FI105773B - Method for measuring of bioimpedance - Google Patents

Description

1 105773

The present invention relates to a method for measuring bioimpedance according to the preamble of claim 1, using a plurality of electrode arrays provided, for example, by computer controlled switches *.

5 Bioimpedance measurement is applied to current of known magnitude through two or more electrodes attached to the body and measures the potential difference caused by the applied current between two or more electrodes. According to Ohm's law, the impedance, or more often only the resistive part, or resistance, is obtained from the ratio of the measured potential difference to the supplied current. A fundamental feature of non-invasive bio impedance measurement methods is the indirect measurement principle that seeks to determine quantities based on body anatomy or physiology. When measuring the impedance of a body or part of a body, the resulting impedance signal can be affected by many factors, which cannot be distinguished by conventional bio impedance measurement methods. These factors include all events that alter the so-called impedance measurement associated with the electrode assembly. switching fields. The switching field in this context refers to the current density field of the current supplied through the power supply and voltage measurement electrodes. The switching fields of the power supply and voltage measurement electrodes together form the measurement sensitivity distribution to estimate how small the volume or volume is; Changes in conductivity affect the signal being measured. The switching fields and the impedance signal thus measured can be altered by any complex, simultaneous change in the electrical, geometric or physiological properties of body tissues. These changes are detected in different ways, depending on the electrode assembly used.

* '* 25 There are several applications of bioimpedance measurements, but this application focuses on the so-called.

«For impedance plethysmographic applications. One of the most desirable parameters to be determined by indirect bioimpedance measurement is the amount of blood pumped (SV), which is a very sensitive indicator in many heart diseases. Conventional bio-impedance based SV measurement methods generally measure either body-wide 2,105,773 or body impedance and cyclical changes in heart pumping activity. SV is defined by a signal measured by empirical formulas. However, the methods have not proven to be very reliable, which would be an important feature when using bioimpedance measurement in clinical work. If the method cannot be trusted, the value of the information it provides 5 is negligible. For this reason, non-invasive measurement methods based on bioimpedance have not achieved widespread clinical acceptance. The SV is determined by the impedance signal measured by only one electrode assembly, which consists of several signal shaping factors. In addition, when a total of eight electrodes (four for power supply and four for 10 voltage measurement) are typically used with two electrodes connected to each terminal of the current and voltage circuits, the contact characteristics of the electrodes affect the measurement sensitivity to measure different body parts and changes. Therefore, the impedance differences in the contacts can change the sensitivity of the measurement to different locations in the body or the volume being measured. Figure 1 a) shows a conventional tetrapolar (current and voltage circuits have their own 15 electrodes) bioimpedance measurement of a body area with a total of eight electrodes (E1, ..., E8) attached to the person to be measured (4). When the skin electrode contacts are exactly the same, the resulting switching fields are evenly distributed between the different electrodes. Figure 1 b) shows a principal current distribution (switching field) associated with the current supply electrodes with their contact impedances being equal to each other. The input current through the electrodes E1 and E2 is then evenly distributed.

If, on the other hand, the impedance of the E1 skin electrode contact is even 10 Ohm and the E2 is 100 Ohm, then according to the resistance distribution 100-10 / (10 + 100) * 100% = 90.9% of the total current. Such a situation is shown in principle in Figure 1 c). Corresponding changes are related to differences in electrode contacts 25 also in voltage measurement. This potential imbalance alters the property of the measuring set to measure, for example, the change in impedance caused by blood flowing in the aorta, and it is possible that such random and 'unpredictable' changes in measurement properties cause errors in parameters determined by bioimpedance, such as SV.

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In order to obtain more accurate and reliable information on the subject measured by bioimpedance, it is desirable to perform multiple measurements with different electrode assemblies that measure the components constituting the bioimpedance in different ways. Several different measurements are used, for example, in general 12-lead electrode array based electrocardiography (ECG), which measures the bio-electrical function of the heart. If the ECG is measured with only one measurement setup, it cannot provide a detailed and reliable diagnosis of cardiac function and condition. Therefore, a 12-lead electrode system is commonly used to measure EKG. A 12-lead ECG provides 12 different signals, some of which, however, are not independent of each other. When multiple signals are available, a more reliable diagnosis can be made.

The ECG thus measures the electrical activity of the heart. However, electrical activity may be normal but the heart still does not mechanically function as a blood pump, which is its primary function. A well-designed 15 bioimpedance measurement can provide a measurement signal that largely reflects this geometric change and blood mass movements due to mechanical action of the heart.

US. U.S. Patent No. 5,720,296 discloses a method for determining body tissue composition by means of impedance measurements. Impedance is measured using eight electrodes at 20 limbs in 20 contacts. By appropriately selecting the electrical connections of the terminals of the current and voltage circuits to the different electrodes, the impedance signals from the body or from each limb can be measured separately. In this way, at least eight different measuring positions can be measured instead of one for the whole body. US. U.S. Pat. No. 5,720,296 discloses an electronic switch that can switch any 25 current electrode pairs (four preselected electrodes out of the eight available. · electrodes) and a pair of voltage electrodes (voltage electrode alternatives four out of the eight electrodes used for measurement) to impedance terminals. Figure 2 shows the US. a diagrammatic view of a device implementing the method of U.S. Patent No. 5,720,296. Voltage can be measured by the method described (i.e., four electrodes are available for voltage measurement, used two at a time) in 12 different ways, including the six ways in which only the polarity of the signal is reversed (e.g., electrodes AB and the second switching electrodes BA give the same signal only signal polarity has changed). Theoretically calculated, the method described can be used to measure 12 * 12 = 144 measurement connections, or, more correctly, 6 * 6 = 36 5 independent measurement connections. The problem with the invention described in that publication is that it is not capable of providing only a limited number of connections. In this case, the sensitivity distribution of the measurement cannot be precisely targeted to particular tissues or organs.

The object of the invention is to eliminate all the above disadvantages and to provide a method and apparatus for bio-impedance 10 which can make measurements with a very large number of different arbitrary measuring connections by connecting more than one electrode per terminal of the impedance device, if necessary. This can provide many different measurements and increase the information used to make the diagnosis. If necessary, the impedance measurement can be performed simultaneously during the bioelectric, for example, ECG measurement, using the same electrodes without interfering with the bioelectric measurement. The introduction of new impedance measurements is thus easier when the clinical measurement situation does not need to be changed, but additional information is obtained. Furthermore, in the method of the invention, the differences in impedances of the skin-electrode contacts of the electrodes used are compensated for by reducing the distortion or errors in the measurement of the bio-impedance caused by the differences in the contact impedances.

The basic idea of the present invention is that any electrode can be used for both power supply and voltage measurement, thus increasing the number of measuring arrangements. By the method according to the invention, more than two electrodes connected to a voltage or circuit can be used simultaneously and thus the number of possible measuring arrangements is multiplied. In addition, by using appropriately selected measuring circuits consisting of several electrodes connected to one terminal, it is possible to obtain more accurate bi-impedance measurements of certain organs or tissues of the body.

More specifically, the method of the invention is characterized in what is set forth in the characterizing part of claim 1. Preferred embodiments of the invention are set forth in the dependent claims.

The invention will now be described in more detail with reference to the accompanying drawings.

5 Figure 1. Principle of bioimpedance measurement and the effect of a different skin electrode contact on the formation of a current density field, i.e. a coupling field, related to the measurement sensitivity distribution.

Figure 2. Schematic view of US. apparatus implementing the invention of patent 5720296.

Figure 3. Schematic diagram of the method according to the invention.

Fig. 3 shows an impedance measuring apparatus with electronically controlled switches (1) from each of the four terminals (U1, U2, L1, I2) of the impedance measuring device (5) to each electrode (EU, ..., EL ') connected to each patient; , capable of independently controlling each electrode connected to a patient to one or more terminals of the impedance measuring apparatus. In this way, all 15 possible different measuring arrangements can be made available, which can be achieved with the available number of electrodes (at least four). This is achieved by electrical control (1), for example by means of a computer, without manual mechanical change of the circuits.

The apparatus further comprises high-pass filters (9) for connections from electrodes (EL, ..., EL,; at least four) connected to each patient, which are controllable by switches (1) of electronically controlled (1) power terminals (L, I2) of the impedance measuring apparatus (5). 2) lets you. When a bio-impedance measuring apparatus employs a high frequency (e.g., 1000 Hz or higher) current signal with a frequency significantly higher than the frequency band of the bioelectric signals (e.g., ECG frequency band generally below 100Hz), current between several electrodes can be supplied simultaneously (e.g., E1, E3, E5, E6 in Figure 1) without causing distortion to the bioelectric measurement. Thus, it is possible to measure bioelectric measurements simultaneously with the power supply included in the bioimpedance measurement 6 105773 without disturbing each other. Without using high-pass filters, for example, when multiple electrodes are connected to a power supply terminal I], a short-circuit is applied between them, that is, their potentials for bio-electrical operation become the same and bio-electrical measurement no longer provides useful information.

The apparatus also includes buffer amplifiers (8) at each of the circuits from the electrodes (EL, ..., ELr ,; at least four) connected to the patient, which are controlled by electronically controlled (1) switches (2) to two average voltage generating portions (6 and 7). , the signals of which are applied to the voltage measuring terminals (U1 and U2) of the impedance measuring device (5) 10. Here, potential differences in the applied current between several electrodes (e.g., E1, E3 - E2, E5, E6 in Figure 1) can be measured without causing distortion to the bioelectric measurement (for example, between the electrodes E1 and E3 in Figure 1). In this way, bioelectric measurements can be measured simultaneously with the voltage measurement included in the bioimpedance measurement, without disturbing each other. Without the buffer amplifiers, the bioelectric measurement no longer provides the appropriate information when connecting several electrodes to the same terminal of an impedance measuring device, since their potentials are then short-circuited, as are the power supply electrodes when connected to multiple terminals.

20 If bioimpedance measurement is used, for example, for cardiological purposes, it may also be possible to use a generic 12-lead ECG electrode system for simultaneous bioimpedance measurement, for example. In this case, the information provided by bioelectric (in this case, ECG) and bioimpedance can be combined. The bio-impedance contributes very well to the use of suitable selective measuring circuits by the actual mechanical action resulting from the bio-electrical function of the heart.

. ·. Combining these two techniques can provide a better chance of making a reliable and comprehensive diagnosis than either measurement alone. In addition, if in any case biofeedback electrodes are used to measure impedances, this increase can be made without significantly increasing or modifying the clinical routine or the workload of the nurse.

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Resistors (3; Ri, .... Rn) are added to the electrical connections from the electrodes (EL1, ELn ', at least four) connected to each patient electronically controlled (1).

These resistors (Ri ..... Rn) thus become connected in series with each electrical resistor associated with the skin electrode contact. These additional resistors (3) are electronically controllable (1) and their resistors can also be electronically adjusted to the desired size. Alternatively, the auxiliary resistors may be implemented with conventional resistors instead of digitally adjustable resistors. If the current and voltage circuits of the impedance measuring apparatus (5) function ideally, whereby the power supply supplies a constant current regardless of the load applied thereto and the input impedance of the voltage measuring circuit is infinite, the additional resistors (3) can be set infinitely large. In practice, depending on the implementation of the impedance measurement instrumentation (5), the additional resistors (3) may be a few hundred or kilo ohms. By adding, for example, additional resistors of 10 kOhm to the wires connected to the electrodes E1 and E2 of Fig. 1, the current is distributed much more uniformly (through E1 by 91.9%), which is useful for performing a measurement and provides a more accurate measurement. So now in our example, with 10 kohm extra resistors, the current is distributed according to the resistance divide: E1 passes 100- (10 + 10,000) / (10 + 10,000 + 100 + 10,000) * 100% = 50.2% of the current supplied, though the skin the electrical resistances of the electrodes between E1 and E2 are very different (E110 ohm, E2 100 ohm).

20 By using as large auxiliary resistors as possible in each conductor connected to the electrode, the reliability of the measurement can be improved. The maximum value of the auxiliary resistor used is determined by the capacity of the impedance measurement instrumentation to operate (I ------- with increased load (resistor)), that is, the idealities of the above power supply circuit and the voltage measurement circuit.

Devices implementing the methods described above may be integrated into an existing impedance measuring device, or may be a stand - alone accessory that may be used. with an existing device.

The invention is not limited to the above embodiments, but may be modified within the scope of the appended claims.

Claims (7)

8 105773 A method for measuring bioimpedance using electronically controlled (1) switches (2) to form a measuring arrangement, characterized in that said switches control patient-connected electrodes (ELI, ..., ELn) 5 independently connecting at least one electrode to each terminal of the impedance measuring device (5). (U1, U2, II, 12) and that the bioelectrical and bioimpedance measurements are performed simultaneously, with the said electrodes used for the measurements being partially or totally identical. 2. Förfarande enligt patentkrav 1, kännetecknat av att electroniciskt (1) styrda förkopplingsmotständ (3) används i alla kopplingar av electroder (EU, ..., Eln) som har fasts pä patienten. Method according to Claim 1, characterized in that 10 additional electronically controlled (1) resistors (3) are used in each connection of the electrodes (EL 1, ..., EL 1) connected to the patient. 3. Förfarande enligt patentkrav 1, kännetecknat av att buffertförstärkare (8) används i alla kopplingar frän elektroder (EU, ..., Eln), a bunch of bunches of bumblebees and bunches of bunches 6 of 6 ) av medelvärdesspänningen account spänningsmätterminaler (Ui, U2) i impedansmätarrangemanget (5). Method according to Claim 1, characterized in that buffer amplifiers (8) are used in each of the connections from the electrodes (EL, ..., EL ") which are connected by said switches, depending on the switch states, via two impedance measuring devices (6 and 7). (5) voltage measuring terminals (U1, U2). 4. Förfarande enligt patentkrav 1, kännetecknat av att högpassfilter (9) används i alla kopplingar frän electroder (EU, .., Eln), pulley copyrights till terminaler (L, I2) i impedansmätarrangemangets (5) strömmatningskrets med hjäl. styrda kopplingar beroende av deras kopplingstillständ. Method according to Claim 1, characterized in that high-pass filters (9) are used in each of the connections from the electrodes (EL 1, ..., EL 1) which are connected to the power supply circuit terminals (L, I 2) 20 of the impedance measuring apparatus (5) by electronically controlled switches depending on the switch states. 5. Apparat för utformning av et mätarrangemang vid bioimpedansmätningen omfattande electronisk styrda (1) kopplingar (2), transducer av att electroder (ELj, ..., Eln) som har fästs pat patienten can oberoende av varandra styras med de nämnda kopplingama account. terminal (U1, U2, li, I2) som Heist vid impedansmätapparaten (5). Apparatus for forming a measuring arrangement for bioimpedance measurement comprising electronically controlled switches (1), characterized in that said switches are capable of controlling patient-connected electrodes (EL, ..., EL ") independently of any of the terminals (Ui, ...) of the impedance measuring device (5). U2, li, I2). Apparat enligt patentkrav 5, a transducer means for integrating this apparatus with bioimpedance methods (5). "105773 Device according to Claim 5, characterized in that said device is integrated: in a bioimpedance measuring device (5). 9 105773 Device according to claim 5, characterized in that said device is operable with a separate bio impedance measuring device (5). * Patent Patent 105773 1 .Förfarande for mätning av bioimpedans genom användning av electronic styrda kopplingar (2) vid utformningen av etmarrangemang, telnet electrospeed (ELI, Eln) som har fästs Pärtand averand n. att copy ätminstone en electrode till vaje terminal (U1, U2, II, 12) i impedance mode (5) och att den bioelectric mätningen och mätningen av bioimpedan utförs samtidigt ä de nd electrode som används för mätningar är delvis hl. Apparat enligt patentkrav 5, a transducer of an apparatus of this type for separation of bioimpedance methods (5). 9