CN112089418B - Thoracic cavity electrical impedance detection method based on human tissue conductivity frequency conversion amplitude modulation method - Google Patents
Thoracic cavity electrical impedance detection method based on human tissue conductivity frequency conversion amplitude modulation method Download PDFInfo
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/053—Measuring electrical impedance or conductance of a portion of the body
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/053—Measuring electrical impedance or conductance of a portion of the body
- A61B5/0531—Measuring skin impedance
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/08—Detecting, measuring or recording devices for evaluating the respiratory organs
- A61B5/085—Measuring impedance of respiratory organs or lung elasticity
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/08—Detecting, measuring or recording devices for evaluating the respiratory organs
- A61B5/091—Measuring volume of inspired or expired gases, e.g. to determine lung capacity
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
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- A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
- A61B2562/02—Details of sensors specially adapted for in-vivo measurements
- A61B2562/0209—Special features of electrodes classified in A61B5/24, A61B5/25, A61B5/283, A61B5/291, A61B5/296, A61B5/053
Abstract
The invention relates to a thoracic cavity electrical impedance detection method based on a human tissue conductivity frequency conversion amplitude modulation method, and belongs to the technical field of medical detection. The method comprises the following steps: s1: constructing a thoracic electrical impedance measurement system, and outputting specific detection amplitude and frequency by using the system; s2: determining the frequency of a measuring current required by thoracic cavity electrical impedance detection according to the relation between the conductivity of biological tissues and the detection frequency; s3: determining the measured current amplitude required by thoracic cavity electrical impedance detection according to the response of a human body to current excitation under different amplitudes; s4: after setting a specific measurement frequency, respectively carrying out detection experiments by using a cross four-electrode method, and solving measurement average values of different amplitudes under the same measurement frequency to finish a group of detection; and determining the fitting relation between the thoracic electrical impedance measured value and the lung expiratory air quantity under different detection frequencies through a plurality of groups of experiments. The invention realizes sustainable online rapid non-invasive pulmonary respiratory capacity detection, and improves the safety, comfort and compliance of detection.
Description
Technical Field
The invention belongs to the technical field of medical detection, and relates to a thoracic cavity electrical impedance detection method based on a human tissue conductivity frequency conversion amplitude modulation method.
Background
In the prior art, common lung function detection schemes mainly have two modes: the first is volume measurement and shaping, and the instrument measures the change of the gas volume of a pontoon or a piston cavity connected with the respiratory tract of a detector, so as to realize the detection of the change rule of the gas volume of the lung; the second is flow measurement and shaping, and the instrument measures the gas flow with a certain flow cross section area and integrates the gas flow with time to obtain the respiratory gas volume, so as to realize the detection of the change of the lung gas volume.
Both of the above detection methods have some non-negligible problems.
1) When the capacity measuring type detector is used for detection, the friction force between the inertia of the pontoon and the piston can cause serious distortion of the measurement result. Furthermore, the gas storage cavities of the pontoon and the piston and the respiratory pipeline can be reused in use, and the risk of cross infection is extremely high.
2) The flow measurement and shaping detector has higher accuracy than the capacity measurement type detector, and the price of the detector and the consumable are more expensive; part of the airway can be reused in the test process, and a certain risk of cross infection is still caused.
Because the two common lung function detectors need to connect the respiratory airway with the instrument measurement airway, the hidden danger of cross infection is unavoidable. Patients also experience discomfort during measurement, resulting in poor compliance in the test. And real-time online monitoring cannot be performed, and the requirements of accuracy, safety and convenience cannot be met.
Aiming at the problems of high detection cost, high risk of cross infection, single detection mode commonly existing in bioelectrical impedance measurement systems and the like existing in the existing lung function detection technology, the invention provides a rapid thoracic cavity impedance measurement method based on a variable frequency amplitude modulation method.
Disclosure of Invention
Therefore, the invention aims to provide a thoracic electrical impedance noninvasive rapid measurement method based on a human tissue conductivity frequency conversion amplitude modulation method, which utilizes the combination 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 safety, accuracy and convenience of a lung function detection device.
In order to achieve the above purpose, the present invention provides the following technical solutions:
a thoracic cavity electrical impedance detection method based on human tissue conductivity frequency conversion amplitude modulation method is realized by constructing a thoracic cavity electrical impedance measurement system, realizing the output of specific measurement current frequency and measurement current amplitude according to the relation between human tissue conductivity and measurement frequency, setting 7 measurement frequencies and 3 measurement amplitudes in the detection process, and realizing the measurement of different measurement current amplitudes under the same measurement current frequency by using a cross four-electrode method, wherein the method comprises the following specific steps:
s1: a thoracic electrical impedance measurement system is constructed, specific detection amplitude and frequency are output by the system according to a signal generation principle, a detection mode is increased, and measurement accuracy is improved;
s2: according to the change relation between the conductivity of biological tissues and the detection frequency, determining the measurement current frequency required by the thoracic cavity electrical impedance detection for comprehensively reflecting the electrical characteristics of thoracic cavity parts, and setting the measurement current frequency range to be 64KHz-1 MHz; in order to realize the operability and accuracy of the detection process, the minimum measurement current frequency is determined to be 64KHz, the maximum measurement current frequency is determined to be 1MHz, the other measurement current frequencies are selected according to the conductivity-measurement current frequency curve of the measurement current frequency section, the frequencies at the left and right ends of the frequency point with the abrupt change of the slope are selected, and the selected frequencies are +/-35% of the frequency of the abrupt change point
S3: according to the response phenomenon of the human body to the current excitation under different amplitudes, determining the measured current amplitude required by the thoracic cavity electrical impedance detection, and selecting the measured current amplitude range to be 500 mu A-1.5 mA; in order to realize operability and rapidity of the detection process, the experimental operation shows that: when the measured current amplitude is lower than 500 mu A, the electric signal measured by the electrode is very weak, the subsequent analysis is difficult to realize, and when the measured current amplitude is higher than 1.5mA, the compliance and the comfort of part of detected personnel in the detection process can be greatly reduced, so that the minimum detection amplitude is selected to be 500 mu A, and the maximum detection amplitude is selected to be 1.5mA; in order to increase the detection mode and improve the measurement precision, 1mA is selected as detection amplitude by equidistant sampling, and 3 detection amplitudes are determined in total: 500 μA,1mA and 1.5mA;
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 measurement average values of different measurement current amplitudes under the same measurement current frequency to finish a group of detection; and (3) finishing all the measurement experiments of the selected measurement current frequency, and determining the fitting relation between the thoracic electrical impedance measurement mean value and the lung expiratory air quantity under different detection frequencies through the measurement experiments.
In step S1, the thoracic electrical impedance measurement system comprises an FPGA with DDS function and a peripheral circuit; the peripheral circuit comprises a singlechip, a signal generation module, a signal conversion module, a signal processing module, a switch array module and a signal acquisition demodulation module;
the signal generation module is used for outputting a signal of digital current 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; and then outputting current required by detection through a signal processing module, controlling the current to be injected into the chest by using a switch array module, obtaining a measured impedance value after signal demodulation, filtering amplification and analog-to-digital conversion by using a signal acquisition demodulation module, controlling a peripheral circuit and an FPGA by using a singlechip to realize amplitude and frequency setting of a detection signal, and transmitting the measured impedance value to an upper computer system.
Further, the signal generation module generates a waveform digital signal by using a DDS technology according to a frequency control word and a waveform control word provided by the STM32F103 singlechip, and the waveform digital signal consists of a phase accumulator, a phase register and a waveform lookup table;
the synthesis frequency of DDS is:
the output lowest frequency is:
wherein ,f1 For the reference clock frequency, f 2 For the output signal frequency, K is the frequency control word, and N is the phase accumulator and register word length.
In step S2, a plurality of measuring current frequencies are selected by judging and sampling from the measuring current frequency range of 64KHz-1MHz, the selection basis of the detection frequency points is the change relation of conductivity and frequency, the irregular change of the detection frequency is realized, and the measuring current frequencies required by the detection of the thoracic cavity electrical impedance are set to 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz in order to ensure the operability, the rapidity and the accuracy of the detection.
In step S3, a plurality of amplitude values are sampled from the measured current amplitude values of 500 mu A-1.5mA, and the measured current amplitude values required by thoracic electrical impedance detection are determined to be 500 mu A,1mA and 1.5mA in order to ensure the operability and the rapidity of detection.
Further, in step S4, the fitting relationship between the thoracic electrical impedance measurement value and the lung expiratory air amount is:
Z＝Ae Bx +Ce Dx
where Z represents 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 invention detects the thoracic electrical impedance by a variable frequency amplitude modulation method, performs more targeted and humanized measurement according to the physical conditions of different individuals, ensures the detection safety, and improves the detection comfort and compliance.
2) The invention realizes the function of continuously and online non-invasively acquiring the lung function status of the detected personnel, namely the lung breathing capacity, and has the monitoring capacity which cannot be realized by the existing detection method.
3) The method provided by the invention detects thoracic resistance impedance through a respiration experiment, fits the thoracic resistance impedance with air injected into the human lung, obtains the air volume change of the lung through system analysis, and can be used for calculating lung function parameters, namely 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 objects and other advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the specification.
Drawings
For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in the following preferred detail with reference to the accompanying drawings, in which:
FIG. 1 is a graph of the corresponding conductivity versus frequency for lung tissue;
fig. 2 is a basic schematic diagram of a DDS;
FIG. 3 is a schematic diagram of a 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 injection current frequency is 64 KHz;
FIG. 6 shows the detection result when the injection current frequency is 96 KHz;
FIG. 7 shows the detection result when the injection current frequency is 128 KHz;
FIG. 8 shows the detection result when the injection current frequency is 256 KHz;
FIG. 9 shows the detection result when the injection current frequency is 512 KHz;
FIG. 10 shows the detection result when the injection current frequency is 700 KHz;
fig. 11 shows the detection result when the injection current frequency was 1MHz.
Detailed Description
Other advantages and effects of the present invention will become apparent to those skilled in the art from the following disclosure, which describes the embodiments of the present invention with reference to specific examples. The invention may be practiced or carried out in other embodiments that depart from the specific details, and the details of the present description may be modified or varied from the spirit and scope of the present invention. It should be noted that the illustrations provided in the following embodiments merely illustrate the basic idea of the present invention by way of illustration, and the following embodiments and features in the embodiments may be combined with each other without conflict.
Wherein the drawings are for illustrative purposes only and are shown in schematic, non-physical, and not intended to limit the invention; for the purpose of better illustrating embodiments of the invention, certain elements of the drawings may be omitted, enlarged or reduced and do not represent the size of the actual product; it will be appreciated 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 numbers in the drawings of embodiments of the invention correspond to the same or similar components; in the description of the present invention, it should be understood that, if there are terms such as "upper", "lower", "left", "right", "front", "rear", etc., that indicate an azimuth or a positional relationship based on the azimuth or the positional relationship shown in the drawings, it is only for convenience of describing the present invention and simplifying the description, but not for indicating or suggesting that the referred device or element must have a specific azimuth, be constructed and operated in a specific azimuth, so that the terms describing the positional relationship in the drawings are merely for exemplary illustration and should not be construed as limiting the present invention, and that the specific meaning of the above terms may be understood by those of ordinary skill in the art according to the specific circumstances.
Referring to fig. 1 to 11, the thoracic cavity electrical impedance detection method based on the human tissue conductivity frequency conversion amplitude modulation method of the invention specifically comprises the following steps:
1) Selecting a working frequency section: when the electrode contacts skin, sinusoidal current on the electrode can generate a time-varying electromagnetic field, and as the organs and tissues of a human body are not general to detect objects, the electrical characteristics of the electrode can change in a nonlinear way along with the change of frequency, and taking the research of important lung tissues as an example, the electrical characteristics of the electrode are easily affected by the frequency, and the corresponding relation between the electrical conductivity and the frequency is shown in figure 1.
It is known that if an excessively low frequency band is selected for detection, the conductivity of the biological tissue is poor and the frequency change is not obvious, a relatively obvious detection signal can be obtained only by a relatively large injection current, and the current amplitude which can be born by a human body is limited. The bioelectrical impedance characteristic displayed on the frequency band is mainly the electrical characteristic change on a cell membrane, and the detection signal can not reflect relatively comprehensive intrathoracic information because the current limited by the frequency can not enter the cell. The detection of the frequency band with too high frequency is limited by skin effect, and the electric conductivity of the chest part in the frequency band is higher and has positive correlation with the frequency change, but the comprehensive internal electric characteristic information of the human body still cannot be obtained due to the poor penetrability of the chest. Therefore, the working frequency band of the excitation current used in the invention is set to be 64KHz-1MHz, the penetration capability of the current in the frequency band to the thoracic cavity is good, the current can pass through the cell membrane to flow through the intracellular fluid, the electrical characteristics of the thoracic cavity part can be comprehensively reflected, and the purpose of obtaining the optimal measurement result can be achieved by selecting proper 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 cavity 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 poor, and the damage to a tested object is likely to be caused. In principle, when the current source is used for excitation, the larger the current which is supplied into the body is, the stronger the electric signal formed by the human body is, so that the current should be as large as possible. However, according to the European Union CE standard, the current applied to the human body must not be greater than 2mA. And in order to ensure that the detected person does not generate uncomfortable feeling in the detection process, the physiological responses of the detected person to the current are considered to be different. Therefore, the amplitude of the experimental injection current is set to be 500 mu A-1.5mA, humanized detection is realized, the safety of detection is ensured, and the comfort and compliance of detection are improved.
3) The multi-amplitude multi-frequency mode is realized by the variable-frequency amplitude modulation method to measure the thoracic impedance. 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 resolution capability, large relative bandwidth, short frequency conversion time, good phase continuity and the like, and the basic principle of the DDS is that the Nyquist sampling theorem is utilized, and waveforms are generated after digital-to-analog conversion and low-pass filtering through 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 number of address line bits 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 searched by a table to finish the conversion from phase to amplitude, 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 by a D/A converter for filtering and amplifying and outputting.
Example 1:
and selecting to perform experiments on the relation between the thoracic cavity electrical impedance and the lung air change by adopting a cross four-electrode measurement method under a plurality of different excitation frequencies. In order to achieve a better measurement result, and considering the operability and rapidity of the experiment, physiological responses of the tested individuals to the current and other factors, the injection current amplitude selected during the experiment is set to 500 mu A,1mA and 1.5mA, and the excitation current working frequency is set to 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz. The method accords with the setting range of the frequency and the amplitude of the injection current, realizes humanized detection, ensures the safety of detection, and improves the comfort and the compliance of detection.
In the experimental process, the experiment requires that the examined personnel take off the clothes of the upper body as far as possible during the detection, so as to prevent the less regular change of the human respiratory value possibly occurring after the general activities or exercises. After sitting still for 3-4 min, standing up the arms and horizontally unfolding, after 10 cycles of calm breath, wiping the joint area and the adjacent parts with alcohol and smearing medical conductive paste before the current injection electrode and the voltage measurement electrode are jointed with a human body, so as to reduce skin contact impedance and improve experimental measurement accuracy. Finally 250ml of air are injected into the lungs each time by an air injector until inhalation is disabled. Recording data one experiment was completed. The experiment uses a cross four-electrode method to detect, and realizes 4 kinds of measurement modes according to the relation between the electrical conductivity of the lung tissue and the frequency in an electrode distribution mode to respectively finish multiple experiments of injection current with the frequencies of 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz and injection current with the amplitudes of 500 mu A,1mA and 1.5mA. And finally fitting the relation between the thoracic cavity electrical impedance and the air gas injected into the human lung to obtain the relation between the thoracic cavity electrical impedance and the lung air volume.
The hard software design of the thoracic electrical impedance parameter acquisition system based on the variable frequency 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 demodulation module. The signal generation module adopts DDS technology to realize the output of waveform digital signals, the frequency of the output signals is directly controlled by the singlechip, the amplitude of the output signals is realized by the singlechip through the signal conversion module by changing the reference voltage, finally, the output current required by the design is realized through the signal processing module, the measurement of thoracic cavity electrical impedance is realized through the signal acquisition demodulation module, and the structure diagram and the specific process are shown in figure 3.
In the embodiment, the thoracic electrical impedance measuring system based on the variable frequency amplitude modulation method is mainly designed by realizing the variable frequency amplitude modulation detection function through the signal generating module, the signal converting module and the signal processing module, and completing the setting output of specific detection current frequency and detection current amplitude.
(1) The signal module generates a waveform digital signal by using a DDS technology according to a frequency control word and a waveform control word provided by an STM32F103 singlechip by using an FPGA, and the waveform digital signal consists of a phase accumulator, a phase register and a waveform lookup table, F 1 For the reference clock frequency, f 2 For the output signal frequency, K is the frequency control word, N is the phase accumulator and register word length, and L is the waveform lookup table and D/A converter word length.
The synthesis frequency of DDS is:
the output lowest frequency is:
the output frequency is selected by the frequency control word K of the FPGA, and the reference clock f 1 The 50MHz crystal oscillator is obtained by frequency division of an FPGA internal phase-locked loop 3: f (f) 1 Approximately 16.67MHz, the phase accumulator word length N is 24 bits, the frequency control word K is a 20 bit binary number. The highest frequency can be 1.041MHz, the frequency is about 0.994Hz, and the frequency range of 64KHz-1MHz required by the invention is satisfied. The current frequency required by the experiment is 7 of 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz, respectively communicatedThe following frequency control word configuration is followed:
when k=64412, f 2 ＝64KHz；
When k= 96617, f 2 ＝96KHz；
When k= 128823, f 2 ＝128KHz；
When k= 257647, f 2 ＝256KHz；
When k= 515293, f 2 ＝512KHz；
When k= 704502, f 2 ＝700KHz；
When k= 1006432, f 2 ＝1MHz；
(2) The signal conversion module comprises a digital-to-analog conversion and amplitude control circuit, and adopts a double D/A mode to realize the amplitude control and digital-to-analog conversion of output signals through a 2-piece DAC chip. The 1 st chip DAC904 provides a reference voltage, the output of the 1 st chip DAC904 is regulated to change the reference voltage of the 2 nd chip AD9744, the amplitude control of IOUTA is realized, the function of injecting human body current is finally 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 digital-to-analog conversion of the waveform data is realized through the chip AD9744 of the 2 nd chip, and the DB0-DB13 is connected with the phase amplitude conversion pins of the system to realize the function of receiving the waveform data; analog signal output is realized, and the reference voltage of the DAC is selected from the inner part and the outer part through the REFLO port. When the port is high, i.e., reflo=avdd, an external reference voltage is selected; when the port is low, i.e., reflo=agnd, the internal reference voltage is selected. The 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 and DAC904 chips are 165M, which is far greater than the signal frequency, so that the quality and accuracy of analog signal output can be ensured, the analog signal is not distorted, and the output signal conduction noise can be effectively restrained.
(3) The signal processing module comprises a low-pass filter amplifying circuit and a voltage-controlled current source circuit, the output signal after digital-to-analog conversion is a differential current signal, the load capacity of the signal processing module is poor, the signal processing module contains more clock components and transition edges, the OPA690 is used as an operational amplifier to form a first-order low-pass filter amplifying circuit, the signal is ensured to be in a frequency section of 1MHz or below, and the filter circuit does not influence the amplitude and the phase of the signal. The open loop gain is designed to be G=2, and the transfer function, the output voltage and the input current relation of the low-pass filter are as follows:
U out1 ＝80×I out1
(4) The signals output by the filtering and amplifying circuit are voltage signals, and when the thoracic cavity electrical impedance measurement is carried out, the voltage excitation amplitude is not easy to control, the safety and the stability are poor, the tested object is possibly damaged, and a current source is required to be selected as an excitation source. Therefore, the invention constructs a voltage-controlled 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 output of the final injection electrode, namely the current I of the voltage control output end, can be obtained out The same digital-to-analog conversion and amplitude control module outputs current I outA The relation is:
the maximum output current required by the thoracic cavity electrical impedance measurement of the experimental design is 1.5mA, and I can be obtained by the two formulas outA =18.75 mA, maximum output current of ad9744 of 20mA, caThe known circuit meets the design and detection experiment requirements.
In order to obtain comprehensive and fine measurement values to reflect the real situation of thoracic impedance, thoracic impedance measurement experiments are required to be carried out in multiple directions, after the detection frequency is set, detection experiments with the amplitudes of 500 mu A,1mA and 1.5mA are measured according to the electrode distribution mode of the crossed four-electrode method in the 4-class measurement mode, and the detection average value is obtained, so that the corresponding thoracic impedance measurement average value is obtained: z is Z 1 、Z 2 、Z 3 、Z 4 。
Integrating the measured thoracic impedance values, substituting the measured thoracic impedance values into the following formula to obtain integrated thoracic impedance values:
wherein Z represents the thoracic electrical impedance value, L represents the thoracic length, W represents the thoracic width, and A, B, C and D are constraint coefficients;
a= 0.2319 (parameter variation range (0.2239,0.2387))
B= 0.3197 (parameter variation range (0.3142,0.3235))
C= 0.1871 (parameter variation range (0.1847,0.1908))
D= 0.1648 (parameter variation range (0.1631,0.1664))
Table 1 shows the average value of the comprehensive thoracic impedance obtained under the conditions of different frequencies and different amplitudes measured by the cross four-electrode method
When the injection current frequency was 64KHz, the injection current amplitude was 500. Mu.A, 1MA,1.5MA, and the average value was obtained, and the detection result was shown in FIG. 5.
The fitting relation is as follows:
Z＝Ae Bx +Ce Dx
a=1.369 (parameter variation range (-0.3896,3.127))
B= -0.6433 (parameter variation range (-1.05, -0.2348))
C= 64.22 (parameter variation range (62.44,65.99))
D= 0.06195 (parameter variation range (0.05237,0.07154))
R-square＝0.9997
RMSE＝0.06207
When the injection current frequency was 96KHz and the injection current amplitude was 500 μa,1ma,1.5ma, the measurement was performed and the average was found, and the detection result was shown in fig. 6.
The fitting relation is as follows:
Z＝Ae Bx +Ce Dx
a=1.998 (parameter variation range (1.857,2.14))
B= -0.001073 (parameter variation range (-0.001242, -0.0009053))
C= 54.18 (parameter variation range (54,54.36))
D= 0.00004545 (parameter variation range (0.00004461,0.00004629))
R-square＝1
RMSE＝0.01854
When the injection current frequency was 128KHz, the injection current amplitude was 500 μa,1ma,1.5ma, and the average value was obtained, the detection result was shown in fig. 7.
The fitting relation is as follows:
Z＝Ae Bx +Ce Dx
a=57.39 (parameter variation range (57.31,57.47))
B= 0.05607 (parameter variation range (0.0544,0.05775))
C= 0.00002285 (parameter variation (-0.0003274,0.0003731))
D= -6.434 (parameter variation range (-16.11,3.242))
R-square＝0.9988
RMSE＝0.12221
When the injection current frequency was 256KHz, the injection current amplitude was 500. Mu.A, 1MA,1.5MA, and the average value was obtained, and the detection result was shown in FIG. 8.
The fitting relation is as follows:
Z＝Ae Bx +Ce Dx
a= 2.785 (parameter variation range (2.055,3.515))
B= -0.0008849 (parameter variation range (0.0544,0.05775))
C= 46.36 (parameter variation range (-0.0013, -0.0004698))
D= 0.00005449 (parameter variation range (0.0000501,0.00005887))
R-square＝0.9996
RMSE＝0.06323
When the injection current frequency was 512KHz and the injection current amplitude was 500 μa,1ma,1.5ma, the measurement was performed and the average was found, and the detection result was shown in fig. 9.
The fitting relation is as follows:
Z＝Ae Bx +Ce Dx
a=1.67 (parameter variation range (1.264,2.076))
B= -0.001208 (parameter variation range (-0.001965, -0.0004517))
C= 43.26 (parameter variation range (42.73,43.79))
D= 0.00005189 (parameter variation range (0.00004865,0.00005512))
R-square＝0.9995
RMSE＝0.06978
When the injection current frequency was 700KHz and the injection current amplitude was 500 μa,1ma,1.5ma, the measurement was performed and the average was found, and the detection result was shown in fig. 10.
The fitting relation is as follows:
Z＝Ae Bx +Ce Dx
a=2.886 (parameter variation range (2.473,3.299))
B= -0.0007342 (parameter variation range (-0.0008922, -0.0005761))
C=38.15 (parameter variation range (37.68,38.62))
D= 0.00006358 (parameter variation range (0.00006096,0.0000662))
R-square＝0.9999
RMSE＝0.02384
When the injection current frequency was 1MHz and the injection current amplitude was 500. Mu.A, 1mA,1.5mA, measurement was performed and the average was found, and the detection result was shown in FIG. 11.
The fitting relation is as follows:
Z＝Ae Bx +Ce Dx
a= 2.992 (parameter variation range (2.727,3.257))
B= -0.0006643 (parameter variation range (-0.0007463, -0.0005823))
C= 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 thoracic electrical impedance, x represents injection air quantity, R-square is a fitting determination coefficient, RMSE is a mean square error, the relation between thoracic electrical impedance and pulmonary air quantity can be accurately described by the model through the fitting determination coefficient and the mean square error, and a thoracic electrical impedance measurement result based on a variable frequency amplitude modulation method can be rapidly obtained in actual use. The calculation and measurement functions of the lung function parameters, namely the lung breathing capacity, based on bioelectrical impedance technology are realized through the relation function of the thoracic cavity electrical impedance measured value and the lung breathing air quantity under specific detection frequency and amplitude obtained through experiments.
Finally, it is noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made thereto without departing from the spirit and scope of the present invention, which is intended to be covered by the claims of the present invention.
Claims (4)
1. The thoracic cavity electrical impedance detection method based on human tissue conductivity frequency conversion amplitude modulation method is characterized in that the method realizes the output of specific measurement current frequency and measurement current amplitude according to the relation between human tissue conductivity and measurement frequency by constructing a thoracic cavity electrical impedance measurement system, sets multiple measurement frequencies and multiple measurement amplitudes in the detection process, and realizes the measurement of different measurement current amplitudes under the same measurement current frequency by using a cross four-electrode method, and the specific steps comprise:
s1: constructing a thoracic electrical impedance measurement 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 conductivity of biological tissues and the detection frequency, determining the measurement current frequency required by the detection of the thoracic cavity electrical impedance for comprehensively reflecting the electrical characteristics of the thoracic cavity part, and setting the measurement current frequency range to be 64KHz-1 MHz; in order to realize the operability and accuracy of the detection process, the minimum measurement current frequency is determined to be 64KHz, the maximum measurement current frequency is determined to be 1MHz, the other measurement current frequencies are selected according to the conductivity-measurement current frequency curve of the measurement current frequency section, the frequencies at the left and right ends of a frequency point with abrupt change of the slope are selected, and the selected frequencies are +/-35% of the frequency of the abrupt change point;
s3: according to the response phenomenon of a human body to current excitation under different amplitudes, determining the measured current amplitude required by thoracic cavity electrical impedance detection, and selecting the measured current amplitude range to be 500 mu A-1.5 mA; in order to realize the operability and rapidness of the detection process, selecting the minimum detection amplitude of 500 mu A and the maximum detection amplitude of 1.5mA; in order to increase the detection mode and improve the measurement precision, 1mA is selected as detection amplitude by equidistant sampling, and 3 detection amplitudes are determined in total: 500 μA,1mA and 1.5mA;
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 measurement average values of different measurement current amplitudes under the same measurement current frequency to finish a group of detection; and (3) finishing all the measurement experiments of the selected measurement current frequency, and determining the fitting relation between the thoracic electrical impedance measurement mean value and the lung expiratory air quantity under different detection frequencies through the measurement experiments as follows:
wherein Z represents the electrical impedance of the thoracic cavity,xthe amount of injected air is indicated,A、B、Cand D is a fitting coefficient.
2. The thoracic electrical impedance detection method of claim 1 wherein in step S1, a thoracic electrical impedance measurement system is constructed, comprising an FPGA with DDS functionality and peripheral circuitry; the peripheral circuit comprises a singlechip, a signal generation module, a signal conversion module, a signal processing module, a switch array module and a signal acquisition demodulation module;
the signal generation 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; and then outputting an analog current signal required by detection through a signal processing module, controlling current to be injected into the chest through a switch array module, obtaining a measured impedance value after signal demodulation, filtering amplification and analog-to-digital conversion by adopting a signal acquisition demodulation module, controlling a peripheral circuit and an FPGA through a singlechip to realize amplitude and frequency setting of a detection signal, and transmitting the measured impedance value to an upper computer system.
3. The thoracic electrical impedance detection method of claim 2 wherein the signal generation module generates a waveform digital signal by using a DDS technique using an FPGA according to a frequency control word and a waveform control word provided by a single chip microcomputer, which is composed of a phase accumulator, a phase register and a waveform lookup table;
the synthesis frequency of DDS is:
the output lowest frequency is:
wherein ,for the reference clock frequency +.>In order to output the signal frequency,Kin order to be a frequency control word,Nis the word length of the phase accumulator and the register.
4. The thoracic electrical impedance detection method of claim 1, wherein in step S2, a plurality of measurement current frequencies are selected by judging and sampling from the measurement current frequency ranges 64KHz to 1MHz, the selection basis of the detection frequency points is the change relation between conductivity and frequency, the irregular change of the detection frequency is realized, and the measurement current frequencies required by thoracic electrical impedance detection are determined to be 64KHz, 96KHz, 128KHz, 256KHz, 512KHz, 700KHz and 1MHz as detection frequencies in order to ensure the operability, rapidity and accuracy of detection.
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