Instrumentation for 12-Lead ECG / ICG

Antti Haapalainen, Pasi K. Kauppinen, Jari Hyttinen, and Jaakko Malmivuo

Ragnar Granit Institute, Tampere University of Technology, Tampere, Finland

Correspondence: A Haapalainen, Ragnar Granit Institute, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland.
E-mail: antti.haapalainen@tut.fi, phone +358 3 3115 2113, fax +358 3 3115 2162

1.  Introduction

Electrical impedance measurement of the human body offers a noninvasive method for cardiac output (CO) measurement. Until now, impedance measurements have been done using whole thorax or whole body as an impedance signal source. New 12-lead impedance measurement concept promises more accuracy and flexibility since the measurement can be localized more precisely to the area of interest [Kauppinen et al, 1999a; Kauppinen et al, 1999b]. This is achieved by measuring all independent impedance information from the 12-lead system and mathematically combining the results.

However, multiplexed body impedance measurement involves problems with measurement accuracy. It has been determined the impedance change of the thorax due to the heart beat is approximately only 1000 ppm of the total body impedance [Kauppinen et al, 2000]. Our initial tests with earlier prototype gave promising results but had problems with the long settling time, which resulted in low sample rate and narrow frequency content of the signal [Haapalainen, 2002]. To overcome these problems, new measurement procedure was developed, which also enables the measurement of ECG data with the same hardware.

2.  Material and Methods

In 12-lead ICG configuration, one current injecting / voltage sensing electrode pair is referred as a channel. A reference measurement value for each channel is needed for removal of the base impedance value, since only the changing part of the signal should be amplified. Two measurement devices have been constructed, with the first being tested in 12-lead configuration. The second one is a prototype of the differential-based measurement device that has preliminarily shown superior performance.

Both instrumentations include sine signal generator, voltage to current converter and multiplexers for channel selection. Voltage to current converter is implemented in order to overcome the effect of the skin-electrode impedance and its changes. Standard PC with a data acquisition card is used to record the signal. In the first instrumentation, a peak detector was used to detect the amplitude of the measured carrier signal. The reference values for each channel were adjusted automatically in the beginning of the measurement so that the total mean output of the device was as close to zero as possible. The second instrumentation is a 2-channel prototype of the differential ICG device. It is designed to test the ideas how to evade the limitations of the first instrumentation, namely the long settling times and the overall accuracy. The instrumentation directly samples the carrier signal changes without any peak detection. Reference values for channels are formed by amplitude and phase modulation of the original carrier signal. Measurement leads are well buffered to minimize the electrode loading effect of the circuit.

3.  Results

Figure 1. Simultaneous ECG and ICG signals measured with two channel prototype instrumentation.

We obtained good results in channel settling times with the differential instrumentation. Digital signal analysis performed off-line was found to be time consuming, since the high samplerate of the system produced very large files. However, ECG and two ICG signals were extracted from the recorded signal showing a good performance of the prototype. The resulting curves can be seen in Fig 1. Both ECG and ICG signals were recorded simultaneously from the same electrodes. In this test measurement, two hand electrodes and one chest electrode were used.

The localization properties of the 12-lead ICG were studied with 12-electrode setup. Results showed that the measurement sensitivity could be mathematically focused to the specific area of the thorax [Haapalainen, 2002].

4.  Discussion

First tests with the prototypes have shown good performance. Body impedance and heart 's electrical activity can be recorded simultaneously from the same electrode setup. Spatial sensitivity of the impedance measurement can be mathematically enhanced to better reflect the impedance changes in tissues, which are considered to be important for CO estimation. Measurement accuracy is important in this process, and needs to be improved further.

The signal processing task is a filtering procedure that takes quite much processing power and therefore has to be performed off-line. For real time impedance recording and displaying, faster algorithms for signal analysis have to be implemented. However, results establish a good initial stage for complete, combined 12-lead impedance- and electrocardiography instrumentation.

References

Haapalainen A. Instrumentation for 12-lead ICG, MSc thesis, Tampere University of Technology, 2002

Kauppinen PK, Hyttinen JA, Kööbi T, Malmivuo J. Lead field theoretical approach in bioimpedance measurements: towards more controlled measurement sensitivity. Annals of the New York Academy of Sciences, 873: 135-142, 1999

Kauppinen P, Kööbi T, Kaukinen S, Hyttinen J, Malmivuo J. Application of computer modelling and lead field theory in developing multiple aimed impedance cardiography measurements. Journal of Medical Engineering & Technology, 23:169-177, 1999

Kauppinen P, Kööbi T, Hyttinen J, Malmivuo J. Segmental composition of whole-body impedance cardiogram estimated by computer simulations and clinical experiments. Clinical Physiology, 20:101-105,2000