JP2001245866A - Electric characteristic measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric characteristic measuring device capable of measuring precisely biological electric impedance or related electric characteristics irrespective of the capacity of measuring cables. SOLUTION: Operation amplifiers 13a and 13b, which are impedance converting circuits, are mounted on conductive clips for holding surface electrodes Hp and Lp for measuring the electric impedance difference. Therefore, a measured signal is inputted to a differential amplifier 81 of a measurement processing part 2 from the surface electrodes Hp and Lp pasted on a human body B through the operation amplifiers 13a and 13b via measuring cables 11b and 11c.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrical characteristic measuring device for measuring various physical quantities of a subject's body based on a bioelectrical impedance method.

[0002]

2. Description of the Related Art The inventor of the present invention has previously applied for an apparatus using an M-sequence signal as a bioelectrical impedance measuring apparatus (Japanese Patent Laid-Open No. 10-14898). In the invention, by performing a Fourier transform on the four-terminal A / D-converted signal, bioelectric impedance at many frequencies is measured to calculate information on the water content inside and outside the cell. Although not described in the specification, this apparatus outputs an M-sequence signal many times and performs synchronous addition of each signal in order to improve the SN ratio of the signal.

[0003] The prior art will be described below. 2. Description of the Related Art In recent years, research on the electrical characteristics of living organisms has been conducted for the purpose of evaluating the body composition of humans and animals. The electrical properties of living organisms vary significantly depending on the type of tissue or organ, for example,
In the case of humans, the electrical resistivity of blood is around 150 Ω · cm, whereas the electrical resistivity of bone and fat is 1 to 5 kΩ · cm.
cm. The electrical characteristics of the living body are called bioelectric impedance, and are measured by passing a small current between a plurality of electrodes attached to the body surface of the living body. The method of estimating the body water distribution, body fat percentage, and body fat mass of the subject from the bioelectric impedance obtained in this way is called the bioelectric impedance method ("Bioelectric impedance method as an evaluation method of body composition", Baumgartner , RN, etc.
Authors, "Bioelectric Impedance and Its Clinical Application", Medical Electronics and Biotechnology, Hiroshi Kanai, 20 (3) Jun 1982, "Estimation of Body Water Distribution by Impedance Method and Its Application",
Medical electronics and biotechnology, Makoto Haeno, 23 (6) 1985, "Long-term measurement of urinary bladder volume by impedance method", Ergonomics, Yasuo Kuchimachi, 28 (3) 1992, etc.) .

[0004] Bioelectric impedance is derived from the resistance of a living body to the current carried by ions in the body (resistance) and the reactance associated with various types of polarization processes created by cell membranes, tissue interfaces, or non-ionized tissue. Be composed. Capacitance, which is the reciprocal of reactance, causes a time delay in current rather than voltage, and creates a phase shift (phase shift). This value is the arctangent of the ratio of reactance to resistance (arctangent). It can be determined geometrically as a phase angle. The bioelectric impedance Z, the resistance R, the reactance X, and the electric phase angle φ depend on the frequency. At very low frequencies f _L , the bioelectrical impedance Z at the cell membrane-tissue interface is too high to conduct electricity. Thus, electricity flows only through the extracellular fluid and the measured bioelectrical impedance Z is purely a resistance R.

[0005] Next, as the frequency increases, the current penetrates the cell membrane, the reactance X increases, and the phase angle φ increases. The magnitude of the bioelectrical impedance Z is equal to the value of the vector defined by Z ² = R ² + X ² . The frequency at which both the reactance X and the phase angle φ are maximized is called a critical frequency f _C , which is one electrical characteristic value of a living body that is a conductive conductor. Beyond this critical frequency f _C , the cell membrane-tissue interface loses its capacitive capacity, and the reactance X decreases accordingly. At very high frequencies f _H , the bioelectrical impedance Z is again purely equivalent to the resistance R.

FIG. 9 is an electrical equivalent circuit diagram (equivalent circuit model) of the human body. In this figure, Cmk represents the cell membrane capacity, and Rik and Re represent the intracellular fluid resistance and the extracellular fluid resistance, respectively. At low frequencies f _L , the current is mainly flowing in the extracellular space, and the impedance Z is equal to the extracellular fluid resistance Re. At high frequencies f _H , the current passes through the cell membrane completely, and the cell membrane capacitance Cmk is substantially equivalent to being short-circuited. Accordingly, the impedance at high frequency f _H Z
Is the combined resistance Ri · Re / (Ri + Re), (1 / Ri
= Σ1 / Rik).

According to the method described above, the intracellular fluid resistance R
i and the extracellular fluid resistance Re can be determined, and based on these, the lean body mass of the subject can be estimated, and a change in body water distribution can be estimated based on a change in these resistances Re and Ri. As a bioelectrical impedance measuring device that performs measurement / estimation of each of these parameters by injecting a small sine wave current of a plurality of frequencies arbitrarily selected into a living body and digitally processing the obtained signal, there is a special table. 6-5
The thing described in 06854 is known. In such a device for measuring bioelectric impedance,
Conventionally, a conductive cable capable of holding a plurality of electrodes and a measuring device body are electrically connected to each other with a measuring cable, and the conductive clip is attached to an electrode attached to a subject by an adhesive method to measure bioelectric impedance. .

[0008]

However, since the measuring cable for connecting the conductive clip and the measuring device main body has a capacity, when a current flows through the measuring cable, the voltage drops, and the bioelectrical impedance can be measured accurately. Can not do it. This tendency becomes more pronounced as the measurement cable becomes longer. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an electric characteristic measuring apparatus capable of accurately measuring bioelectric impedance or electric characteristics related thereto regardless of the length of a measurement cable. And

[0009]

According to the electric characteristic measuring apparatus of the present invention, a measurement signal is applied to a subject's body via a conductive clip which holds an electrode which is conductively attached to a surface portion of the subject's body. Measuring a bioelectric impedance by measuring a voltage value generated between surface portions of the body, wherein at least one of the conductive clips includes:
It has an impedance conversion circuit. Further, since the impedance conversion circuit is an operational amplifier, it can be realized with a simple configuration.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings. The case where the present invention is used for a bioelectrical impedance measuring device will be described in detail. FIG. 1 is a block diagram showing an electrical configuration of the measuring device. As shown in FIG. 1, a bioelectrical impedance measuring device of this example includes a keyboard 1 and a body B of a subject.
The probe current Ia is sent out as a measurement signal to thereby control the measurement processing unit 2 for digitally processing the voltage / current information obtained from the subject's body B, and each unit of the apparatus. A CPU (Central Processing Unit) 3 for calculating various quantities related to bioelectric impedance, body fat, and body water distribution of the human body based on the
A display unit 4 for displaying the bioelectric impedance, body fat mass, body water content, and the like of the body B of the subject calculated by the CPU 3, a ROM 6 for storing a processing program of the CPU 3, and various data (for example, RAM in which a data area for temporarily storing height, weight, sex, amount of extracellular fluid and intracellular fluid, and a work area of the CPU 3 are set.
5 and the schematic configuration.

The keyboard 1 is used by a measurer to input a measurement start switch for instructing the start of measurement, a human body characteristic item such as the height, weight, sex, and age of the subject, and to measure the total measurement time T and measurement interval t. The operation data of each key supplied from the keyboard 1 is converted into a key code by a key code generation circuit (not shown), and
Supplied to The measurement processing unit 2 includes a PIO (parallel interface) 71, a measurement signal generator 72,
An output processing circuit including a low-pass filter (hereinafter, referred to as LPF) 73, a coupling capacitor 74, and a surface electrode Hc attached to a predetermined part of the body, and surface electrodes Hp, Lp,
Lc, coupling capacitors 80a, 80b, 90,
Differential amplifier 81, I / V converter (current / voltage converter) 9
1. L composed of an analog anti-aliasing filter
PFs 82 and 92, an A / D converter 83.93, and an input processing circuit including sampling memories (ring buffers) 84 and 94.

In the measurement processing section 2, a measurement signal generator 7
2 is equal to or more than 10 kΩ over the entire signal frequency range in which the output resistance is generated, and every time a signal generation instruction signal is supplied from the CPU 3 via the PIO 71 at a predetermined cycle t during the entire measurement time T, The longest linear code (maximal li
LPF7 that repeatedly generates the probe current Ia of the (near codes) series (M series) a predetermined number of times and uses the generated probe current Ia as a measurement signal to remove high-frequency noise.
3 and a coupling capacitor 74 that removes the DC component from flowing to the subject's body B so as not to flow to the surface electrode Hc. The value of the probe current Ia is, for example, 500 to
800 μA. The supply period of the signal generation instruction signal is set to a measurement interval t set by the measurer using the keyboard 1.
Matches. Further, in this example, the number of repetitions of the probe current (measurement signal) Ia is 1 to 256 per signal generation instruction signal. The number of repetitions may be arbitrarily set by the measurer using the keyboard 1.
The number of repetitions increases as the number of repetitions increases. However, although it is a minute current, it may adversely affect the human body when continuously applied to the human body for a long time. Therefore, the number of repetitions is preferably 1 to 256.

Now, the M-sequence signal will be described. The M-sequence signal is a code signal generally used in a spread-spectrum communication system or a spread-spectrum ranging system, and is the longest code sequence generated by a shift register or a delay element having a certain length. Say. A binary M-sequence generator that generates an M-sequence signal having a length of (2 ⁿ -1) bits (n is a positive integer) is composed of an n-stage shift register and a logical combination of the n-stage states. And a logic circuit (exclusive logic circuit) that feeds back to the input of the shift register. M at a certain sample time (clock time)
The output of the sequence generator and the state of each stage is a function of the output of the feedback stage at the immediately preceding sample time. In this embodiment, an M-sequence generator having eight stages (n = 8) of shift registers is used. Further, the frequency of the shift clock of the shift register is set to 2 MHz.

When an impulse signal is used, energy concentrates in a short time interval (0.1 μsec), whereas a probe current using an M-sequence signal contains many frequency components, Since the energy is dispersed to about 1 msec, it does not damage the living body, and occurs at a time interval sufficiently shorter than the pulse or respiratory cycle. Absent. Furthermore, for example, in the case of a rectangular wave signal with a duty of 50%, the amplitude of the frequency spectrum is large at low frequencies,
Since the frequency characteristic of the SN ratio deteriorates in the high frequency region because it is small at high frequencies, the M-sequence signal has a substantially flat frequency characteristic of the SN ratio because the amplitude of the frequency spectrum is substantially flat over the entire frequency region. is there. Note that M
For details of the sequence signal, see “Spread Spectrum Communication System” by RCDixon (p.56 to p.89).

FIG. 2 is a diagram schematically showing a state of use of the electrical characteristic measuring device of the present embodiment. Surface electrode Hc
At the time of measurement, the (first electrode) is connected to the connection terminal 12a by the measurement cable 11a, and is attached to the right back part H of the subject by a conductive adhesive method, and the surface electrode Lc (second electrode) is measured. It is connected to the connection terminal 12b by the cable 11b, and is conductively attached to the right instep L by an adhesive method. Therefore, the measurement signal (probe current) Ia is
The subject enters the body B from the right hand. In addition, the surface electrode H
p (third electrode) is connected to the connection terminal 12 by the measurement cable 11c.
c, is electrically conductively attached to the right back part H of the subject by an adhesive method, the surface electrode Lp (fourth electrode) is connected to the connection terminal 12d by the measuring cable 11d, and the right foot part is connected. It is electrically conductively attached to L by an adhesive method. At this time, the surface electrodes Hc and Lc are
It is attached to a part farther from the center of the human body than p and Lp.

FIG. 3 is a diagram showing a more specific configuration between the surface electrode and the measurement processing unit. Between the surface electrode Hc and the connection terminal 12a, and between the surface electrode Lc and the connection terminal 12d.
Are directly connected by measuring cables 11a and 11d, respectively. The measurement cable 11 is connected between the surface electrode Hp and the connection terminal 12b and between the surface electrode Lp and the connection terminal 12c via operational amplifiers 13a and 13b, respectively.
b, 11c. These operational amplifiers 13a and 13b
Is for impedance conversion, the amplification factor may be 1, or may be larger or smaller than 1. Since the operational amplifiers 13a and 13b perform impedance conversion, even if the measuring cables 11b and 11c are long and some leakage current flows through the cables, the surface electrodes Hp and Lp
Can be accurately transmitted to the connection terminals 12b and 12c.

FIG. 4 is a circuit diagram for explaining the operation of the impedance conversion circuit. Cb and Cc indicate the capacities of the measurement cables 11b and 11c, respectively. Operational amplifier 13a,
13b does not exist, the leakage current is
Since a part of the current of the constant current source Ia flows to the capacitors Cb and Cc of the capacitors b and 11c, the detection voltage Vp becomes smaller than the original voltage. On the other hand, the presence of the operational amplifiers 13a and 13b maintains the voltage even if a leakage current flows, so that the original voltage can be measured faithfully. Here, since the voltage applied to the capacitor Cb is much higher than the voltage applied to the capacitor Cc, the leakage current also increases accordingly. Therefore, the operational amplifier 13a
Has a greater effect than the operational amplifier 13b, and even if only the operational amplifier 13a is provided, a considerable effect can be obtained.

FIGS. 5 to 7 are diagrams for explaining the details of the conductive clip and the surface electrode. FIG. 5A is a perspective view of the upper surface of the electrode sheet in a state where the conductive gel is separated, FIG. 5B is a perspective view of the lower surface, FIG. 6 is a schematic perspective view of the conductive clip, and FIG.
FIG. 4 is a perspective view of a state before attaching an electrode sheet to a back part H and a back part L and connecting them with a conductive clip. The electrode sheet 20 is attached to the back part H and the back part L of the subject. The electrode sheet 20 is made of a plastic sheet having a thickness of 50 μm or more, such as polyethylene terephthalate. Contact pieces 21 and 21 are formed at the center by cutting, and these contact pieces are bent upward. The electrode sheet 20 has electrodes 22, 22 formed on the lower surfaces at both ends in the longitudinal direction, and these electrodes extend to the contact pieces 21, 21. Then, conductive gels 23 having adhesiveness are attached to the lower surfaces of the electrodes 22. The conductive gels 23 contain a NaCl solution in order to obtain electrical conduction. The electrodes 22, 22 are formed, for example, by printing a conductive paste.

The contact pieces 21 and 21 of the above-mentioned electrode sheet 20
As shown in FIG. 6, the conductive clip 30 connected to the main body 31 includes a main body 31 made of plastic, and sandwiching pieces 33, which are swingably supported by the main body 31 by shafts 32, 32.
33, the two holding pieces are urged toward the main body 31 by a spring 34. Electrodes 35, 35 are located at portions of the main body facing the sandwiching pieces 33, 33, and one of these electrodes (current supply side) is directly connected to the measurement cable 11 and the other (voltage measurement side). Is connected to the measurement cable 11 via the operational amplifier 13. The electrode portions 35, 35 are formed of a conductor that is hardly deteriorated. In this embodiment, the electrode portions 35, 35 are formed of a stainless steel wire having a diameter of about 1.5 mm, and slightly projecting from the surface of the main body 31 to facilitate contact. desirable. Moreover, you may comprise from the plate material of stainless steel.

As shown in FIG. 7, the electrode sheet 20 is adhered to the back part H and the foot part L by conductive gels 23, 23, and the electrodes 22, 22 are connected to the back part H and the foot part L. It becomes conductive. Connect the contact pieces 21 and 21 to the conductive clip 3
0, the electrode portion 3
The connection between the measuring cable 11 and the back part H and the foot part L is established.
In FIG. 7A, the electrode 22 on the hand side is a surface electrode H.
c, the measurement cable 11a is connected, the electrode 22 in the center direction of the body B is equivalent to the surface electrode Hp, and the measurement cable 11b is connected. In FIG. 7B, the electrode 22 on the toe side corresponds to the surface electrode Lc, and the measurement cable 1
1d are connected, and the electrode 22 in the center direction of the body B is the surface electrode L
p, and the measurement cable 11c is connected. And
The conductive clip 30 of the back part H has two electrode parts 35 connected to the first and third electrodes, and the conductive clip 30 of the foot part L has two electrode parts 35 connected to the second and fourth electrodes. By having the electrode portion 35 and connecting the two conductive clips 30, 30 to the two electrode sheets 20, 20, conduction between the four surface electrodes and the measurement cable can be achieved.

The operation of the bioelectrical impedance measuring apparatus having the above configuration will be described. The differential amplifier 81
The potential (potential difference) between the two surface electrodes Hp and Lp is detected. That is, the differential amplifier 81 has the probe current I
When a is inserted into the body B of the subject, the voltage Vp between the right limbs of the subject is detected and input to the LPF 82.
This voltage Vp is a voltage drop between the surface electrode Hp and the surface electrode Lp due to the bioelectric impedance of the body B of the subject. Here, since the operational amplifiers 13a and 13b are connected to the surface electrode Hp and the surface electrode Lp, respectively, the voltage Vp obtained by the impedance conversion action of the operational amplifiers 13a and 13b is equal to the capacity of the measuring cables 11b and 11c. The effect can be small.

The LPF 82 removes high frequency noise from the voltage Vp and supplies it to the A / D converter 83. LPF
The cutoff frequency 82 is lower than half the sampling frequency of the A / D converter 83. Thereby, aliasing noise generated in the A / D conversion processing by the A / D converter 83 is removed. The A / D converter 83 converts the noise-removed voltage Vp into a digital signal at a predetermined sampling cycle every time the digital conversion signal Sd is supplied from the CPU 3, and converts the digitized voltage Vp to the sampling cycle. The data is supplied to the sampling memory 84 every time.

The I / V converter 91 has two surface electrodes H
The current flowing between c and Lc is detected and converted into a voltage. That is, when the probe current Ia is applied to the body B of the subject, the I / V converter 91 detects the probe current Ia flowing between the right limb of the subject, converts the probe current Ia to the voltage Vc,
Supply to F92. The LPF 92 receives the input voltage Vc.
And removes high-frequency noise from the A / D converter 93. The cutoff frequency of the LPF 92 is lower than half the sampling frequency of the A / D converter 93. Also in this case, aliasing noise generated in the A / D conversion processing by the A / D converter 93 is removed. The A / D converter 93 has a CP
Each time the digital conversion signal Sd is supplied from U3, the voltage Vc from which the noise has been removed is converted into a digital signal at a predetermined sampling period, and the digitized voltage Vc is supplied to the sampling memory 94 at each sampling period. .

The CPU 3 starts the measurement by the measurement processing unit 2 according to the processing program stored in the ROM 6, and detects the detection voltages Vp and V at a predetermined sampling cycle.
After sampling c for a predetermined number of times, the following processing is performed in addition to the control for stopping the measurement. That is, the CPU 3
First, the voltages Vp and Vc, which are functions of time, stored in the sampling memories 84 and 94 are sequentially read out, and the voltages Vp (f) and Vc (f) (f Is converted to a frequency), and averaging is performed to calculate bioelectric impedance Z (f) {= Vp (f) / Vc (f)} for each frequency.

Next, based on the obtained bioelectrical impedance Z (f) for each frequency, the CPU 3 obtains an impedance locus D as shown in FIG. From the obtained impedance locus D, the bioelectric impedance R0 of the body B of the subject at a frequency of 0 and the bioelectric impedance R at a frequency of infinity are obtained.
∞ is calculated, and from the calculation result, the intracellular fluid resistance and the extracellular fluid resistance of the body B of the subject are calculated. In an actual human body tissue, cells of various sizes are irregularly arranged, so that an electrical equivalent circuit that is close to actual, as shown in FIG.
It is represented by a distributed constant circuit in which elements connected in series with a capacitor and a resistor having a time constant τ = Cmk · Rik (Re is the extracellular fluid resistance, Rik is the intracellular fluid resistance of each cell, and Cmk is each Cell membrane volume of the cell). Therefore, the impedance locus D of the human body is an arc whose center is higher than the real axis as shown in FIG.

Next, the calculated intracellular fluid resistance and extracellular fluid resistance, and the height of the subject input from the keyboard 1,
Based on human body characteristic data such as weight, gender and age,
By making full use of the body composition estimation formula incorporated in the processing program in advance, the body fat percentage, fat weight,
The amount of lean body mass, the amount of intracellular fluid, the amount of extracellular fluid, and the sum of these amounts of body water (body fluid) are calculated. And
Each of the calculated data is displayed on a display unit 4 including a display controller and a display (for example, an LCD). Next, CP
U3 determines whether or not the entire measurement time T has elapsed, and if it is concluded that the elapsed time has elapsed, terminates the subsequent measurement processing,
If it has not elapsed, after waiting for the time t corresponding to the measurement interval to elapse, the same measurement processing is started again. Then, the above process is repeated until the entire measurement time T has elapsed.

As described above, the bioelectrical impedance measuring apparatus according to the present embodiment can accurately measure the bioelectrical impedance regardless of the capacity of the measuring cable. Therefore, the measurement cable can be made relatively long. Such a device is suitable for dialysis water management and edema management. For example, when measuring the water content of a plurality of dialysis patients, the measurement processing unit is used in common, a conductive clip serving as a detection unit is prepared for a plurality of patients, and the plurality of detection units are processed in a time sharing manner. This makes it possible to collectively handle a plurality of patients without moving the measuring device. As a result, only code management is required around the bed, which is effective in terms of space.

The embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to these embodiments, and the design can be changed without departing from the scope of the present invention. Even if there is, it is included in the present invention. For example, the impedance conversion circuit may be an active circuit other than the operational amplifier.

[0029]

As described above, the electric characteristic measuring apparatus of the present invention can accurately measure bioelectric impedance or electric characteristics related thereto regardless of the capacity of the measuring cable.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an electrical configuration of a bioelectrical impedance measuring device according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a state of use of the bioelectrical impedance measuring device of the present embodiment.

FIG. 3 is a diagram showing a more specific configuration between a surface electrode and a measurement processing unit.

FIG. 4 is a circuit diagram illustrating an operation of the impedance conversion circuit.

FIG. 5A is a perspective view of the upper surface side in a state where the conductive gel of the electrode sheet is separated, and FIG. 5B is a perspective view of the lower surface side.

FIG. 6 is a schematic perspective view of a conductive clip.

7A is a schematic perspective view showing a state before connection between a conductive clip and an electrode sheet on a back part, and FIG. 7B is a schematic view showing a state before connection between a conductive clip and an electrode sheet on a foot part. It is a perspective view.

FIG. 8 is a diagram showing an impedance locus of a human body.

FIG. 9 is a close-to-actual electrical equivalent circuit diagram of cells in a tissue of a human body.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Keyboard 3 CPU 4 Display part 10 Bioelectric impedance measuring device 11a-11d Measurement cable 12a-12d Connection terminal 13a, 13b Operational amplifier (impedance conversion circuit) 30 Conductive clip 31 Differential amplifier 72 Measurement signal generator 73 LPF 81 Differential amplifier 82 LPF 83 A / D converter 84, 94 Sampling memory 91 I / V converter 92 LPF Hc, Hp, Lc, Lp Surface electrode

Claims (2)

[Claims] 1. A measurement signal is applied to a body of a subject via a conductive clip that holds an electrode that is conductively attached to a surface portion of the body of the subject, and a voltage value generated between the surface portions of the body of the subject is measured. An electrical property measuring apparatus for measuring bioelectrical impedance, wherein at least one of the conductive clips includes an impedance conversion circuit. 2. The electrical characteristic measuring device according to claim 1, wherein the impedance conversion circuit is an operational amplifier.