New Synchronous Measurement Technique for
Intracardiac Impedance Analysis

A Kuusik, R. Land, M. Min, and T. Parv

Institute of Electronics, Tallinn Technical University, Tallinn, Estonia

Correspondence: M Min, Institute of Electronics, Tallinn Technical University, 19086 Tallinn, Estonia.
E-mail: min@edu.ttu.ee, phone +372 620 2156, fax +372 620 2151

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1.    Introduction

Electrical bioimpedance gives information about physiological processes in living organisms. The information carrier is not only the base value Ż₀ of bioimpedance Ż(t), but namely its variations or biomodulation ΔŻ(t) is more informative in cardiac applications.

The bioimpedance Ż (Fig.1a) has an equivalent circuit consisting of resistors and capacitors, the parameters of which are time-variant due to physiological processes (heart beating, breething). It is assumed that in the 3-element equivalent circuit (Fig.1b) the active resistance r_ext represents the extracellular liquid between the cells, and the parallel branch characterizes the cells [Grimnes and Martinsen, 2000], wheras the capacitance C presents cell membranes, and the serially connected r_int describes intracellular fluid. The serial equivalent circuit (Fig.1c) and the corresponding vector presentation can be obtained as a result of the complex impedance Ż = R + jX_C measurement (Fig.2).

Figure 1. Bioimpedance and its electrical equivalents.	Figure 2. Vector diagram of bioimpedance.

2.    Lock-In Measurement of Cardiac Bioimpedance

Electronic instruments for the measurement of bioimpedance Ż(t), and especially for demodulation of its variations ΔR(t) and ΔX_C(t) must be designed with special care In principle, measurement of the complex impedance must be principally carried out using pure sine waves [Meade, 1989]. In practice, however, the lock-in demodulation technique is used (Fig.3), where the synchronous detector SD demodulates the voltage response by multiplying the response with rectangular pulses instead of sine (0°) and/or cosine (90°) wave reference, and extracts the inphase (I) and quadrature (Q) components using low pass filtering.

Switching circuits are commonly used in implantable devices instead of continuous mode ones [Webster, 1995]. Accordingly, the stepwise approximations replace the sine wave signals [Min et al., 2000]. When the simplest two-level (+A/–A) rectangular pulses are used, the odd higher harmonics cause errors. The relative error of magnitude |Ż| estimation lies between +23% and –13%, and it depends on the actual phase angle Φ of the impedance vector [Min et al., 1996]. The phase error ΔΦ can reach ±4.1°. Multilevel approximations of sine waves are used for reducing errors [Min et al., 2000]. However, this approach requires setting of several precise signal levels and gain values, which complicates circuits excessively. But there still exists a simpler way for handling the pulse waveforms. Introducing the third, zero- level value during certain time interval (Fig.3) enables to suppress several higher odd harmonics and to achieve the accuracy, which is closer to that for true sine/cosine wave signals [Min et al., 2002b].

3.    Results Figure 3. Lock-in measurement of bioimpedance using synchronous detection. Using of different zero-level time intervals for the excitation and reference signals (Fig.3) one can achieve that only some of coinciding, and therefore error causing higher harmonics excist in these signals. The most favorite approximation is found to be in case, where the zero-level values exist during the phase intervals φ1=18°, and φ2=30° [Min et al., 2002a]. The lowest coinciding higher harmonic is the 7th, which is followed by the 11th and 13th, 17th and 19th, 23rd and 25th, etc. Impact of these coinciding spectral lines is now so low that they can cause only 2.4% magnitude error, and the phase error remains within ±1° [Min, 2002]. Such results are fully acceptable for medical applications. 4.    Discussion and Conclusions Evidently, the lock-in demodulation method based on differently modified pulse signals, opens a possibility to design simple but precise enough low voltage/power circuits for intracardiac impedance measurement. The devices can be designed as mixed-signal application specific integrated circuits (ASICs), which operate using low voltage (3V and lower) power supply and consume only microamperes of current operating at the frequencies lower than 100 kHz. Acknowledgements This work was supported by Estonian Science Foundation, grant no. 4859, and St. Jude Medical AB, USA-Sweden.

References

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Meade ML. Lock-in Amplifiers: Principles & Applications. Peregrinus, London, 1989.

Min M, Parve T. Improvement of the vector analyser based on two-phase switching mode synchronous detection. Measurement, 19(2): 103-111, 1996.

Min M, Märtens O, Parve T. Lock-in measurement of bioimpedance variations. Measurement, 27(1): 21-28, 2000.

Min M. Electrical bioimpedance measurement: Means and methods for applying in cardiac monitoring and pacing. In proc. of the 1^st World Congress on Biomimetics and Artificial Muscles, 2002 (6 p., in press).

Min M, Kink A, Land R, Parve T. A method of electrical bio-impedance analysis and a device corresponding to the method. Patent application No. 0677/02 (Estonia), Dec 6, 2002a.

Min M, Parve T, Kukk V, Kuhlberg A. An Implantable Analyzer of Bio-Impedance Dynamics: Mixed Signal Approach. IEEE Trans. Instrum. & Meas., 51(4): 674-678, 2002b.