Bioimpedance measuring device

Abstract

Bioimpedance measuring device for determining composition data of the human body with several electrodes attachable to limbs and electrical circuits connectable thereto, including an alternating current source, a voltage measuring circuit, and a control and evaluation unit configured to control the operation of the alternating current source and voltage measuring circuit and their selective connection to the electrodes, and according to predetermined measurement programs, to induce an alternating current from the alternating current source into the body via two electrodes specific to the respective measurement program, and to detect the resulting voltage with two electrodes on different limbs using the voltage measuring circuit, and from this to determine the impedance of the current-carrying body component between the electrodes detecting the voltage, characterized in that the control and evaluation unit is configured toIn connection with determining the impedance, an interference test is performed by not applying alternating current from the alternating current source to the body for a certain period of time, recording the voltage signal between the electrodes connected to the voltage measuring circuit during this period, checking it against a threshold criterion, and if this criterion is exceeded, the presence of an electromagnetic interference is determined.

Description

[0001] The present invention relates to a bioimpedance measuring device for determining composition data of the human body with several electrodes attachable to limbs and associated electrical circuits including an AC power source, a voltage measuring circuit and a control and evaluation unit, which is configured to control the operation of the AC power source and voltage measuring circuit and their selective connection to the electrodes and, according to predetermined measurement programs, to induce an alternating current from the AC power source into the body via two electrodes specific for the respective measurement program and to detect the resulting voltage with two electrodes on different limbs using the voltage measuring circuit and to determine from this the impedance of the current-carrying body component between the electrodes detecting the voltage.

[0002] Typical body composition data relate to body fat content, which is often of interest in the context of determining body weight. The conductivity of the human body is highly dependent on its water content. Since the fat-free components of the body, such as muscles and body fluids, contain a large proportion of the body's water, whereas adipose tissue has a very low water content due to its hydrophobic properties, determining the conductivity of the body or a body segment allows inferences to be drawn about the relative proportion of fat, taking into account other body-specific data such as the height and weight of the individual being examined. Equivalent to determining conductivity is, of course, determining the corresponding resistance or impedance of the body segment under consideration.

[0003] Bioimpedance measurements can thus be used to derive representative values ​​for body fat percentage, which includes not only the body fat percentage itself, but also proportional or complementary body parameters, for example the fat-free mass (FFM) of the body (which is complementary to fat mass when body weight is known), fat mass (FM), total body water mass (TBW), extracellular water mass (ECW) or lean soft tissue mass (LST).

[0004] A bioimpedance measuring device according to the preamble of claim 1 is described, for example, in WO 97 / 01303. The described device has eight electrodes, which are connected by eight leads to a control and evaluation unit. The eight electrodes include four foot electrodes arranged on a platform on which the person being examined must stand. Furthermore, four hand electrodes are provided on two handles, which the person being examined must grasp with their hands. An alternating current is then applied to two electrodes located on different limbs, and the resulting voltage is measured at two other electrodes, also located on different limbs.The control and evaluation unit determines, according to predetermined measurement programs, which two electrodes on which two limbs receive alternating current via connection to the AC power source, and which two other electrodes are connected to the voltage measuring circuit to record the resulting voltage between the electrodes. By switching to different pairs of current-inducing and voltage-measuring electrodes, various measurement programs can be successively executed and different body segments examined. Furthermore, by applying current to one hand and one foot, and by measuring voltage at the same hand and foot, an entire side of the body can be measured. The measuring platform on which the person being examined stands can also include a weighing device, so that, in addition to the impedance measurement, the body weight of the person being examined can also be recorded.

[0005] There are also devices where current is applied and voltage is measured simultaneously via a pair of electrodes. In these cases, the resistance of the circuit located outside the body of the person being examined must be taken into account, and its contribution to the resistance or impedance measurement must be corrected. Furthermore, the impedance of the skin also contributes to this and must be corrected for in the measurement result.

[0006] The technical background and variations of bioimpedance measurements and their use in determining body composition parameters are described, for example, in the article "Bioelectrical Impedance Analysis - Part I: Review of Principles and Methods" by Ursula G. Kyle et al., Clinical Nutrition (2004) 23, 1226-1243. Different operating principles underlie the procedures and devices. There are simpler devices with only two or four electrodes, which are attached, for example, to a hand and a foot on one side of the body, thus integrally recording the impedance of one side of the body. With other devices, such as the one described in the introduction, the impedance values ​​of individual body segments can also be recorded. Furthermore, there are devices that operate with a single frequency for the alternating current, for example, with a frequency in the range of 1 kHz to 1 MHz.In addition, there are so-called multi-frequency bioimpedance analysis methods that work with a range of measurement frequencies, e.g. with the measurement frequencies 0, 1, 5, 50, 100, 200 to 500 kHz.

[0007] In addition to the bioimpedance measurement device with a platform and handles described in the introduction, which allows for examinations of individuals while standing, there are devices better suited for lying or sitting subjects. A bioimpedance measurement device for supine subjects is described, for example, in US 2013 / 0102873 A1. This device has a central unit that is positioned separately from a bed in which the subject lies. Leads extend from the central unit, each with electrodes at its end, designed for connection to measurement positions on the subject's left and right hands and feet. The leads are sufficiently long to be positioned over the subject from the central unit and routed to the designated measurement positions on the hands and feet. Such bioimpedance measurement devices are generally easily portable.

[0008] To ensure the safety of the person being tested, the amplitude of the applied alternating current is kept low. Typical bioimpedance measuring devices, for example, use measuring currents with an amplitude of 100 µA. Due to the low level of the measuring current, the resulting voltages are correspondingly small; that is, the voltage measuring circuit must be able to detect small voltage signals sensitively and accurately. However, the human body can, to a certain extent, act as an "antenna," meaning it can receive interfering electromagnetic waves that lead to an alternating voltage signal between the two measuring electrodes. These can be, for example, radio waves and alternating electromagnetic fields generated by electrical equipment and installations in the vicinity. Such interference can superimpose itself on the voltage signal being measured, thus distorting the measurement or rendering it completely unusable.Even if a bioimpedance measuring device passes an EMC test (electromagnetic compatibility test) at the manufacturer, interference can still occur at the point of use. For example, medium-wave transmitters operate at frequencies that are not tested via irradiation fields in EMC tests. Another possibility is that interfering electrical devices are operating in the vicinity of the bioimpedance measuring device. This can lead to faulty measurements or even interference so severe that no meaningful measurements can be taken at all. Typically, the operator of the bioimpedance measuring device is unaware of such electromagnetic interference at the point of use and can only measure it independently with considerable effort. As a result, the operator of the bioimpedance measuring device may become dissatisfied with its performance due to frequent errors or interference during measurements.This problem is particularly acute with portable bioimpedance measurement devices that are operated at various locations. In such cases, malfunctions are usually unknown, and any errors or malfunctions are difficult to reproduce.

[0009] The object of the present invention is to make the effects of electromagnetic disturbances on a bioimpedance measuring device more manageable, in particular to enable a more accurate measurement of the impedance.

[0010] The bioimpedance measuring device with the features of claim 1 serves to solve this problem. Advantageous embodiments of the invention are listed in the dependent claims.

[0011] According to the present invention, the control and evaluation unit of the bioimpedance measuring device is configured to perform a disturbance test in connection with the determination of the impedance by not inducing any alternating current from the alternating current source into the body during a time interval, recording the voltage signal between the electrodes connected to the voltage measuring circuit during this time interval and checking it against a threshold criterion, and if this criterion is exceeded, the presence of an electromagnetic disturbance is detected.

[0012] In an advantageous embodiment, the fault test in connection with impedance measurement involves the control and evaluation unit being configured to perform the fault test immediately before applying the alternating current for impedance determination. However, a fault test can also be performed immediately after impedance determination or at an intermediate interval within the measurement period, provided the measuring alternating current is switched off during that interval.

[0013] The threshold criterion applied to the voltage signal in the disturbance test is intended to provide a measure of the signal activity of AC voltage signals during the time interval. For example, the mean magnitude amplitude, the intensity, or the maximum amplitude of the voltage signal within the time interval can be determined as the disturbance level. This disturbance level can then be compared to a predefined limit value, and the threshold criterion is considered exceeded if the disturbance level is above the limit value.

[0014] In an advantageous embodiment, the control and evaluation unit is configured to check for exceeding the threshold criterion only for voltage signal frequencies within a frequency interval around the frequency of the alternating current to be applied for measurement. Checking for disturbances within a frequency interval around the measurement frequency is sufficient to detect relevant electromagnetic interference that impairs measurement quality.

[0015] In an advantageous embodiment, the control and evaluation unit is configured to subject the voltage signal acquired during the time interval of the disturbance test to a Fourier transform and to check, as a threshold criterion, whether the magnitude of Fourier coefficients in a frequency interval around the frequency of the alternating current applied for the measurement exceeds a predetermined value. The Fourier transform converts the voltage signal from its time-domain representation into a frequency spectrum during the disturbance test interval. The disturbance test then uses a threshold criterion of checking the amplitude in a frequency interval around the measurement frequency and comparing this amplitude with a threshold value.If the frequency spectrum of the voltage signal does not exhibit a significant amplitude (above a predetermined value) in the frequency interval around the measurement frequency during the time interval of the interference test, then there is no significant interference with the measurement.

[0016] The fact that the presence of electromagnetic interference has been detected in a disturbance test can be used in various ways to improve the quality of the bioimpedance measurement.

[0017] Firstly, an error message can be displayed on an output device, for example, on a screen that is usually present. This alerts the operator of the bioimpedance measurement device that the measurement is impaired and faulty. The operator is then prompted by this error message to discard the measurement and its results and to perform another measurement, possibly at a different measurement frequency.

[0018] Alternatively, the control and evaluation unit can be configured to change the frequency of the alternating current used for measurement if the threshold criterion is exceeded during the interference test, and to perform another interference test. The two aforementioned steps are then repeated until no further interference test reveals that the threshold criterion has been exceeded. Afterward, the measurement to determine the impedance is performed at the frequency at which the interference test did not result in an exceedance of the threshold criterion. In this way, the bioimpedance measuring device automatically performs an interference-free measurement. The frequency is changed, for example, by selecting a frequency close to the original measurement frequency, such as 19 or 21 kHz, if the original measurement frequency of 20 kHz was affected by interference.The number of frequency changes while searching for a noise-free frequency may be limited to restrict the search time. If no noise-free frequency is found, the measurement is marked as erroneous.

[0019] The invention is described below with reference to an exemplary embodiment in the drawings, in which: Fig. Figures 1 to 4 show schematic circuit diagrams of a bioimpedance measuring device during the execution of different measurement programs. Fig. Figure 5 shows a flowchart illustrating the measurement process of a conventional bioimpedance measuring device, Fig. Figure 6 shows a flowchart to explain the operating mode of a first embodiment of a bioimpedance measuring device according to the invention, Fig. Figure 7 shows a flowchart to explain the operating mode of a second embodiment of a bioimpedance measuring device according to the invention.

[0020] A typical bioimpedance measurement device includes, for example, a standing platform on which the person being measured stands with both feet. Two electrodes are positioned on the platform for each foot, for example, one in the heel area and one in the forefoot area. Furthermore, there are two hand grips which the user grasps in a specific way so that each hand is in contact with two electrodes, for example, one on the little and ring fingers and the other on the middle and index fingers.

[0021] Various measurement programs can be performed with such a bioimpedance measuring device. In the Fig. In the measurement program shown in Figure 1, the control and evaluation unit connects the schematically depicted alternating current source to an electrode on the foot and an electrode on the hand on the same side of the body. The voltage resulting from the applied measuring current is measured by the control and evaluation unit bringing the voltage measuring circuit into contact with the two other electrodes on the hand and foot on the same side of the body. This measurement program measures the impedance of the entire side of the body across which the alternating current is applied and the voltage is measured.

[0022] At the in Fig. In the measurement program shown in Figure 2, the control and evaluation unit connects the alternating current source to an electrode on one hand and an electrode on the foot of the same side of the body, inducing an alternating current across that side. The voltage measuring circuit is connected to the electrodes on the two opposite hands. This measurement program is sensitive to the impedance of the current-carrying arm (the left arm in Figure 2). Fig. 2).

[0023] At the in Fig. In the measurement program shown in Figure 3, the control and evaluation unit connects the AC power source to electrodes on the hand and foot on the same side of the body in the same way as before. Furthermore, the control and evaluation unit connects the voltage measurement circuit to an electrode on each of the subject's opposite feet. This impedance measurement is sensitive to the impedance of the current-carrying leg (the left leg in Figure 3). Fig. 3).

[0024] At the in Fig. In the measurement program shown in Figure 4, the control and evaluation unit connects the alternating current source to one electrode each on a hand and foot on the same side of the body, while the voltage measurement circuit is connected to electrodes on the hand and foot on the opposite side of the body. This measurement program is sensitive for measuring the impedance of the subject's torso.

[0025] Furthermore, measurement programs for a body segment can be carried out on the opposite side of the body.

[0026] In Fig. Figure 5 shows a flowchart illustrating the operating procedure of a conventional bioimpedance measurement device. After the measurement is started, a body segment to be measured is selected, and one of the measurement programs is chosen accordingly. Fig. 1 to 4 (or their permutations on the other side of the body) were selected.

[0027] The measurement frequency of the alternating current to be applied is then selected. This is done by first selecting the first frequency from a predetermined sequence (e.g., 1, 5, 10, 20, and 50 kHz) and then selecting the next frequency in each subsequent iteration. The alternating current source is then connected to the current-imposing electrodes, so that alternating current flows through the selected body segment of the subject. The resulting voltage is measured with two electrodes using a voltage measurement circuit, the result is recorded, and the alternating current is switched off by disconnecting the connection between the alternating current source and the current-imposing electrodes.

[0028] The system then checks whether all desired measurement frequencies from the predetermined sequence have already been used. If not, it returns to selecting the next measurement frequency and performs the next impedance measurement at that frequency. This process is repeated until all desired measurement frequencies have been processed.

[0029] The system then checks whether all body segments have been measured, i.e., it verifies whether all measurement programs were used as described in the instructions. Fig. Measurements 1 to 4 (and their permutations) have been performed, and if not, a next, as yet unmeasured body segment is selected and the measurement process is repeated for it. This process is repeated until all body segments have been measured and the results displayed.

[0030] With a bioimpedance measuring device operating in this way, it is not possible to determine whether individual or multiple measurements of body segments have been affected by electromagnetic interference. For example, a disturbance that occurs only temporarily may have influenced only the impedance measurement of the torso, while the measurement results for the arms and legs remain unaffected and appear plausible. However, including the erroneous result for the torso distorts the overall result.

[0031] Fig. Figure 6 shows a corresponding flowchart for a bioimpedance measuring device according to the invention, operating according to a first embodiment. Some steps correspond to those previously mentioned in connection with Fig. The five described above are not described again here.

[0032] According to this embodiment of the invention, after selecting the measurement frequency, a disturbance test is performed in which no alternating current from the AC power source is applied for a specific time interval, and the voltage signal is recorded at the electrodes designated for voltage measurement in the measurement program. The voltage signal during the time interval without applied measurement current is then examined for a threshold criterion. This threshold criterion can be verified, for example, by subjecting the voltage signal during the disturbance test interval to a Fourier transform and subsequently checking the magnitude of Fourier coefficients in the frequency spectrum of the voltage signal within a frequency interval around the measurement frequency to see if they exceed a predetermined value.If a significant interference signal is found in the measurement frequency range during the interference test by detecting that the threshold criterion has been exceeded, an error marker is set for this measurement.

[0033] The measurements are then continued as before until all measurement frequencies have been measured and all body segments have been measured.

[0034] Once all measurements are complete, a list of results is displayed. Measurements marked as erroneous can be displayed in this list, either with an error message instead of the result itself, or with a note indicating the potential error next to the result.

[0035] In Fig. Figure 7 shows a flowchart of the operating mode of a bioimpedance measuring device according to a further embodiment. Those steps that correspond to the previously described embodiment are not shown again in detail.

[0036] After selecting the measurement frequency from the sequence of predetermined frequencies, a disturbance test is performed as before by interrupting the connection between the AC power source and the current-generating electrodes for a specific time interval. During this interval, the voltage signal is recorded via the two voltage measuring electrodes designated for the measurement program using the voltage measuring circuit. The voltage signal recorded during this interval is checked against a threshold criterion, and if the threshold criterion is met, the measurement is classified as disturbed. If electromagnetic interference is detected and the measurement is classified as disturbed, an attempt is made to find a frequency close to the disturbance frequency at which no interference is present. For this purpose, the control and evaluation unit is configured to switch to a modified frequency close to the disturbance frequency. For example, ifIf interference is detected at the measurement frequency of 20 kHz, the frequency is changed to 19 kHz, then to 21 kHz, then to 18 kHz, and so on, until an undisturbed measurement is obtained. The number of attempts with different frequencies in the vicinity of the interfering measurement frequency may be limited to restrict the time required to search for an undisturbed frequency. Fig. 7 refers to the transition to a changed frequency as the selection of an alternative frequency.

[0037] Otherwise, the measurement continues as in the previously described example.

[0038] Finally, after all measurement frequencies and all segments have been processed, the results are displayed. In this embodiment, successful and correct measurement results can generally be displayed for all body segments, since for each body segment, the measurement could usually be carried out undisturbed at the measurement frequency itself for each measurement frequency, or, in the case of interference at a measurement frequency, a successful measurement was at least achieved at a modified frequency in the vicinity of the measurement frequency. If interference is detected at a measurement frequency, either the undisturbed measurement result is displayed at the modified frequency if a successful measurement was carried out undisturbed in the vicinity of the disturbed measurement frequency, or, if no undisturbed measurement could be carried out at the modified frequency, the result at the measurement frequency is displayed with a marker indicating interference.

[0039] It can be seen that the design of the impedance measuring device according to the invention reduces the effects of electromagnetic interference on the measurements, since the operator of the bioimpedance measuring device can detect any interference as soon as the measurement results are displayed and can repeat the measurement at a later time or, if necessary, eliminate the cause of the interference. According to the second embodiment, in which the selection of the measurement frequency is repeated until a measurement frequency is found at which no interference is detected, the bioimpedance measuring device automatically leads to measurements without interference.

Claims

[1] Bioimpedance measuring device for determining composition data of the human body with several electrodes attachable to limbs of the body and electrical circuits connectable thereto, including an alternating current source, a voltage measuring circuit and a control and evaluation unit, which is configured to control the operation of the alternating current source and voltage measuring circuit and their selective connection with the electrodes and, according to predetermined measurement programs, to induce an alternating current from the alternating current source into the body via two electrodes specific for the respective measurement program and to detect the resulting voltage with two electrodes on different limbs by means of the voltage measuring circuit and to determine from this the impedance of the current-carrying body component between the electrodes detecting the voltage, characterized by, that the control and evaluation unit is designed to perform a disturbance test in connection with the determination of the impedance by not inducing any alternating current from the alternating current source into the body during a time interval, recording the voltage signal between the electrodes connected to the voltage measuring circuit during this time interval and checking it against a threshold criterion, and if this criterion is exceeded, the presence of an electromagnetic disturbance is determined. [2] Bioimpedance measuring device according to claim 1, characterized by that the control and evaluation unit is designed to perform the fault test immediately before applying the alternating current to determine the impedance. [3] Bioimpedance measuring device according to claim 1 or 2, characterized by, that the control and evaluation unit is set up to determine the mean intensity of the voltage signal or the maximum amplitude of the voltage signal in the time interval as a threshold criterion to determine the disturbance level and to compare it with a predetermined limit value and to evaluate the threshold criterion as exceeded if the disturbance level is above the limit value. [4] Bioimpedance measuring device according to one of the preceding claims 1 or 2, characterized by , that the control and evaluation unit is set up to check for exceeding the threshold criterion only for voltage signal frequencies in a frequency interval around the frequency of the alternating current to be impressed for the measurement. [5] Bioimpedance measuring device according to claim 4, characterized by, that the control and evaluation unit is set up to subject the voltage signal recorded in the time interval to a Fourier transformation and to check, as a threshold criterion, whether the magnitude of Fourier coefficients in a frequency interval around the frequency of the alternating current to be impressed for the measurement exceeds a predetermined value. [6] Bioimpedance measuring device according to any of the preceding claims, characterized by , that the control and evaluation unit is connected to an output device and is further configured to issue an error message on the output device if the threshold criterion is exceeded in the fault test. [7] Bioimpedance measuring device according to any of the preceding claims, characterized by, that the control and evaluation unit is further configured to change the frequency of the alternating current to be impressed for the measurement if the threshold criterion is exceeded in the disturbance test and to perform another disturbance test, to repeat the two aforementioned steps until no exceedance of the threshold criterion is detected in a disturbance test, and then to perform the measurement to determine the impedance at the frequency at which the disturbance test did not show an exceedance of the threshold criterion. [8] Bioimpedance measuring device according to claim 7, characterized by , that the control and evaluation unit is further configured to change the frequency only a predetermined number of times when a disturbance is detected, and if no undisturbed measurement could be carried out by then, to mark the measurement as faulty.

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