A FPGA based Measurement System for Estimation of the Stroke Volume of the Heart by measuring Bioimpedance Changes - First Results Steffen Kaufmann, Ankit Malhotra and Martin Ryschka Centre of Excellence for Technology and Engineering in Medicine (TANDEM), Lübeck University of Applied Sciences, Lübeck, Germany
Introduction
Results
Bioimpedance measurements have proven to be a very vital and noninvasive tool for real-time analysis of various physiological parameters. For the measurement of the bioimpedance, small well-known alternating currents (AC) are injected via body surface electrodes into the object under test. The resulting voltage drop is measured and together with the known current used to calculate the complex impedance over time [1]. In impedance plethysmography (IPG) bioimpedance measurements are used to determine changes in tissue volumes [1]. These measured volume changes can be used to subsequently estimate the stroke volume of the human heart [3, 4]. This work presents a FPGA based multi-frequency bioimpedance measurement system for real-time estimation of the stroke volume.
Figure 2 show the populated PCB of the developed measurement system.
Methods Figure 1 shows the system architecture of the developed multi-frequency FPGA based bioimpedance measurement system. The object under test is connected via skin-surface electrodes to the measurement system. The FPGA controls signal excitation and data acquisition, as well as preprocessing of the measurement data. The embedded system is connected via a galvanic isolated USB link to a host PC, which is responsible for further data processing and analysis.
PGA
VSSC
DAC
Figure 2. Populated PCB of the developed measurement system
Figure 3 shows the measured impedance variation, as well as its first derivative measured across the thorax on a healthy subject. The stroke volume and cardiac output where estimated using the first derivative curves [3, 4]. This leads to reasonable cardiac output and stroke volume calculations of about 5.5 l / min and 70 ml, respectively.
Signal Generation (DDS) FFT & Averaging
Down sampling
Electrodes PGA
USB
ADC
PC Control logic
PGA
Electrodes
Subject
ADC FPGA
Embedded System
Figure 1. Principle system architecture of the developed multi-frequency bioimpedance measurement system (PPG and ECG not shown)
The data acquisition is accomplished via a 14 bit, 25 MSPS Analog to Digital Converter (ADC, LTC2296 - Linear Technology). In order to maintain the ADC full-scale usage for varying input voltages, Programmable Gain Amplifiers (PGA, AD8250 - Analog Devices) are employed. The required excitation current is generated via Direct Digital Synthesis (DDS) techniques in combination with a 14 bit, 50 MSPS Digital to Analog Converter (DAC, LTC1668 - Linear Technology) and an AD8130 (Analog Devices) based voltage controlled current source (VCCS) [2]. In order to allow fast measurements, in addition to conventional sinusoidal excitation signals also linear chirps or any other signal-overlay can be used, whereby the frequency is adjustable in a range of 10 kHz to 300 kHz with excitation currents from 60 µA to 5 mA. Furthermore the system is in compliance with the IEC60601-1 safety requirements. The employed impedance demodulation is FFT based and has a temporal resolution of about 1 ms. In the impedances range between 20 Ω and 1 kΩ the overall uncertainty depending on load and frequency is about 0.05 % to 1 % for magnitudes and 0.01 ° to 1° for phases [2]. The data transmission between the PC and the FPGA (LPX2-17E-QN208C Lattice Semiconductor) is realized with a high speed USB connection. To allow further comparisons the developed measurement system also contain possibilities to simultaneously capture an ECG and four different PPG signals. Corresponding Author Steffen Kaufmann Lübeck University of Applied Sciences, TANDEM Mönkhofer Weg 239, 23562 Lübeck, Germany
[email protected];
[email protected]
Figure 3. Impedance variation (-ΔZ) and its first derivative (-dZ) measured across the thorax on a healthy subject with sinusoidal excitation at f ≈ 48 kHz
Summary and Outlook A multi-frequency bioimpedance measurement system for the estimation of stroke volume and cardiac output has been developed, manufactured and tested. In future different frequencies, as well as different electrode positions will be investigated and correlated with PPG and ECG measurements. Moreover the device will be developed further to make it portable and wireless.
References [1] Malmivuo, J.; Plonsey, R., Bioelectromagnetism: Principles and applications of bioelectric and biomagnetic fields. Oxford University Press, USA, first edition, 1995. [2] Kaufmann, S.; Ardelt, G.; Ryschka, M., A high accuracy Bioimpedance Measurement System - System Design and first Measurements, Proceedings of the 5th International Workshop on Impedance Spectroscopy, 2012 [3] Suttner, S.; Schöllhorn, T.; Boldt, J.; Mayer, J.; Röhm, K. D.; Lang, K.; Piper, S. N., Noninvasive assessment of cardiac output using thoracic electrical bioimpedance in hemodynamically stable and unstable patients after cardiac surgery: a comparison with pulmonary artery thermodilution, Intensive Care Medicine 32, 2053–2058, 2006 [4] Zoremba, N. et al., Comparison of electrical velocimetry and thermodilution techniques for the measurement of cardiac output, ACTA Anesthesiologica Scandinavica 51, 1314–1319, 2007
Acknowledgment The authors would like to thank Analog Devices, Lattice Semiconductor and Linear Technology for their support in terms of free samples during the development process.
