Volumen 5 (2014): Heft 1 (January 2014)

The non-invasively measured initial systolic time interval (ISTI) reflects a time difference between the electrical and pumping activity of the heart and depends on cardiac preload, afterload, autonomic nervous control and training level. However, the duration of the ISTI has not yet been compared to other time markers of the heart cycle. The present study gauges the duration of the ISTI by comparing the end point of this interval, the C-point, with heart cycle markers obtained by echocardiography. The heart rate of 16 healthy subjects was varied by means of an exercise stimulus. It was found that the C-point, and therefore the end point of ISTI, occurred around the moment of the maximum diameter of the aortic arch in all subjects and at all heart rates. However, while the time difference between the opening of the aortic valves and the maximum diameter of the aortic arch decreased significantly with decreasing RR-interval, the time difference with respect to the moment of the C-point remained constant within the subjects. This means that the shortening of the ISTI with increasing heart rate in response to an exercise stimulus was caused by a shortening of the pre-ejection period (PEP). It is concluded that the ISTI can be used as a non-invasive parameter indicating the time difference between the electrical and mechanical pumping activity of the heart, both inside and outside the clinic.

In this paper we show why the poorly conducting cytoplasmic membranes have little effect on the overall impedance of the tissue above a certain frequency, and derive an estimate of this upper frequency. It is further shown that the induced transmembrane potentials at different sites over the membrane can be found through a simple formula for frequencies above the threshold, without the need to analytically or theoretically model the membranes directly. The findings are validated for an irregular cell shape through rigorous numerical modeling.

During the last decades, Electrical Bioimpedance Spectroscopy (EBIS) has been applied mainly by using the frequency-sweep technique, across a range of many different applications. Traditionally, the tissue under study is considered to be time-invariant and dynamic changes of tissue activity are ignored by treating the changes as a noise source. A new trend in EBIS is simultaneous electrical stimulation with several frequencies, through the application of a multi-sine, rectangular or other waveform. This method can provide measurements fast enough to sample dynamic changes of different tissues, such as myocard. This high sampling rate comes at a price of reduction in SNR and the increase in complexity of devices. Although the frequency-sweep technique is often inadequate for monitoring the dynamic changes in a variant system, it can be used successfully in applications focused on the time-invariant or slowly-variant part of a system. However, in order to successfully use frequency-sweep EBIS for monitoring time-variant systems, it is paramount to consider the effects of aliasing and especially the folding of higher frequencies, on the desired frequency e.g. DC level. This paper discusses sub-Nyquist sampling of thoracic EBIS measurements and its application in the case of monitoring pulmonary oedema. It is concluded that by considering aliasing, and with proper implementation of smoothing filters, as well as by using random sampling, frequency-sweep EBIS can be used for assessing time-invariant or slowly-variant properties of time-variant biological systems, even in the presence of aliasing. In general, undersampling is not always a problem, but does always require proper consideration.

Phantoms are widely used in medical imaging to predict image quality prior to clinical imaging. This paper discusses the possible use of bolus material, as a conductivity phantom, for validation and interpretation of electrical impedance tomography (EIT) images. Bolus is commonly used in radiation therapy to mimic tissue. When irradiated, it has radiological characteristics similar to tissue. With increased research interest in CT/EIT fusion imaging there is a need to find a material which has both the absorption coefficient and electrical conductivity similar to biological tissues. In the present study the electrical properties, specifically resistivity, of various commercially available bolus materials were characterized by comparing their frequency response with that of in-vivo connective adipose tissue. It was determined that the resistivity of Gelatin Bolus is similar to in-vivo tissue in the frequency range 10 kHz to 1MHz and therefore has potential to be used in EIT/CT fusion imaging studies.

In this paper, we used impedance spectroscopy and gold electrodes to detect the presence of yeast cells and monitor the attachment of these cells to the electrodes. We analyzed the effect of conductivity changes of the medium and the attachment on the electrode-electrolyte interface impedance. A three-electrode cell was designed to produce a uniform electric field distribution on the working electrode and to minimize the counter electrode impedance. Moreover, we used a small AC overpotential (10 mV) to keep the system within the linear impedance limits of the electrode-electrolyte interface. This study proposes a new method to differentiate the impedance changes due to the attachment of yeast cells from those due to conductivity changes of the medium. The experiments showed that when the difference between the cell suspension and base solution conductivities is within the experimental error, the impedance changes are only due to the attachment of yeast cells to the electrodes. The experiments also showed a strong dependence (decrease) of the parallel capacity of the electrode electrolyte interface with the yeast cell concentration of suspension. We suggest that this decrease is due to an asymmetrical redistribution of surface charges on both sides of cell, which can be modeled as a biologic capacity connected in series with the double layer capacity of the interface. Our results could help to explain the rate of biofilm formation through the determination of the rate of cell adhesion.

Non-invasive estimation of bladder volume could help patients with impaired bladder volume sensation to determine the right moment for catheterisation. Continuous, non-invasive impedance measurement is a promising technology in this scenario, although influences of body posture and unknown urine conductivity limit wide clinical use today. We studied impedance changes related to bladder volume by simulation, in-vitro and in-vivo measurements with pigs. In this work, we present a method to reduce the influence of urine conductivity to cystovolumetry and bring bioimpedance cystovolumetry closer to a clinical application.

Mechanotransduction is of fundamental importance in cell physiology, facilitating sensing in touch and hearing as well as tissue development and wound healing. This study used an impedance sensor to monitor the effective resistance and permittivity of artificial tissues, alginate hydrogel with encapsulated fibroblasts, which were kept viable through the use of a bespoke microfluidic system. The observed transient impedance responses upon the application of identical compressive normal loads differed between acellular hydrogels and hydrogels in which fibroblasts were encapsulated. These differences resulted from changes in the conductivity and permeability of the hydrogel due to the presence of the encapsulated fibroblasts, and transient changes in ion concentrations due to mechanotransduction effects.

Impedance spectroscopy is a useful tool for non-invasive and real time measurements of cell suspensions and a variety of biological tissues. The objective of this study was the investigation of the dielectric properties of living aquatic worms (Lumbriculus variegatus) using impedance spectroscopy in a frequency range between 100 Hz and 10 MHz. We demonstrate a linear relation between the worm biomass and the phase response of the signal thereby providing a quick and precise method to determine the biomass of aquatic worms in situ. Possible applications for non-destructive online biomass monitoring of aquatic worms and other aqueous organisms are discussed. Furthermore, we show that groups of worms fed different diets can be distinguished by the method presented. These results reveal a close relationship between the nutritional composition of the worms and the measured phase response. We also demonstrate that the phase response at 90 kHz does not depend on the worm size. In contrast, the response function for the signal at 440 Hz reveals a linear correlation of average individual worm size and phase. Therefore, we conclude that the measured phase response at 90 kHz qualifies as a measure of the total amount of worm biomass present in the measuring cell, whereas the phase measurement at 440 Hz can be used to estimate the average individual worm size.

The accurate assessment of body fluid volume is important in many clinical situations, especially in the determination of “dry weight” in a dialysis setting. Currently, no clinically applicable diagnostic system exists to determine the mechanical properties that accurately characterize peripheral edema in an objective and quantitative manner. We have developed a method for quantifying the impact of compression on the electrical properties of tissue by measuring stress-induced changes in bioimpedance (BIS). Using this method, we simultaneously measured the impedance and mechanical response of a tissue mimicking material (tofu) under both quasi-static and dynamic loading conditions. Our results demonstrate a temporal quantification of viscoelastic properties using a viscoelastic phantom tissue model.

Despite the long history of rheoencephalography (REG), some important aspects of the method are still debatable. Bioimpedance measurements offer great potential benefit for study of the human brain, but the traditional four or six electrode method suffers from potential misinterpretations and lack of accuracy. The objective of this paper is to study the possible mechanism of REG formation by means of numerical modelling using a realistic finite element model of the human head. It is shown that the cardiac related variations in electrical resistivity of the scalp contributes more than 60% to the REG amplitude, whereas the brain and cerebrospinal fluid are mutually compensated by each over.

Lab-on-chip systems (LOCs) can be used as in vitro systems for cell culture or manipulation in order to analyze or monitor physiological cell parameters. LOCs may combine microfluidic structures with integrated elements such as piezo-transducers, optical tweezers or electrodes for AC-electrokinetic cell and media manipulations. The wide frequency band (<1 kHz to >1 GHz) usable for AC-electrokinetic manipulation and characterization permits avoiding electrochemical electrode processes, undesired cell damage, and provides a choice between different polarization effects that permit a high electric contrast between the cells and the external medium as well as the differentiation between cellular subpopulations according to a variety of parameters. It has been shown that structural polarization effects do not only determine the impedance of cell suspensions and the force effects in AC-electrokinetics but can also be used for the manipulation of media with inhomogeneous temperature distributions. This manuscript considers the interrelations of the impedance of suspensions of cells and AC-electrokinetic single cell effects, such as electroorientation, electrodeformation, dielectrophoresis, electrorotation, and travelling wave (TW) dielectrophoresis. Unified models have allowed us to derive new characteristic equations for the impedance of a suspension of spherical cells, TW dielectrophoresis, and TW pumping. A critical review of the working principles of electro-osmotic, TW and electrothermal micropumps shows the superiority of the electrothermal pumps. Finally, examples are shown for LOC elements that can be produced as metallic structures on glass chips, which may form the bottom plate for self-sealing microfluidic systems. The structures can be used for cell characterization and manipulation but also to realize micropumps or sensors for pH, metabolites, cell-adhesion, etc.

This paper gives a basic overview of relevant statistical methods for the analysis of bioimpedance measurements, with an aim to answer questions such as: How do I begin with planning an experiment? How many measurements do I need to take? How do I deal with large amounts of frequency sweep data? Which statistical test should I use, and how do I validate my results? Beginning with the hypothesis and the research design, the methodological framework for making inferences based on measurements and statistical analysis is explained. This is followed by a brief discussion on correlated measurements and data reduction before an overview is given of statistical methods for comparison of groups, factor analysis, association, regression and prediction, explained in the context of bioimpedance research. The last chapter is dedicated to the validation of a new method by different measures of performance. A flowchart is presented for selection of statistical method, and a table is given for an overview of the most important terms of performance when evaluating new measurement technology.