Bioimpedance dispersion width as a parameter to monitor living tissues

Quantitative comparison of EGFR expression levels of optically trapped individual cells using a capacitance biosensor

Cellular endocytosis is an essential phenomenon which induces cellular reactions, such as waste removal, nutrient absorption, and drug delivery, in the process of cell growth, division, and proliferation. To observe capacitance responses upon endocytosis on a single-cell scale, this study combined an optical tweezer that can optically place a single cell on a desired location with a capacitance sensor and a cell incubation chamber. Single HeLa cancer cell was captured and moved to a desired location through optical trapping, and the single-cell capacitance change generated during the epidermal growth factor (EGF) molecule endocytosis was measured in real time. It was found that single HeLa cells showed a larger increase in capacitance values compared to that of the single NIH3T3 cells when exposed to varying EGF concentrations. In addition, the capacitance change was in proportion to the cell's EGF receptor (EGFR) level when cells of different levels of EGFR expression were tested. An equation derived from these results was able to estimate the EGFR expression level of a blind-tested cell. The biosensor developed in this research can not only quickly move a single cell to a desired location in a non-invasive manner but also distinguish specific responses between cancer and normal cells by continuous measurement of real-time interactions of a single cell in culture to the external ligands.

Comparison of gastric reactance with commonly used perfusion markers in a swine hypovolemic shock model

Background

The gut has been hypothesized to be a protagonist tissue in multiple organ dysfunction syndrome (MODS) for the past three decades. Gastric reactance (XL) is a potential perfusion marker derived from gastric impedance spectroscopy (GIS), which is an emerging tool through which living tissue can be continuously measured to determine its pathophysiological evolution. This study aimed to compare the performance of XL [positive predictive values (PPV), negative predictive values (NPV), and area under the curve (AUC)] against commonly used perfusion markers before and during hypovolemic shock in swine subjects.

Methods

Prospective, controlled animal trial with two groups, control group (CG) N = 5 and shock (MAP ≤ 48 mmHg) group (SG) N = 16. Comparison time points were defined as T-2 (2 h before shock), T-1 (1 h before shock), T0 (shock), T1 (1 h after shock), and T2 (2 h after shock). Shock severity was assessed through blood gases, systemic and hemodynamic variables, and via histological examination for assessing inflammation-edema and detachment in the gastric mucosa. Macroscopic assessment of the gastric mucosa was defined in five levels (0—normal mucosa, 1—stippling or epithelial hemorrhage, 2—pale mucosa, 3—violet mucosa, and 4—marmoreal mucosa). Receiver Operating Characteristic (ROC) curves of perfusion markers and XL were calculated to identify optimal cutoff values and their individual ability to predict hypovolemic shock.

Results

Comparison among the CG and the SG showed statistically significant differences in XL measurements at T-1, T0, T1, and T2, while lactate showed statistically significant differences until T1 and T2. Statistically significant differences were detected in mucosa class (p < 0.001) and in inflammation-edema in the gastric body and the fundus (p = 0.021 and p = 0.043). The performance of the minimum XL value per subject  per event (XL_Min) was better (0.81 ≤ AUC ≤ 0.96, 0.93 ≤ PPV ≤ 1.00, 0.45 ≤ NPV ≤ 0.83) than maximum lactate value (Lac_Max) per subject per event (0.29 ≤ AUC ≤ 0.82, 0.82 ≤ PPV ≤ 0.91, 0.24 ≤ NPV ≤ 0.82). Cutoff values for XL_Min show progressive increases at each time point, while cutoff values for Lac_Max increase only at T2.

Conclusions

XL proved to be an indirect and consistent marker of inadequate gastric mucosal perfusion, which shows significant and detectable changes before commonly used markers of global perfusion under the hypovolemic shock conditions outlined in this work.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-022-00476-1.

Uncertainty Quantification and Sensitivity Analysis for the Electrical Impedance Spectroscopy of Changes to Intercellular Junctions Induced by Cold Atmospheric Plasma

The influence of pertinent parameters of a Cole-Cole model in the impedimetric assessment of cell-monolayers was investigated with respect to the significance of their individual contribution. The analysis enables conclusions on characteristics, such as intercellular junctions. Especially cold atmospheric plasma (CAP) has been proven to influence intercellular junctions which may become a key factor in CAP-related biological effects. Therefore, the response of rat liver epithelial cells (WB-F344) and their malignant counterpart (WB-ras) was studied by electrical impedance spectroscopy (EIS). Cell monolayers before and after CAP treatment were analyzed. An uncertainty quantification (UQ) of Cole parameters revealed the frequency cut-off point between low and high frequency resistances. A sensitivity analysis (SA) showed that the Cole parameters, R₀ and α were the most sensitive, while R_inf and τ were the least sensitive. The temporal development of major Cole parameters indicates that CAP induced reversible changes in intercellular junctions, but not significant changes in membrane permeability. Sustained changes of τ suggested that long-lived ROS, such as H₂O₂, might play an important role. The proposed analysis confirms that an inherent advantage of EIS is the real time observation for CAP-induced changes on intercellular junctions, with a label-free and in situ method manner.

Development of a tissue discrimination electrode embedded surgical needle using vibro-tactile feedback derived from electric impedance spectroscopy

Some tumours may not be detected by ultrasound during biopsy, but recent evidence has shown that different tissues can be discerned by electric impedance. This paper explores the application of vibrotactile feedback in an electrode embedded needle to help classify tissue during fine-needle aspiration biopsy from bioimpedance measurements. The process uses electric impedance spectroscopy from 10 Hz to 349 kHz to fit the double-dispersion Cole model through the Newton-Raphson algorithm. A Naive Bayes classifier is then used on the equivalent circuit parameters to estimate the tissue at the needle tip. The method is validated through a series of experiments and user trials. The results show that the vibrotactile feedback is able to help the operator in determining the tissue the needle is in, suggesting that this may prove to be a useful supplement to the standard procedure used today. Graphical Abstract This paper explores the application of vibrotactile feedback for an electrode embedded needle to help classify tissue from electric impedance measurements. The data is fit to an equivalent circuit using Newton- Raphon's method. A Naive Bayes classifier is trained and later used in an experiment to estimate the tissue at the needle tip and provide an assigned vibrotacticle feedback to the user.

Impedance Measurement System for Assessing the Barrier Integrity of Three-Dimensional Human Intestinal Organoids

Human pluripotent stem cell (hPSC)-derived intestinal organoids (HIOs) hold unprecedented promise for basic biology and translational applications. However, developing a quantitative method to evaluate the epithelial cell membrane integrity of HIOs as an in vitro intestinal barrier model is a major challenge because of their complex three-dimensional (3D) structure. In this study, we developed an impedance system to measure the change in electrical resistance of 3D HIOs depending on the integrity of the intestinal epithelial cell membrane, which can reflect functionality and maturity. The expression of intestinal maturation- and tight junction-related markers was significantly higher in HIOs matured in vitro by treatment with IL-2 than in control HIOs. Analysis of gap junction size indicated that mature HIOs have greater integrity, with approximately 30% more compact gaps than immature HIOs. We designed a multi-microchannel system controlled by the inhalation pressure where the HIO is loaded, which enhances the stability and sensitivity of the impedance signal. We demonstrated the applicability of the impedance system by showing the difference in resistance between control and mature HIOs, reflecting the expression of tight junction proteins and their maturation status. We also validated the impedance system by monitoring its resistance in real time during junctional damage to HIOs induced by a digestive agent. In summary, we suggest a quantitative method to directly quantify the physiological changes in complex 3D organoid structures based on impedance spectroscopy, which can be applied to noninvasively monitor live cells and therefore enable their use in subsequent experiments.

Quantification of non-alcoholic fatty liver disease progression in 3D liver microtissues using impedance spectroscopy

Non-alcoholic fatty liver disease (NAFLD) has become a global pandemic. However, a pharmacological cure has not been approved for NAFLD treatment. The greatest barriers to the development of new treatments are the ambiguous criteria among the NAFLD stages and the lack of quantitative methodologies for its disease assessment in a translatable preclinical model. In this study, we developed impedance assessment systems to quantify NAFLD progression in three-dimensional (3D) liver microtissue (hMT). The hMT model undergoing NAFLD represents clinical-like characteristics for a range of stages, such as lipid accumulation, cell ballooning, and stiffening. Each stage can be quantitatively assessed by an impedance system with microchannels under constant or dynamic pressure, depending on the relevant mechanical and morphological changes used in the clinical assessment of NAFLD. We determined a correlation between the impedance parameters and pathophysiological characteristics, such as gap widening and cytoplasmic deformation associated with NAFLD progression using bioimpedance simulation, showing hMTs struggling to return to normal states. In addition, we identified the relative stiffness to assess fibrogenesis from the correlation of resistance change and elongation length into the smaller channel of hMTs. We hope this methodology will have a significant impact on drug development by facilitating improved NAFLD assessment.

Monitoring the quality of frozen-thawed venous segments using bioimpedance spectroscopy

OBJECTIVE

Storage at temperatures as low as -80 °C and below (cryopreservation) is considered a method for long-term preservation of cells and tissues, and especially blood vessel segments, which are to be used for clinical operations such as transplantation. However, the freezing and thawing processes themselves can induce injuries to the cells and tissue by damaging the structure and consequently functionality of the cryopreserved tissue. In addition, the level of damage is dependent on the rate of cooling and warming used during the freezing-thawing process. Current methods for monitoring the viability and integrity of cells and tissues after going through the freezing-thawing cycle are usually invasive and destructive to the cells and tissues. Therefore, employing monitoring methods which are not destructive to the cryopreserved tissues, such as bioimpedance measurement techniques, is necessary. In this study we aimed to design a bioimpedance measurement setup to detect changes in venous segments after freezing-thawing cycles in a noninvasive manner.

APPROACH

A bioimpedance spectroscopy measurement technique with a two-electrode setup was employed to monitor ovine jugular vein segments after each cycle during a process of seven freezing-thawing cycles.

MAIN RESULTS

The results demonstrated changes in the impedance spectra of the measured venous segments after each freezing-thawing cycle.

SIGNIFICANCE

This indicates that bioimpedance spectroscopy has the potential to be developed into a novel method for non-invasive and non-destructive monitoring of the viability of complex tissue after cryopreservation.

Bioimpedance Analysis of Epithelial Monolayers after Exposure to Nanosecond Pulsed Electric Fields

Exposures to pulsed electric fields (PEFs) are known to affect cell membranes and consequently also cell-cell interactions as well as associated characteristics. Bioimpedance analysis offers direct and non-invasive insights into structural and functional changes of cell membranes and extracellular matrices through a rigorous evaluation of electrical parameters. Accordingly, the multi-frequency impedance of confluent monolayers of rat liver epithelial WB-F344 cells was monitored in situ before and after exposure to nanosecond PEFs (nsPEFs). The results were fitted by two Cole models in series to obtain the Cole parameters for the monolayer. For an interpretation of the results, dielectric parameters, were correlated with changes of the TJ protein zonula occludens (ZO-1) and the paracellular permeability of the monolayer Cole parameters in general change as a function of pulse number and time. The findings demonstrate that impedance analysis is an effective method to monitor changes of TJs cell-cell contacts and paracellular permeability and relate them to exposure parameters.

Localised impedance monitoring device for the remote clinical assessment of home-based dialysis patients

BioImpedance Spectroscopy (BIS) has been clinically used to determine the hydrational status of patients undergoing haemodialysis (HD). In the present project we are developing a calf-localised, integrated impedimetric device to periodically and conveniently measure and transmit information on the hydrational status of home-based patients to a remote clinic. Surprisingly, we have found that simple postural changes before or during measurement lead to significant fluid shifts in the lower leg that are as important and as long lasting as the effects of haemodialysis. These must be taken into account if potentially hazardous errors are not to be made in assessing a patient's hydrational status.

Age-related differences of intraischemic gap junction uncoupling in hearts during ischemia

OBJECTIVE

Myocardial ischemia leads to energetic, morphologic, metabolic, and functional alterations. To evaluate differences in ischemia tolerance between neonatal and adult hearts, we investigated gap junction uncoupling (GJU) and its correlation to myocardial intracellular edema formation during normothermic ischemia.

METHODS

Hearts of landrace piglets (neonates, 7.4 ± 1.9 days of age, body weight 2.9 ± 0.5 kg, n = 5 and adults, 84 ± 9 days of age, body weight 30.5 ± 3.9 kg, n = 5) were investigated. After we harvested the hearts, the bioelectrical impedance spectra were measured continuously during normothermic global ischemia (35°C). Spectra of the dielectric permittivity, ε'(frequency), and conductivity, σ(frequency), were calculated from the impedance measurements, and GJU was identified in the sigmoidal time course of ε' (13 kHz). The extracellular volume was estimated by the ratio σ (100Hz)/σ (1MHz). Dielectric data were correlated with electron-microscopical images.

RESULTS

Intraischemic GJU was observed in neonates after 54 ± 9 minutes of ischemia and thus significantly earlier than in adults (90 ± 7 minutes, P < .05). A more than 20% increase of intercalated water was found in tissue samples of neonates after 20 ± 2 minutes, in contrast to adults after 137 ± 8 minutes (P < .05).

CONCLUSIONS

Intraischemic formation of edema and earlier GJU indicate faster intraischemic changes in neonates compared with adults. Intraischemic GJU and determination of intracellular water shifts are an experimental approach to establish the period of life-threatening damage. Because both parameters are linked and occur significantly earlier in neonates, they distinctly demonstrate the lower ischemia tolerance of neonatal hearts as both events interact.

The Feasibility of a Smart Surgical Probe for Verification of IRE Treatments Using Electrical Impedance Spectroscopy

SIGNIFICANCE

Irreversible electroporation (IRE) is gaining popularity as a focal ablation modality for the treatment of unresectable tumors. One clinical limitation of IRE is the absence of methods for real-time treatment evaluation, namely actively monitoring the dimensions of the induced lesion. This information is critical to ensure a complete treatment and minimize collateral damage to the surrounding healthy tissue.

GOAL

In this study, we are taking advantage of the biophysical properties of living tissues to address this critical demand.

METHODS

Using advanced microfabrication techniques, we have developed an electrical impedance microsensor to collect impedance data along the length of a bipolar IRE probe for treatment verification. For probe characterization and interpretation of the readings, we used potato tuber, which is a suitable platform for IRE experiments without having the complexities of in vivo or ex vivo models. We used the impedance spectra, along with an electrical model of the tissue, to obtain critical parameters such as the conductivity of the tissue before, during, and after completion of treatment. To validate our results, we used a finite element model to simulate the electric field distribution during treatments in each potato.

RESULTS

It is shown that electrical impedance spectroscopy could be used as a technique for treatment verification, and when combined with appropriate FEM modeling can determine the lesion dimensions.

CONCLUSIONS

This technique has the potential to be readily translated for use with other ablation modalities already being used in clinical settings for the treatment of malignancies.

Gastrointestinal ischemia monitoring through impedance spectroscopy as a tool for the management of the critically ill

Impedance spectroscopy (IS) has been proposed as a tool for monitoring mucosal tissue ischemia and damage in the gut of critically ill patients resulting from shock and hypoperfusion. A specific device and system have been developed and tested for this specific application over the past 12 years by our research group. This paper reviews previously published studies as well as unpublished experimental results, and puts the whole in context and perspective to help understand this technology. Results presented include summaries of gastric reactance measurement understanding, in vivo measurements in animal models, clinical significance of the measurement, and future perspectives of clinical use of this technology. All of the experimental work done to date has been designed to determine the evolving device prototypes' performance and limitations from an instrumentation point of view. Although there are still questions to be answered with regard to the IS measurement, we conclude that we have reached enough confidence in the measurement and the device's performance and safety to begin clinically oriented research to learn how this technology may be useful in the diagnosis and management of different populations of the critically ill.

Gastric tissue damage analysis generated by ischemia: bioimpedance, confocal endomicroscopy, and light microscopy

The gastric mucosa ischemic tissular damage plays an important role in critical care patients' outcome, because it is the first damaged tissue by compensatory mechanism during shock. The aim of the study is to relate bioimpedance changes with tissular damage level generated by ischemia by means of confocal endomicroscopy and light microscopy. Bioimpedance of the gastric mucosa and confocal images were obtained from Wistar male rats during basal and ischemia conditions. They were anesthetized, and stain was applied (fluorescein and/or acriflavine). The impedance spectroscopy catheter was inserted and then confocal endomicroscopy probe. After basal measurements and biopsy, hepatic and gastric arteries clamping induced ischemia. Finally, pyloric antrum tissue was preserved in buffered formaldehyde (10%) for histology processing using light microscopy. Confocal images were equalized, binarized, and boundary defined, and infiltrations were quantified. Impedance and infiltrations increased with ischemia showing significant changes between basal and ischemia conditions (P < 0.01). Light microscopy analysis allows detection of general alterations in cellular and tissular integrity, confirming gastric reactance and confocal images quantification increments obtained during ischemia.

Electro interstitial scan system: assessment of 10 years of research and development

BACKGROUND

Ten years of research and development have allowed an understanding of how the electro interstitial scan (EIS) works and what its clinical applications may be.

MATERIALS AND METHODS

The EIS is a galvanic skin response device. The measurements are performed by electrical stimulation of the post sympathetic cholinergic fiber with weak DC current and voltage 1.28V applied during 2 minutes and in bipolar mode. CURRENT SCIENTIFIC KNOWLEDGE: EIS ELECTRICAL MEASUREMENTS ARE RELATED TO: (1) the concentration of free chloride ions in the interstitial fluid, which affects the transfer of electrical current and the ratio intensity/voltage; (2) the morphology of the interstitial fluid, which is related to the electrical dispersion calculated from the Cole equation (α parameter); (3) electrical stimulation, which causes a change in sweat rate at the passive electrodes - post sympathetic cholinergic fiber electrical stimulation appears to be responsible for activating M2 receptors, which regulate nitric oxide (NO) production in the endothelial cell and cause vasodilation and a released sweat response; and (4) the electrochemical redox reactions (electrolysis) of the released sweat on electrodes, which are different on the bulk of the metal electrodes (O2 + [4H(+)] + [4e(-)]) and on the Ag/AgCl disposable electrodes (AgCl precipitation).

RESULTS

FOR EACH OF THE EIS CLINICAL RESULTS, VARIOUS EXPLANATIONS WERE POSITED, SUCH AS: (1) electrical stimulation of the postsympathetic cholinergic fiber-activating NO production in the endothelial cell, which causes vasodilation and a released sweat response (diabetes detection); (2) estimation of interstitial fluid's acid-base balance, which is reflected in an electrochemical reaction on the bulk of the electrodes through the released sweat (prostate cancer detection); (3) estimation of cerebral interstitial fluid chloride ions (detection of ADHD in children); and (4) estimation of the morphology of the interstitial fluid (selective serotonin reuptake inhibitor treatment response).

CONCLUSION

After 10 years of development, the analysis of current scientific knowledge and results of clinical investigations have allowed a better understanding of EIS electrical measurements.

Bioimpedance in monitoring of effects of selective serotonin reuptake inhibitor treatment

Background

Bioimpedance has been shown to be a safe technique when used in a number of biomedical applications. In this study, we used the Electro Interstitial Scan (EIS) to perform bioimpedance measurements to follow up the efficacy of selective serotonin reuptake inhibitor (SSRI) treatment in subjects diagnosed to have major depressive disorder.

Methods

We recruited 59 subjects (38 women, 21 men) aged 17–76 (mean 47) years diagnosed with major depressive disorder by psychiatric assessment at the Botkin Hospital according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV). Baseline Clinical Global Impression scores and EIS (electrical conductivity and dispersion α parameter) measurements were done before starting SSRI therapy. Treatment follow-up was undertaken using EIS bioimpedance measurements and by treatment response based on the Hamilton Depression Scale and Clinical Global Impression, every 15 days for 60 days. At day 45, we classified the patients into two groups, ie, Group 1, including treatment responders, and Group 2, including nonresponders. At day 60, patients were classified into two further groups, ie, Group 3, comprising treatment responders, and Group 4, comprising nonresponders.

Results

Comparing Group 1 and Group 2, electrical conductivity measurement of the pathway between the two forehead electrodes had a specificity of 72% and a sensitivity of 85.3% (P < 0.0001), with a cutoff >4.32. Comparing Group 3 and Group 4, electrical conductivity measurements in the same pathway had a specificity of 47.6% and a sensitivity of 76.3% (P < 0.16), with a cutoff >5.92. Comparing Group 1 and Group 2, the electrical dispersion α parameter of the pathway between the two disposable forehead electrodes had a specificity of 80% and a sensitivity of 85.2% (P < 0.0001) with a cutoff >0.678. Comparing Group 3 and Group 4, the electrical dispersion α parameter of the same pathway had a specificity of 100%, a sensitivity of 89.5% (P < 0.0001), and a cutoff >0.692.

Conclusion

Electrical conductivity measurement of the forehead pathway using EIS has a high specificity and sensitivity at day 45 when comparing treatment responders and nonresponders, but decreases at day 60. The EIS electrical dispersion α parameter of the forehead pathway has a high specificity and sensitivity at day 45 when comparing treatment responders and nonresponders, and increases at day 60. The EIS system may be a noninvasive, easily administered, low-cost technique that could be used as an adjunct to DSM-IV and Clinical Global Impression scores for monitoring of efficacy of treatment in patients with major depressive disorder.

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.

Extracting cancer cell line electrochemical parameters at the single cell level using a microfabricated device

Cancer cells have distinctive electrochemical properties. This work sheds light on the system design aspects and key challenges that should be considered when experimentally analyzing and extracting the electrical characteristics of a tumor cell line. In this study, we developed a cellularbased functional microfabricated device using lithography technology. This device was used to investigate the electrochemical parameters of cultured cancer cells at the single-cell level. Using impedance spectroscopy analyses, we determined the average specific capacitance and resistance of the membrane of the cancer cell line B16-F10 to be 1.154 +/- 0.29 microF/cm(2), and 3.9 +/- 1.15 KOmega.cm(2) (mean +/- SEM, n =14 cells), respectively. The consistency of our findings via different trails manifests the legitimacy of our experimental procedure. Furthermore, the data were compared with a proposed constructed analytical-circuit model. The results of this work may greatly assist researchers in defining an optimal procedure while extracting electrical properties of cancer cells. Detecting electrical signals at the single cell level could lead to the development of novel approaches for analysis of malignant cells in human tissues and biopsies.

In vivodetection of liver steatosis in rats based on impedance spectroscopy

Hepatic steatosis is a widespread condition of high prevalence in Western populations, and its asymptomatic nature represents a hefty problem in liver surgery and transplantation. Current diagnostic methods rely mainly on biopsy and blood tests, and are thus time consuming and expensive. Here we report the use of direct impedance measurements on liver tissue as a promising alternative to conventional diagnostic methods in surgery and transplantation. Working on a dual Zucker Fat (ZF), Zucker Lean (ZL) rat experimental model, we show that certain parameters extracted from multi-frequency impedance measurements correlate well with the presence of steatosis and that these results can be adequately approximated with bi-frequency measurements extracting the impedance modulus at 1 kHz and the impedance phase angle at 5.7 kHz. We further support our findings on a theoretical model of tissue impedance, and the simulations carried out suggest a possible mechanism to expound the negative effect of steatosis in post-transplant graft function.

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.