Impedance Cardiography Signals Research Articles

Textrode-enabled transthoracic electrical bioimpedance measurements – towards wearable applications of impedance cardiography

Abstract During the last decades the use of electrical bioimpedance (EBI) in the medical field has been the subject of extensive research, as an affordable, harmless and non-invasive technology. In some specific applications, such as body composition assessment where EBI has proven a good degree of effectiveness and reliability, the use of textile electrodes and measurement garments have shown good performance and reproducible results. Impedance cardiography (ICG) is a modality of EBI that can benefit from the implementation and use of wearable sensors. ICG is based on continuous impedance measurements of a longitudinal segment across the thorax taken at a single frequency. The need for a specific electrode placement on the thorax and neck can be easily ensured with the use of a garment with embedded textile electrodes, also known as “textrodes.” The first step towards the implementation of garment-based ICG is to determine the quality of ICG measurements with textile sensors to allow estimation of fundamental ICG parameters. In this work, the measurement performance of a 2-belt set with incorporated textrodes for thorax and neck is compared against ICG measurements obtained with Ag/AgCl electrodes. The analysis is based on the quality of the fundamental ICG signals (ΔZ, dZ/dt and ECG), systolic time intervals and other ICG parameters. The results indicate the feasibility of using textrodes for ICG measurements with consistent measurements and relatively low data dispersion. Thus, enabling the development of measuring garments for ICG measurements.

Detecting variations of blood volume shift due to heart beat from respiratory inductive plethysmography measurements in man

The simultaneous study of the cardiac and respiratory activities and their interactions is of great physiological and clinical interest. For this purpose, we want to investigate if respiratory inductive plethysmography (RIP) can be used for cardiac functional exploration. We propose a system, based on RIP technology and time-scale approaches of signal processing, for the extraction of cardiac information. This study focuses on the monitoring of blood volume shift due to heart beat, noted ▵Vtr_c and investigates RIP for the detection of ▵Vtr_c variations by comparison to stroke volume (SV) variations estimated by impedance cardiography (IMP). We proposed a specific respiratory protocol assumed to induce significant variations of the SV. Fifteen healthy volunteers in the seated and supine positions were asked to alternate rest respiration and maneuvers, consisting in blowing into a manometer. A multi-step treatment including a variant of empirical mode decomposition was applied on RIP signals to extract cardiac volume signals and estimate beat-to-beat ▵Vtr_c. These were averaged in quasi-stationary states at rest and during the respiratory maneuvers, and analysed in view of SV estimations from IMP signals simultaneously acquired. Correlation and statistical tests over the data show that RIP can be used to detect variations of the cardiac blood shift in healthy young subjects.

Simulation of 'pathologic' changes in ICG waveforms resulting from superposition of the 'preejection' and ejection waves induced by left ventricular contraction

The impedance cardiography (ICG) is widely used for beat-to-beat noninvasive evaluation of the left ventricular stroke volume and contractility. It implies the correct determination of the ejection start and end points and the amplitudes of certain peaks in the differentiated impedance cardiogram. An accurate identification of ejection onset by ICG is often problematic, especially in the cardiologic patients, due to peculiar waveforms. Using a simple theoretical model, we tested the hypothesis that two major processes are responsible for the formation of impedance systolic wave: (1) the changes in the heart geometry and surrounding vessels produced by ventricular contraction, which occur during the isovolumic phase and precede ejection, and (2) expansion of aorta and adjacent arteries during the ejection phase. The former process initiates the preejection wave WpE and the latter triggers the ejection wave WEj. The model predicts a potential mechanism of generating the abnormal shapes of dZ/dt due to the presence of preejection waves and explains the related errors in ICG time and amplitude parameters. An appropriate decomposition method is a promising way to avoid the masking effects of these waves and a further step to correct determination of the onset of ejection and the corresponding peak amplitudes from 'pathologically shaped' ICG signals.

High spatial and temporal resolution 4D FEM simulation of the thoracic bioimpedance using MRI scans

In this work, a finite element model was created using MRI scans of the main author to analyze sources of the dynamic thoracic bioimpedance. This model can be used to identify limitations of impedance cardiography (ICG) in practice. Heart beat (8.3 ms temporal resolution) and aortic wave propagation (2.6 ms temporal resolution) were implemented. The static volume contains all major organs of the thorax in high spatial resolution. Simulations were successfully conducted and a high correlation (r = 0.9) between the simulated aortic ICG signal and a measured signal of the same subject was obtained.

Person perception and autonomic nervous system response: The costs and benefits of possessing a high social status

This research was designed to investigate the relationship between sympathetic and parasympathetic autonomic nervous system (ANS) responses to the perception of social targets varying in social status. Participants varying in subjective financial status were presented with faces assigned with either a low, average, or high financial status. Electrocardiographic and impedance cardiography signals were recorded and measures of sympathetic (pre-ejection period; PEP) and parasympathetic (high frequency heart rate variability; HF HRV) cardiac control were derived. These measures associated with the presentation of each face condition were examined in relation to the subjective status of the perceivers. Participants with high subjective financial status showed reduced sympathetic activity when viewing low- and medium-status targets as compared to high-status targets, and lower parasympathetic response when viewing high- and medium-status targets relative to low-status targets.

Arterial tension time reflects subclinical atherosclerosis, arterial stiffness and stroke volume

To introduce and evaluate a new haemodynamic parameter known as arterial tension time (ATT) and study whether ATT is associated with traditional cardiovascular risk factors as well as with indices of arterial stiffness, cardiac pump function and subclinical atherosclerosis. Arterial tension time was measured from the whole-body impedance cardiography (ICG) signal and defined as the time difference between the onset of arterial distension induced by stroke volume (SV) and maximal integrated arterial distension. As measures of subclinical atherosclerosis and arterial stiffness, carotid artery intima-media thickness (IMT), Young's elastic modulus (YEM), arterial stiffness index (ASI) and carotid artery compliance (CAC) were assessed with ultrasound in 336 Finnish adults (aged 46-76 years, 43·2% men) participating in the Health 2000 Survey. In addition, pulse wave velocity (PWV) and stroke volume index (SI), as indices of arterial stiffness and cardiac pump function, were assessed with ICG. Arterial tension time was associated inversely with PWV, IMT, YEM and ASI (P<0·002 for all) and directly with SI and CAC (P<0·001 for both). Age, systolic blood pressure, diastolic blood pressure and fasting glucose were independent determinants of decreased ATT (P<0·04 for all). Moreover, accumulation of cardiovascular risk factors was associated with the decrease in ATT (P for trend <0·001). Decreased ATT was associated with increased arterial stiffness, increased subclinical atherosclerosis and decreased SV. Current results suggest that ATT provides simultaneous information on several aspects of cardiovascular structure and function and could possibly serve as a new integrated parameter for cardiovascular risk stratification.

Hemodynamics assessed via approximate entropy analysis of impedance cardiography time series: effect of metabolic syndrome

The metabolic syndrome (MS), a predisposing condition for cardiovascular disease, presents disturbances in hemodynamics; impedance cardiography (ICG) can assess these alterations. In subjects with MS, the morphology of the pulses present in the ICG time series is more irregular/complex than in normal subjects. Therefore, the aim of the present study was to quantitatively assess the complexity of ICG times series in 53 patients, with or without MS, through a nonlinear analysis algorithm, the approximate entropy, a method employed in recent years for the study of several biological signals, which provides a scalar index, ApEn. We correlated ApEn computed from ICG times series data during fasting and postprandial phase with the presence of alterations in the parameters defining MS [Adult Treatment Panel (ATP) III (Grundy SM, Brewer HB Jr, Cleeman JI, Smith SC Jr, Lenfant C; National Heart, Lung, and Blood Institute; American Heart Association. Circulation 109: 433-438, 2004) and the International Diabetes Federation (IDF) definition]. Results show that ApEn was significantly higher in subjects with MS compared with those without (1.81 ± 0.09 vs. 1.65 ± 0.13; means ± SD; P = 0.0013, with ATP III definition; 1.82 ± 0.09 vs. 1.67 ± 0.12; P = 0.00006, with the IDF definition). We also demonstrated that ApEn increase parallels the number of components of MS. ApEn was then correlated to each MS component: mean ApEn values of subjects belonging to the first and fourth quartiles of the distribution of MS parameters were statistically different for all parameters but HDL cholesterol. No difference was observed between ApEn values evaluated in fasting and postprandial states. In conclusion, we identified that MS is characterized by an increased complexity of ICG signals: this may have a prognostic relevance in subjects with this condition.

Detection of left ventricular systolic dysfunction using a newly developed, laptop based, impedance cardiographic index

Congestive heart failure (CHF) is a major cause of morbidity and mortality worldwide [1]. The development of left ventricular systolic dysfunction (LVSD) is a marker of poor prognosis. Mild reduction in EF progresses with time and when EF gets to b40%, most patients develop CHF and their prognosis is dismal [2,3]. Simple, cheap techniques that will reliably detect mild reduction of the EF, typical for ALVSD,will thus be of enormous value to reduce the occurrence of CHF and cardiac mortality [3–7]. Impedance Cardiography (ICG) is a noninvasive method of determining hemodynamic status and reliable estimates of myocardial contractility (and EF) can be obtained using indices based on systolic time intervals [8–10]. We have recently developed an index— Granov Goor Index (GGI)—that combines time interval and impedance parameters in order to identify subjects with LVSD. The GGI is obtained from a regional ICG signal—measured using wrist and ankle electrodes [10–13]. In this manuscript we determined the accuracy of GGI in identifying subjects with LVSD (in a “training” cohort of 100 individuals) and validated the findings in an additional cohort of 201 subjects. The regional ICG signal is obtained from 2 pairs of tetra-polar electrodes: one on the wrist, above the radial artery, and the other on the contra-lateral ankle above the posterior tibial artery. The detailed description of the system and measured parameters can be reviewed in previous publications [10–13]. The GGI is designed to assess the systolic contractile function of the LV and is obtained from the following formula:

The Influence of Heart and Lung Dynamics on the Impedance Cardiogram — A Simulative Analysis

Impedance cardiography (ICG) is a simple and cheap method for acquiring hemodynamic parameters. Unfortunately, not all physiological influences on the ICG signal have yet been identified. In this work, the influence of heart and lung dynamics is analyzed using a simplified model of the human thorax with high temporal resolution. Simulations are conducted using the finite integration technique (FIT) with temporal resolution of 103 Hz. It is shown that changes in heart volume as well as conductivity changes of the lung have a high impact on the ICG signal, if analyzed separately. Considering the sum signal of both physiological sources, it can be shown that they compensate each other and thus do not contribute to the signal. This finding supports Kubicek’s model.

Analysis of the Aortic Influence on the Impedance Cardiography Signal by a Simple Model using Finite Integration Technique

Analysis of the Aortic Influence on the Impedance Cardiography Signal by a Simple Model using Finite Integration Technique

Wavelet-based correlations of impedance cardiography signals and heart rate variability

The wavelet-based correlation analysis is employed to study impedance cardiography signals (variation in the impedance of the thorax z(t) and time derivative of the thoracic impedance (- dz/dt)) and heart rate variability (HRV). A method of computer thoracic tetrapolar polyrheocardiography is used for hemodynamic registrations. The modulus of wavelet-correlation function shows the level of correlation, and the phase indicates the mean phase shift of oscillations at the given scale (frequency). Significant correlations essentially exceeding the values obtained for noise signals are defined within two spectral ranges, which correspond to respiratory activity (0.14-0.5 Hz), endothelial related metabolic activity and neuroendocrine rhythms (0.0095-0.02 Hz). Probably, the phase shift of oscillations in all frequency ranges is related to the peculiarities of parasympathetic and neuro-humoral regulation of a cardiovascular system.

Influence of low frequency noise in adaptive estimation using the LMS algorithm

Adaptive systems are employed in the cancelation of noises and estimation of periodic and quasiperiodic signals. Amongst these signals are the electrocardiogram (ECG), impedance cardiography (ZCG), brain evoked potentials and modulated signals in telecommunication applications. In this paper we study the behavior of the weights of the LMS algorithm when used to estimate the coefficients of the discrete Fourier transform (DFT) of a signal under influence of low frequencies. We show theoretically that low frequency noise affects the estimation of the weights at higher frequencies. The simulation results obtained are in agreement with theoretical results. Moreover, we exemplify the problem with impedance cardiography (ZCG) signals.

Automated Impedance Cardiography for Detecting Ischemic Left Ventricular Dysfunction during Exercise Testing

Automated impedance cardiography (ICG) is an attractive method for noninvasive hemodynamic evaluation. The objective of our study was to evaluate the feasibility and diagnostic value automated ICG in patients with suspected coronary artery disease (CAD). We measured stroke index (SI) and cardiac index (CI) in 65 patients with suspected CAD at rest and during bicycle exercise testing. All patients underwent subsequent cardiac catheterization including coronary angiography (CA). Depending on the results of CA, patients were divided into three groups, patients without significant CAD (group 0), single vessel disease (group 1) or multivessel disease (group 2–3). In a subset of 20 patients, automated ICG was compared to measurements of CI by the thermodilution (TD) method. Results: There were no significant differences in SI and CI at baseline between the three groups. At 75- and 100-watt exercise, patients in group 2–3 showed significantly lower mean values of SI and CI as compared to patients of group 0 and group 1 (all p < 0.05), indicating exercise-induced ischaemic left ventricular (LV) dysfunction. Three patients had to be excluded because of inappropriate quality of the ICG signals during exercise. Comparison of automated ICG with TD measurements of CI showed good correlations between both methods at rest (r = 0.73) and during exercise (r = 0.89–0.91). Conclusions: We conclude that hemodynamic monitoring by automated ICG is both feasible and practical during exercise testing. Automated ICG can provide reliable and valuable additional diagnostic information on LV function during exercise which is helpful for selecting those patients for angiography who are likely to benefit from coronary interventions.

Impedance cardiographic waveforms in children: Involvement in left ventricular ejection time determination during anesthesia.

OBJECTIVE: The physiologic basis of the impedance cardiographic (ICG) signal is still a matter of debate. An accurate determination of left ventricular ejection time (LVET) is needed to calculate stroke volume. In children, several shapes of the ICG signal are observed during anesthesia or in critical care. The aim of this study was to determine the relationship between the shape of the ICG signal and various morphologic and hemodynamic determinants obtained with Doppler echocardiography to highlight the effect of the various shapes in the accuracy of LVET determination. DESIGN: Prospective study. SETTING: Pediatric surgery in a university hospital. PATIENTS: 103 children, ASA physical status I or II. INTERVENTIONS: General anesthesia for elective surgery. Measurements: Electrocardiography, ICG, and Doppler echocardiography were recorded simultaneously. Classic hemodynamic variables, such as heart rate, LVET, stroke volume, or systemic vascular resistance, were measured or calculated. A mathematical model of the ICG signal was used to determine the shape of the ICG signal and its potential relationship to various morphologic and hemodynamic determinants. The ICG signal shape was then modeled with the normalized amplitudes (NHA) and phase (NHP) of the three harmonics of the fundamental signal. Analysis was performed with a multiple correspondence analysis. RESULTS: ICG method LVET did not significantly correlate with LVET determined with Doppler or with heart rate. NHA and NHP of the ICG signal were not significantly related to LVET determined with Doppler or to stroke volume. NHA were linked to heart rate and the relative length of the cardiac diastolic period. CONCLUSION: The absence of relationship between LVET determined with Doppler and the main variables defining the shape of the ICG signal highlight the relative limit of stroke volume determination with ICG method. The involvement of the arterial compliance in the ICG signal shape can be discussed in regard to the links between NHA and both heart rate and the relative length of the cardiac diastolic period, respectively.

Impedance pneumography: noise as signal in impedance cardiography.

Thoracic impedance is modulated by events within the respiratory cycle, which represents a source of "noise" in impedance cardiography. Respiration itself, however, is a physiological rhythm of interest to psychophysiologists. We report here methods and validation for deriving impedance pneumographic measures of respiration from impedance cardiography signals, based on standard tetrapolar band electrodes. We recorded the change in impedance (delta Z), the first derivative of the change in impedance (dZ/dt), output from a strain-gauge respirometer, and criterion spirometry from eight healthy adults during rest, paced breathing, abdominal breathing, thoracic breathing, and a mental arithmetic task. Transfer function analyses revealed that a delta Zd signal (derived by integration of the dZ/dt signal) provided the best estimate of the criterion spirometric measure for all parameters (coherence, phase, and gain), accounting for almost 90% of the variance in respiratory waveform morphology. The results document the potential utility of impedance pneumography, as derived from standard impedance cardiography signals.

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.

Filtering noncorrelated noise in impedance cardiography.

Impedance cardiography (ICG) may be altered by noises as respiration and movement artifacts, mainly during exercise. In this work, a scaled Fourier linear combiner (SFLC) event-related to the R-R interval of ECG is proposed. It estimates the deterministic component of the impedance cardiographic signal and removes the noises uncorrelated to this interval. The impedance cardiographic signal is modeled as Fourier series with the coefficients estimated by the least mean square (LMS) algorithm. Simulations have been carried out to evaluate the filter performance for different noise conditions. Moreover, the method capability to remove uncorrelated noises was also examined in physiological data obtained in rest and exercise, by synchronizing respiration and pedaling with a metronome. Analyzing the ICG power spectrum, it was concluded that the proposed filter could remove the noises that are not synchronized with heart rate.

Multiple sources of the impedance cardiogram based on 3-D finite difference human thorax models.

Two 3-D electrical models of the human thorax, each consisting of 216,000 control volumes, were constructed based upon MR images taken at end diastole and end systole. Using the finite difference method, the contributions of various sources to the impedance cardiogram were studied for the traditional band electrode configuration. The contributions were categorized into three areas: 1) the structural changes between end diastole and end systole, 2) the flow-induced blood resistivity changes in major arteries and veins, and 3) the lung resistivity variation due to the lung blood volume change. Based on the models, Zo and delta Z between end diastole and end systole were 24.4 omega and -0.132 omega, as compared with the measurements of 21.8 omega and -0.123 omega made on the same subject from whom the images were taken. Arterial and venous blood resistivity changes caused approximately 57% of the total impedance change. The lung resistivity change and the structural changes contributed 39% and 4%, respectively. The structural changes inside the thorax included the dimensional changes of blood vessels, the blood volume changes of the heart chambers, and heart movement. Although the net impedance change due to the structural changes was relatively small, the individual variation of various factors was large, with significant cancellation occurring. The results suggest that the thoracic impedance cardiographic signal is a mixed representation of many inseparable factors and may not be reliable for the stroke volume calculation. Also, the O-wave, which is clinically observed in various cardiac conditions, may be linked to the diastolic blood flow in the central veins.

An advanced signal processing technique for impedance cardiography.

A new design using the latest technique in signal processing, the time-frequency analysis method, was developed to process impedance cardiography signals. This technique, when used to determine the relevant calculation parameters, was found to be more accurate than conventional methods. It was shown to be advantageous in reducing ventilation artifacts and motion noise, resulting in greater accuracy. Its cardiac output values had a much better correlation coefficient when compared in the clinical setting to the standard thermodilution technique than did the values from conventional impedance cardiography devices.

Thoracic impedance changes following saline injection into right and left ventricles.

ArticleThoracic impedance changes following saline injection into right and left ventricles.L E Geddes, and L E BakerL E Geddes, and L E BakerPublished Online:01 Aug 1972https://doi.org/10.1152/jappl.1972.33.2.278MoreSectionsPDF (933 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByAn overview of the impedance models of the thorax and the origin of the impedance cardiography signal for modelling of the impedance signalsEstimation of left ventricular stroke volume by impedance cardiography: its relation to the aortic reservoir13 May 2013 | Experimental Physiology, Vol. 98, No. 7Studies on separating the impedance change components of blood vessels and ventricles in thorax from mixed impedance signals on chest surface1 June 2011 | Medical Physics, Vol. 38, No. 6Part1Mechanism of the Formation for Thoracic Impedance Change16 January 2010 | Annals of Biomedical Engineering, Vol. 38, No. 3Electrical impedance plethysmography technique in estimating pulmonary function status9 July 2009 | Journal of Medical Engineering & Technology, Vol. 31, No. 1The relationship between vascular expansion of the aorta and pulmonary artery and the genesis of the impedance cardiogram using the technique of sonomicrometry9 July 2009 | Journal of Medical Engineering & Technology, Vol. 31, No. 6Noninvasive assessment of hemodynamics: an emphasis on bioimpedance cardiographyCurrent Opinion in Cardiology, Vol. 15, No. 3Evaluation of a new transcardiac conductance method for continuous on-line measurement of left ventricular volumeCritical Care Medicine, Vol. 28, No. 5Endotracheal Cardiac Output MonitorAnesthesiology, Vol. 92, No. 1Multiple sources of the impedance cardiogram based on 3-D finite difference human thorax modelsIEEE Transactions on Biomedical Engineering, Vol. 42, No. 2Evaluation of left ventricular function by impedance cardiography: A reviewProgress in Cardiovascular Diseases, Vol. 36, No. 4Impedance Cardiography for Cardiac Output Estimation in Pacemaker Patients: Review of the LiteraturePacing and Clinical Electrophysiology, Vol. 16, No. 7Respiratory effects on cardiac related impedance indices measured under voluntary cardio-respiratory synchronisation (VCRS)Medical & Biological Engineering & Computing, Vol. 29, No. 5On the source of peak first time derivative (dZ/dt) during impedance cardiographyAnnals of Biomedical Engineering, Vol. 17, No. 5Origins of the impedance change in impedance cardiography by a three-dimensional finite element modelIEEE Transactions on Biomedical Engineering, Vol. 35, No. 12Possible technique to measure ventricular volume using electrical impedance measurements with an oesophageal electrodeMedical & Biological Engineering & Computing, Vol. 25, No. 6The use of impedance cardiography in heart failureInternational Journal of Cardiology, Vol. 12, No. 1Non Invasive Evaluations of Intracranial Pressure, Cerebral Blood Flow, Vascular Compliance and Oxygen Consumption Using Laser Infrared Spectroanalysis and the Skull Impedance Plethysmographic MethodComputer-Automated Impedance-Derived Cardiac IndexesIEEE Transactions on Biomedical Engineering, Vol. BME-33, No. 1Sources of the thoracic cardiogenic electrical impedance signal as determined by a modelMedical & Biological Engineering & Computing, Vol. 23, No. 5Simplified electrode array for impedance cardiographyMedical & Biological Engineering & Computing, Vol. 23, No. 1The effects of pulmonary circulation pulsatility on the impedance cardiogram26 September 2008 | Archives Internationales de Physiologie et de Biochimie, Vol. 91, No. 4Surface mapping of thoracic impedance cardiogramsMedical & Biological Engineering & Computing, Vol. 20, No. 3Mitral regurgitation and characteristic changes in impedance cardiogram.Heart, Vol. 45, No. 5Impedance cardiographyIntensive Care Medicine, Vol. 5, No. 4A model for the cardiac-associated changes of the thoracic electrical impedance in man24 May 2002 | Physics in Medicine and Biology, Vol. 24, No. 1Studies on the effect of controlled volume change on the thoracic electrical impedanceMedical & Biological Engineering & Computing, Vol. 16, No. 5Impedance CardiographyPsychophysiology, Vol. 15, No. 1Blood resistivity and its implications for the calculation of cardiac output by the thoracic electrical impedance techniqueIntensive Care Medicine, Vol. 3, No. 2The effect of a progressive decrease in the circulating blood volume of the dog on the transthoracic impedanceEuropean Journal of Intensive Care Medicine, Vol. 2, No. 3An analysis of the relationship between transthoracic impedance variations and thoracic diameter changesMedical & Biological Engineering, Vol. 12, No. 4Comparison of several pulse-pressure techniques for monitoring stroke volumeMedical & Biological Engineering, Vol. 12, No. 1The use of the electrical-impedance technique for the monitoring of cardiac output and limb bloodflow during anaesthesiaMedical & Biological Engineering, Vol. 11, No. 5 More from this issue > Volume 33Issue 2August 1972Pages 278-81 https://doi.org/10.1152/jappl.1972.33.2.278PubMed5054437History Published online 1 August 1972 Published in print 1 August 1972 Metrics