On-line electrical impedance measurement for monitoring blood viscosity during on-pump heart surgery.

BACKGROUND: The viscosity of blood (eta) as well as its electrical impedance at 20 kHz at high shear rate depends on hematocrit, temperature, concentration of macromolecules and red cell deformability. The aim of our study was to investigate the relation between viscosity and electrical impedance in a heart-lung machine-like set-up, because during on-pump heart surgery considerable viscosity changes occur. METHODS: Blood of 10 healthy volunteers was examined under temperature variation between 18.5 and 37 degrees C at four different levels of hemodilution. Blood viscosity was examined with a golden-standard technique, i.e. a Contraves LS 30 Couette viscometer, and the results were compared with measurements of the electrical resistivity (R) at 20 kHz by a specially designed device in series with the tubing system of a heart-lung machine. All measurements were performed at a shear rate of 87 s(-1). RESULTS: Using stepwise multiparameter regression analysis (SPSS) a highly significant correlation was found (r(2) = 0.882) between viscosity (eta) and resistivity (R). Adding the variables sodium ([Na(+)]) and fibrinogen ([Fibr]) concentration the coefficient of correlation further improved to r(2) = 0.928 and the relation became: eta = -0.6844 + 0.038 R + 0.038 [Na(+)] + 0.514 [Fibr]. All coefficients showed a statistical significance of p < 0. 001. CONCLUSIONS: Electrical impedance measurement is feasible in a heart-lung machine-like set-up and allows accurate continuous on-line estimation of blood viscosity; it may offer an adequate way to record and control viscosity changes during on-pump heart surgery.

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.

A meta-analysis of three decades of validating thoracic impedance cardiography.

OBJECTIVE: To provide a meta-analysis of current literature concerning the validation of thoracic impedance cardiography (TIC) and to explain the variations in the reported results from the differences in the studies. DATA SOURCES: A computer-assisted search of English-language, German, and Dutch literature was performed for the period January 1966 to April 1997. Moreover, references from review articles were obtained. STUDY SELECTION: A total of 154 studies comparing measurements of cardiac output or related variables obtained from TIC and a reference method were analyzed. DATA EXTRACTION: Articles were classified by differences in TIC methodology, reference method, and subject characteristics. Fisher's Zf transformed correlation coefficients were used to compare results. Data were pooled using the random-effects method. DATA SYNTHESIS: An overall pooled r2 value of .67 (95% confidence interval, 0.64-0.71) was found. However, the correlation was higher in repeated-measurement designs than in single-measurement designs (r2 = .53; 95% confidence interval, 0.43-0.62). Further research using analysis of variance revealed a significant influence of the reference method and the subject characteristics on the correlation coefficient. The correlation was significantly better in animals than in cardiac patients. Subgroup analysis revealed that TIC correlated significantly better to the indirect Fick method than to echocardiography in healthy subjects. No significant influence of the applied TIC methodology was found. DISCUSSION: The overall r2 value of .67 indicates that TIC might be useful for trend analysis of different groups of patients. However, for diagnostic interpretation, a r2 value of .53 might not meet the required accuracy of the study. Great care should be taken when TIC is applied to the cardiac patient. However, because the applied reference method was of significant influence, differences between TIC and the reference method are incorrectly attributed to errors in TIC alone.

Estimation of extracellular volume by a two-frequency measurement.

An approach to determine intra- and extracellular conduction on the basis of Bode analysis is presented. Estimation of the ratio between intra- and extracellular conduction could be performed by phase measurement only, midrange in the bandwidth of interest. An important feature is that the relation between intra- and extracellular conduction can be continuously monitored by phase measurement and no curve fitting whatsoever is required. Based on a two-frequency measurement determining Re at 4 kHz and phi max at 64 kHz, it proved possible to estimate extracellular volume (ECV) in 23 patients. Reference values on ECV were determined by sodium bromide. The results show a good correlation (r = 0.90) with the reference method. The average error of ECV estimation was -3.6% (SD 8.4).

A meta-analysis of published studies concerning the validity of thoracic impedance cardiography.

UNLABELLED: Our aim was to provide a meta-analysis of the literature concerning the validation of thoracic impedance cardiography (TIC) and to explain variations in reported results from differences in the studies. One hundred fifty-four studies (164 Fisher's Z-transformed correlation coefficients) comparing measurements of cardiac output or related parameters from TIC and a reference method were analyzed. Papers were classified according to differences in TIC methodology, reference method, and subject characteristics. Pooling using the random-effects method yielded an overall correlation of r = 0.82 (95% confidence interval: 0.80-0.84). ANOVA revealed a significant influence of the reference method and the subject characteristics on the correlation coefficient. In cardiac patients, the correlation was significantly decreased. No influence of the applied TIC methodology was found. CONCLUSION: TIC might be useful for trend analysis of different groups of patients. However, since the reference method was of significant influence, differences between TIC and the reference method are incorrectly attributed to TIC alone.

Towards a theoretical understanding of stroke volume estimation with impedance cardiography.

In electrical impedance cardiography, Kubicek's formula is often used to measure stroke volume from thoracic impedance variations synchronously to heart activity. To calculate stroke volume from impedance variations, the so-called outflow problem should be adequately solved. This outflow problem refers to the joint causes of impedance change due to blood entering the aorta from the heart, as well as blood leaving the aorta due to arterial runoff. The aim of this study was to investigate the Kubicek formula as a solution of the outflow problem. Kubicek's formula was theoretically investigated using a simple model of the volume-conducting properties of the thorax (two-cylinder model), as well as the hemodynamics of the systemic circulation (three-element "windkessel" model). The mathematical analysis showed that the outflow problem was not solved by the Kubicek formula. Moreover, this theoretical result was experimentally confirmed.

The influence of extravascular lung water on cardiac output measurements using thoracic impedance cardiography.

The purpose of this study was to investigate the influence of pulmonary oedema as measured with the double indicator dilution technique on the accuracy of cardiac output (CO) measurement using thoracic impedance cardiography (TIC) compared with thermodilution in thirteen sepsis patients. Differences in the Kubicek and Sramek-Bernstein equation with respect to pulmonary oedema were explored theoretically and experimentally. From a parallel two cylinder model a hypothesis can be derived that CO determined with the Kubicek equation is oedema independent, whereas CO determined using the Sramek-Bernstein equation is oedema dependent. Experimentally, CO determined using Kubicek's equation correlated better with thermodilution CO (r = 0.75) than CO determined with the Sramek-Bernstein equation (r = 0.25). The effect of oedema on the accuracy of TIC was investigated by comparing the differences in the CO of impedance and thermodilution to the extravascular lung water index. For the Kubicek equation the difference was not influenced by oedema (r = 0.04, p = 0.84), whereas for the Sramek-Bernstein equation the difference was affected by oedema (r = 0.39, p = 0.05). Thus, the effects of pulmonary oedema on the accuracy of TIC measurements can better be understood with the parallel cylinder model. Moreover, the Kubicek equation still holds when pulmonary oedema is present, in contrast to the Sramek-Bernstein equation.

Extra-cellular volume estimation by electrical impedance--phase measurement or curve fitting: a comparative study.

In order to determine body fluid shifts between the intra- and extra-cellular spaces, multifrequency impedance measurement is performed. According to the Cole-Cole extrapolation, lumped values of intra- and extra-cellular conduction can be estimated which are commonly expressed in resistances Ri and Re respectively. For this purpose the magnitude and phase of the impedance under study are determined at a number of frequencies in the range between 5 kHz and 1 MHz. An approach to determine intra- and extra-cellular conduction on the basis of Bode analysis is presented in this article. On this basis, estimation of the ratio between intra- and extra-cellular conduction could be performed by phase measurement only, midrange in the bandwidth of interest. An important feature is that the relation between intra- and extra-cellular conduction can be continuously monitored by phase measurement and no curve fitting whatsoever is required. Based on a two frequency measurement determining Re at 4 kHz and phi(max) at 64 kHz it proved possible to estimate extra-cellular volume (ECV) more accurately compared with the estimation based on extrapolation according to the Cole-Cole model in 26 patients. Reference values of ECV were determined by sodium bromide. The results show a correlation of 0.90 with the reference method. The average error of ECV estimation was -3.6% (SD 8.4), whereas the Cole-Cole extrapolation showed an error of 13.2% (SD 9.5). An important feature of the proposed approach is that the relation between intra- and extra-cellular conduction can be continuously monitored by phase measurement and no curve fitting whatsoever is required.

Thoracic geometry and its relation to electrical current distribution: consequences for electrode placement in electrical impedance cardiography.

In thoracic impedance cardiography (TIC) measurements the neck electrodes are often positioned at the basis of the neck, close to the neck-thorax transition. Theoretically, this neck-thorax transition will cause inhomogeneities in the current density and potential distribution. This was simulated using a 3D finite element method, solely representing the geometrical neck-thorax transition. The specific conductivity was 7 10(-3) (omega cm)-1 and the injected current was 1 mA. As expected, the model generated inhomogeneities in the current distribution at the neck-thorax transition, which reached as far as 5 cm into the neck and 20 cm into the thorax. These results are supported by in vivo measurements performed in 10 young male subjects, in which the position of the neck electrodes was varied. A two-way ANOVA revealed that the stroke volume of the lowest neck position was significantly different from the other positions. Small shifts in the position of the neck electrode resulted in large changes in impedance and stroke volume (127 to 82 ml for the Kubicek equation). To standardise the electrode position, the authors strongly recommend placement of the neck electrodes at least 6 cm above the clavicula.

An electrically isolated balanced wideband current source: basic considerations and design.

At relatively high frequencies, the application of an alternating current through the body or a body segment results in electromagnetic stray fields which reduce the amount of current actually injected into the tissue under study. This radiation effect can be reduced by use of a symmetrical configuration current source. The symmetry of such an arrangement, however, depends on the stray capacitances of the source with respect to surrounding equipment. To minimise these effects, it is required that the source is electrically isolated from the surrounding equipment and the subject under study. In this manner stray capacitances with respect to elements of the current source are reduced. In such a configuration common mode voltages to the input amplifier of the measuring system are also reduced. The paper describes design considerations and the implementation of a wideband current source capable of injecting alternating current in the order of 300 microARMS into biological tissue having impedances up to 1 k omega. Current stabilisation is obtained by means of a control circuit which measures the actual current passing through the tissue under study. Leakage currents arising from shielding and stray capacitances are compensated for. The usable frequency range is between 4 kHz and 1024 kHz and current stability is better than 0.2%. Through the use of a symmetrical, floating circuit a configuration is obtained which substantially reduces stray effects. The current source is connected to other circuits by means of two isolation ports: (1) a transformer coupling for the carrier frequency; and (2) an opto-coupler to transfer a phase reference signal obtained from current measurement. The current amplitude can be modulated by controlling the reference input to the control loop by means of a third auxiliary isolation port for transfer of the modulating signal.

Estimation of non-cardiogenic pulmonary oedema using dual-frequency electrical impedance.

The study investigates the effects of non-cardiogenic oedema, especially the accumulation of protein in extracellular fluid, on thoracic impedance and proposes a new method of oedema measurement based on an impedance ratio from a dual-frequency measurement. In vitro measurements in a cell containing an albumin-in-saline solution yield a resistance increase when the albumin concentration increases. Subsequently, 13 patients having acute respiratory failure are measured. The single-frequency Z0 measurements and the proposed impedance ratio are compared with extravascular lung water (EVLW) determined by the double indicator dilution method. The single-frequency measurement correlates poorly with EVLW (r = -0.24, p = 0.56). In some patients, a total thoracic impedance increase is found with increasing EVLW. The correlation between the impedance ratio and EVLW is r = -0.79 (p < 0.0005). The ratio decreases as EVLW increases. Thus, when oedema is measured using bio-impedance, cardiogenic and non-cardiogenic oedema yield different results. It is well recognised that cardiogenic oedema decreases total thoracic impedance. In non-cardiogenic oedema, however, protein accumulation causes an impedance increase. The decrease in the impedance ratio as EVLW increases can be explained by the accumulation of albumin in the extracellular compartment.

A wideband high common mode rejection ratio amplifier and phase-locked loop demodulator for multifrequency impedance measurement.

Design considerations and implementation of a multifrequency measuring channel for application in the field of bio-impedance measurement are discussed in this paper. The input amplifier has a differential configuration which is electrically isolated from the remaining circuits. Transformer coupling provides improved common mode rejection when compared to non-isolated input stages. The frequency characteristic of the section between input and demodulator is flat within +/- 0.1 dB between 4 kHz and 1024 kHz. The synchronous demodulator is based on a wideband switched video amplifier. In contrast to commonly used lock--in techniques, the carrier for demodulation is recovered from the input signal by means of a phase-locked loop. This method ensures zero phase shift with respect to the input signal and improves the accuracy of measurement. The system has been developed primarily for thoracic impedance cardiography (TIC) but has also successfully been applied in the field of total body bio-impedance analysis (BIA). At present an electrical impedance tomograph is under development based on the instrumentation described. Results regarding the measurement range and accuracy are given and some recordings of patient data are shown.

The inaccuracy of Kubicek's one-cylinder model in thoracic impedance cardiography.

The validity of a one- and a two-cylinder model, underlying thoracic impedance cardiography (TIC), was investigated by studying the length dependence of the impedance parameters Z0, (dZ/dt)min, and stroke volume (SV). It can be shown that, within a one-cylinder model, all parameters are directly proportional to the length, whereas, if the volume conduction of the thorax and the neck are modeled separately, Z0 and (dZ/dt)min are expected to be linear dependent and SV will be nonlinear upon the length. The expectations were compared to results from in vivo measurements. Two electrode arrays were studied, in which the caudal recording electrode position was varied; SV was calculated using Kubicek's equation. Except for small distances, the results showed a nearly linear relation between the parameters and the length. Regression analysis of the linear part revealed statistically significant intercepts (p < 0.05). Neither the intercept nor the nonlinear part can be explained by a one-cylinder model, whereas a model consisting of two cylinders serially connected describes the experimental results accurately. Thus SV estimation based on a one-cylinder model is biased due to the invalid one-cylinder model. Corrections for the Kubicek-equation need to be developed in future research using this two-cylinder model.

A comparative clinical study of fetal phono- and movement-sensors from Amsterdam, Cambridge and Edinburgh.

In a European collaboration, a joint project to conduct an experimental and clinical investigation of recently developed sensors from three centres (Amsterdam, Cambridge and Edinburgh) has been carried out. The Amsterdam sensor was based on an inductive principle whereas the Cambridge and Edinburgh transducers used a piezo-electric material (PVDF) as transducing element. Nine patients with varying gestational age (29-38 weeks) were measured in a clinical investigation. Recordings of fetal heart sounds (FHS) and fetal breathing movements (FBM) were made using three sensors; one from each centre. These recordings were digitized directly into a computer using a purpose-built data acquisition system. For each patient 3 min of FBM data, and 1 min of FHS data were recorded by each sensor. The FBM recordings were carried out simultaneously with ultrasound, so as to enable a correlation to be made between both recordings. The FHS recordings were carried out simultaneously with the maternal heart pulse, to discount any maternal heart influences on the resulting signals. Of the nine patients analysed, FHS were recorded in seven patients. On the other hand, it appeared difficult to identify fetal breathing movements in the FBM recordings due to the dominance of the maternal breathing component. The analysis of the FBM signals and its correlation with ultrasound could not be carried out due to the relatively poor quality of the signals detected by the sensors, given the present techniques of analysis. The evaluation of the FHS recordings showed that although there is relatively little difference between the sensors, the inductive sensor performed best.

Recording and processing of fetal movements and sounds obtained with the Inpho inductive transducer.

A recently developed transducer based on an inductive principle allows recording of fetal displacement signals on the maternal abdominal wall. The transducer is a relatively passive device, in contrast to commonly applied ultrasound transducers. This permits long-term observation of fetal movements and sounds. The bandwidth of the system is DC-200 Hz (+/- 3 dB), and signal-to-noise ratios of more than 96 dB have been measured in a laboratory setup, whereas in the practical situation a signal-to-noise ratio of 78 dB has been established. The transducer has been applied to study fetal respiratory sinus arrhythmia, which means that fetal breathing movements have to be extracted from the transducer's output. This proved possible by digital filtering of the displacement signal as detected by the transducer. The transducer has also been applied in a study where the signal-to-noise ratio of fetal heart sounds as a function of location of the fetus and position of the transducer on the maternal abdominal wall has been studied. It proved possible to adequately record fetal heart sounds for measurement of fetal heart rate. Also uterine activity could be recorded using the sensor's DC output.

Coherent average technique--a tutorial review.

This paper highlights the effects of digitization in relation to the effective numberical range over which the signal to be averaged has to be resolved. The effect of quantization noise on the number of averaging cycles is assessed and it is shown that for averaging weak signals such as the HIS bundle electrogram, high resolution analogue-to-digital (A/D) conversion is required so as to prevent a substantial increase in the number of averaging cycles. Also the requirement for adequate low-pass filtering is discussed and relationships between the numerical range of the A/D conversion process, the order of the low-pass filter, its cut-off frequency and sampling frequency are formulated.

Recording fetal breathing movements with a passive transducer based on an inductive principle.

A new transducer has been developed, based on an inductive principle, for recording fetal movements and sounds. The compliance of this transducer, the INductive PHOno-sensor (INPHO), can be matched to that of the maternal abdomen to provide an optimal transfer of displacement between maternal abdomen and transducer. In this manner, it is possible to detect fetal breathing movements by digital filtering of the INPHO signal in a frequency band between 0.5 and 2.0 Hz. Singular breathing movements can be detected and this was verified by real time ultrasound imaging. The INPHO transducer shows a flat (+/- 1.5 dB) frequency response between 0.2 and 200 Hz. The signal-to-noise ratio of the transducer system is better than 95 dB, and enables very weak movements and sounds to be detected. Spectral analysis of the processed signal shows that modulation of fetal breathing by maternal breathing takes place. The measuring setup allows for the quantitative assessment of fetal respiratory sinus arrhythmia.

A comparative experimental study of fetal phono- and movement-sensors from Amsterdam, Cambridge and Edinburgh.

This paper describes the experimental studies and results of a collaborative investigation between three European institutes: Amsterdam, Cambridge and Edinburgh. The object of this investigation and the collaborative exchanges was to evaluate and compare different sensors for recording fetal sounds and movements from each centre. This would be carried out in a series of experimental and clinical tests involving researchers from each of the European institutes, and using three sensors: one from each centre. Experimental measurements have been performed regarding the conversion gain, frequency response, equivalent input noise and dynamic range of the transducers. The measurements were carried out using a vibration-free table testing rig to evaluate these characteristics of each of the sensors. The equivalent input noise ranged from 50 nm to 1600 nm for the transducers studied.

Method for the measurement of susceptibility to decubitus ulcer formation.

A method for measuring the susceptibility of a patient to develop decubitus ulcers is described and initially evaluated. It is based on an indirect, noninvasive measurement of the transient regional blood flow response after a test pressure load which simulates the external stimulus for pressure-sore formation. This method was developed to determine the individual risk of a patient and to study the subfactors which contribute to the susceptibility. This would also offer the possibility of evaluating the effect of preventive treatment aimed at reducing the susceptibility. The method was found to discriminate between preselected elderly patients at risk on the one hand, and non-risk patients and healthy young adults on the other hand. No differences in blood flow responses were found between the non-risk elderly patients and the healthy young adults. This suggests that age per se is not a factor in the formation of pressure sores. In the risk group the recovery time after pressure relief was found to be three times as long as the duration of the pressure exercise. This indicates that the recovery time after pressure exercise may be as important as the period of pressure exercise in deducing the risk of developing decubitus ulcers.

A transducer for detection of fetal breathing movements.

The paper describes a new transducer for detection of fetal movements and sounds from the maternal abdominal wall. This transducer is based on an inductive principle. The compliance of the transducer should match the compliance of the maternal abdominal wall in order to detect the very weak acoustic signal caused by individual fetal breathing movements. The contact area of the transducer is supported on a membrane, the tension of which can be adjusted so as to match the compliances of tissue and transducer. The goal of this paper is to investigate detection of fetal movements in order to monitor fetal condition and motility. The more specialized field of investigating fetal heart rate variability by studying the relationship between fetal breathing movements and fetal heart rate may become of more interest in the future. The inductive phonometer (INPHO) shows a flat (+/- 1.5 dB) frequency response between 0.2 and 200 Hz which is not affected by the compliance adjustment. Some in vivo measurements have been performed. The transducer signal is stored on a 20MB hard disk of an Olivetti M24 Personal Computer (PC) simultaneously with a control signal indicating the presence of breathing movements. Ultrasound imaging is exclusively used for verification of breathing movements. The images are stored on videotape simultaneously with control signals from the PC for synchronization of data and image. Substantial digital filtering is necessary to discriminate between actual breathing movements and other fetal activities. It is shown that optimal adjustment of the transducer's compliance to that of the maternal abdominal wall is possible and that individual breathing movements can indeed be measured.