Helmut Hutten

Assessing abdominal fatness with local bioimpedance analysis: basics and experimental findings.

OBJECTIVE: Abdominal fat is of major importance in terms of body fat distribution but is poorly reflected in conventional body impedance measurements. We developed a new technique for assessing the abdominal subcutaneous fat layer thickness (SFL) with single-frequency determination of the electrical impedance across the waist (SAI).SUBJECTS AND MEASUREMENTS: The method uses a tetrapolar arrangement of surface electrodes which are placed symmetrically to the umbilicus in a plane perpendicular to the body axis. Twenty-four test subjects (12 male, 12 female) underwent SAI and abdominal magnetic resonance imaging (MRI). The SFL below the sensing electrodes was determined from MRI and correlated with the SAI data at four different frequencies (5, 20, 50 and 204 kHz).RESULTS: A highly significant linear correlation (r2=0.99) between SFL and SAI over a wide range of the abdominal SFL was found. Separate regression models for female and male subjects did not differ significantly, except at 50 kHz.CONCLUSION: SAI represents a good predictor of the SFL and provides an excellent tool for the assessment of central obesity.International Journal of Obesity (2001) 25, 502–511

Direct estimation of Cole parameters in multifrequency EIT using a regularized Gauss–Newton method

A major drawback of electrical impedance tomography is the poor quality of the conductivity images, i.e., the low spatial resolution as well as large errors in the reconstructed conductivity values. The main reason is the necessity for regularization of the ill-conditioned inverse problem which results in excessive spatial low-pass filtering. A novel regularization method (SMORR (spectral modelling regularized reconstructor)) is proposed, which is based on the inclusion of spectral a priori information in the form of appropriate tissue models (e.g. Cole models). This approach reduces the ill-posedness of the inverse problem, when multifrequency data are available. An additional advantage is the direct reconstruction of the (physiological) tissue parameters of interest instead of the conductivities. SMORR was compared with posterior fitting of a Cole model to the conductivity spectra obtained with a classical iterative reconstruction scheme at various frequencies. SMORR performed significantly better than the reference method concerning robustness against noise in the data.

Accurate reconstruction algorithm of the complex conductivity distribution in three dimensions

In electrical impedance tomography, an inverse problem has to be solved to reconstruct the complex conductivity distribution /spl kappa/=/spl sigma/+j/spl omega//spl epsiv/. The problem is ill posed, and therefore, a regularization has to be used. The aim is to reconstruct, as accurately as possible, both the electric conductivity /spl sigma/ and the electric permittivity /spl epsiv/ in three dimensions using finite elements of the second order for solving the forward problem. To this end, a new reconstruction algorithm based on a priori information has been developed.

Ventricular Intramyocardial Electrograms and Their Expected Potential for Cardiac Risk Surveillance, Telemonitoring, and Therapy Management

Ventricular intramyocardial electrograms are recorded with electrodes directly from the heart either in intraventricular or epimyocardial position and may be acquired either from the spontaneously beating or from the paced heart. The morphology of these signals differs significantly from that of body surface ECG recordings. Although the morphology shows general characteristics, it additionally depends on different individual impacts. This problem of individual evaluation is briefly discussed. As an appropriate methodology for its solution, personalized referencing based on similarity averaging has been employed. A more general approach may be model-based signal interpretation, which is still under investigation. The preliminary results reveal a promising potential of intramyocardial electrograms for cardiac risk surveillance, e.g., for arrhythmia detection, recognition of rejection events in transplanted hearts, and assessment of hemodynamic performance. Employing implants with telemetric capabilities may render possible permanent and even continuous cardiac telemonitoring. Furthermore, the signals can be utilized for supporting therapy management, e.g., in patients with different kinds of cardiomyopathies. This paper shall demonstrate some preliminary results and discuss the expected potential.

Some Interesting Properties of Cardiac Fusion Beats

Ventricular fusion beats are the result if the depolarizing wavefronts of excitations that originate from different sites collide within the ventricular functional syncitium. Fusions have been a matter of concern since 1944 [1]. The region of collision functions like a barrier that prevents the further spreading of the depolarizing wavefronts. As a consequence of the change in the spatio-temporal excitation spreading fusions have a complex signal morphology. Hence, fusions can cause problems for proper signal analysis. Erroneous signal analysis and pattern recognition, however, may be a major problem for the timing control of cardiac pacemakers and implantable defibrillators. Furthermore, fusions have also an impact on the spatio-temporal process of contraction since the contraction waves follow the depolarizing wavefronts and collide in the same region. As a consequence, ventricular fusions affect the hemodynamic efficiency of the cardiac pump mechanism. The occurrence of fusion beats has been used for diagnostic purposes, e.g. for the diagnosis of disturbances of the regular excitation spreading like the Wolff-Parkinson-White syndrome or bundle branch blocks. More recently interest is growing in the therapeutic employment of provoked fusions, e.g. for treatment in certain forms of cardiomyopathy and for reversed cardiac remodeling by multisite pacing. Fusions can be detected in records both of surface electrocardiograms and intramyocardial electrograms. Until now, however, the basic understanding of the signal morphology of intramyocardial electrograms is rather poor. For that reason, the analysis and interpretation of the signal morphology of intramyocardially recorded fusion beats needs further examination. This paper presents some results of the analysis of intramyocardial electrograms.

Geometric multigrid to accelerate the solution of the quasi-static electric field problem by tetrahedral finite elements

The acceleration of the solution of the quasi-static electric field problem considering anisotropic complex conductivity simulated by tetrahedral finite elements of first order is investigated by geometric multigrid.

Dynamical Control of the Dialysis Process. Part II: An Improved Algorithm for the Solution of a Tracking Problem - Dynamische Optimierung des Dialyseprozesses. Teil II: Ein verbesserter Algorithmus zur Lösung eines Verfolgungsproblems

An efficient algorithm for the optimization of process parameters during dialysis has been developed. By solving a tracking-problem for prescribed time courses of distinguished variables, it is possible to compute optimal concentrations of electrolytes in dialysate as well as an optimal rate of ultrafiltration. These variables are indirectly influencing the status of the patient and can be directly modelled. They are describing the important exchange processes between blood and dialysate as well as between the different distribution spaces within the patient during dialysis. Their time courses are determined by an individually identifiable patient model. The tracking problem was treated as a dynamic optimization problem, and a continuous descent procedure which is usually employed for solving unconstrained static optimization problems has been adapted in such a manner that it is applicable for the solution of this problem. The used method is characterized by its simple mode of application, short solution time and moderate storage need. Especially in cases of contradictional requirements for desired time courses of model outputs the used optimization method performs well.

Non-invasive monitoring of the dynamic compliance in the carotid artery

A device has been developed to monitor non-invasively, continuously and simultaneously both the intravascular pressure and the vessel diameter of the carotid artery. The intravascular pressure is obtained by using the applanation method. The pressure sensor utilizes the principle that the extravascular pressure necessary to flatten the vessel wall is equal to the intravascular pressure. The vessel diameter is determined by employing the four-electrode bioimpedance method. This approach is based on the principle that the conductivity of blood is much higher than for the surrounding tissue including the vessel wall. The device has been successfully applied in 12 healthy persons of different age and gender, and in a patient with strong atherosclerosis during ballon dilatation of the carotid artery. The results can be utilized for estimating the visco-elastic behaviour of the vessel wall. Pathological changes of the vessel wall structure can be detected.

EBCT imaging of heart and lungs triggered by ECG and ventilation

A device has been developed to monitor both the bioimpedance across the thorax using the four-electrode method and the ECG. The transthoracic bioimpedance signal depends on the alveolar air volume. Coincidence of the ECG related trigger signal (e.g. R-wave) with a set tolerance band for this bioimpedance signal is determined and employed to trigger the electron beam computerised tomography (EBCT) imaging. The coincidence rate of the R-wave with the alveolar air volume depends on the width of the tolerance band. Furthermore, the coincidence rate depends on the position of the tolerance band between the endinspiratory and the endexpiratory volume. When the tolerance band is set to 10% of the tidal volume (approx. 50 ml) the displacement effect is estimated to be about 2 mm. Under these conditions, the total cycle time is prolonged to about 200 s. The trigger device has been successfully used to control the EBCT machine.

EIN ANSATZ ZUR OPTIMIERUNG DER PROZESSSTEUERUNG BEI DER HAMODIALYSE

Trotz entscheidender Verbesserungen in der Dialysetechnik kommt es heute bei etwa 25 % aller Behandlungen zum Auftreten von Akutkomplikationen. Es wird daher ein patientenindividueller Ansatz zur Optimierung dieser Therapieform präsentiert. Dieser Ansatz führt zur Behandlung eines off-line Steuerungsproblems für die relevanten Dialyseaustauschvorgänge. Aufgrund der Komplexität dieses Problems wurde das effiziente Kontrollverfahren PSEUDYGALG entwickelt, das hinsichtlich der Relation zwischen zu tätigendem Lösungsaufwand und erzielter Lösungsqualität zufriedenstellende Resultate liefert. Die Kontrollnebenbedingungen sind durch die Beschreibung der Dialyseaustauschvorgänge mittels eines Multikompartimentmodells gegeben. Zur Einbeziehung von Expertenwissen hinsichtlich des klinischen Optimierungsziels wird ein Ansatz eines Expertensystems präsentiert. Das Multikompartiment-Modell, das Expertensystem und der Controller PSEUDYGALG sind in einem globalen Optimierungsschema miteinander verknüpft. Erste Ergebnisse des Controllers zeigen eine hinreichende Realisierbarkeit fiktiver Zieldynamiken für die Elektrolytkonzentrationen im Patientenblut und verschiedener Volumendynamiken mittels entsprechend berechneter Dialysatund Infusionseinstellungen.