METHOD AND ELECTRODES FOR BIOIMPEDANCE MEASUREMENTS

Abstract: Electrodes useful for electric measurements from or injecting current to the human body are disclosed, which are flexible disposable electrodes with a surface area in the range of about 18-36 cm<2>. A particular use of such electrodes is in bioimpedance measurement, particularly such performed by utilizing the same electrodes for passage of current and measurement of potential. Also disclosed are processes for bioimpedance measurement making use of such electrodes.

Description: METHOD AND ELECTRODES FOR BIOIMPEDANCE MEASUREMENTS FIELD OF THE INVENTION The present invention is in the field of electrical measurements made on the human body in general and bioimpedance measurements in particular. Provided by the present invention are novel disposable electrodes for such a measurement as well as a process for bioimpedance measurements in which such electrodes are used. BACKGROUND OF THE INVENTION Measurements and monitoring of a variety of electrical parameters from the human body, such as ECG, bioimpedance measurements, EEG, or the like, involve placing electrodes on the human body and then measuring of electrical signals, at times during or following an electrical stimulus. Electrodes which are used in such measurements usually constitute either small disposable circular metal plates, typically having a diameter of about 1 cm., or needles. Among the advantages of using such small electrodes is the ability to apply them to practically any desired part of the human body. In addition, electrodes having small dimensions are advantageous in intensive care units where many functions of the human body are continu- ously monitored and accordingly patients are covered, in almost literal sense, with many wires, electrodes, needles, tubes of infusion systems, etc. It should be mentioned, however, that small non-invasive electrodes usually provide non-stable electπcal contact with the patient's skin, thus affecting accuracy of the measurements. U.S. Patent No. 4,117,846, discloses a disposable skin conductive electrode assembly which comprises an electrolyte pad in contact with a metal electrode. The purpose of the electrolyte pad is to improve electrical contact between the electrode and the skin. This electrode is intended for use either as a return electrode during electrosurgery, or as an active electrode in electrotherapy. An electrode widely used as a grounding plate in the field of electrosurgery, is the Arbo ™ electrode manufactured by Arbo ™ Medizintechnologie GmbH Pfl230 D38002, Braunschweig, Germany. This electrode constitutes a silver-silver chloride plate and avoids the need to use electrolyte gels for improving the contact. Such electrodes are relatively large and cannot be used on portions of the patient's body where there is only a restricted area for electrode placement, e.g., the arms or legs. GENERAL DESCRIPTION OF THE INVENTION It is an object of the invention to provide a disposable electrode for reliable electrical measurements from the human body, particularly bioimpedance measurements. It is particularly an object of the present invention to provide disposable electrodes for bioimpedance measurements in a bipolar system, i.e., a system in which current is injected and potential is measured through the same electrodes. It is furthermore an object of the invention to provide disposable electrodes for use in the above measurements. It is furthermore an object of the present invention to provide an assembly comprising the above electrode and means for holding an infusion needle, a catheter tube, or the like. It is still further an object of the invention to provide a process for bioimpedance measurements making use of such electrodes. It has been found in accordance with the invention that reliable bioimpedance measurements are obtained by the use of electrodes having a relatively large surface area, typically in the range of 19-22 cm 2 . In the following, the term "bioimpedance measurement" will be used to denote a measurement in which the relationship between an injected current and a measured voltage of the entire body or part thereof is tested; the term "bipolar bioimpedance measurement" to denote bioimpedance measurement performed by passing current and measuring voltage through the same electrodes. The bioimpedance measurement in accordance with the invention is particularly one aimed at testing the cardiac, vascular and respiratory parameters of a tested individual, e.g. as in the method described in European Patent Application No. 0575984A2 and its corresponding U.S. Patent No. 5,469,859. The present invention provides in accordance with one of its aspects, electrodes for measurement of electrical signals from the human body, injecting electric current to the human body, or both, particularly for bioimpedance measurement, being flexible disposable electrodes with a surface area in the range of about 18-36 cm 2 , preferably in the range of about 19-22 cm 2 . The present invention provides, by another of its aspects, a process for bipolar bioimpedance measurements in an individual comprising attaching two electrodes to the individual's body, the electrodes being flexible, disposable electrodes having a surface area in the range of about 18-36 cm 2 , preferably in the range of about 19-22 cm 2 , injecting an alternating current through said electrodes, measuring current-voltage relationship and processing thereof to obtain bioimpedance parameters. In accordance with the preferred embodiment of the invention, the electrodes are attached to arms and legs of an individual for whole body bioimpedance measurement. The electrodes are preferably provided within a device comprising means for securing the electrode to the skin, e.g. a pad overlaying the electrode and having a peripheral portion with an adhesive bottom face, extending beyond the edges of the electrode for adherence to underlaying skin portions; an adhesive, electrically conducting substance coating the - 4 - bottom surface of the electrode, that which comes in contact with the individual's skin; or a combination thereof. Also provided by the invention is an assembly comprising electrodes for bioimpedance measurements, means for securing the electrodes onto the skin, and means for holding and securing catheter tubes or infusion needles. In case of electrodes intended for attachment to arms and legs, particularly preferred are such having an elongated flat body with broader ends and a narrower middle portion. The electrode is configured and then fixed onto the skin of the arms and legs such that the middle portion of the electrode overlays the area of the arm and leg comprising the veins, and the broader ends are placed over the arteries. The distance between the two main arteries in the wrist area, which are the radial artery and the ulinar artery, is different in each individual; the same applies to the distance between two major groups of arteries in the leg, one group consisting of the anterior and the tibial arteries, and the other group consisting of the posterior and the peroneal arteries. However, it has been found that an electrode configured as above, and placed at the inner surface of the wrist or leg, having a total length of about 0.4 of the average circumference of the wrist or leg respectively, arid thus have its broader ends overlaying the arteries and its narrower middle portion overlaying the veins. For grown up individuals the values of average circumferences of the wrist and leg are given by Leslie W. Organ, Gilbert B. Bradham, Dwight T. Core and Susan L. Lozier in their paper "Bioelectrical impedance analysis: theory and application of a new technique", Physiologic, 77(1):98-112 (1994). The disclosed values are as follows: Wrist Leg 23.3±1.3 - for men 30.7+2.4 - for men 19.4±1.0 - for women 27.9H.7 - for women According to the invention, the total length of the electrode for grown up individuals is preferably about 10 cm. Such an electrode is suitable for application on both the wrist and the leg. In accordance with an embodiment of the invention, the electrode is combined into an assembly adapted also to hold and secure catheter tubes or infusion needles. The electrode may be made of a silver alloy covered with a layer of silver chloride (Ag/AgCl electrode), may be made of gold, aluminum, platinum, etc. The electrode may also be provided with a layer of an electrolytic gel or an electrolytic adhesive or with a pad impregnated with an electrolytic fluid or gel which serves as an interface between the electrode and the skin. The electrode is typically provided with an exposed portion at one of its ends for connecting the electrode to an electrical lead which is in turn connectable to a bioimpedance measurement device. The invention will now be illustrated, by way of example only, in the following non-limiting specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates an electrode device in accordance with an embodiment of the invention; Fig. 2 is a cross-section through lines II-II in Fig. 1 (the cross- sectional dimensions are exaggerated for clearer view and understanding); Fig. 3 shows the manner of attachment of the electrode of Fig. 1 to an individual's wrist; Fig. 4 shows the manner of attachment of the electrode of Fig. 1 to an individual's leg; Fig. 5 shows an electrode device in accordance with another embodi¬ ment of the invention, wherein Fig. 5A is a top view with an electrode device being attached to a carrier sheet and Fig. 5B shows a bottom view of the device being detached from the carrier member prior to use; Fig. 6 shows a cross-section through lines VI-VI in Fig. 5B (similarly as in Fig. 2, the cross-sectional dimensions are exaggerated for a clear view and understanding); Figs. 7 and 8 show two embodiments of an assembly comprising an electrode and means to hold and secure catheter tubes or infusion needles; Fig. 9 shows the manner of attachment of the assembly of Fig. 7 to an individual's wrist and its use in securing an infusion needle and tube; and Fig. 10 shows the resistive and capacitance impedance components as a function of the electrode's area. DESCRIPTION OF SPECIFIC EMBODIMENTS Reference is first being made to Figs. 1 and 2 showing an electrode device 10 in accordance with an embodiment of the invention. Device 10 comprises an electrode 12 which is preferably made of a thin foil, e.g. an aluminum foil having a thickness of about 0.007" (however, as can be appreciated, this is an example only and the electrode body can have a different thickness or made of a different metal such as gold, silver, platinum, etc.). The electrode is an elongated flat member with a narrow middle portion 14 and two broader end portions 16 and 18. The electrode 12 further comprises an exposed connector portion 20 for connection to an electrical lead (not shown), which is in turn connectable to a bioimpedance measuring device. The device 10 further comprises a cover pad 22, made of an electrically non conducting material (examples of such materials are various plastic materials such as polyethylene, e.g. a polyethylene film), which overlays electrode 12 and which has peripheral portions 23 extending beyond the edges of the electrode. The bottom face 24 of the peripheral portions 23 is adhesive providing_for a firm adherence of the electrode device to the skin. (The cross-section in Fig. 2 is schematical and the vertical dimensions are enlarged for ease of description; the thickness of electrode 12 is typically a fraction of a millimeter and thus bottom face 24 of the pad can easily come into tight contact with the skin). The bottom face of the electrode 12 may be covered by an electrolytic adhesive layer 26 which serves both as an electrical interface between electrode 12 and the skin and for improving mechanical contact between the two. Layer 26 may for example be a conductive, adhesive gel¬ like matrix, e.g. Lectec LT 3300 ™ conductive hydrogel (manufactured by Katecho, Inc., U.S.A.), or ConMed ™ #45, (manufactured by ConMed Corp., U.S. A). In storage, the bottom face 28 of the device is covered by a protective layer 30 (represented by a broken line) which is removed prior to use. Flaps 32, of the device, which are integral with pad 22, have also an adhesive bottom face and serve to secure the contact between the wire and connector portion 20. In use, the device 10 is attached to the skin and then a wire is connected to connector portion 20, e.g. by means of an electrical clip (not shown in Figs. 1 or 2) and the contact thereof with portion 20 is fixed by a firm attachment of flaps 32 to the skin. Such a tight mechanical attachment secures electrical contact and eliminates electrical noise. Figs. 3 and 4 show the manner of attachment of the device 10 to an individual's wrist and leg, respectively. As can be seen, the device 10 is secured to the wrist in a manner that the broad portions 16 and 18 of the electrode body overlay portions comprising the radial and the ulinary arteries 34 and 34'. Similarly, the device in the leg is fixed such that the broad portions 16 and 18 of the electrode overlay the portions comprising the main artery groups 36 and 36'. The middle portion 14 of the electrode overlays an area comprising the veins. This design of the electrode thus permits insertion of catheter tubes, infusion needles, etc. into the veins. As can be seen in Figs. 3 and 4, connector portion 20 is connected to a wire 38 by means of an electrical clip 40. Clip 40 is in fact secured against the skin by the mechanical force provided by flaps 32. Reference is now being made to Fig. 5 showing an electrode device 42 in accordance with another embodiment of the invention. Fig. 5A shows the device 42 in a storage state, attached to a carrier sheet 43; the device 42 is shown in Fig. 5B removed from the carrier sheet prior to use. The electrode of this embodiment has a general structure similar to that of the electrode device shown in Fig. 1 and is constituted by an elongated flat member comprising wide peripheral portions 45 and 46 and a narrow middle portion 47. The cross-section in Fig. 6 shows that device 42 has a top cover pad 48 made of an electrically non conductive material, e.g. polyeth- ylene foam, an intermediate electrode 49, e.g. made of aluminum foil, and a bottom layer 50 made of a conductive adhesive gel. The electrode 49 covers the entire surface area of the device save for a peripheral portion 51. The adhesive electrolyte layer 50 covers most of the surface of electrode 49 save for a peripheral portion 52 at the other end which is thus exposed and serves for connection to an electrical lead similarly as in the embodiment shown in Figs. 1 and 2. The device is provided with a tab 53 which is an independent member having an adhesive coating on its bottom face and its purpose is to be placed over peripheral portion 52 to secure the contact with the electric lead. Tight attachment of the electrode device 42 to the skin is ensured by means of the adhesive conductive gel 50, which serves also as an interface between electrode 49 and the skin, by means of adhesive peripheral portion 51 and by means of tab 53. Layer 48 may for example be a 1/16" polyethylene closed cell white foam with an acrylic adhesive coating the exposed portion 51. Layer 50 may for example be ConMed ™ #45 conductive/adhesive gel or Lectec LT 3300 ™ conductive hydrogel, mentioned above. An electrode device 60 in accordance with another embodiment of the invention is shown in Fig. 7. The assembly 60 comprises an electrode 61 which is essentially similar to electrode 12 of the embodiment of Figs. 1 and 2 and is an elongated flat member comprising a narrow middle portion 62, broad end portions 63 and 64 and a connector portion 65. Furthermore, similarly as in the embodiment shown in Fig. 1, the electrode device comprises an adhesive pad 66 with end flaps 67. One difference between device 60 and device 10 of Fig. 1, lies in the addition of an adhesive loop member 68 which serves for securing needles and catheters. Member 68, which has an adhesive bottom face comprises a loop portion 69 and three openings 70, enabling a physician, a nurse, etc. to choose a suitable place for insertion of a needle or a catheter. Window 72 of loop member 68 is typically constituted by a transparent film although it may also be left open. Member 68 may be detachable by means of a perforated tearline 74. The device 60 also optionally comprises detachable tab portions 76 and 78 which may be implemented for independent use as fixation elements of catheter tubes or the like. An electrode device 90 in accordance with another embodiment of the invention, which is in fact a modification of the device 60 of Fig. 7 is shown in Fig. 8. The major difference between the two embodiments is in that in device 80 member 82 consisting of a loop 84 and a window 86 is arranged to be at the opposite side of the electrode 88 as compared to the arrangement in device 60 of Fig. 7. Similarly as in the embodiment of Fig. 7, member 82 has a number of openings 87 at its point of attachment with the major portion 88 of the device, and is detachable by means of a perforated tearline 89. The manner of use of a device such as that of the embodiment of Fig. 7 is shown in Fig. 9. As can be seen, a needle 90 at the end of a catheter tube 92 is inserted into vein 94 through one of the openings 70, the needle being fixed and covered by loop 69 and window 72. One of the peripheral tab portions, e.g. portion 76, is used to secure catheter tube 92. In the following Examples, experiments performed in accordance with the invention will be described. - 10 - Example 1: Dependency of impedance components as a function of an electrode surface area Electrodes having a structure similar to that shown in Fig. 1 were used for bipolar bioimpedance measurement. In order to improve the contact between the electrode and the skin, an electrolytic gel (e.g. a gel comprising about 1% or less NaOH, was placed between the electrode and the skin. The bioimpedance was measured using an NICAS 2001 ™ apparatus (N.I. Medical, Tel Aviv, Israel) disclosed in European Patent Application No. 0575984A2 (and in the corresponding U.S. Patent 5,469,859), the contents of which is incoφorated herein by reference. Electrodes with different surface areas were tested, and in each case an active, resistive impedance component and a reactive, capacitance impedance component were extracted from the bioimpedance measurement. The results are shown in Fig. 10. As can be seen, both impedance components were linear when the electrode area was above about 19 cm 2 , but the linearity, particularly of the capacitance component was impaired below the mentioned surface area. These results show that in surface areas below about 19 cm 2 , limitations imposed by the electrodes start to play a role. Therefore it is desired to maintain the electrode area above about 19 cm 2 . For other considerations, in particular skin area availability for the electrode, it is desired to have a surface area below about 25 cm 2 , particularly below about 22 cm 2 . Example 2: Stability of electrodes Electrodes similar to those tested in Example 1, having a surface area of about 19 cm 2 were used and bioimpedance was measured in a bipolar mode in a similar manner to that described in Example 1. The resistive component of a bioimpedance measurement was measured over a six hour period and the results are shown in the following Table I: - 11 - Table I Individual Sex Age Height Weight Bioimpedance after ΗME (hours) Years cm kg 0 1 2 3 4 5 6 1 M 43 178 80 564 564 548 554 550 548 552 2 M 41 186 86 484 492 492 476 472 480 482 3 M 56 157 65 472 454 444 440 438 436 438 4 F 47 156 78 526 496 504 508 508 510 502 5 F 38 167 80 590 566 576 580 572 574 566 The above results demonstrate the very high stability of the electrodes of the invention over an extended time period. Example 3: Comparison of measurements provided in a bipolar mode in accordance with the invention to measurements using other electrodes operating in a tetrapolar mode Whole body bipolar bioimpedance measurement was performed, using a NICAS 2001 ™ device, with electrodes in accordance with the invention placed on one arm and a contralateral leg, similarly as in Examples 1 and 2. 30 male volunteers and 36 female volunteers were tested. In each case the resistance component of the whole body bioimpedance measurement (Rwb) was calculated. The results which were obtained, compared to results obtained in two prior art documents, wherein the bioimpedance measurement was performed in a widely used tetrapolar mode, i.e. using separate electrodes for passage of current and for voltage measurement, are shown in the following Table II: Table II Study N% (M/F) Age Height Weight Rwb (3) (cm) Segal K.R. et al., 1988 (1) Lab A 96 M 32±9 178±8 75±12 485+63 64 F 35±9 165±8 59± 8 587+58 Lab B 99 M 26±8 179±7 79±12 459+47 81 F 29±10 165+7 71+23 551±68 Chumlea W.C. et al, 29 M 18-62 171±7 83±12 474±52 1988 (2) 44 F 18-58 165±6 64±16 588+72 NICAS 2001 30 M 62±10 171±8 75±18 464±62 36 F 50± 9 158±7 70±16 573+65 ( 1) Lean body mass estimation by bioelectrical impedance analysis, a four-site cross-validation study; Karen R. Segal, Ed.D; Maria Van Loan, Ph.D; Patricia I. Fitzgerland, Ph.D; (2) James A. Hodgdon, Ph.D; and Theodore B. Van Itallie, MD, American Clinical Nutrition, 47:7-14, 1988; Specific resistivity used to estimate fat-free mass from segmental body measures of bioelectric impedance; W. Cameron Chumlea, Ph.D; Richard N. Baumgartner, Ph.D, and Alex F. Roche, MD.Ph.D., Am. Cl. Nutri¬ tion, 48:7-15 (3) Rwb - Whole body resistance The above results clearly show that the resistance component of the bioimpedance measurement is essentially the same as the measurements obtained hitherto in a tetrapolar measurement mode.

Claims: CLAIMS: 1. An electrode for measurement of electric signals from or injecting current to the human body, being flexible a disposable electrode with a surface area in the range of about 18-36 cm 2 . 2. An electrode according to Claim 2, having a surface area in the range of about 19-22 cm 2 . 3. An electrode according to Claim 1 or 2, being an elongated electrode, having broad end portions and a narrow intermediate portion. 4. An electrode according to any one of Claims 1-3, having the total length of about 10 cm. 5. An electrode according to any one of Claims 1-4, comprising an exposed connector portion for connection to an electric lead. 6. An electrode according to any one of Claims 1-5, being made essentially of aluminum, gold, platinum, silver or silver with a bottom layer consisting of silver chloride. 7. An electrode device comprising an electrode according to any one of Claims 1-5 with its bottom face, which comes in contact with the skin being coated by an electrically conducting adhesive substance. 8. An electrode device comprising an electrode according to any one of Claims 1-5 and a pad made of an electrically non-conducting material covering its top face, being the face other than that which comes in contact with the skin, the pad having peripheral portions extending beyond edges of said electrode and which portions have an adhesive bottom face. 9. An assembly comprising electrodes for bioimpedance measure¬ ment, means for securing the electrode onto the skin, and a member for securing a catheter tube or infusion needle. 10. An assembly according to Claim 9, wherein the electrode is an electrode according to any one of Claims 1-7. 11. A process for bipolar bioimpedance measurement in an individual comprising attaching two electrodes to the individual's body, the electrodes being flexible, disposable electrodes having a surface area in the range of about 18-36 cm 2 , injecting an alternating current through said electrodes, measuring current-voltage relationship and processing thereof to obtain bioimpedance parameters. 12. A process according to Claim 11, wherein the electrodes have a surface area in the range of about 19-22 cm 2 . 13. A process according to Claim 11 or 12, for whole body bioimpedance measurement, comprising attaching one electrode to an individual's arm and another electrode to an individual's leg.

Inventors: FRINERMAN EFIM, TSOGLIN ALEXANDER, FRINERMAN, EFIM, TSOGLIN, ALEXANDER
Applicants: FRINERMAN EFIM, N I MEDICAL LTD, TSOGLIN ALEXANDER, FRINERMAN, EFIM, N.I. MEDICAL LTD., TSOGLIN, ALEXANDER
Application year: 1997
Main classification: A61B5
IPC codes: A61B5, A61N1