Impedance spectroscopy on living tissue for determination of the state of organs

Automatic error detection in the clinical measurement of gastric impedance spectra

Gastric impedance spectroscopy has been proposed as a method of monitoring mucosal injury due to hypoperfusion and ischemia in the critically ill. During validation tests for this procedure, it was found that 60% of the measurements had errors by factors inherent to the clinical setting, indicating that some kind of automatic error detection should be incorporated to potentially avoid the loss of measurements. This paper presents an algorithm developed to detect errors due to bad connection, bad location or bad contact of the electrode probe. A labeled database with 20,908 sets of 92 spectral measurements each, obtained from critically ill patients was used as training/testing data. To reduce the dimensionality, the database was resized by dividing the spectral range into four bands, and then by computing mean and standard deviation in magnitude, phase, resistance and reactance for each band and measurement. Initial exploration into the data space was performed by k-means clustering, establishing the number of classes. Sequential Forward Selection was performed to determine best features from the reduced data set. Finally, Support Vector Machine classifiers were designed in a one-vs-rest hierarchical scheme to classify the quality of the spectra. Each classifier gave a hit rate greater than 95% and an area under the relative operating characteristic curve of 0.99. In a validation run with cardiac surgery and intensive care unit patient spectra, the error rates were 2.3% and 8.4% respectively.

The predictive value of gastric reactance for postoperative morbidity and mortality in cardiac surgery patients

No useful method to directly monitor the level of end organ tissue injury is currently available clinically. Gastric reactance has been proposed to measure changes in a tissue structure caused by ischemia. The purpose of this study was to assess whether gastric reactance is a reliable, clinically relevant predictor of complications and a potentially useful tool to assess hypoperfusion in cardiovascular surgery patients. The value of gastric reactance measurements, standard hemodynamic and regional perfusion variables, and scores to predict postoperative complications were compared in 55 higher risk cardiovascular surgery patients with cardiopulmonary bypass. Low frequency gastric reactance, X(L), had a significant predictive value of postoperative persistent shock requiring more than 48 h of vasopressors and associated complications, before, during and after surgery (p < 0.05). Results suggest that reactance is an earlier predictor of patients at risk than all other variables tested. Patients with a high reactance (X(L) > 26) before surgery had a significantly higher incidence of complications, higher mortality and more days in the ICU than patients with a low reactance (X(L) < 13). X(L) was found to be a reliable and clinically relevant measurement. These results justify further clinical research to explore how this information may be used to improve patient management.

The detection of brain ischaemia in rats by inductive phase shift spectroscopy

Ischaemia in the brain is an important clinical problem that is often monitored and studied with expensive devices such as MRI and PET, which are not readily available in low economical resource parts of the world. We have developed a new less expensive tool for non-invasive monitoring of ischaemia in the brain. This is a first feasibility study describing the concept. The system is based on the hypothesis that electromagnetic properties of the tissue change during ischaemia and that measuring the electromagnetic properties of the bulk of the brain with non-contact means can detect these changes. The apparatus we have built and whose design we describe here consists of two electromagnetic coils placed around the head. The system measures the bulk change in time of the phase difference between the electromagnetic signal on the two coils in a range of frequencies. A mathematical model simulating the device and the measurement is also introduced. Ischaemia was induced in the brain of rats by occlusion of the right cerebral and carotid arteries. Experimental subjects were monitored for 24 h. Inductive phase shift measurements were made at five frequencies in the range of 0.1-50 MHz eight times during the observation period. An ex vivo estimation of the percentage of necrosis in the ischemic subjects at t = 24 h was done. The mathematical model was also applied to the experimental tested situation. The results of both experiments and theory show significant phase shifts increase as a function of frequency and ischaemia time. The theoretical and experimental results suggest that the tested technique has the potential to detect the processes and level of ischaemia in the brain by non-invasive, continuous, bulk volumetric monitoring with a simple and inexpensive apparatus.

Implantable bioimpedance monitor using ZigBee

In this paper, a novel implantable bioimpedance monitor using a free ZigBee protocol for the transmission of the measured data is described. The application field is the tissue and organ monitoring through electrical impedance spectroscopy in the 100 Hz - 200 kHz range. The specific application is the study of the viability and evolution of engineered tissue in cardiac regeneration. Additionally to the telemetric feature, the measured data are stored in a memory for backup purposes and can be downloaded at any time after an RF link break. In the debugging prototype, the system autonomy exceeds 1 month when a 14 frequencies impedance spectrum is acquired every 5 minutes. In the current implementation, the effective range of the RF link is reduced and needs for a range extender placed near the animal. Current work deals with improving this range.

Imaging cryosurgery with EIT: tracking the ice front and post-thaw tissue viability

Cryosurgery employs freezing for targeted destruction of undesirable tissues such as cancer. Ice front imaging has made controlled treatment of deep body tumors possible. One promising method, recently explored for this task, is EIT, which recovers images of electrical impedance from measurements made at boundary electrodes. However, since frozen tissue near the ice front survives, ice front imaging is insufficient. Monitoring treatment effect would enable iterative cryosurgery, where extents of ablation and need for further treatment are assessed upon thawing. Since lipid bilayers are strong barriers to low frequency electrical current and cell destruction implies impaired membranes, EIT should be able to detect the desired effect of cryosurgery: cell death. Previous work has tested EIT for ice front imaging with tank studies while others have simulated EIT in detecting cryoablation, but in vivo tests have not been reported in either case. To address this, we report 3D images of differential conductivity throughout the freeze-thaw cycle in a rat liver model in vivo with histological validation, first testing our system for ice front imaging in a gel and for viability imaging post-thaw in a raw potato slice.

Algorithm for characteristic parameter estimation of gastric impedance spectra in humans

Impedance spectroscopy has been proposed as a method of monitoring mucosal injury due to hypoperfusion and ischemia in the critically ill. The present paper presents an algorithm developed to calculate the characteristic electrical values that best describe human gastric impedance measurements and simplify the information obtained with this method. An impedance spectroscopy probe and nasogastric tube (ISP/NGT) was placed into the stomach of healthy volunteers, cardiovascular surgery and critically ill patients, and a database with 16199 spectra was obtained. The gastric spectrum forms two semi circles in the complex domain, divided into low frequency (F < 10 kHz) and high frequency (F > 10 kHz). A fitting algorithm was developed based on the Cole model, and central characteristic parameters were calculated. The parameters were validated using the normalized mean squared error and 0.66% of the spectra were discarded. From the experimental data obtained in humans, the greatest changes observed as the gastric mucosa becomes ischemic occur at low frequencies, which are specific and sensitive to tissue damage, and vary with the degree of hypoperfusion.

Electrical Impedance Spectroscopy Study of Biological Tissues

The objective of this study was to investigate the electrical impedance properties of rat lung and other tissues ex vivo using Electrical Impedance Spectroscopy. Rat lungs (both electroporated and naïve (untreated)), and mesenteric vessels (naïve) were harvested from male Sprague-Dawley rats; their electrical impedance were measured using a Solartron 1290 impedance analyzer. Mouse lung and heart samples (naïve) were also studied. The resistance (Real Z, ohm) and the reactance (Im Z, negative ohm)) magnitudes and hence the Cole-Cole (Real Z versus Im Z) plots are different for the electroporated lung and the naive lung. The results confirm the close relationship between the structure and the functional characteristic. These also vary for the different biological tissues studied. The impedance values were higher at low frequencies compared to those at high frequencies. This study is of practical interest for biological applications of electrical pulses, such as electroporation, whose efficacy depends on cell type and its electrical impedance characteristics.

Video rate electrical impedance tomography of vascular changes: preclinical development

Peripheral vasculature disease is strongly correlated with cardiovascular-associated mortality. Monitoring circulation health, especially in the peripheral limbs, is vital to detecting clinically significant disease at a stage when it can still be addressed through medical intervention. Electrical impedance tomography (EIT) maps the electrical properties of tissues within the body and has been used to image dynamically varying physiology, including blood flow. Here, we suggest that peripheral vasculature health can be monitored with EIT by imaging the hemodynamics of peripheral vessels and the surrounding tissues during reactive hyperemia testing. An analysis based on distinguishability theory is presented that indicates that an EIT system capable of making measurements with a precision of 50 microV may be able to detect small changes in vessel size associated with variations in blood flow. An EIT system with these precision capabilities is presented that is able to collect data at frame rates exceeding 30 fps over a broad frequency range up to 10 MHz. The system's high speed imaging performance is verified through high contrast phantom experiments and through physiological imaging of induced ischemia with a human forearm. Region of interest analysis of the induced ischemia images shows a marked decrease in conductivity over time, changing at a rate of approximately -3 x 10(-7) S m(-1) s(-1), which is the same order of magnitude as reported in the literature. The distinguishability analysis suggests that a system such as the one developed here may provide a means to characterize the hemodynamics associated with blood flow through the peripheral vasculature.

Gastric Impedance Spectroscopy in Cardiovascular Surgery Patients vs. Healthy Volunteers

Impedance spectroscopy has been proposed as a method of monitoring mucosal injury due to hypoperfusion and ischemia in the critically ill. The present study evaluates spectral differences in elective cardiovascular surgery patients compared with the impedance spectra of the gastric mucosa in healthy adults. An impedance spectroscopy probe and nasogastric tube (ISP/NGT) was placed into the stomach of 77 heart surgery and 21 healthy volunteers. The recorded impedance spectra were classified into 3 groups: group 1 for healthy volunteers; group 2 for patients with neither ischemia nor complications; and group 3 for patients with ischemia and complications during the first 3 days in the recovery unit. There were statistical differences in resistance (p<0.001) and reactance (p<0.001) among the 3 groups. The results show that impedance spectra are distinct and significantly different in patients with probable ischemic mucosal injury compared to healthy subjects, and therefore this technology may be a useful prognostic and diagnostic monitoring tool.

Assessment of the Viability of Transplant Organs with 3D Electrical Impedance Tomography

Methods that can determine the extent of tissue damage in transplant organs, before the decision to transplant has been made, have the potential to improve the outcome of the procedure by preventing unfit organs from being transplanted into the patient. The raised confidence in the organ state with such a technique would also increase availability. Now restricted due to the fear of introducing a failed organ resulting from the relative lack of viability data during transport, stringent criteria for donation would relax. Electrical impedance tomography is an imaging modality that recovers the spatial variation of the complex impedivity in the body from electrical measurements made on the periphery. In this study, we apply 3D EIT with the complete electrode model to a sample conductivity distribution that might result from an organ that is losing its viability due to prolonged ischemia. The reconstructed images show that EIT has the potential to become an adjuvant method for the field of organ transplantation.

Modification of pulse wave signals in electrical bioimpedance analyzers for implantable medical devices

The problems of application of pulse wave signals in electrical bioimpedance analyzers foreseen for using in implantable medical devices as diagnostical means are discussed in this paper. The main problem arises at measurement of phasor parameters by the aid of rectangular pulse wave signals. The specific measurement errors appear due to presence of higher harmonics in the spectra of pulse waveforms. These errors are discussed in two cases, in the case of full cycle rectangular waveform, and in the case of using the shortened pulses introduced specially for reduction of errors.

A sampling multichannel bioimpedance analyzer for tissue monitoring

The paper focuses on principles of designing of a multichannel bioimpedance analyzer based on simultaneous multisine measurement. The measurement task arises due to the need to monitor patients during and after heart surgery operation performing MIMO (multiple-input-multiple-output) bioimpedance measurement. Frequencies of the simultaneously applied sinusoidal excitations must be close but simultaneously varied in a larger range (e.g. from 1 kHz up to 10 MHz). The main idea of the proposed approach is that the use of a rather specific signal system (frequencies of sinusoidal excitations are related as integers and sampling frequencies are properly related/adapted to them) makes it possible to separate responses to different excitations from the measured summary signals by means of a quite simple filter and different (under) sampling rates.

Therapy guided by gastric impedance spectroscopy in a septic shock model in pigs

The disruption of the gastric mucosa plays a key role in the evolution of shock and it is the "motor of multiple organ failure"; in this sense, no clinically useful method to directly monitor the level of mucosal ischemic injury is available yet to guide appropriate therapy for the critically ill. An experimental model was developed in order to demonstrate that resuscitation therapy by gastric impedance spectroscopy, using a minimally invasive gastric catheter, is feasible and therefore, to establish its potential as a practical future clinical monitoring tool for the critically ill. A septic shock model in pigs was developed, which reproduces the hemodynamic conditions of the critically ill patient. Intervention therapies were designed in order to promote changes in hemodynamic response as well as in splanchnic perfusion, conventionally guided and by impedance spectroscopy. At the end of experiments, survival in each intervention therapy was compared and the outcome was better in the therapy group guided by gastric electric impedance spectroscopy.

A SiC microdevice for the minimally invasive monitoring of ischemia in living tissues

Monitoring of ischemia in living tissues is a field of increasing interest in many clinical settings. In this work we report for the first time anywhere the development of needle-shaped, minimally-invasive impedance probes based on silicon carbide (SiC) substrates. An in-vitro comparison of these new devices with Si-based impedance probes demonstrates that their effective operation range extends up to the 100 kHz range, thus allowing a wide-spectrum multi-frequency analysis of impedance modulus and phase angle. Furthermore, we show that, when applied to in-vivo settings, this kind of analysis yields to an accurate monitoring of ischemia, while making possible the application of more robust mathematical methods for the study of impedance in living tissues.

Gastric impedance spectroscopy in elective cardiovascular surgery patients

Impedance spectroscopy has been proposed as a method of monitoring mucosal injury due to hypoperfusion and ischemia in critically ill patients. The present study characterizes human gastric impedance spectral changes under gastric hypoperfusion in patients undergoing cardiovascular surgery, and evaluates spectral differences between patients with no evidence of gastric ischemia and complications, and patients who developed ischemia and complications. Cole and Kun parameters were calculated over time to characterize changes as tissue injury progresses. Gastric ischemia was determined by air tonometry. Impedance spectroscopy spectra were obtained from 63 cardiovascular surgery patients. The recorded spectra were classified into three groups: group 1 for patients without ischemia or complications, group 2 for patients with a short period of ischemia (less than 2 h) and group 3 for patients with more than 4 h of gastric ischemia and complications. Two distinct dispersion regions of the spectra centered at about 316 Hz and 215 kHz become clearer as tissue damage develops. The average spectrum in group 3 shows a significant difference in tissue impedance at all frequencies relative to group 1. The parameters obtained for human gastric tissue show significant changes that occur at different times and at different frequencies as ischemia progresses, and could be correlated with patient outcome. This confirms our hypothesis that hypoperfusion and ischemia cause evident changes in the impedance spectra of the gastric wall. Therefore, this technology may be a useful prognostic and diagnostic monitoring tool.

Temperature Dependence of Tissue Impedivity in Electrical Impedance Tomography of Cryosurgery

The temperature-dependent impedivity of rat liver, transverse abdominal muscle and full skin was determined in vitro as a function of frequency across the temperature range 5 degrees C to 37 degrees C and from 100 Hz to 10 kHz. This study was motivated by an increasing interest in using electrical impedance tomography (EIT) for imaging of cryosurgery and a lack of applicable data in the hypothermic range. Using a controlled-temperature impedance analyzer, it was found that as the temperature is reduced the resulting increase in tissue impedivity is more pronounced at low frequencies and that the beta dispersion, resulting from cell membrane polarization, shifts to lower frequencies. With these new data a simple case study of EIT of liver cryosurgery was examined, using a finite-element model incorporating the Pennes bio-heat equation, to determine the impact of this behavior on imaging accuracy. Overestimation of the ice-front position was found to occur if the EIT system ignored the effects of the low-temperature zone surrounding the frozen tissue. This error decreases with increasing blood perfusion and with higher measurement frequencies.

Electrical bioimpedance measurement during hypothermic rat kidney preservation for assessing ischemic injury

Non-heart-beating donors sustain an ischemic insult of unknown severity and duration, which can compromise the viability of the graft. This preliminary study aimed to assess whether electrical bioimpedance monitoring of cold preserved organs could be useful to identify kidneys that have suffered previous warm ischemia (WI). Two rat groups were studied during 24 h of preservation in University of Wisconsin solution (UW): a control cold ischemia group and another group subjected previously to 45 min of WI. Multi-frequency bioimpedance was monitored during preservation by means of a miniaturized silicon probe and the results were modeled according to the Cole equation. Tissular ATP content, lactate dehydrogenase in UW solution and histological injury were assessed. Renal function and cell injury, evaluated during 3 h of ex vivo reperfusion using the isolated perfused rat kidney model, demonstrated differences between groups. Bioimpedance results showed that the time constant and the high frequency resistivity parameters derived from the Cole equation were able to discriminate between groups at the beginning of the preservation (Deltatau approximately 78%, DeltaRinfinity approximately 36%), but these differences tended to converge as preservation time advanced. Nevertheless, another of the Cole parameters, alpha, showed increasing significant differences until 24 h of preservation (Deltaalpha approximately 15%).

Multikanalsystem zur nichtinvasiven Funktions- und Zustandsanalyse von Organen und Gewebe / A Multichannel Device for the Noninvasive Analysis of the State and Function of Organs and Tissues

The subject of this paper is to present a technical description of a high-performance, modular research device, designed and constructed with the aim of enabling the non-invasive investigation of the passive electrical properties of biological tissue in the frequency range 1 kHz-1 MHz. The modular hardware- and software of the device makes the system highly flexible when it comes to applying a wide range of methods for the non-invasive analysis of the state and function of biological tissues and evaluating them in a clinical setting. Furthermore, the concept also makes it possible to combine the modules in new ways for new applications, to accommodate future developments in bio-impedance research.

Methode zur Verbesserung der Meßgenauigkeit bei der Multikanal-Impedanz-Spektroskopie (MIS) / A method for Improving Measuring Accuracy in Multi-channel Impedance Spectroscopy (MIS)

The use of impedance spectroscopy as a diagnostic tool for the investigation of biological objects involves the consideration of numerous parameters impacting on measuring accuracy. This paper describes a calibration method for multichannel instruments that reduces the non-inconsiderable influence of frequency response variations between the channels, thus significantly increasing measuring accuracy. The method is tested in a recently developed, high-resolution, multi-channel bio-impedance analyser. Reduction of the measuring error is demonstrated, and the magnitude and phase resolution is quantified. The advantage of this method lies in its applicability to existing systems. Furthermore, an additional calibration impedance is not needed.

Minimally invasive silicon probe for electrical impedance measurements in small animals

It is commonly accepted that electrical impedance provides relevant information about the physiological condition of living tissues. Currently, impedance measurements are performed with relatively large electrodes not suitable for studies in small animals due to their poor spatial resolution and to the damage that they cause to the tissue. A minimally invasive needle shaped probe for electrical impedance measurements of living tissues is presented in this paper. This micro-probe consists of four square platinum electrodes (300 microm x 300 microm) on a silicon substrate (9 mm x 0.6 mm x 0.5 mm) and has been fabricated by using standard Si microelectronic techniques. The electrodes are not equally spaced in order to optimise the signal strength and the spatial resolution. Characterisation data obtained indicate that these probes provide high spatial resolution (measurement radius <4 mm) with a useful wide frequency band going from 100 Hz to 100 kHz. A series of in vivo experiments in rat kidneys subjected to ischemia was performed to demonstrate the feasibility of the probes and the measurement system. The impedance modulus and phase were measured at 1 kHz since this frequency is sufficiently low to permit the study of the extracellular medium. The extracellular pH and K+ were also simultaneously measured by using commercial miniaturised Ion Selective Electrodes. The induced ischemia period (45 min) resulted in significant changes of all measured parameters (Delta/Z/ approximately 65%; DeltapH approximately 0.8; DeltaK+ approximately 30 mM).

Monitoring of water content and water distribution in ischemic hearts

We determined water content and water distribution by fitting dielectric spectra of ischemic canine hearts between 5 MHz and 3 GHz with a newly developed model which describes heart cells and subcellular organelles as rotational ellipsoids filled with electrolyte enclosed by an isolating membrane. The fraction of dry material is modelled by spherical particles with a small dielectric permittivity. Free model parameters were water content, cell volume fraction, and the conductivity of the electrolytes. Resulting model parameters were compared to data from tissue desiccation and to conductivity changes produced by protons and lactate ions. We investigated hearts in two states: during ischemia after interruption of blood flow (pure ischemia, PI, n=5) and during ischemia after resuscitation with Tyrode's solution (IAR, n=14). The difference between water content determined by tissue desiccation and by dielectric spectroscopy was less than 0.5%. During 360 min of ischemia, water content in IAR decreased from 85+/-1.6% to 83+/-2.2% and in PI from 80+/-0.8% to 78+/-1.5%. Cellular volume fraction in IAR increased from 0.47+/-0.045 to 0.63+/-0.031 and in PI from 0.62+/-0.014 to 0.73+/-0.013, which is consistent with published morphometric data. After 180 min of ischemia, the increase of the cytosolic conductivity was 0.14+/-0.02 S/m as calculated from the dielectric spectrum and was similar to the conductivity increase which was roughly estimated on the basis of tissue lactate concentration. In conclusion, dielectric spectroscopy combined with our model analysis facilitates the monitoring of water content and distribution by means of nondestructive surface probes.

Impedance spectroscopy for monitoring ischemic injury in the intestinal mucosa

This work evaluates the feasibility of monitoring ischemic injury in the gastrointestinal mucosa by impedance spectroscopy, using a minimally invasive intestinal catheter. The disruption of the intestinal mucosa plays a key role in the evolution of shock and is the 'motor of multiple organ failure'. Different technologies have been developed to monitor mucosal perfusion, oxygenation and/or ischemia, but no practical method exists to assess tissue damage, which may be crucial for preventing multiple organ failure. The experimental protocol of this study relied on an isobaric model of hypovolemic shock in 16 anaesthetized rabbits assigned to three groups: sham (n = 6), ischemia (n = 5) and ischemia + reperfusion (n = 5). Complex impedance spectra were recorded in the range of 0.05 to 300 kHz, with simultaneous measurements of tonometric pHi in the ileum every 30 min for 4 h. Impedance spectra were reproducible, and those of tissue under prolonged ischemia were clearly differentiable from those of normally perfused tissue. The dynamic changes in impedance did not correlate directly with either tissue perfusion or pHi, but instead correlated well with the duration of ischemia. It is concluded that impedance spectroscopy does indeed measure changes in tissue injury, and could be a very useful tool to guide therapy of patients in shock.

Multiparametric monitoring of ischemia-reperfusion in rat kidney: effect of ischemic preconditioning

BACKGROUND

Microelectrode technology is a promising tool for monitoring kidney ischemia and the changes induced by its therapeutic management. Ischemic preconditioning, that is, brief ischemic periods before sustained ischemia, has been shown to protect several organs, including the kidney, from ischemia-reperfusion injury. We tested whether the effect of preconditioning could be appraised by real-time measurement of parameters representative of tissue hypoxia.

METHODS

In a sample of pentobarbital-anesthetized and mechanically ventilated rats, we studied the effect of renal ischemic preconditioning (10-min ischemia and 10-min reflow interval) on subsequent ischemia-reperfusion (45 min and 60 min). Renal tissue electrical impedance, extracellular pH, and potassium concentration [K+] were measured continuously by implanted microelectrodes.

RESULTS

Ischemia induced an early, rapid rise in extracellular potassium and impedance module, followed by a phase of slower increase, whereas pH decreased rapidly, reaching a plateau. Preconditioning treatment did not cause significant changes in interstitial pH and [K+] but increased ischemic tissue impedance. During reperfusion, the three variables recovered progressively; however, after a decline, electrical impedance showed a clear postischemic increase. This rise was suppressed by preconditioning.

CONCLUSIONS

Real-time measurement of any of the three parameters showed capability for early detection of ischemia. In contrast with findings in myocardial tissue, preconditioning in the kidney did not increase potassium cell loss during ischemia or improve ischemic acidosis or tissue impedance. Electrical impedance increased for a second time during reperfusion, indicating the presence of a postischemic cellular edema; concealing this episode was the most noticeable effect of the preconditioning treatment.

The complex dielectric spectrum of heart tissue during ischemia

INTRODUCTION

Because of the variety of tissue structures, the interpretation of the passive complex dielectric permittivity spectrum epsilon (omega) of the heart is still a problem. The aim of this work was to correlate epsilon (omega) of heart tissue with physical processes on cellular level.

METHODS

epsilon (omega) of canine hearts was continuously measured in the range from 10 Hz to 400 MHz during cardioplegic perfusion and during following ischemia. Cardioplegic perfusion was performed with HTK (Custodiol) without or with heptanol, in order to produce electrical cell uncoupling via the closure of gap junctions. To analyse epsilon (omega), we present two heart models which consider cell shape, electrical cell coupling, and dielectric polarisation of cell membranes and membranes of intracellular structures.

RESULTS

epsilon (omega) of heart tissue shows an alpha-, beta-, and gamma-dispersion. epsilon (omega) remains unchanged during cardioplegic perfusion with HTK, but if heptanol is added, there is an immediate decrease in the region of alpha-dispersion and an increase in the low frequency part of beta-dispersion. Similar changes are observed during ischemia following HTK perfusion without heptanol; additionally, the beta-dispersion shifts to higher frequencies. Using our models, we obtain analogue changes of epsilon (omega) by fitting model parameters which describe water content, water distribution, extra- and intracellular conductivity, and gap junction resistance.

DISCUSSION

Changes of these tissue properties as calculated by our models based on the measurement of epsilon (omega) are consistent with intraischemic changes of heart tissue known from immunohistochemical, biochemical, and histological investigations. The next step will be to use our models for the prognosis of irreversible tissue damage.

Monitoring of cell and tissue responses to photodynamic therapy by electrical impedance spectroscopy

Electrical impedance spectroscopic (EIS) monitoring of photodynamic therapy (PDT) was investigated in vivo in rat liver and in vitro in multicellular spheroids. Liver impedance was continuously measured with two needle electrodes before, during and up to 3 hours following Photofrin-PDT. EIS spectra were altered immediately after PDT, with significant changes in conductivity at approximately 10 kHz, and in permittivity at approximately 30 kHz and 1 MHz. The change in permittivity at high frequencies was related to oedema, while low-frequency effects were attributed to cell necrosis and vascular changes. Photofrin-PDT-treated spheroids showed dose-dependent decreases in permittivity and conductivity at frequencies above 10 and 100 kHz, respectively. Histology showed concomitant development of a damaged rim containing sparsely distributed cells with compromised membranes and lightly staining cytoplasm. Different EIS responses to apoptotic versus necrotic modes of cell death further verified the sensitivity of impedance to purely cellular changes in the spheroid model. In conclusion, EIS sensitivity to PDT-induced damage, at both the cell and tissue level, varies with dose and time, and can be correlated qualitatively to biological changes.

Multifrequency method for dielectric monitoring of cold-preserved organs

To answer a growing need for non-invasive monitoring of biological organs, we have developed an automated system capable of repeated dielectric measurements over the frequency range 10 kHz-100 MHz. Further, we propose a novel method of data analysis that may convert the acquired, individual dispersion curves into a diagram of the time course of specific phenomenological parameters, such as the characteristic frequency. By using this new procedure, unattended, long-term monitoring of temporal changes in the dielectric behaviour of excised liver lobes stored at 4 degrees C was successfully realized. The 'multifrequency' method presented here was definitely superior to the conventional 'fixed-frequency' method in providing reliable results.

The electrical properties of leg skin in normal individuals and in patients with ischemia

Measurements of skin electrical admittance were performed on the leg in healthy controls and patients with ischemia. The experiments were carried out using sinusoidal wave voltage at seven frequencies from 100 Hz to 100 kHz applied using two electrodes. The character of the frequency dependencies of admittance for both groups arises from stratum corneum--in the frequencies up to 10 kHz, and the underlying tissue of skin, above this frequency. Legs change their passive electrical properties due to ischemia. The measured data are interpreted as cell membranes become more permeable and thus an efflux of intracellular electrolyte.