Bioimpedance For Physicians

Centre Nacional de Microelectrònica

Biomedical Applications Group

Bioimpedance Monitoring for physicians: an overview

Antoni Ivorra
July, 2002
(revised and expanded July, 2003)

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Bioimpedance Monitoring for physicians: an overview

Sometimes I have had to explain the basics of electrical Bioimpedance Monitoring (BM) to physicians, biologists or veterinarians, all of them fromthe biomedical community and familiarized with the cell biology. In general, these professionals are not skilled in circuit theory or electromagnetics and I found difficulties to explain concepts such as impedance phase, real part of the impedance, complex numbers... In these cases, I would have liked to provide them some written material to clarify those concepts. Unfortunately, the literatureabout bioimpedance is mostly written by physicists for physicists and a background in electromagnetic theory is assumed. On the other hand, the basic circuit theory texts are too much dense and remote from the bioimpedance field. Therefore, I thought that a short paper trying to fill this gap could be profitable. I have not tried to write a scientific review about BM. My objective has been to bedidactic and I have specially focussed the electrical concepts. 0. INTRODUCTION Electrical Bioimpedance Monitoring is an emerging tool for biomedical research and for medical practice. It constitutes one of the diagnostic methods based on the study of the passive electrical properties1 of the biological tissues. These properties have been object of study since Luigi Galvani (1737-1789) discoveredthat while an assistant was touching the sciatic nerve of a frog with a metal scalpel, the frog's muscle moved when he drew electric arcs on a nearby electrostatic machine. However, it was not until the end of XIX (McAdams et Jossinet, 1995) that these properties started to be measured thanks to the development of new instrumentation and the set up of the electromagnetic field theory by JamesClerk Maxwell (1831-1879). The practical use of the electrical passive properties started in the middle of the XX century. Different properties and techniques resulted in a collection of methods that are now used for multiple applications. Usually, these methods have three advantages in common: require low-cost instrumentation. are easily applicable in practice. enable on-line monitoring. Excellentreviews about the applications of Bioimpedance (BI) methods can be found in (Grimnes et Martinsen, 2000), (Bourne, 1996) and (Scharfetter, 1999). Here some of these applications are listed in order to show the BI potentiality. Cellular Measurements Coulter counter. This method is the best known application of impedance methods in the cellular field. It is used to count the amount of cells in asuspension. The measuring principle is quite simple: cells are forced, or enabled, to pass trough a capillary (~100 µm) that changes its electrical impedance at each cell passage. Then, the concentration of cells is estimated from the rate of impedance fluctuations and, in some cases, it is even possible to extract information about the cell sizes from the impedance peak values at each cellpassage. Measurement of the hematocrit. The concentration of dielectric particles in a conductive solution can be estimated if the shape and size of the particles is known. This fact has been used in commercial blood analyzers to determine the hematocrit.
The passive electrical properties are determined by the observation of the tissue electrical response to the injection of external electricalenergy. That is, the tissue is characterized as it was an electrical circuit composed by resistors, capacitors, inductors... Some biological tissues also show active electrical properties since they are capable of generating currents and voltages (e.g. the nerves).
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Bioimpedance Monitoring for physicians: an overview

Monitoring of cell cultures. BI can be used to quantify the biomass in...