Two portable long-term measurement devices for ECG and bioimpedance Timo Vuorela, Jukka Vanhala Institute of Electronics Tampere University of Technology Tampere, Finland timo.vuorela@tut.fi Ville-Pekka Seppä, Jari Hyttinen Ragnar Granit Institute Tampere University of Technology Tampere, Finland ville-pekka.seppa@tut.fi Abstract—This paper presents an implementation of two portable measurement device prototypes. Both prototypes are targeted for measuring ECG and bioimpedance signals. The first prototype is a wireless type, which sends data in real-time to a PC. The second prototype is a logging device, which stores data to a memory card. After a measurement session data from memory card is transferred to a PC through USB cable. One drawback in the first prototype is a poor usability caused by the limited range of the wireless data link and another drawback is a crosstalk between measured signals. The drawbacks of the first prototype were improved in the second prototype. Keywords: ECG, bioimpedance, portable measurement device I. INTRODUCTION ECG (Electrocardiogram) –signals are nowadays widely utilized in both clinical and consumer applications. Many wireless prototypes for measuring ECG are developed [1],[2] and e.g. many amateur athletes use commercial heart rate monitors during an exercise. ECG is a very useful physiological signal and it is also quite easy to measure. Even through many physiological parameters, such as energy consumption, can be calculated from ECG, these calculations are always based on mathematical models which are not accurate for every possible user. If more accurate results are needed other measurement techniques must be applied. One possible candidate for a new measurement technique is a bioimpedance measurement. Bioimpedance of a tissue is measured in a same way than the conventional impedance. A small alternating current is supplied to the tissue and the alternating voltage generated by this excitation current is measured. The absolute value and phase angle of the bioimpedance is then calculated from current and voltage signals. Just like from ECG many physiological parameters can be calculated from bioimpedance signal as well. For example different kind of body composition values e.g. amount of body fat, fat free mass in the body or body’s fluid balance can be calculated. Furthermore with different measurement and electrode setup also some cardiovascular and respiration related signals can be measured. Portable devices for measuring bioimpedance have been developed [3, 4]. Many times these devices measure only bioimpedance and are targeted for measuring only one physiological quantity e.g. TBW (Total Body Water). A measurement device capable to measure both the bioimpedance and ECG could produce information about the correlation of these two signals. Furthermore in order to perform measurements during people’s normal daily activities or during sporting the measurement device should be portable and as unnoticeable as possible. Target of this research is to develop a portable measurement device described in previous paragraph. So far two prototypes of the devices have been implemented and are described in this paper. Rest of this paper is organized as follows; section two presents some basics of the bioimpedance measurement. Sections three and four present the implemented prototypes. In section five measurement results achieved from implemented prototypes are presented. Finally sections six and seven draw some conclusion and present future work. II. BIOIMPEDANCE BASICS Bioimpedance can be measured with two, three or four electrodes. In two and three electrode setups the impedance of the electrodes can affect to the accuracy of the measurement. Therefore, in the implemented measurement device a tetrapolar, four electrodes, measurement method is utilized. [5] In four electrode measurement two electrodes are supplying the excitation current to the tissue and other two electrodes are measuring the generated voltage. If the input impedance of the measurement amplifier is high enough the impedance of electrodes shouldn’t affect the measurement. Furthermore with four electrode setup it is easier to control the actual measurement zone. Measurement principle is presented in the figure one. In order to simplify the electronic design, the measurement devices measure only the dynamic changes in the absolute value of the bioimpedance. Measuring of both the absolute value and the phase angle accurately, would make electronics more complicated. This is due to the additional blocks needed for detecting the difference in phase between current and voltage signals, and for calibrating the devices e.g. for different temperatures.