Biomedical subjects

T J Faes

Publications and source records attributed to T J Faes.

At least 19 recordsLinked to original sources

PubMed2000

A histopathological contribution to supratentorial glioma grading, definition of mixed gliomas and recognition of low grade glioma with Rosenthal fibers.

BACKGROUND: Previous glioma studies have described separate grading systems for oligodendrogliomas and astrocytomas. Many of these gliomas contain mixtures of neoplastic astrocytes and oligodendrocytes. Prognosis may be related to the percentages of these neoplastic components. Previous survival/grading studies have been limited to histopathological features but have not evaluated the importance of percentages of neoplastic components. This study attempted to perceive the relative importance of percentages of neoplastic astrocytes and oligodendrocytes for definition of astroglial, oligodendroglial and mixed oligoastroglial tumors. After determination of these limits we explored the possibility to develop a grading system for common supratentorial gliomas based on reproducible histopathological features. METHODS: A retrospective study was performed of 362 cases of unselected supratentorial glioma. One hundred and thirty-eight binary and nine continuous histopathological variables, amongst which percentages of neoplastic astrocytes and oligodendrocytes, were scored and related to survival. Only well reproducible histological features were accepted in Cox regression to define glioma grades. RESULTS AND CONCLUSIONS: Supratentorial gliomas appeared to be composed of variable percentages of neoplastic oligodendrocytes and astrocytes, but this spectrum did not correspond to a continuous change in prognosis. Gliomas containing 30% or more neoplastic oligodendrocytes had a slightly better outcome (p < 0.0432) but higher percentages did not further improve prognosis. Percentages of neoplastic astrocytes were not correlated to survival. We therefore propose to designate gliomas containing 30% or more neoplastic oligodendrocytes as oligodendroglial tumors, and others as astroglial tumors. From a prognostic point of view there is no need to recognize mixed oligoastrocytomas. An interesting finding was the recognition of a low grade glioma group with Rosenthal fibers, which had the longest postoperative survival. Another prognosticator of interest concerns the mitotic rate as a continuous variable. Atypical mitoses indicated the worst survival, after necrosis. It was possible to develop a grading system for all supratentorial gliomas using six reproducible histological parameters: necrosis, atypical mitoses, the mitotic rate, endothelial proliferative activity, percentage of neoplastic oligodendrocytes and Rosenthal fibers. This resulted in four grades for astroglial tumors (p < 0.002) and three grades for oligodendroglial tumors (p < 0.008) which differed significantly within each group with respect to survival.

Adolescent↗

PubMed2000

Determination of stroke volume by means of electrical impedance tomography.

ECG-gated electrical impedance tomography (EIT) is a non-invasive imaging technique, developed to monitor blood volume changes. This study is the first in comparing this non-invasive technique in measuring stroke volume with established techniques. The objective of this study was to validate EIT variables derived from the EIT images with paired obtained stroke volume measurements by thermodilution and MRI. After right cardiac catheterization, EIT measurements were performed in 25 patients. Regression analysis was used to analyse the relation between the EIT results and stroke volume determined by thermodilution. From the regression line an equation was derived to estimate stroke volume (in ml) by EIT. A strong correlation was found between EIT and stroke volume measured by the thermodilution method (r = 0.86). In a group of 11 healthy subjects this equation was validated to MRI. The mean and standard deviation of the difference between EIT and MRI was 0.7 ml and 5.4 ml respectively. These data indicate that EIT is a valid and reproducible method for the assessment of stroke volume.

Aged↗

PubMed2000

Monitoring of recruitment and derecruitment by electrical impedance tomography in a model of acute lung injury.

OBJECTIVE: To evaluate a noninvasive system for obtaining information about alveolar recruitment and derecruitment in a model of acute lung injury. DESIGN: Prospective experimental study. SETTING: Animal research laboratory. SUBJECTS: Nine anesthetized pigs. INTERVENTIONS: Electrical impedance tomography measurements were performed. Electrical impedance tomography is an imaging technique that can register the ventilation-induced impedance changes in different parts of the lung. In nine anesthetized pigs, repeated lung lavages were performed until a PaO2 of <80 mm Hg was reached. Thereafter, the lungs were recruited according to two different recruitment protocols: the open lung approach and the open lung concept. Five time points for measurements were chosen: healthy (reference), lavage (atelectasis), recruitment, derecruitment, and maintain recruited (final). MEASUREMENTS AND MAIN RESULTS: After lavage, there was a significant increase in the impedance ratio, defined as the ventilation-induced impedance changes of the anterior part of the lung divided by that of the posterior part (from 1.75 +/- 0.63 to 4.51 +/- 2.22; p < .05). The impedance ratio decreased significantly after performing the recruitment protocol (from 4.51 +/- 2.22 to 1.18 +/- 0.51). During both recruitment procedures, a steep increase in baseline impedance change was seen. Furthermore, during derecruitment, a decrease in the slope in baseline impedance change was seen in the posterior part of the lung, whereas the anterior part showed no change. CONCLUSION: Electrical impedance tomography is a technique that can show impedance changes resembling recruitment and derecruitment of alveoli in the anterior and posterior parts of the lung. Therefore, electrical impedance tomography may help in determining the optimal mechanical ventilation in a patient with acute lung injury.

Animals↗

PubMed2000

The boundary element method in the forward and inverse problem of electrical impedance tomography.

In this paper, a new formulation of the reconstruction problem of electrical impedance tomography (EIT) is proposed. Instead of reconstructing a complete two-dimensional picture, a parameter representation of the gross anatomy is formulated, of which the optimal parameters are determined by minimizing a cost function. The two great advantages of this method are that the number of unknown parameters of the inverse problem is drastically reduced and that quantitative information of interest (e.g., lung volume) is estimated directly from the data, without image segmentation steps. The forward problem of EIT is to compute the potentials at the voltage measuring electrodes, for a given set of current injection electrodes and a given conductivity geometry. In this paper, it is proposed to use an improved boundary element method (BEM) technique to solve the forward problem, in which flat boundary elements are replaced by polygonal ones. From a comparison with the analytical solution of the concentric circle model, it appears that the use of polygonal elements greatly improves the accuracy of the BEM, without increasing the computation time. In this formulation, the inverse problem is a nonlinear parameter estimation problem with a limited number of parameters. Variants of Powell's and the simplex method are used to minimize the cost function. The applicability of this solution of the EIT problem was tested in a series of simulation studies. In these studies, EIT data were simulated using a standard conductor geometry and it was attempted to find back this geometry from random starting values. In the inverse algorithm, different current injection and voltage measurement schemes and different cost functions were compared. In a simulation study, it was demonstrated that a systematic error in the assumed lung conductivity results in a proportional error in the lung cross sectional area. It appears that our parametric formulation of the inverse problem leads to a stable minimization problem, with a high reliability, provided that the signal-to-noise ratio is about ten or higher.

Algorithms↗

PubMed1999

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).

Body Composition↗

PubMed1999

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.

Age Factors↗

PubMed1999

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.

Adult↗

PubMed1999

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.

Body Water↗

PubMed1999

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.

Analysis of Variance↗

PubMed1998

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.

Cardiography, Impedance↗

PubMed1998

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.

Adult↗

PubMed1998

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.

Cardiography, Impedance↗

PubMed1998

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.

Cardiography, Impedance↗

PubMed1998

Pulmonary perfusion measured by means of electrical impedance tomography.

Electrical impedance tomography (EIT) is a recent imaging technique based on electrical impedance, offering the possibility of measuring pulmonary perfusion. In the present study the influence of several pulmonary haemodynamical parameters on the EIT signal were investigated. First, the influence on the systolic wave of the EIT signal (delta Zsys) of stroke volume, large pulmonary artery distensibility (both assessed by means of MRI) and the extent of the pulmonary peripheral vascular bed in 11 emphysematous patients (reduced peripheral vascular bed) and 9 controls (normal peripheral vascular bed) was investigated. Second, the influence of hypoxic pulmonary vasoconstriction on delta Zsys was examined in 14 healthy subjects. Finally, the origin of the diastolic wave was examined in three patients with atrioventricular dissociation. Multiple regression analysis showed that delta Zsys was only dependent on the variable emphysema (p < 0.02), but not dependent on stroke volume (p < 0.3) or pulmonary artery distensibility (p > 0.9). The mean value of delta Zsys for emphysematous patients (131 +/- 32 arbitrary units (AU)) was significantly lower (p < 0.001) than in the control group (200 +/- 39). In the group of healthy subjects delta Zsys decreased significantly (p < 0.001) during hypoxia (193 +/- 38 AU) compared with rest measurements (260 +/- 62 AU). The absence of the diastolic wave in the cardiological patients suggests the influence of reverse venous blood flow on the EIT signal. It is concluded that volume changes in the small pulmonary vessels contribute significantly to the EIT signal. Moreover, the hypoxia induced decrease in delta Zsys indicates the potential of EIT for measuring pulmonary vascular responses to external stimuli.

Aged↗

PubMed1998

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.

Adult↗

PubMed1998

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.

Electric Impedance↗

PubMed1997

Validity and reproducibility of electrical impedance tomography for measurement of calf blood flow in healthy subjects.

The Sheffield electrical impedance tomography; (EIT) system produces images of changes in the distribution of resistivity within tissue. The paper reports on the application of electrical impedance tomography in monitoring volume changes in the limb during venous occlusion. The aim of the study is to assess the feasibility, reproducibility and validity of calf blood flow measurements by EIT. In 14 healthy volunteers calf blood flow is compared, as determined in a calf segment by strain-gauge plethysmography (SGP), with the impedance changes measured by EIT during rest and post-ischaemic hyperaemia. The measurements are repeated to assess reproducibility. The reproducibility for the EIT, assessed from the repeated measurements and expressed as a reproducibility coefficient, is 0.88 during rest and 0.89 during hyperaemia. The reproducibility coefficient for SGP data is 0.83 at rest and 0.67 during hyperaemia. Flow measurements, assessed by means of two methods, correlate well at rest (r = 0.89), but only moderately during hyperaemia (r = 0.51). The correlation coefficient for the pooled flow measurements is 0.98. It is concluded that EIT is a valid and reliable method for assessing blood flow in the limb. Possible applications of EIT in localising fluid changes are discussed.

Adult↗

PubMed1997

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.

Adult↗