Selection of measurement frequencies for optimal extraction of tissue impedance model parameters

Effect of Feeding and Suction on Gastric Impedance Spectroscopy Measurements

A specific device and system has been developed and tested for clinical monitoring of gastric mucosal reactance in the critically ill as an early warning of splanchnic hypoperfusion associated with shock and sepsis. This device has been proven effective in clinical trials and is expected to become commercially available next year. The system uses a combination nasogastric tube and impedance spectroscopy probe as a single catheter. Because this device has a double function, the question is: Does enteral feeding or suction affect the gastric reactance measurements? This study was designed to evaluate the effect of feeding and suction on the measurement of gastric impedance spectroscopy in healthy volunteers. Impedance spectra were obtained from the gastric wall epithelia of 18 subjects. The spectra were measured for each of the following conditions: postinsertion of gastric probe, during active suction, postactive suction, and during enteral feeding (236 ml of nutritional supplement). Impedance spectra were reproducible in all volunteers under all conditions tested. There was a slight increase in impedance parameters after suction, and a decrease in impedance after feeding; however, these observed differences were insignificant compared to patient-to-patient variability, and truly negligible compared with previously observed changes associated with splanchnic ischemia in critically ill patients. Our results demonstrate that suction or feeding when using the impedance spectro-metry probe/nasogastric tube does not significantly interfere with gastric impedance spectrometer measurements.

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 This paper reviews previously published studies as well as unpublished experimental results, and puts the whole in context and perspective to help understand this technology.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.

In silico validation procedure for cell volume fraction estimation through dielectric spectroscopy

Dielectric spectroscopy has proved to be a good tool for analyzing the passive electrical properties of biological tissues as well as those of inhomogeneous materials. This technique promises to be a valid alternative to the classical ones based on metabolites to monitor the growth and cell volume fraction of cell cultures in a simple and minimally invasive way. In order to obtain an accurate estimation of the cell volume fraction as a function of the permittivity of the suspension, a simple in silico procedure is proposed. The procedure is designed to perform homogenization from the micro-scale to the macro-scale using simple analytical models and simulation setups hypothesizing the properties of diluted suspension (cell volume fraction less than 0.2). Results obtained show the possibility to overcome some trouble involving the analytical treatment of the cellular shape by considering a sphere with the same permittivity in the quantitative analysis of the cell volume fraction. The entire study is based on computer simulations performed in order to verify the correctness of the procedure. Obtained data are used in a cell volume fraction estimation scenario to show the effectiveness of the procedure.

Influence of electrode mismatch on Cole parameter estimation from total right side electrical bioimpedance spectroscopy measurements

Applications based on measurements of Electrical Bioimpedance (EBI) spectroscopy analysis, like assessment of body composition, have proliferated in the past years. Currently Body Composition Assessment (BCA) based in Bioimpedance Spectroscopy (BIS) analysis relays on an accurate estimation of the Cole parameters R(0) and R(∞). A recent study by Bogonez-Franco et al. has proposed electrode mismatch as source of remarkable artefacts in BIS measurements. Using Total Right Side BIS measurements from the aforementioned study, this work has focused on the influence of electrode mismatch on the estimation of R(0) and R(∞) using the Non-Linear Least Square curve fitting technique on the modulus of the impedance. The results show that electrode mismatch on the voltage sensing electrodes produces an overestimation of the impedance spectrum leading to a wrong estimation of the parameters R(0) and R(∞), and consequently obtaining values around 4% larger that the values obtained from BIS without electrode mismatch. The specific key factors behind electrode mismatch or its influence on the analysis of single and spectroscopy measurements have not been investigated yet, no compensation or correction technique is available to overcome the deviation produced on the EBI measurement. Since textile-enabled EBI applications using dry textrodes, i.e. textile electrodes with dry skin-electrode interfaces and potentially large values of electrode polarization impedance are more prone to produce electrode mismatch, the lack of a correction or compensation technique might hinder the proliferation of textile-enabled EBI applications for personalized healthcare monitoring.

Incidence of gastric mucosal injury as measured by reactance in critically ill patients

Gastric reactance has been proposed as a measure of mucosal ischemic injury in the critically ill. The purpose of this study was to evaluate the incidence of gastric mucosal injury as measured by gastric reactance in different subgroups of critical patients. We studied 100 adult patients admitted to 7 different hospital intensive care units, requiring a nasogastric tube. Gastric impedance measurements were continuously obtained from each patient for 24 hours. Patients were managed based on conventional protocols by hospital staff, blinded to the changes in gastric impedance parameters. The low-frequency central reactance (X L) reflects tissue edema caused by prolonged ischemia. The previously reported threshold of X L ≥ 13 - jΩ was used to classify injured mucosa; 80% of all patients had mean X L above this threshold. No significant differences were found in the incidence of mucosal ischemia between medical versus surgical, hemodynamic versus respiratory or neurological patients. Significant lower urine output was found in patients with X L above threshold (P < .01); also, there was a significant effect of fluid balance in those patients (P < .05). More complicated patients had higher average reactance. This study shows that gastric ischemia as estimated by gastric reactance has a very high incidence in the critically ill, independently of the reason for admission. High reactance is related with higher morbidity in agreement with other reports using different methods of assessing splanchnic hypoperfusion in this patient population.

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.

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.

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.

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.

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.

Algorithm for tissue ischemia estimation based on electrical impedance spectroscopy

The purpose of this paper is to present an algorithm developed for real-time estimation of skeletal muscle ischemia, based on parameters extracted from in vivo obtained electrical impedance spectra. A custom impedance spectrometer was used to acquire data sets: complex impedance spectra measured at 27 frequencies in the range of 100 Hz-1 MHz, and tissue pH. Twenty-nine in vivo animal studies on rabbit anterior tibialis muscle were performed to gather data on the behavior of tissue impedance during ischemia. An artificial neural network (ANN) was used to quantitatively describe the relationship between the parameters of complex tissue impedance spectra and tissue ischemia via pH. The ANN was trained on 1249, and tested on 946 ischemic tissue impedance data sets. A correlation of 94.5% and a standard deviation of 0.15 pH units was achieved between the ANN estimated pH and measured tissue pH values.

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.

Measuring lower leg swelling: Optimum frequency for impedance method

The optimum measuring frequency of the impedance plethysmograph is determined for the estimation of leg volume increase caused by a sitting work load. Ten subjects sit for 1 h without leg movement. Right calf impedance is measured by the four-electrode method. The detecting electrodes are fixed on the calf with an inter-electrode distance of 15 cm. The measuring frequencies are 5, 10, 50 and 100 kHz. As the indices of leg swelling, three values are calculated: percentage change in impedance, volume change and percentage change in volume. The mean, standard deviation and coefficient of variation (CV) are calculated from the mean values of indices for each subject and frequency. The CV of all three indices increases when measurements are taken at higher frequencies. Among the three indices across all measuring frequencies, the volume change shows the highest CV (0.42 at 5 kHz, 0.82 at 100 kHz), and the percentage change in impedance shows the lowest CV (0.32 at 5 kHz, 0.72 at 100 kHz). It is concluded that the measuring frequency of 5 to 10 kHz is the best to obtain accurate data for the evaluation of lower leg swelling.