock frequency, f₂To output the signal frequency, K is the frequency control word and N is the phase accumulator and register word length.

Further, in step S2, a plurality of measured current frequencies are determined and sampled from the measured current frequency band of 64KHz to 1MHz, the selection of the detection frequency point is based on the variation relationship between the conductivity and the frequency, so as to realize the irregular change of the detection frequency, and the determined measured current frequencies required by the thoracic electrical impedance detection are set to 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz, in order to ensure the operability, rapidity and accuracy of the detection at the same time.

Further, in step S3, a plurality of amplitudes are sampled from the measurement current amplitude segment of 500 μ a to 1.5mA, and the measurement current amplitudes required for detecting the thoracic electrical impedance are determined to be 500 μ a, 1mA, and 1.5mA in order to ensure the operability and rapidity of detection.

Further, in step S4, the fitting relationship between the measured thoracic electrical impedance and the amount of the expired air in the lung is:

Z＝Ae^Bx+Ce^Dx

where Z represents the thoracic electrical impedance, x represents the amount of injected air, and A, B, C, D is a fitting coefficient.

The invention has the beneficial effects that:

1) the thoracic electrical impedance detection is carried out by a frequency conversion and amplitude modulation method, more targeted and humanized measurement is carried out according to the physical conditions of different individuals, the detection safety is ensured, and the comfort and compliance of detection are improved.

2) The invention realizes the function of continuously obtaining the lung function status of the detected person, namely the lung breathing capacity on line in a non-invasive way, and has the monitoring capacity which cannot be realized by the existing detection method.

3) The method detects the resistance impedance of the thoracic cavity through a respiration experiment, fits the resistance impedance with air injected into the lung of a human body, obtains the volume change of the air in the lung through system analysis, and can be used for calculating lung function parameters, namely the lung respiration capacity.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.

Drawings

For the purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a graph of electrical conductivity versus frequency for lung tissue;

FIG. 2 is a basic schematic diagram of a DDS;

FIG. 3 is a schematic diagram of the thoracic electrical impedance detection system of the present invention;

FIG. 4 is a pin diagram of a signal conversion module;

FIG. 5 shows the detection result when the frequency of the injection current is 64 KHz;

FIG. 6 shows the result of the detection when the frequency of the injection current is 96 KHz;

FIG. 7 shows the result of the detection when the frequency of the injected current is 128 KHz;

FIG. 8 shows the detection result when the frequency of the injection current is 256 KHz;

FIG. 9 shows the detection result when the injection current frequency is 512 KHz;

FIG. 10 shows the result of the test when the frequency of the injection current is 700 KHz;

FIG. 11 shows the result of detection when the frequency of the injection current is 1 MHz.

Detailed Description

The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.

Wherein the showings are for the purpose of illustrating the invention only and not for the purpose of limiting the same, and in which there is shown by way of illustration only and not in the drawings in which there is no intention to limit the invention thereto; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "front", "rear", etc., based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not an indication or suggestion that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes, and are not to be construed as limiting the present invention, and the specific meaning of the terms may be understood by those skilled in the art according to specific situations.

Referring to fig. 1 to 11, the method for detecting thoracic electrical impedance based on the human tissue conductivity frequency conversion amplitude modulation method of the present invention specifically includes the following steps:

1) selecting a working frequency section: when the electrode contacts the skin, the sinusoidal current on the electrode can generate a time-varying electromagnetic field, and because human organs and tissues are not general detection objects, the electrical characteristics of the electrode can change nonlinearly along with the change of frequency, taking the lung tissue as an example of the research focus of the invention, the electrical characteristics of the electrode are easily affected by the frequency, and a graph corresponding to the relationship between the electrical conductivity and the frequency is shown in fig. 1.

It can be known that if the frequency band is too low for detection, the electrical conductivity of the biological tissue is poor and the frequency change is not obvious, so that a larger injection current is required to obtain a more obvious detection signal, and the current amplitude that the human body can bear is limited. The bioelectrical impedance characteristic displayed on the frequency band is mainly the electrical characteristic change on the cell membrane, and the detection signal can not reflect more comprehensive thoracic cavity internal information because the current limited by the frequency can not enter the cell. The detection of the excessively high frequency band is subject to the skin effect, and although the conductivity of the chest part under the frequency band is high and is in positive correlation with the frequency change, the comprehensive human body internal electrical characteristic information cannot be obtained due to the poor penetrating performance of the chest part. The working frequency band of the exciting current is set to be 64KHz-1MHz, the penetrating capacity of the current to the thoracic cavity is good in the frequency band, the current can pass through the cell membrane and flow through the intracellular fluid, the electrical characteristics of the thoracic cavity part can be comprehensively reflected, and the purpose of obtaining the optimized measurement result can be achieved by selecting the appropriate detection frequency in the frequency band and using a variable frequency detection method.

2) Excitation source selection and excitation current amplitude selection: when the thoracic electrical impedance measurement is carried out, a current source is selected as an excitation source, and the voltage excitation amplitude is not easy to control, so that the safety and the stability are not good, and the damage to a tested object is possibly caused. In principle, when using current sources for excitation, the greater the current flowing in the body, the stronger the electrical signal generated by the human body, and therefore the current should be as large as possible. However, according to the CE standard of European Union, the current passed into human body must not be greater than 2 mA. And in order to ensure that the testee does not feel uncomfortable in the detection process, the physiological response of the testee to the current is considered to be different. Therefore, the amplitude of the experimental injection current is set to be 500 muA-1.5 mA, humanized detection is realized, the detection safety is ensured, and the comfort and compliance of detection are improved.

3) The thoracic cavity electrical impedance is measured in a multi-amplitude and multi-frequency mode through a frequency conversion and amplitude modulation method. The invention uses direct digital frequency synthesis technology, namely DDS technology to realize the change of detection amplitude and frequency, the DDS technology has the advantages of high frequency-phase discrimination capability, large relative bandwidth, short frequency conversion time, good phase continuity and the like, and the fundamental principle of the DDS is to generate waveform by using Nyquist sampling theorem and through digital-to-analog conversion and low-pass filtering by a table look-up method. The basic circuit principle is shown in fig. 2:

sampling the phase by a reference frequency source, wherein the output data of the phase accumulator is the phase of the synthesized signal; the frequency of the output signal depends on the frequency control word; the frequency resolution depends on the accumulator bit number; the phase resolution depends on the address line bit number of the waveform memory ROM; the data output by the phase accumulator is used as the phase sampling address of the waveform memory. The waveform sampling value in the waveform memory is found out through a table, the conversion from phase to amplitude is completed, the waveform required by the experiment can be generated by modifying the data stored in the waveform memory ROM, and the digital signal is converted into the analog signal required to be used through a D/A converter, filtered and amplified and then output.

Example 1:

the relationship between the thoracic electrical impedance and the change of the lung air is tested by adopting a cross four-electrode measurement method under various different excitation frequencies. In order to achieve a better measurement result, and simultaneously considering factors such as operability, rapidness, different physiological responses of a tested individual to current and the like, the amplitude of the injection current selected in the experiment is set to be 500 muA, 1mA and 1.5mA, and the working frequency of the excitation current is set to be 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1 MHz. The method conforms to the set range of the frequency and the amplitude of the injection current, realizes humanized detection, ensures the detection safety, and improves the comfort and the compliance of the detection.

In the experimental process, the experiment requires that the person to be examined takes off the clothes of the upper part of the body as far as possible when in detection, in order to prevent the breathing value of the human body from more irregular change which may occur after general activities or sports. Standing for 3-4 minutes, then standing the two arms to horizontally unfold, after breathing for 10 cycles, wiping the attaching area and the adjacent parts with alcohol and smearing medical conductive paste before the current injection electrode and the voltage measurement electrode are attached to the human body, so as to reduce the skin contact impedance and improve the experimental measurement precision. Finally, 250ml of air is injected into the lungs each time by an air injector until inspiration is disabled. Data were recorded to complete one experiment. The experiment uses a cross four-electrode method for detection, and realizes 4 types of measurement modes according to an electrode distribution mode and the relationship between the lung tissue conductivity and the frequency to respectively finish multiple experiments with injection current frequencies of 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz and injection current amplitudes of 500 muA, 1mA and 1.5 mA. And finally fitting the relation between the thoracic electrical impedance and the air gas injected into the human lung to obtain the relation between the thoracic electrical impedance and the air volume of the lung.

The hardware and software design of the thoracic electrical impedance parameter acquisition system based on the frequency conversion amplitude modulation method is completed, and the specific process is as follows:

and the FPGA with the DDS function and a peripheral circuit are used for jointly realizing the parameter measurement and acquisition function. The system consists of a singlechip, a signal generation module, a signal conversion module, a signal processing module, a switch array module and a signal acquisition and demodulation module. The signal generation module adopts DDS technology to realize the output of waveform digital signals, the frequency of output signals is directly controlled by a single chip microcomputer, the amplitude of the output signals is realized by the single chip microcomputer through a signal conversion module by changing reference voltage, finally, the output current required by design is realized through a signal processing module, the measurement of thoracic cavity electrical impedance is realized through a signal acquisition demodulation module, and the structure diagram and the specific process are shown in figure 3.

In the present embodiment, the thoracic electrical impedance measurement system based on the frequency-variable amplitude-modulation method is mainly designed to achieve the frequency-variable amplitude-modulation detection function through the signal generation module, the signal conversion module and the signal processing module, so as to complete the set output of the specific detection current frequency and the detection current amplitude.

(1) The signal module generates waveform digital signals by using FPGA (field programmable Gate array) by utilizing DDS (direct digital synthesizer) technology according to frequency control words and waveform control words provided by an STM32F103 singlechip, and the signal module consists of a phase accumulator, a phase register and a waveform lookup table, F₁Is a reference clock frequency, f₂For output signal frequency, K is the frequency control word, N is the phase accumulator and register word length, and L is the waveform look-up table and D/A converter word length.

The synthesis frequency of the DDS is:

the lowest frequency of output is:

the output frequency is selected by the frequency control word K of the FPGA, and the reference clock f₁The frequency division is carried out by a 50MHz crystal oscillator through an FPGA internal phase-locked loop 3 to obtain: f. of₁Approximately equal to 16.67MHz, the word length N of the phase accumulator is 24 bits, and the frequency control word K is 20 bits binary number. The highest frequency of the obtained product is 1.041MHz, the lowest frequency is about 0.994Hz, and the required frequency section of the invention is 64KHz-1 MHz. The current frequencies required by the experiment are 7 in total of 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz, and are respectively configured by the following frequency control words:

when K is 64412, f₂＝64KHz；

When K is 96617, f₂＝96KHz；

When K is 128823, f₂＝128KHz；

When K is 257647, f₂＝256KHz；

When K is 515293, f₂＝512KHz；

When K is 704502, f₂＝700KHz；

When K is 1006432, f₂＝1MHz；

(2) The signal conversion module comprises a digital-to-analog conversion and amplitude control circuit, adopts a double D/A mode, and realizes amplitude control and digital-to-analog conversion of output signals through 2 DAC chips. The 1 st chip DAC904 provides reference voltage, the output of the 1 st chip DAC904 is adjusted to change the reference voltage of the 2 nd chip AD9744, the amplitude control of IOUTA is realized, and finally the function of adjustable injected human body current is achieved, and the output formula is as follows:

I_OUTFS＝32×I_REF

IOUTA＝(DAC CODE/16384)×I_OUTFS

IOUTB＝(16384-DAC CODE)/16384×I_OUTFS

the 2 nd chip AD9744 realizes the digital-to-analog conversion of waveform data, and the DB0-DB13 is connected with a phase amplitude conversion pin of a system to realize the function of receiving waveform data; and realizing analog signal output, and selecting the inside and outside of the reference voltage of the DAC through a REFLO port. When the port is high, i.e., REFLO ═ AVDD, the external reference voltage is selected; when the port is low, i.e., REFLO — AGND, the internal reference voltage is selected. So REFLO is set high and the REFIO pin is connected to the output voltage port of DAC904 as shown in fig. 4.

The working frequency ranges of the AD9744 chips and the DAC904 chips are 165M, which are far greater than the signal frequency, so that the quality and accuracy of analog signal output can be ensured, the analog signal output is not distorted, and the conducted noise of the output signal can be effectively inhibited.

(3) The signal processing module comprises a low-pass filtering amplifying circuit and a voltage-controlled current source circuit, an output signal after digital-to-analog conversion is a differential current signal which is poor in carrying capacity and contains more clock components and transition edges, in the embodiment, an OPA690 is used as an operational amplifier to form a first-order low-pass filtering amplifying circuit, and the filter circuit is ensured not to influence the amplitude and the phase of the signal in a frequency range of 1MHz or below. The open loop gain is designed to be G-2, and the relation between the transfer function of the low-pass filter, the output voltage and the input current is as follows:

U_out1＝80×I_out1

(4) the signal output by the filtering and amplifying circuit is a voltage signal, and when the thoracic electrical impedance is measured, the voltage excitation amplitude is not easy to control, the safety and the stability are not good, the object to be measured is possibly damaged, and a current source is required to be selected as an excitation source. Therefore, the invention constructs a voltage control current source with negative feedback through ADA4898 design, realizes the current output of the final injection electrode, and the relation between the input voltage and the output current is as follows:

the current I of the final injection electrode, namely the voltage-controlled output end, can be obtained_outOutput current I of same digital-to-analog conversion and amplitude control module_outAThe relationship is as follows:

the maximum output current required by the thoracic electrical impedance measurement is 1.5mA, and the maximum output current I can be obtained by the two formulas_outAThe maximum output current of the AD9744 is 20mA which is 18.75mA, and the circuit can meet the requirements of design and detection experiments.

In order to obtain a comprehensive and detailed measured value to reflect the true condition of the thoracic cavity electrical impedance, thoracic cavity electrical impedance measurement experiments need to be carried out in multiple directions, after the detection frequency is set, the detection experiments with the amplitudes of 500 muA, 1mA and 1.5mA are measured in 4 measurement modes according to the electrode distribution mode of the cross four-electrode method, and the detection mean value is calculated to obtain the corresponding thoracic cavity electrical impedance measurement mean value: z₁、Z₂、Z₃、Z₄。

And integrating the measured thoracic electrical impedance value, and substituting into the following formula to obtain the integrated thoracic electrical impedance value:

wherein Z represents the thorax electrical impedance value, L represents the thorax length, W represents the thorax width, and A, B, C and D are constraint coefficients;

a is 0.2319 (parameter variation range (0.2239,0.2387))

0.3197 (parameter variation range (0.3142,0.3235))

0.1871 (parameter variation range (0.1847,0.1908))

0.1648 (parameter variation range (0.1631,0.1664))

Table 1 shows the mean value of the integrated thoracic electrical impedance measured by the cross four-electrode method under different frequencies and different amplitudes

When the frequency of the injection current is 64KHz and the amplitude of the injection current is 500 muA, 1mA and 1.5mA, the measurement is carried out and the average value is obtained, and the detection result is shown in FIG. 5.

The fitting relation is as follows:

Z＝Ae^Bx+Ce^Dx

a is 1.369 (parameter variation range (-0.3896,3.127))

B-0.6433 (parameter variation range (-1.05, -0.2348))

64.22 (parameter variation range (62.44,65.99))

0.06195 (parameter variation range (0.05237,0.07154))

R-square＝0.9997

RMSE＝0.06207

When the frequency of the injection current is 96KHz, the amplitude of the injection current is 500 muA, 1mA, 1.5mA, the measurement is carried out and the average value is obtained, and the detection result is shown in FIG. 6.

The fitting relation is as follows:

Z＝Ae^Bx+Ce^Dx

a is 1.998 (parameter variation range (1.857,2.14))

B-0.001073 (parameter variation range (-0.001242, -0.0009053))

54.18 (parameter variation range (54,54.36))

0.00004545 (parameter variation range (0.00004461,0.00004629))

R-square＝1

RMSE＝0.01854

When the frequency of the injection current is 128KHz and the amplitude of the injection current is 500 muA, 1mA and 1.5mA, the measurement is carried out and the average value is obtained, and the detection result is shown in FIG. 7.

The fitting relation is as follows:

Z＝Ae^Bx+Ce^Dx

a57.39 (parameter variation range (57.31,57.47))

0.05607 (parameter variation range (0.0544,0.05775))

0.00002285 (parameter variation range (-0.0003274,0.0003731))

D-6.434 (parameter variation range (-16.11,3.242))

R-square＝0.9988

RMSE＝0.12221

When the frequency of the injection current is 256KHz and the amplitude of the injection current is 500 muA, 1mA and 1.5mA, the measurement is carried out and the average value is obtained, and the detection result is shown in FIG. 8.

The fitting relation is as follows:

Z＝Ae^Bx+Ce^Dx

a is 2.785 (parameter variation range (2.055,3.515))

B-0.0008849 (parameter variation range (0.0544,0.05775))

46.36 (parameter variation range (-0.0013, -0.0004698))

0.00005449 (parameter variation range (0.0000501,0.00005887))

R-square＝0.9996

RMSE＝0.06323

When the injection current frequency is 512KHz and the injection current amplitude is 500 muA, 1mA, 1.5mA, the measurement is carried out and the average value is obtained, and the detection result is shown in FIG. 9.

The fitting relation is as follows:

Z＝Ae^Bx+Ce^Dx

a is 1.67 (parameter variation range (1.264,2.076))

B-0.001208 (parameter variation range (-0.001965, -0.0004517))

43.26 (parameter variation range (42.73,43.79))

0.00005189 (parameter variation range (0.00004865,0.00005512))

R-square＝0.9995

RMSE＝0.06978

When the frequency of the injection current is 700KHz and the amplitude of the injection current is 500 mua, 1mA, 1.5mA, the measurement is performed and the average value is obtained, and the detection result is shown in fig. 10.

The fitting relation is as follows:

Z＝Ae^Bx+Ce^Dx

a2.886 (parameter variation range (2.473,3.299))

B-0.0007342 (parameter variation range (-0.0008922, -0.0005761))

38.15 (parameter variation range (37.68,38.62))

0.00006358 (parameter variation range (0.00006096,0.0000662))

R-square＝0.9999

RMSE＝0.02384

When the injection current frequency is 1MHz and the injection current amplitude is 500 μ a, 1mA, 1.5mA, the measurement is performed and the average value is obtained, and the detection result is shown in fig. 11.

The fitting relation is as follows:

Z＝Ae^Bx+Ce^Dx

a is 2.992 (parameter variation range (2.727,3.257))

B-0.0006643 (parameter variation range (-0.0007463, -0.0005823))

34.72 (parameter variation range (34.42,35.01))

D-0.00006834 (parameter variation range (0.00006662,0.00007))

R-square＝1

RMSE＝0.01235

Wherein, Z represents the thoracic electrical impedance, x represents the injected air quantity, R-square is a fitting determination coefficient, RMSE is a mean square error, the relation between the thoracic electrical impedance and the air quantity of the lung can be accurately described by the model through the fitting determination coefficient and the mean square error of the above formulas, and the thoracic electrical impedance measurement result based on the frequency conversion amplitude modulation method can be rapidly obtained in the actual use. The calculation and measurement functions of the lung function parameters, namely the lung breathing capacity, based on the bioelectrical impedance technology are realized through the relation function of the thoracic cavity electrical impedance measurement value and the lung breathing air quantity under the specific detection frequency and amplitude obtained by experiments.

Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.

Claims

1. A thoracic electrical impedance detection method based on a human tissue conductivity frequency conversion amplitude modulation method is characterized in that a thoracic electrical impedance measurement system is constructed, output of specific measurement current frequency and measurement current amplitude is achieved according to the relationship between human tissue conductivity and measurement frequency, multiple measurement frequencies and multiple measurement amplitudes are set in the detection process, and measurement of different measurement current amplitudes under the same measurement current frequency is achieved by using a cross four-electrode method, and the method comprises the following specific steps:

s1: constructing a thoracic electrical impedance measuring system, and outputting specific detection amplitude and frequency by using the system according to a signal generation principle;

s2: according to the change relation between the biological tissue conductivity and the detection frequency, determining the measurement current frequency required by the thoracic electrical impedance detection for comprehensively reflecting the electrical characteristics of the thoracic part, and setting the measurement current frequency band to be 64KHz-1 MHz; in order to realize operability and accuracy of the detection process, firstly, the minimum measurement current frequency is determined to be 64KHz, the maximum measurement current frequency is 1MHz, and the frequencies of the left end and the right end of a frequency point with a sudden change of slope are selected according to a conductivity-measurement current frequency curve of a measurement current frequency section according to the rest measurement current frequencies, wherein the selection frequency is +/-35% of the frequency of the sudden change point;

s3: determining the measuring current amplitude required by the thoracic cavity electrical impedance detection according to the reaction phenomenon of the human body to current excitation under different amplitudes, and selecting the measuring current amplitude section to be 500 muA-1.5 mA; in order to realize operability and rapidness of the detection process, the minimum detection amplitude is 500 muA, and the maximum detection amplitude is 1.5 mA; in order to increase the detection mode and improve the measurement precision, 1mA is selected as the detection amplitude in equidistant sampling, and 3 detection amplitudes are determined: 500 μ A, 1mA and 1.5 mA;

2. The thoracic electrical impedance detecting method according to claim 1, wherein in step S1, the thoracic electrical impedance measuring system is constructed to include an FPGA having a DDS function and a peripheral circuit; the peripheral circuit comprises a single chip microcomputer, a signal generation module, a signal conversion module, a signal processing module, a switch array module and a signal acquisition and demodulation module;

the signal generating module is used for outputting a digital current signal with a specific frequency; the frequency of the output signal is directly controlled by the singlechip, and the amplitude of the output signal is realized by changing the reference voltage by the singlechip through the signal conversion module; then, a signal processing module outputs an analog current signal required by detection, a switch array module is used for controlling current to be injected into the thoracic cavity, a signal acquisition and demodulation module is used for realizing signal demodulation, filtering amplification and analog-to-digital conversion to obtain a measured impedance value, a single chip microcomputer is used for controlling a peripheral circuit and an FPGA (field programmable gate array) to realize amplitude and frequency setting of the detection signal, and the measured impedance value is transmitted to an upper computer system.

3. The thoracic electrical impedance detection method according to claim 2, wherein the signal generation module generates a waveform digital signal by using an FPGA (field programmable gate array) by utilizing DDS (direct digital synthesizer) technology according to a frequency control word and a waveform control word provided by the singlechip, and the waveform digital signal is composed of a phase accumulator, a phase register and a waveform lookup table;

the synthesis frequency of the DDS is:

the lowest frequency of output is:

4. The thoracic electrical impedance detection method of claim 1, wherein in step S2, a plurality of measured current frequencies are selected by performing judgment sampling from the measured current frequency band of 64KHz to 1MHz, the detection frequency points are selected according to the variation relationship between the conductivity and the frequency, so as to realize irregular change of the detection frequency, and the measured current frequencies required for detecting the thoracic electrical impedance are determined to be 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz, and 1MHz as the detection frequencies, in order to ensure the operability, rapidity, and accuracy of the detection.

5. The method for detecting thoracic electrical impedance of claim 1, wherein in step S4, the fitting relationship between the measured thoracic electrical impedance and the amount of expired air in the lung is:

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