Computation of intraluminal impedance

The measurement of the electrical impedance inside the esophagus provides information about its status, and is being explored in the study of the gastroesophageal reflux. This paper presents theoretical computation of impedance inside the esophagus. The results of the numerical solution for a simple geometry are compared against the solution formulated from the Green's function approach. The effect of the electrode configuration on the resulting impedance is studied as an application of the methodology developed in this paper. The results of this paper will be useful in the design of an intraluminal impedance catheter as well as in the interpretation of the resulting impedance signals.     

Effect of esophagus status and catheter configuration on multiple intraluminal impedance measurements

Multiple intraluminal impedance measurement is used to investigate the esophagus condition. This procedure provides information about the esophagus status, reflux occurrence and clearance mechanism. This paper presents finite-element approximation of the forward problem of the catheter inside the esophagus, relating the real data obtained from multiple intraluminal impedance to the solution obtained from the finite-element approximation. Investigation of the effect of the esophagus status on the impedance, and the correlation between various factors that affect the impedance are presented. The results of this paper provide theoretical bases for relating the esophagus condition to the impedance waveforms.     

Multichannel intraluminal impedance: general principles and technical issues

Multichannel intraluminal impedance (MII) is a new technology that allows detection of bolus movement without the use of external radiation or radiolabeled substances. The principles of MII are based on changes in resistance to alternating electrical current (impedance) induced by the presence of various boluses within the esophagus. The timing of changes in multiple impedance-measuring segments in the esophagus allows determination of the direction of bolus movement. Combined MII and manometry (MII-EM) provides simultaneous information on intraesophageal pressures and bolus transit, offers the ability to monitor all types of reflux, and allows the detection of the physical (liquid, gas, or mixed) and chemical (acid, nonacid) characteristics of the gastroesophageal refluxate.     

Reflux monitoring: role of combined multichannel intraluminal impedance and pH

Combined multichannel intraluminal impedance and pH (MII-pH) is a promising technique that identifies liquid, gas, and mixed gastroesophageal reflux and categorizes it into acid and nonacid types. Gastroesophageal monitoring using combined MII-pH reconfirms previously reported data that there is a difference between refluxate volume presence time in the esophagus and time pH less than 4. Only one in five patients with persistent symptoms on acid-suppressive therapy have symptoms associated with persistent acid reflux, whereas two of five patients have symptoms associated with nonacid reflux; the remaining patients do not have any type of GER related to their symptoms. Combined MII-pH is a useful tool in evaluating patients with persistent symptoms on acid-suppressive therapy, patients with atypical symptoms, and infants.     

Esophageal function testing: role of combined multichannel intraluminal impedance and manometry

Combined multichannel intraluminal impedance and manometry (MII-EM) is a relatively new technique that allows simultaneous measurement of intraesophageal pressures and bolus transit. Combined MII-EM has the ability to identify what percentage of manometric normal/peristaltic, ineffective, and simultaneous swallows have complete or incomplete bolus transit. Predictors of normal bolus transit in patients with ineffective esophageal motility are the distal esophageal amplitude, the number of sites with low amplitudes, and the overall number of manometric ineffective contractions. Outcome studies are necessary to evaluate whether combined MII-EM is superior to traditional manometry in evaluating patients with nonobstructive dysphagia and in identifying patients at risk for developing dysphagia after antireflux surgery.     

Physiological interpretation of electrical impedance epigastrography measurements

Measurement of the electrical impedance of the gastric region is carried out with the epigastrograph. This generates and applies alternating current around the abdominal area and measures the potential difference in order to determine the impedance externally, via electrodes. The change of epigastric impedance for a subject, given a meal after fasting, depends on the conductivity of the meal compared to the stomach and surrounding tissues. Typically a conductive meal has conductivity >7 mS cm(-1), non-conductive <2 mS cm(-1) and neutral about 4.5 mS cm(-1). Half-emptying times (T50s) from gastric emptying studies in volunteers using three test meals of 450 ml volume were obtained and found to be shorter than expected from the literature. The meals were a 10% glucose solution and two milk shakes of energy 1,300 kJ and 2,850 kJ, respectively. These electrical impedance epigastrography (EIE) measurements were carried out with scintigraphy. The T50 values of the latter were significantly longer. The direct comparison of the normalized experimental data obtained by both methods led to the concept that EIE measurements are mainly influenced by gastric secretion. Thus the EIE trace of a 'neutral' meal suggests the hypothesis that the volume of the meal is not the significant factor but is influenced by gastric acid secretions. Physiology of the gastric mucosa during the digestion of a meal and intragastric pH values also suggests this. Gastric function studies using EIE measurements may therefore reflect gastric ionic concentration rather than the volume of the contents of the stomach. In turn this could lead to the development of a non-invasive method for the continuous recording of gastric acid secretions.     

Determination of extracellular fluid volume using impedance measurements

Currently, accurate determination of body fluid compartments depends on the sodium bromide method (NaBr), an invasive measurement requiring venipuncture with infusion of a foreign substance. Impedance (Z) measurements may provide a practical noninvasive alternative for estimating fluid compartments in sick, premature neonates. To validate the impedance method, we compared it with the NaBr technique in nine anesthetized rabbits. Electrodes were placed for impedance measurement. Vascular catheters were inserted into the femoral artery and vein. Baseline impedance data were collected at 1.0 kHz and blood samples were drawn for NaBr standard assay. Using conventional assay techniques for determination of extracellular fluid volume (ECFV), we correlated NaBr data with impedance measurements. A linear relationship between ECFV by NaBr assay and the previously developed impedance equation rho L2/Z1.0 was established using regression analysis. A correlation value of r = .95 was obtained. These data suggest the potential for impedance to estimate ECFV.     

SVM for prostate cancer using electrical impedance measurements

Biopsies are currently the 'gold standard' method for identifying cancer of the prostate. While biopsies yield very accurate information regarding the area they sample, they are performed at discrete points and provide no information on the adjacent tissue. To enhance procedural accuracy, biopsies of a large number of sites are routinely carried out. Although more accurate, this method is both more complex and nevertheless remains discrete. In this paper, we evaluate the advantages of using bio-impedance information as the input for a support vector machines (SVMs) classifier to overcome these limitations. In this method, the biopsy probes are used as electrodes to obtain electrical impedance data during each biopsy sample. Using a computer model of the prostate, a SVM was trained and tested. Different tumor shapes and conductivity values, and the classifier's ability to generalize to these different properties, were examined. We demonstrate that by using this classifier the number of biopsies can be reduced and valuable information concerning the adjacent tissue which was not biopsied can be generated.     

Application of mechanical impedance measurements to the study of dysfibrinogens

Most techniques of clot detection do not give information about the mechanical properties of the forming clot. A procedure was developed by which the dynamic loss modulus, or mechanical impedance, of a solution of fibrinogen could be continuously monitored during clot development. This method was then applied to the analysis of purified preparations of normal fibrinogen and two dysfibrinogens. Both thrombin and reptilase were used as clotting agents, and unique tracings of clot impedance vs. time were generated for each dysfibrinogen. The functional defect was known in each case, and the clot impedance tracings were able to distinguish between abnormalities in the release of fibrinopeptides A and B. Mechanical impedance measurements were shown to complement other types of analyses now used in the characterization of dysfibrinogens. In particular, they distinguished between dysfibrinogens that appeared similar by more conventional laboratory methods.     

Cardiorespiratory monitoring equipment interferes with whole body impedance measurements

Bioelectrical impedance measurements are widely used for the study of body composition. Commonly measurements are made at 50 kHz to estimate total body water or at low frequencies (<10 kHz) to estimate extracellular fluid volume. These measurements can be obtained as single measurements at discrete frequencies, or as fitted data interpolated from plots of measurements made at multiple frequencies. This study compared single frequency and multiple frequency (MF) measurements taken in the intensive care environment. MF bioimpedance (4-1000 kHz) was measured on an adult with and without cardiorespiratory monitoring, and on babies in the neonatal intensive care unit. Measurements obtained at individual frequencies were plotted against frequency and examined for the presence of outlying points. Fitted data for measurements obtained at 5 kHz and 50 kHz with and without cardiorespiratory monitoring were compared. Significant artefacts were detected in measurements at approximately 50 kHz and at integral divisions of this frequency as a result of interference from cardiorespiratory monitors. Single frequency measurements taken at these frequencies may be subject to errors that would be difficult to detect without the aid of information obtained from MF measurements.     

Myocardial electrical activity does not affect myocardial electrical impedance measurements

Objective  Myocardial electrical impedance (MEI) has shown to be an effective indicator of myocardial ischemia. We have previously developed a novel monitor for measuring MEI in near-real time. The object of this study was to test whether drug-induced changes in the frequency of the periodic myocardial electrical activity, as measured by the heart rate (HR), affect MEI measurements made with our monitor. Methods  Thirty dogs were randomly assigned to one of three study groups (placebo, isoproterenol or esmolol) and then anesthetized with sodium thiamylal, intubated, ventilated, given isoflurane, and had venous, arterial, and pulmonary artery catheters placed. Median sternotomy was performed to facilitate myocardial exposure and to allow the left anterior descending (LAD) coronary artery to be isolated. Following baseline measurements, saline (control), isoproterenol or esmolol was administered and the LAD coronary artery was occluded in a timed sequence in order to study the effects of heart rate changes and demonstrate induced myocardial ischemia on MEI. Results  Isoproterenol raised the HR and esmolol lowered the HR without affecting our MEI measurements. Myocardial electrical impedance increased during LAD coronary artery occlusion in all groups, as previously shown. Conclusion  These results demonstrate that our MEI monitor is unaffected by the frequency of the periodic myocardial electrical activity that generates the HR. Dzwonczyk R, del Rio C, McSweeney TD, Zhang X, Howie MB. Myocardial electrical activity does not affect myocardial electrical impedance measurements.     

Diurnal and position-induced variability of impedance cardiography measurements in healthy subjects

Cardiovascular (CV) parameters and their measurements are subject to variation. In this study, we evaluated the reproducibility of impedance cardiography (ICG) measurements following orthostatic and diurnal challenges for a set of 22 CV parameters in ten randomly selected healthy nonpregnant women. A standard protocol was used to record a consecutive series of measurements for each parameter before and after three position changes. This series of measurements was performed twice (AM and PM sessions). For each parameter, measurement-shift following position change was evaluated at 5% cutoff and compared between sessions. Intra- and intersession intraclass correlation (ICC) was calculated for individual measurements per position using repeated-measures analysis of variance. Intra- and intersession Pearson's correlation coefficient (PCC) was calculated for mean values per position. Intersession correlation for measurement-shift following position change was 0·42 (5/12) for pressure parameters, whereas this was 0·96 (52/54) for other parameters. Inter- and intrasession ICC for individual measurements varied between 0·02 and 1 for all parameters, however inter- and intrasession PCC for mean values was consistently >0·80 for stroke volume (SV), stroke index (SI), cardiac output (CO), acceleration and velocity index (ACI and VI), thoracic fluid content (TFC), TFC index (TFCI) and heart period duration (HPD). We conclude that in healthy subjects under standardised conditions, reproducibility of means of multiple ICG measurements is high for SV, SI, CO, ACI, VI, TFC, TFCI and HPD. From our data, we cannot draw conclusions on trends in diseased subjects.     

Filtering artefacts in measurements of forced oscillation respiratory impedance in young children

Respiratory mechanical impedance measured at a single frequency is of great interest to describe change in airways dimensions with time in young children with asthma. Adequate filtering is mandatory in order to eliminate spurious values at rapid flow transient or during glottis closure. The aim of the study was to test a new filtering procedure. Respiratory impedance was measured at 12 Hz using the head generator technique in 33 asthmatic children aged 3.5-7.5 years. Two separate measurements were obtained at baseline and one 10 min after salbutamol inhalation. Assuming corrupted data would correspond to major distortion in flow oscillation, the tested procedure calculated an index expressing departure of each flow oscillation from an ideal sinusoid (flow shape index filter (FSIF)). Cut-offs ranging from 5 to 20% were compared to the visual identification and 'by hand' elimination of corrupted data (visual filtering). Sensitivity and specificity of FSIF versus visual filtering indicated an optimal range of cut-off values between 10 and 15% (FSIF10 and FSIF15). Compared with visual filtering, respiratory conductance in inspiration (Grs) was marginally but significantly lower with FSIF10 (p < 0.01) but not FSIF15 and had slightly larger (within measurement) coefficient of variation with either FSIF10 or FSIF15 (p < 0.01). Between-measurement repeatability of Grs and Grs change induced by salbutamol were similarly estimated by FSIF10, FSIF15 or visual filtering. It is concluded that FSIF appears as a valid alternative to visual filtering. The 10% cut-off is a good compromise considering sensitivity (72%), amount of rejected data (<20%) and effect on Grs and variability.     

Doppler sonography of renal obstruction: value of venous impedance index measurements.

OBJECTIVE: Arterial resistive index values have poor sensitivity and specificity for alterations in renal perfusion induced by collecting system obstruction. We aimed to determine whether the intrarenal venous impedance index values could be useful in evaluating renal parenchymal compliance in cases of obstruction and in differentiating acute obstruction from chronic cases. METHODS: Fifteen patients with acute renal colic having unilateral stone disease and another 15 patients having unilateral chronic obstruction due to various causes were evaluated sonographically. The diagnosis was confirmed either by computed tomography or intravenous urography in all cases. Fifteen subjects with normal kidneys were investigated as a control group. All patients were examined prospectively by conventional and Doppler sonography. The impedance indices and peak flow signals of the interlobar arteries and veins of both kidneys were calculated from spectral Doppler waveforms in all 3 groups. RESULTS: The mean venous impedance index on the acutely obstructed side was lower than the index on the unobstructed side: 0.25 +/- 0.07 and 0.53 +/- 0.3 (mean +/- SD), respectively (P = .005). The mean venous impedance index on the acutely obstructed side was less than the indices both on the chronically obstructed side and in the control subjects (P > .0001). In acute cases, the mean arterial resistive index on the obstructed side was higher than the index on the unobstructed side: 0.62 +/- 0.06 and 0.57 +/- 0.06, respectively (P = .001). No statistically significant difference was detected between other parameters evaluated for the test and control groups. CONCLUSIONS: Renal venous impedance index values may be helpful in evaluating renal hemodynamics in obstruction and in differentiating acute obstruction from chronic cases when used in conjunction with the arterial resistive index.     

Skin impedance measurements for acupuncture research: development of a continuous recording system

Skin impedance at acupuncture points (APs) has been used as a diagnostic/therapeutic aid for more than 50 years. Currently, researchers are evaluating the electrophysiologic properties of APs as a possible means of understanding acupuncture's mechanism. To comprehensively assess the diagnostic, therapeutic and mechanistic implications of acupuncture point skin impedance, a device capable of reliably recording impedances from 100 kΩ to 50 MΩ at multiple APs over extended time periods is needed. This article describes design considerations, development and testing of a single channel skin impedance system (hardware, control software and customized electrodes). The system was tested for accuracy against known resistors and capacitors. Two electrodes (the AMI and the ORI) were compared for reliability of recording over 30 min. Two APs (LU 9 and PC 6) and a nearby non-AP site were measured simultaneously in four individuals for 60 min. Our measurement system performed accurately (within 5%) against known resistors (580 kΩ–10 MΩ) and capacitors (10 nF–150 nF). Both the AMI electrode and the modified ORI electrode recorded skin impedance reliably on the volar surface of the forearm (r = 0.87 and r = 0.79, respectively). In four of four volunteers tested, skin impedance at LU 9 was less than at the nearby non-AP site. In three of four volunteers skin impedance was less at PC 6 than at the nearby non-AP site. We conclude that our system is a suitable device upon which we can develop a fully automated multi-channel device capable of recording skin impedance at multiple APs simultaneously over 24 h.     

Influence of electrode impedance changes on the common-mode rejection ratio in bioimpedance measurements

Bioelectrical impedance in vivo measurements suffer from many potential sources of error due to the patient-instrument interface. The total common-mode rejection ratio (CMRR(T)) was investigated experimentally for three measurement channel circuit versions, including electrode-skin impedance imbalance. The first version was of the 'classical' type. The second one makes use of a differential filter at the input of the instrumentation amplifier. The third circuit was a frequency-converting structure, where the signal was demodulated before being amplified. The differential demodulator was based on synchronous sampling using floating capacitors. The experiments were accomplished with simulated imbalance of the real and imaginary parts of electrode-skin impedances. To reduce unwanted common-mode voltage, a differential accurately balanced current source was used. Considering an application in impedance cardiography, the experiments were carried out at a single frequency of 40 kHz. The results showed the advantage of the circuits using frequency conversion and differential input filter, rendering at least 15 dB higher CMRR(T). The most significant reduction of CMRR(T) resulted from imbalance of the capacitance component of voltage-sensing electrode impedances. The third circuit showed an unexpected behaviour of CMRR(T) improvement with higher imbalance of the electrode-skin impedance resistance component.     

Detection of L-band electron paramagnetic resonance in the DPPH molecule using impedance measurements

Detection of electron paramagnetic resonance (EPR) using a microwave cavity resonating at a fixed frequency (between 9 and 10 GHz) remains the most popular method to date. Here, we report a cavity-less technique which makes use of only an impedance analyzer and a copper strip coil to detect L-band EPR (f = 1–3 GHz) in the standard EPR marker 2,2-diphenyl-1-picrylhydrazyl (DPPH). Our method relies on measuring the magnetoimpedance (MI) response of DPPH through a copper strip coil that encloses DPPH. In contrast to commercial EPR which measures only the field derivative of power absorption, our method enables us to deduce both absorption and dispersion. Changes in resistance (R) and reactance (X) of the copper strip while sweeping an external dc magnetic field, were measured for different frequencies (f = 0.9 to 2.5 GHz) of radio frequency current in the coil. R exhibits a sharp peak at a critical value of the dc magnetic field, which is identified as the resonance field and X shows a dispersion at the same frequency. The data were analyzed to obtain line width and resonance field parameters. The resonance field increased linearly with frequency and the obtained Landé g factor of 1.999 ± 0.0197 is close to the accepted value of 2.0036, measured in the X-band. The simplicity of this technique can be exploited to study paramagnetic centers in catalysis and other materials.

(a) Schematic diagram of our experimental set up. (b) Resistance and reactance of the DPPH molecule for 2 GHz current in the strip coil.