Sensitivity distributions of impedance cardiography using band and spot electrodes analyzed by a three-dimensional computer model

Non-invasive monitoring of cardiac contractility: Trans-radial electrical bioimpedance velocimetry (TREV)

We describe methods and software resources for a bioimpedance measurement technique, 'trans-radial electrical bioimpedance velocimetry' (TREV) that allows for the non-invasive monitoring of relative cardiac contractility and stroke volume. After reviewing the relationship between the measurement and cardiac contractility, we describe the general recording methodology, which requires impedance measurements of the forearm. We provide open-source Jupyter-based software (operable on most computers) for deriving cardiac contractility from the impedance measurements. The software includes tools for removing variance associated with heart rate and respiration. We demonstrate the ability of this bioimpedance measurement for tracking beat-to-beat changes of contractility in a maximal grip force production task. Critically, the results demonstrate both a reactive increase in contractility with force production, and suggest there is a learned increase in contractility prior to grip onset, consistent with anticipatory allostatic autonomic regulation mediated by sympathetic inotropy. The method and software should be of broad utility for investigations of event-related cardiac dynamics in psychophysical studies.

Sensitivity Analysis Highlights the Importance of Accurate Head Models for Electrical Impedance Tomography Monitoring of Intracerebral Hemorrhagic Stroke

OBJECTIVE

Electrical impedance tomography (EIT) has been proposed as a novel tool for diagnosing stroke. However, so far, the clinical feasibility is unresolved. In this study, we aim to investigate the need for accurate head modeling in EIT and how the inhomogeneities of the head contribute to the EIT measurement and affect its feasibility in monitoring the progression of a hemorrhagic stroke.

METHODS

We compared anatomically detailed six- and three-layer finite element models of a human head and computed the resulting scalp electrode potentials and the lead fields of selected electrode configurations. We visualized the resulting EIT measurement sensitivity distributions, computed the scalp electrode potentials, and examined the inverse imaging with selected cases. The effect of accurate tissue geometry and conductivity values on the EIT measurement is assessed with multiple different hemorrhagic perturbation locations and sizes.

RESULTS

Our results show that accurate tissue geometries and conductivity values inside the cranial cavity, especially the highly conductive cerebrospinal fluid, significantly affect EIT measurement sensitivity distribution and measured potentials.

CONCLUSIONS

We can conclude that the three-layer head models commonly used in EIT literature cannot depict the current paths correctly in the head. Thus, our study highlights the need to consider the detailed geometry of the cerebrospinal fluid (CSF) in EIT.

SIGNIFICANCE

The results clearly show that the CSF should be considered in the head EIT calculations.

Fitting the determined impedance in the guinea pig inner ear to randles circuit using square error minimization in the range of 100 Hz to 50 kHz

OBJECTIVE

A number of lumped and distributed parameter models of the inner ear have been proposed in order to improve the vestibular implant stimulation. The models should account for all significant physical phenomena influencing the current propagation: electrical double layer (EDL) and medium polarization. The electrical properties of the medium are reflected in the electrical impedance, therefore the aim of this study was to measure the impedance in the guinea pig inner ear and construct its equivalent circuit.

APPROACH

The electrical impedance was measured from 100 Hz to 50 kHz between a pair of platinum electrodes immersed in saline solution using sinusoidal voltage signals. The Randles circuit was fitted to the measured impedance in the saline solution in order to estimate the EDL parameters (C, W, and R_ct) of the electrode interface in saline. Then, the electrical impedance was measured between all combinations of the electrodes located in semicircular canal ampullae and the vestibular nerve in the guinea pig in vitro. The extended Randles circuit considering the medium polarization (R_i, R_e, C_m) together with EDL parameters (C, R_ct) obtained from the saline solution was fitted to the measured impedance of the guinea pig inner ear. The Warburg element was assumed negligible and was not considered in the guinea pig model.

MAIN RESULTS

For the set-up used, the obtained EDL parameters were: C=27.09*10^-8F, R_ct=18.75 kΩ. The average values of intra-, extracellular resistances, and membrane capacitance were R_i=4.74 kΩ, R_e=45.05 kΩ, C_m=9.69*10^-8F, respectively.

SIGNIFICANCE

The obtained values of the model parameters can serve as a good estimation of the EDL for modelling work. The EDL, together with medium polarization, plays a significant role in the electrical impedance of the guinea pig inner ear, therefore, they should be considered in electrical conductivity models to increase the credibility of the simulations.

Modified electrode placements for measurement of hemodynamic parameters using impedance cardiography

Measuring and monitoring hemodynamic parameters has brought many benefits in supporting diagnosis and treatment for cardiovascular patients. There are many advantages to measuring hemodynamic parameters by non-invasive technique based on impedance cardiography (ICG) such as simplicity, real-time and low cost. However, the electrode positions of this method are very difficult to implement in cases where the patient has to use multiple medical devices at the same time, especially for patients on active treatment and resuscitation. This paper presents the results of the study proposing new three locations of ICG electrodes to overcome the above limitation. Accordingly, we measured and evaluated 10 volunteers on the Niccomo device. The results show that all three positions of proposed electrode can be used to replace standard electrode position. In particular, the 1st proposed position, can be used to measure all five hemodynamic parameters HR, SV, LVET, Z_o, CO and ICG waveforms, expressed by the average correlation and the relative average difference of five parameters, R 2 ¯ = 0.9641 and Mean ¯ = 3.31%. The 2nd proposed position can be used to measure four parameters HR, SV, LVET, CO and ICG waveforms shown by R 2 ¯ = 0.9091 and Mean ¯ = 8.67%. The 3rd proposed position can be used to measure three parameters HR, Z_o and LVET, expressed by R 2 ¯ = 0.8485 and Mean ¯ = 9.26%.

Surface Potential Simulation for Robust Electrode Placement by MRI Based Human Phantom with FEM Based Quasi-Static Solver for Bioimpedance Measurement

Estimating cardiac output (CO) and thoracic fluid content (TFC) by non-invasive measurement of thoracic electrical bioimpedance (TEB) is on the rise of becoming clinically established. Dynamic fluid management and detecting and trending excess quasi-static thoracic fluids are of particular interest to benefit the critically ill patient. While the advantages such as easy application and non-invasive assessment are intriguing, there are some challenges. In addition to artifacts due to the patient's movement, instability of the electrode-skin interfaces, the accuracy of the applied current and varying cable capacity because of motion, exact placement of electrodes - or lack thereof - must be considered. In particular the robustness of the electrode placement, i.e., the insensitivity to inaccuracy of electrode placement (actual sensor from the nominal). We propose a new technique for evaluation of electrode placement by simulation with MRI based human phantoms with FEM based quasi-static solver for bioimpedance measurement. Results: We identified alternative electrode positions which lead to an increase in robustness of bioimpedance measurements of up to 9.5 times compared to the standard electrode placement. Simulations suggested an improvement in ECG signal quality which was confirmed by a subject measurement.

Evaluation of Electrode Setups by MRI Based Human Phantom with FEM Based Quasi-Static Solver for Bioimpedance Measurement. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2019;2019:3978-3982. [PMID: 31946743 DOI: 10.1109/embc.2019.8856693]

Transthoracic electrical bioimpedance (TEB) measurement for acquiring of hemodynamic parameters e.g. stroke volume (SV) and cardiac output (CO) becomes commonly used. For precise and reliable measurement, a good understanding of the measurement system is needed to provide robust electrode placement, reproducible results, and large signal amplitudes. We propose an evaluation by MRI based human phantom with FEM based quasi-static solver for electrode placement in bioimpedance measurement. Placements according to Osypka, Cheetah and Bernstein et al. were simulated and compared with measurements taken from a (real) human subject. As a parameter for evaluation of signal quality, the percentage of current passing through the descending aorta from the overall injected current is measured. The placement according to Osypka results in 1.46 % of current flow through the descending aorta (Cheetah placement 1.12 %, Bernstein et al. placement 0.877 %) for the male human phantom DUKE. The simulation was compared with a real human subject (with comparable age, height, weight) by calculating the baseline impedance (Z₀). The simulation seems to fit at best with Osypka placement, where the deviation between simulated baseline impedance and real human subject is 19.9 % (deviation for Cheetah 34.1 %, deviation for Bernstein 62.5 %). The simulation with MRI based human phantoms seems to be a very good basis for further investigations of current injection and bioimpedance measurement comprehension.

Analysis of Consistency of Transthoracic Bioimpedance Measurements Acquired with Dry Carbon Black PDMS Electrodes, Adhesive Electrodes, and Wet Textile Electrodes

The detection of intrathoracic volume retention could be crucial to the early detection of decompensated heart failure (HF). Transthoracic Bioimpedance (TBI) measurement is an indirect, promising approach to assessing intrathoracic fluid volume. Gel-based adhesive electrodes can produce skin irritation, as the patient needs to place them daily in the same spots. Textile electrodes can reduce skin irritation; however, they inconveniently require wetting before each use and provide poor adherence to the skin. Previously, we developed waterproof reusable dry carbon black polydimethylsiloxane (CB/PDMS) electrodes that exhibited a good response to motion artifacts. We examined whether these CB/PDMS electrodes were suitable sensing components to be embedded into a monitoring vest for measuring TBI and the electrocardiogram (ECG). We recruited N = 20 subjects to collect TBI and ECG data. The TBI parameters were different between the various types of electrodes. Inter-subject variability for copper-mesh CB/PDMS electrodes and Ag/AgCl electrodes was lower compared to textile electrodes, and the intra-subject variability was similar between the copper-mesh CB/PDMS and Ag/AgCl. We concluded that the copper mesh CB/PDMS (CM/CB/PDMS) electrodes are a suitable alternative for textile electrodes for TBI measurements, but with the benefit of better skin adherence and without the requirement of wetting the electrodes, which can often be forgotten by the stressed HF subjects.

Do mathematical model studies settle the controversy on the origin of cardiac synchronous trans-thoracic electrical impedance variations? A systematic review

Impedance cardiography (ICG) is a method to evaluate cardiac-stroke volume and cardiac-output by measuring the cardiac-synchronous changes in the dynamic trans-thoracic electrical impedance (ΔZ). Clinical evaluations on the accuracy of ICG showed varying results. Consequently, the classic assumption in ICG-the aorta as a main source of ΔZ-is questioned and subsequently investigated in simulation studies using mathematical models of the electrical resistivity of the human body. The aim is to review the consensus in mathematical modelling studies that investigate the origin of the ΔZ as measured in ICG. In a systematic literature search, studies were identified and surveyed with reference to characteristics, such as included organs and their resistivity and geometries, electrode positions and calculation of ΔZ, to review the consensus between mathematical modelling studies that investigate the origin of the ΔZ as measured in ICG. Thirteen papers showed considerable variation in the model's characteristics with varying or contradicting outcomes for the ΔZ 's origin. For instance, 11 studies excluded perfused muscle tissue, implying implicitly their insignificance, while 3 other studies included muscle tissue and indicated it as the most important origin of ΔZ. In conclusion, the reviewed papers show a lack of consensus with respect to both the modelled characteristics as well as the model outcomes and, as a result, these studies failed to settle the controversy on ΔZ 's origin. Recommendations have been added to improve future mathematical model studies.

Development of an Anatomically Realistic Forward Solver for Thoracic Electrical Impedance Tomography

Electrical impedance tomography (EIT) has the potential to provide a low cost and safe imaging modality for clinically monitoring patients being treated with mechanical ventilation. Variations in reconstruction algorithms at different clinical settings, however, make interpretation of regional ventilation across institutions difficult, presenting the need for a unified algorithm for thoracic EIT reconstruction. Development of such a consensual reconstruction algorithm necessitates a forward model capable of predicting surface impedance measurements as well as electric fields in the interior of the modeled thoracic volume. In this paper, we present an anatomically realistic forward solver for thoracic EIT that was built based on high resolution MR image data of a representative adult. Accuracy assessment of the developed forward solver in predicting surface impedance measurements by comparing the predicted and observed impedance measurements shows that the relative error is within the order of 5%, demonstrating the ability of the presented forward solver in generating high-fidelity surface thoracic impedance data for thoracic EIT algorithm development and evaluation.

ZMIND - an interactive environment for electrical modeling of the thorax

Interest in the electrical modeling of the thorax is motivated by various desires ranging from determining cardiac function, optimizing defibrillation efficacy, monitoring pulmonary edema, etc. However, existing models represent the thorax with rather coarse anatomical details, limiting their utilizations for accurately simulating small electrodes which typically occurs in pacing and defibrillation clinical practices. In this paper, we describe an anatomically realistic finite difference modeling software environment, referred as ZMIND. Segmented image-based finite difference models of a male adult at the end of systole and the end of diastole were constructed based on ECG-gated MRI scans. Up to 36 types of tissues were identified and included in the model, providing fine anatomical details in the heart and lung regions. The environment enables placing electrodes interactively and also provides a library of clinically-based, user-configurable electrodes. The analysis module of this environment allows performing sensitivity analysis and visualizing the computed electric fields, current density, and sensitivity distribution.

Computational modelling of blood-flow-induced changes in blood electrical conductivity and its contribution to the impedance cardiogram

Studies have shown that blood-flow-induced change in electrical conductivity is of equal importance in assessment of the impedance cardiogram (ICG) as are volumetric changes attributed to the motion of heart, lungs and blood vessels. To better understand the sole effect of time-varying blood conductivity on the spatiotemporal distribution of trans-thoracic electric fields (i.e. ICG), this paper presents a segmented high-resolution (1 mm(3)) thoracic cardiovascular system, in which the time-varying pressures, flows and electrical conductivities of blood in different vessels are evaluated using a set of coupled nonlinear differential equations, red blood cell orientation and cardiac cycle functions. Electric field and voltage simulations are performed using two and four electrode configurations delivering a small alternating electric current to an anatomically realistic and electrically accurate model of modelled human torso. The simulations provide a three-dimensional electric field distribution and show that the time-varying blood conductivity alters the voltage potential difference between the electrodes by a maximum of 0.28% for a cardiac output of about 5 L min(-1). As part of a larger study, it is hoped that this initial model will be useful in providing improved insights into blood-flow-related spatiotemporal electric field variations and assist in the optimal placement of electrodes in impedance cardiography experiments.

An optimal spot-electrodes array for electrical impedance cardiography through determination of impedance mapping of a regional area along the medial line on the thorax

Electrical impedance or admittance cardiography is a simple method for non-invasive, continuous measurement the stroke volume and cardiac output. For the electrical impedance cardiography, the band-electrodes array proposed by Kubicek et al has been widely used, and various spot-electrodes array have been experimented in search of a less uncomfortable and equally reliable electrodes array that is easier to attach. From the uniformity of current distribution on the thorax, we have reinvestigated focusing on the measurement of contour maps of static and pulsatile components of a regional area along the medial line on the frontal part of the thorax. Consequently, the appropriate electrodes locations for current injection were determined as the back of an ear and on the lower abdomen, while those for voltage pick-up was on the medial portion at the level of clavicle and on the portion above the xiphisternum. Preliminary comparison experiments between the cardiac output values obtained by the electrical impedance cardiography and by a pulse dye-densitometry showed a fairy good agreement.

Assessment of stroke volume index with three different bioimpedance algorithms: lack of agreement compared to thermodilution

OBJECTIVE

The accuracy of bioimpedance stroke volume index (SVI) is questionable as studies report inconsistent results. It remains unclear whether the algorithms alone are responsible for these findings. We analyzed the raw impedance data with three algorithms and compared bioimpedance SVI to transpulmonary thermodilution (SVI(TD)).

DESIGN AND SETTING

Prospective observational clinical study in a university hospital.

PATIENTS

Twenty adult patients scheduled for coronary artery bypass grafting (CABG).

INTERVENTIONS

SVI(TD) and bioimpedance parameters were simultaneously obtained before surgery (t1), after bypass (t2), after sternal closure (t3), at the intensive care unit (t4), at normothermia (t5), after extubation (t6) and before discharge (t7). Bioimpedance data were analyzed off-line using cylinder (Kubicek: SVI(K); Wang: SVI(W)) and truncated cone based algorithms (Sramek-Bernstein: SVI(SB)).

MEASUREMENTS AND RESULTS

Bias and precision between the SVI(TD) and SVI(K), SVI(SB), and SVI(W) was 1.0+/-10.8, 9.8+/-11.4, and -15.7+/-8.2 ml/m2 respectively, while the mean error was abundantly above 30%. Analysis of data per time moment resulted in a mean error above 30%, except for SVI(W) at t2 (28%).

CONCLUSIONS

Estimation of SVI by cylinder or truncated cone based algorithms is not reliable for clinical decision making in patients undergoing CABG surgery. A more robust approach for estimating bioimpedance based SVI may exclude inconsistencies in the underlying algorithms in existing thoracic bioimpedance cardiography devices.

Experimental and numerical study on optimal spot-electrodes arrays in transthoracic electrical impedance cardiography

Transthoracic electrical impedance (or admittance) cardiography is a simple technique for the non-invasive and continuous monitoring of stroke volume or cardiac output by detecting the electrical impedance of a thorax which is roughly assumed to be a two-compartment coaxial cylindrical model composed of the aorta and its surrounding thoracic tissues. A tetra-polar band-electrodes method by Kubicek et al. has been widely used for the detection of the electrical impedance. However, this band-electrodes attachment makes a subject troublesome. Replacement of the band- to a spot-electrodes array is therefore highly required for practical use. In our previous reports, we have confirmed that a thorax is nearly assumed as an electrical cylinder model when a pair of current injection spot-electrodes are placed far from the thorax like a placement of the forehead - left medial knee or the mastoid process - lower right abdomen, and that the changes in current distributions associated with cardiac blood ejection are roughly homogeneous around the medial line of the thorax. The present study concerns with determination of an optimal spot-electrodes array for voltage pick-up through the detailed measurements of pulsatile components of the thoracic impedance along the medial line of the thorax using an 11x2 channels impedance mapping system. Additionally, we have investigated the influence of blood volume change in the heart itself by a finite element method.

Impedance cardiography revisited

Previously reported comparisons between cardiac output (CO) results in patients with cardiac conditions measured by thoracic impedance cardiography (TIC) versus thermodilution (TD) reveal upper and lower limits of agreement with two standard deviations (2SD) of approximately +/-2.2 l min(-1), a 44% disparity between the two technologies. We show here that if the electrodes are placed on one wrist and on a contralateral ankle instead of on the chest, a configuration designated as regional impedance cardiography (RIC), the 2SD limit of agreement between RIC and TD is +/-1.0 l min(-1), approximately 20% disparity between the two methods. To compare the performances of the TIC and RIC algorithms, the raw data of peripheral impedance changes yielded by RIC in 43 cardiac patients were used here for software processing and calculating the CO with the TIC algorithm. The 2SD between the TIC and TD was +/-1.7 l min(-1), and after annexing the correcting factors of the RIC formula to the TIC formula, the disparity between TIC and TD further declined to +/-1.25 l min(-1).

CONCLUSIONS

(1) in cardiac conditions, the RIC technology is twice as accurate as TIC; (2) the advantage of RIC is the use of peripheral rather than thoracic impedance signals, supported by correcting factors.

Maternal hemodynamics during cesarean delivery assessed by whole-body impedance cardiography

BACKGROUND

This descriptive study was designed to evaluate maternal hemodynamics and cardiovascular responses to delivery during cesarean section (CS) under spinal anesthesia. We also assessed the feasibility of a noninvasive and continuous method of measuring cardiac output, namely whole-body impedance cardiography (ICG(WB)), during elective CS. Because of the techniques used in previous studies, only fractionated data on maternal hemodynamics during CS are available to date.

METHODS

We studied 10 healthy women with normal pregnancies and two pregnant women with heart disease undergoing elective CS. Mean arterial pressure (MAP), heart rate (HR), stroke index (SI), cardiac index (CI) and systemic vascular resistance index (SVRI) were recorded continuously during CS, during the period of dissipation of anesthesia and on the second to fifth postpartum day. Analysis of variance for repeated measurements (anova) and the paired sample t-test were used in statistical analysis.

RESULTS

The hemodynamic parameters could be registered continuously during the whole procedure. At the point of delivery, a 47% increase in CI and a 39% decrease in SVRI were recorded, while MAP remained stable. These changes occurred within 2 min of delivery of the newborn and persisted on average for 10 min.

CONCLUSION

Sudden and significant hemodynamic changes take place at the moment of delivery. Intact physiological cardiovascular compensation mechanisms are needed to adapt to these challenges. Whole-body impedance cardiography may offer a useful noninvasive tool to monitor hemodynamics during cesarean section.

Monitoring lung edema using the pacemaker pulse and skin electrodes

Previous clinical studies have shown that impedance measurements using right ventricular (RV) leads can monitor congestion due to heart failure. We previously reported on a three-fold advantage of bipolar left ventricular (LV) leads, which are near the lung, over RV leads in detecting pulmonary edema with impedance. A combined system of internal and external electrodes is now investigated using computer models, for use with conventional cardiac resynchronization (CRT) systems with unipolar LV leads. The system uses the normal LV pacing pulse as current source, and the resultant voltage at two skin electrodes to obtain a lung edema impedance (Z) measurement. Using gated MRIs, thoracic computer models of 3.8 million control volumes were constructed. Changes of Z with edema were simulated with a conventional totally implanted system, as well as with combined implanted-external systems. Right atrial (RA), RV, RV defibrillator coil and LV leads were used. Per cent Z responses to edema were compared. The all implanted responses were RA: 11.8%, RV: 8.6%, RVcoil: 11.3%, LV: 23.8%. The combined system responses were LV-ext: 21.45%, RA-ext: 10.13%, LV-arm leg: 26.08%. The computer models suggest that combined internal-external systems can be as sensitive as the totally implanted ones. Lung edema may be monitored at follow up or home for LV paced patients with only two external electrodes. Using very low impedance configurations optimized by computer can greatly maximize the response, with a cost of poor stability.

Design of electrode array for impedance measurement of lesions in arteries

Use of impedance catheters can provide additional information about the composition and the morphology of early plaques in arteries. However, for a correct interpretation of the impedance data recorded inside a vessel, the extra-vessel conditions should not influence the measurement results. In this paper, we estimate the influence of the extra-vessel conditions on the impedance measurement of a vessel wall by using FEM simulation and a two-layer model. Therefore sensitivity fields are simulated. The simulations are validated by experiments and compared to analytical solutions. Further, the influence of the inner radius of a vessel on the measurement result is determined by FEM simulations. From experiments based on the two-layer model, it is found that the apparent resistance depends on the thickness of the first layer and the separation distance of the electrode structure. The measured result corresponds to the results of the FEM simulations, whereas the analytical solution assuming point electrodes is different from the measurement and simulation results. Under the assumption of homogenous and linear volume conductors, the FEM simulated distributions of sensitivity fields are determined. The inner diameter of the artery has no influence on the measurement results. The FEM simulation can support the design of electrode configuration and geometries for impedance catheters.

Improved lung edema monitoring with coronary vein pacing leads: a simulation study

This computer simulation study compared the ability of left ventricular coronary vein (LV) pacemaker leads against right ventricular (RV) and right atrial (RA) leads to monitor lung edema using electrical impedance measurements. MRI images were used to construct electrical models of the thorax. Four lead configurations were tested with increases of pulmonary edema, intravascular fluids and heart dilation. The impedance changes observed at end systole with severe lung edema were 8.5%, 11.2%, 12.3% and 26.8% for the RA, RV, RV coil and LV configurations, respectively. Sensitivities in ohms per litre of lung fluid were 19.15, 19.15, 25.07 and 52.11 for the same configurations. The impedance changes for intravascular fluid overload with constant lung status were 1%, 1.3%, 9.2% and 6.4% while the sensitivities were 2, 2, 17 and 11 ohms per litre of intravascular fluid, respectively. Regional analysis of the thoracic sources of impedance revealed a high sensitivity near pacing electrodes and generator, and a low sensitivity to the right lung and all pulmonary vessels. Simulations showed that LV leads have a threefold advantage in sensitivity when monitoring lung edema in comparison to conventional RV leads. To monitor vascular and lung fluids independently, combined impedance configurations may be used. Regional sensitivities must be taken into account for proper clinical interpretation of impedance changes.

Imaging of thoracic blood volume changes during the heart cycle with electrical impedance using a linear spot-electrode array

Electrical impedance (EI) measurements conducted on the thorax contain useful information about the changes in blood volume that occur in the thorax during the heart cycle. The aim of this paper is to present a new (tomographic-like) method to obtain this relevant information with electrical impedance measurements, using a linear electrode array. This method is tested on three subjects and the results are compared with results, obtained from magnetic resonance cine-images showing the cross-sectional surface area changes of the aorta, the vena cava, the carotid arteries, and the heart. This paper shows that the different sources of the thoracic EI waveform may be separated in time and location on the thoracic surface and that aortic volume changes may be estimated accurately.

Comparing spot electrode arrangements for electric impedance cardiography

This study investigates whether an arrangement with nine spot electrodes, for thoracic bio-impedance cardiography, can be replaced by an arrangement with five spot electrodes. The study was conducted on 15 healthy subjects, six females and nine males, in supine rest. The variables obtained from the measurements were the mean of the impedance of the thorax segment between the recording electrodes, the maximum negative deflection of the first derivative of the thoracic impedance, the left ventricular ejection time and an estimate of left ventricular stroke volume. An analysis of variance for a randomized complete block design was used to determine whether significant differences exist in the group means of the observed variables between six different electrode arrangements. If no statistically significant differences were found in these group means between pairs of arrangements, Bland and Altman analyses were used to determine the differences in the observed variables between pairs of arrangements for individual subjects. This study concludes that reducing the number of spot electrodes from nine to five, does not yield significant differences in the group means of the observed variables, but it could result in large differences in the values of these variables for individual subjects.

Correlation of esophageal conductance measurements with aortic and left ventricular diameters and stroke volume

Esophageal conductance measurements were correlated with hemodynamic events in 9 dogs chronically instrumented for measurement of left ventricular (LV) and aortic pressures, LV short axis and descending aortic diameters, and aortic blood flow. A four-electrode conductance catheter was positioned in the esophagus. Both an internal and an internal/external configuration were examined during anesthesia with hemodilution, pulmonary lavage and dobutamine infusion. LV stroke volume was altered by caval occlusion at each intervention. Stroke conductance was highly correlated to aortic or LV diameters and stroke volume over a range of diameters depending on the electrode configuration. Esophageal conductance measurements are directly influenced by local hemodynamic events adjacent to the site of measurement.

Segmental composition of whole-body impedance cardiogram estimated by computer simulations and clinical experiments

Whole-body impedance cardiography (ICGWB) has been proposed as a feasible means of measuring cardiac output (CO). However, the source distribution of heart-related impedance variations in the whole body is not known. To establish how much of a signal originates in each segment of the body and what the contribution of each is to stroke volume (SV) in ICGWB, impedance in the extremities and trunk were investigated in 15 healthy volunteers. In addition, the theoretical measurement properties of ICGWB were studied using a computer model of the whole-body anatomy as a volume conductor. The model confirmed the expected result that most of the basal impedance originates from the extremities. Clinical experiments revealed that the heart-related amplitude variations in the ICGWB signal originate more evenly from various body segments, the trunk slightly more than the arms or legs. The heart-related ICGWB signal represents a weighted sum of segmental pulsatile events in the body yielding physiologically meaningful data on almost the whole circulatory system.

The electric resistivity of human tissues (100 Hz-10 MHz): a meta-analysis of review studies

The electric resistivity of various human tissues has been reported in many studies, but on comparison large differences appear between these studies. The aim of this study was to investigate systematically the resistivities of human tissues as published in review studies (100 Hz-10 MHz). A data set of 103 resistivities for 21 different human tissues was compiled from six review studies. For each kind of tissue the mean and its 95% confidence interval were calculated. Moreover, an analysis of covariance showed that the calculated means were not statistically different for most tissues, namely skeletal (171 omega cm) and cardiac (175 omega cm) muscle, kidney (211 omega cm), liver (342 omega cm), lung (157 omega cm) and spleen (405 omega cm), with bone (> 17,583 omega cm), fat (3,850 omega cm) and, most likely, the stratum corneum of the skin having higher resistivities. The insignificance of differences between various tissue means could imply an equality of their resistivities, or, alternatively, could be the result of the large confidence intervals which obscured real existing differences. In either case, however, the large 95% confidence intervals reflected large uncertainties in our knowledge of resistivities of human tissues. Applications based on these resistivities in bioimpedance methods, EEG and EKG, should be developed and evaluated with these uncertainties in mind.

Application of computer modelling and lead field theory in developing multiple aimed impedance cardiography measurements

Conventional impedance cardiography (ICG) methods estimate parameters related to the function of the heart from a single waveform that reflects an integrated combination of complex sources. We have previously developed methods and tools for calculating measurement sensitivity distributions of ICG electrode configurations. In this study, the methods were applied to investigate the prospects of recording multiple aimed ICG waveforms utilizing the 12-lead electrocardiography (ECG) electrode locations. Three anatomically realistic volume conductor models were used: one based on Visible Human Man cryosection data and two on magnetic resonance (MR) images representing end diastolic and end systolic phases of the cardiac cycle. Based on the sensitivity distributions obtained, 236 electrode configurations were selected for preliminary clinical examination on 12 healthy volunteers and 9 valvular patients. The model study suggested that a variety of configurations had clearly enhanced sensitivity to the cardiovascular structures as compared to conventional ICGs. Simulation data and clinical experiments showed logical correspondence supporting the theoretically predicted differences between the configurations. Recorded 12-lead ICG signals had characteristic waveforms and landmarks not coinciding with those of conventional ICG. Furthermore, configurations showing resemblance to invasive data and morphological variations in disease are of interest. The results indicate the applicability of the modelling approach in developing ICG measurement configurations. However, the level of clinical relevance and potential of the 12-lead method remains to be explored in studies employing dynamic modelling and acquisition of invasive reference data.

Multiple lead recordings improve accuracy of bio-impedance plethysmographic technique

We have developed the theory and instrumentation of multiple multi-electrode bio-impedance (BI) measurements based on lead field theoretical approach. To derive reliable information based on BI data, a quantity of measurements should be taken with electrode configurations possessing regional measurement sensitivity. An apparatus has been developed with an eye to the requirements imposed by the theoretical aspects of achieving multiple multi-electrode BI measurements. It has features compensating electrode-contact related errors and errors due to imbalance between the conductive pathways when multiple electrodes are utilised for BI measurement. The proposed design allows simultaneous multi-electrode BI and bioelectric recording with the same electrode system. Initial operation experiences in clinical environment indicate that the device functions as intended, and allows user-friendly utilisation of multiple BI measurements. Contributions presented to BI methodology and instrumentation improve the reliability of BI measurements.

Non-invasive cardiac output determination: state of the art

This review deals with recent developments in non-invasive cardiac output measurement. In the past few years significant progress has been made with semi-invasive transoesophageal echocardiography; the method now provides advanced facilities to measure cardiac output and other important characteristics of cardiac function. The method is, however, operator-dependent and the equipment used is expensive, which means that large-scale use on intensive care patients is not feasible. Whole-body impedance cardiography has recently shown good accuracy and flexibility in use, and seems to be the most promising method for the non-invasive measurement of cardiac output.