Method and apparatus for digital demodulation and further processing of signals obtained in the measurement of electrical bioimpedance or bioadmittance in an object

Abstract

Methods and apparatus for digital demodulation of signals obtained in the measurement of electrical bioimpedance or bioadmittance of an object. One example comprises: generating an excitation signal of known frequency content; applying the excitation signal to the object; sensing a response signal of the object; sampling and digitizing the response signal to acquire a digitized response signal representing the response signal with respect to frequency content, amplitude and phase; correlating, for each frequency fAC of the excitation signal applied, digitized samples of the response signal, with discrete values representing the excitation signal; calculating, using the correlated signals for each frequency fAC of the excitation signal applied, complex values for the bioimpedance Z(fAC); providing, over time, a set of digital bioimpedance waveforms Z(fAC,t)); separating the base bioimpedance Z0(fAC), from the waveforms; and separating the changes of bioimpedance ΔZ(fAC,t), from the waveforms.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] Priority for this patent application is being claimed to European Patent Application No. EP05017871, filed Aug. 17, 2005, titled “Method and apparatus for digital demodulation and further processing of signals obtained in the measurement of electrical bioimpedance or bioadmittance in an object”, which is incorporated herein by this reference. BACKGROUND [0002] 1. Technical Field [0003] This invention is related to the field of digital demodulation and further processing of signals obtained from the measurement of electrical bioimpedance or bioadmittance in a biological object, for instance an animal or a human due to cardiac and / or respiratory activity, for instance in cardiometry, in particular to the monitoring through measurement of the change in thoracic electrical bioimpedance (TEB) or bioadmittance, and pertains to the processing of the excitation, response and / or reference signals obtained through sensing and measuring excitation...

AI Technical Summary

Benefits of technology

[0025] With the sampling rate being significantly higher than the highest frequency component of the excitation signal, particularly an alternating current (AC), applied, the method and apparatus according to the invention provides measurement results at not only a sufficient resolution but a very high amplitude resolution. Unlike common approaches proposed for the display of multi-frequency bioimpedance (Withers P.O.: U.S. Pat. No. 5,280,429) and in a real-time electrical impedance tomography system (Brown B. H. and Barber D. C.: U.S. Pat. No. 5,311,878), bioimpedance or bioadmittance cardiometry requires a high resolution and accur

Problems solved by technology

However, conventional demodulation by diodes and subsequent low-pass filtering exhibit certain drawbacks.

Moreover, even a full-wave rectified sinusoidal signal is difficult to smooth; the time constant of the smoothing low-pass filter cannot be chosen appropriately high because the bandwidth of the desired demodulated signal will be limited, and critical waveform detail can be lost.

Digitization of a signal with ripple can produce unstable data, due to the fact that the values of the samples of the demodulated signal depend on the position of the sampling within the period of the carrier signal.

For the purpose of determining the change in thoracic electrical bioimpedance, the theoretical sound approach has significant practical limitations.

First, circuit design will compromise on the theoretically available resolution of 20 bits.

Images