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At least 631 records · Page 35Linked to original sources
PubMedDate not supplied

Electrical current distribution under transthoracic defibrillation and pacing electrodes.

The known effect of high current density under the perimeter of defibrillation electrodes, leading to skin damage and even severe burns in some cases, has been considered by many investigators. Two main approaches for improvement were proposed: (i) interfacing with layers of varying and high resistivity and (ii) lengthening and shaping the perimeter line. Using finite element and physical modelling, it is shown that the second approach does not yield significant improvement in the distribution uniformity. Moreover, the application of high resistivity layers is unacceptable in dibrillation. The use of a low resistance layer with a diameter covering and extending over the metal plate by at least 2.5 mm results in better uniformity. A similar effect can be obtained by recessing the metal plate in an isolating support--an approach adopted from implantable neurostimulation electrodes. These two versions can be applied in combination.

Cardiac Pacing, Artificial↗
PubMedDate not supplied

Analysis of the operation of the SCD Response intermittent compression system.

The work assessed the performance of the Kendall SCD Response intermittent pneumatic compression system for deep vein thrombosis prophylaxis, which claimed to set its cycle according to the blood flow characteristics of individual patient limbs. A series of tests measured the system response in various situations, including application to the limbs of healthy volunteers, and to false limbs. Practical experimentation and theoretical analysis were used to investigate influences on the system functioning other than blood flow. The system tested did not seem to perform as claimed, being unable to distinguish between real and fake limbs. The intervals between compressions were set to times unrealistic for venous refill, with temperature changes in the cuff the greatest influence on performance. Combining the functions of compression and the measurement of the effects of compression in the same air bladder makes temperature artefacts unavoidable and can cause significant errors in the inter-compression interval.

Artifacts↗
PubMedDate not supplied

On the variability of QRS time-duration in magnetocardiographic recordings.

High resolution electrocardiography (HRECG) recordings have already shown an increased beat-to-beat microvariability of the QRS duration of the terminal QRS in patients with a history of ventricular tachycardia (VT). The purpose of this study is to detect QRS-duration microvariability with magnetocardiographic (MCG) recordings in normals, patients with coronary heart disease (CHD), patients with a history of myocardial infarction (MI), and VT patients. QRS microvariability is calculated as the variance of time-shifts of single beats respectively to the average of all beats. The average over all channels of the MCG is performed. QRS microvariability was evaluated from 55-channel MCG in 15 normal persons, in 12 patients with CHD, in 13 patients with MI, and in 10 patients with VT. We found a significantly higher microvariability in patients with MI compared to normals. The highest microvariability was found in VT patients.

Adult↗
PubMedDate not supplied

Sampling frequency of the RR interval time series for spectral analysis of heart rate variability.

Spectral analysis of heart rate variability (HRV) is an accepted method for assessment of cardiac autonomic function and its relationship to numerous disorders and diseases. Various non-parametric methods for HRV estimation have been developed and extensive literature on their respective properties is available. The RR interval time series can be seen as a series of non-uniformly spaced samples. To analyse the power spectra of this series using the discrete Fourier transform (DFT), we need to interpolate the series for obtaining uniformly spaced intervals. The selection of sampling period plays a critical role in obtaining the power spectra in terms of computational efficiency and accuracy. In this paper, we shall analyse the RR interval time series from selected subjects for different sampling frequencies to compare the error introduced in selected frequency-domain measures of HRV at a constant frequency resolution for a specific duration of electrocardiogram (ECG) data. It should be pointed out that, although many other error causes are possible in the frequency-domain measures, our attention will be confined only to the performance comparison due to the different sampling frequencies. While the choice of RR interval sampling frequency (f(s)) is arbitrary, the sampling rate of RR interval series must be selected with due consideration to mean and minimum RR interval; f(s = )4 Hz was proposed for a majority of cases. This is an appropriate sampling rate for the study of autonomic regulation, since it enables us to compute reliable spectral estimates between dc and 1 Hz, which represents the frequency band within which the autonomic nervous system has significant response. Furthermore, resampled RR intervals are evenly spaced in time and are synchronized with the samples of the other physiologic signals, enabling cross-spectral estimates with these signals.

Algorithms↗
PubMedDate not supplied

Mathematical model of chloride concentration in human blood.

This paper deals with mathematical modelling of blood chloride concentration. The main features of the model are that it reveals mathematically the physiological relationship between blood chloride and other electrolytes and serves as an accurate indirect method for chloride measurements with accuracy fulfilling clinical requirements. The main advantages of the method based on this model are that it is more comfortable than traditional methods and clinically less harmful for the patient under study. Experimental verification of the developed model ensures that the results of chloride measurements obtained using this model are significantly correlated with the results for the blood samples obtained from standard chloride analysers.

Algorithms↗
PubMedDate not supplied

A new strategy for easy volume conductor modelling in magnetocardiography.

Modelling the electromagnetic properties of the thorax in magnetocardiographic (MCG) studies is usually performed by the Boundary Element Method (BEM). Magnetic Resonance Imaging (MRI) scans are generally used as the basis for extracting the coordinates for BEM. As MRI is a (time) expensive technique and scanners have a high use demand, in this work a strategy is presented that reduces the costs and the need for additional MRI images. This strategy is based on the use of low resolution and incomplete MRI image sets of the thorax.

Body Surface Potential Mapping↗
PubMedDate not supplied

Development of a flow simulator to study haemodynamic behaviour of natural and artificial blood vessels under physiologic flow conditions.

A new computer-controlled flow simulator has been designed to study the haemodynamic behaviour of natural and artificial blood vessels under physiologic flow conditions. The simulator can generate well characterized and fully developed laminar flow properties. It includes a unique perfusion case that imposes an axial tension on the vessel segment, and a commercial programmable pump to reproduce pulsatile flow rates. Response to high frequency commands was greatly attenuated and displayed a frequency dependent phase angle. Thus, for complex pulsating flow rates containing different frequency components, the system response was significantly distinct from the command. To reproduce physiologic waveforms, the transfer function of the whole system was determined for different amplitudes and frequencies of flow rate excitations. Each input command was compared to the measured flow rate, and the values of the gain and phase angle were evaluated. If the desired flow rate was composed of a sum of n sine wave components, each has a frequency fj and an amplitude Aj, a corrected command signal was then reconstructed by amplifying the attenuated components and advancing those lagged in time. The corrected signal was finally applied as the new command to the pump. The results showed an excellent agreement with physiologic waveforms. Examples of different pulsatile flow experiments to investigate the effects of frequency, pressure, and wall elasticity are presented.

Blood Flow Velocity↗
PubMedDate not supplied

Analysis of regurgitation, mean systolic pressure drop and energy losses for two artificial aortic valves.

The work reported here is related to the hydrodynamic performance of a Jellyfish valve and St Vincent valve in terms of total energy losses, mean systolic pressure drop and regurgitation. The in vitro experimental investigation was conducted at cardiac outputs of 3.5, 4.5 and 6.51 min-1 across the two valves and under pulsatile flow condition. It was found that the closure volume of the St Vincent valve was about 2.5 times higher than that of the Jellyfish valve. The total back flow losses on the other hand were found to be in the range of 36.5 to 107.1 and 85.5 to 192.5 mJ for the Jellyfish valve and St Vincent valve respectively. Moreover, the mean systolic pressure drop of the St Vincent valve was found to be higher than that of the Jellyfish valve. However, for all the operating conditions tested here, the Jellyfish valve showed superior hydrodynamic performance in terms of backflow and mean systolic pressure as well as energy losses.

Aortic Valve↗
PubMed2005

Solving the heart mechanics equations with Newton and quasi Newton methods--a comparison.

The non-linear elasticity equations of heart mechanics are solved while emulating the effects of a propagating activation wave. The dynamics of a 1 cm(3) slab of active cardiac tissue was simulated as the electrical wave traversed the muscular heart wall transmurally. The regular Newton (Newton-Raphson) method was compared to two modified Newton approaches, and also to a third approach that delayed update only of some selected Jacobian elements. In addition, the impact of changing the time step (0.01, 0.1 and 1 ms) and the relative non-linear convergence tolerance (10(-4), 10(-3) and 10(-2)) was investigated. Updating the Jacobian only when slow convergence occurred was by far the most efficient approach, giving time savings of 83-96%. For each of the four methods, CPU times were reduced by 48-90% when the time step was increased by a factor 10. Increasing the convergence tolerance by the same factor gave time savings of 3-71%. Different combinations of activation wave speed, stress rate and bulk modulus revealed that the fastest method became relatively even faster as stress rate and bulk modulus was decreased, while the activation speed had negligible influence in this respect.

Action Potentials↗
PubMed1989

A computer controlled flow phantom for generation of physiological Doppler waveforms.

A flow phantom for the generation of physiological Doppler waveforms is described. The suspension of scattering particles is driven by a gear pump powered by a stepping motor. The speed of the stepping motor is controlled by a BBC microcomputer. The waveform shape is selected from a library of waveforms from disc. Use of the microcomputer allows the waveform shape and mean flow to be easily changed. Sephadex particles suspended in a solution of glycerol were used as artificial blood. Thin walled heat shrink tubing which had been moulded around metal rods was used. Distortions in the waveforms caused by reflections from the end of the tubing were largely removed by reducing the pipe diameter to half of its value for 30 cm from the end of the pipe. There was good agreement between the control waveforms and the Doppler waveforms over a wide range of waveform pulsatility.

Blood Circulation↗
PubMed1994

Biomagnetic localization of electrical current sources in the human heart with realistic volume conductors using the single-current-dipole model.

The boundary element method was applied in order to investigate the localization accuracy for focal sources measured from MCG data. Various homogeneous volume conductor models were composed: the individually shaped torso, a scaled standard torso, an unscaled standard torso, a scaled cuboid and a scaled ellipsoid. We implemented these models in single-dipole inverse solution techniques. High resolution multichannel data were analysed from two patients showing ventricular extrasystoles and two patients suffering from Wolff-Parkinson-White syndrome. Moreover, we report the localization of shallow- and deep-lying catheters (depth 9 cm and depth 17.5 cm below the measurement grid). Using an individually shaped homogeneous torso yields a localization error of less than 3 cm even for the deepest sources (mean error 2.4 cm). Probability-based dipole localization shows that the remaining error could only partly be explained by data noise statistics. Therefore it seems to be due to either inner inhomogeneities or the inadequacy of the single current dipole or a combination of the two. Thus clinically useful localization accuracy in the millimetre range requires more sophisticated volume conductor and source models. The evaluation of measurement data and simulation study shows that a scaled cuboid model can provide nearly the same localization accuracy as the individually shaped torso model. Single dipole reconstruction with this model is computationally faster than that with the individually shaped model of the human body and is fast enough for use in clinical applications.

Arrhythmias, Cardiac↗
PubMed1995

Elastic properties of human aortas in relation to age and atherosclerosis: a structural model.

A new structural model is described for the tension-radius relationship of blood vessels, taking into account their mechanically important constituents: collagen, elastin and smooth muscle. The model has four characteristic parameters: EC, the Young's modulus of the collagen fibres; ESE, the Young's modulus of the combined smooth-muscle/elastin network; epsilon mu, the amount of strain at which the high stiffness region on the tension-radius curve is reached, and eta an indicator for the degree of collagen fibre stretching. The structural stiffness of the wall constituents is reflected by EC and ESE whereas the global stiffness of the entire blood vessel is described by epsilon mu and eta. All these elasticity parameters are pressure independent, in contrast to generally quoted values for the incremental modulus or vascular compliance which are strongly pressure dependent. Hence, an objective comparison of the mechanical properties for various types of blood vessel, based on the present model parameters, has been made possible. The model was successfully fitted to tension-radius data of 65 human aortas, age range 30-88 years, with moderate or severe atherosclerosis. The structural as well as the global stiffness changes with age, e.g. collagen stiffness shows a ninefold increase over 60 years. Global stiffness depends on atherosclerosis.

Adult↗
PubMed1995

Large blood vessel cooling in heated tissues: a numerical study.

Large blood vessels can produce steep temperature gradients in heated tissues leading to inadequate tissue temperatures during hyperthermia. This paper utilizes a finite difference scheme to solve the basic equations of heat transfer and fluid flow to model blood vessel cooling. Unlike previous formulations, heat transfer coefficients were not used to calculate heat transfer to large blood vessels. Instead, the conservation form of the finite difference equations implicitly modelled this process. Temperature profiles of heated tissues near thermally significant vessels were calculated. Microvascular heat transfer was modelled either as an effective conductivity or a heat sink. An increase in perfusion in both microvascular models results in a reduction of the cooling effects of large vessels. For equivalent perfusion values, the effective conductivity model predicted more effective heating of the blood and adjacent tissue. Furthermore, it was found that optimal vessel heating strategies depend on the microvascular heat transfer model adopted; localized deposition of heat near vessels could produce higher temperature profiles when microvascular heat transfer was modelled according to the bioheat transfer equation (BHTE) but not the effective thermal conductivity equation (ETCE). Reduction of the blood flow through thermally significant vessels was found to be the most effective way of reducing localized cooling.

Animals↗
PubMed1997

Accuracy of geometrical modelling of heat transfer from tissue to blood vessels.

We have developed a thermal model in which blood vessels are described as geometrical objects, 3D curves with associated diameters. Here the behaviour of the model is examined for low resolutions compared with the vessel diameter and for strongly curved vessels. The tests include a single straight vessel and vessels describing the path of a helix embedded in square tissue blocks. The tests show the excellent behaviour of our discrete vessel implementation.

Blood Vessels↗
PubMed1997

Tests of the geometrical description of blood vessels in a thermal model using counter-current geometries.

We have developed a thermal model, for use in hyperthermia treatment planning, in which blood vessels are described as geometrical objects; 3D curves with associated diameters. For the calculation of the heat exchange with the tissue an analytic result is used. To arrive at this result some assumptions were made. One of these assumptions is a cylindrically symmetric temperature distribution. In this paper the behaviour of the model is examined for counter-current vessel geometries for which this assumption is not valid. Counter-current vessel pairs intersecting a circular tissue slice are tested. For these 2D geometries vessel spacing, tissue radius and resolution are varied, as well as the position of the vessel pair with respect to the discretized tissue grid. The simulation results are evaluated by comparison of the different heat flow rates with analytical predictions. The tests show that for a fixed vessel configuration the accuracy is not a simple decreasing function of the voxel dimensions, but is also sensitive to the position of the configuration with respect to the discretized tissue grid.

Blood Vessels↗
PubMed1998

Tissue motion assessment from 3D echographic speckle tracking.

The potential of ultrasonic image speckle tracking to characterize tissue dynamics has been illustrated and validated elsewhere. In this paper we wish to extend this speckle tracking methodology to 3D. To investigate the feasibility of such an approach we first model the image formation process and simulate the 3D speckle motion inherent to tissue linear transformations (translation, rotation and deformation). It is shown that tissue axial rotation and translation are perfectly correlated with the tissue speckle motion while tissue deformation and non-axial rotations corrupt the speckle pattern with a motion-induced noise and are therefore more difficult to track when large motions are concerned. Furthermore, in the framework of our model, our results indicate that short ultrasound pulses with low frequencies and small beamwidths are more desirable for a speckle tracking methodology. The feasibility of speckle tracking is illustrated with an optical flow algorithm. A theoretical study of the correlation between various linear transformations of the tissue and the corresponding ultrasonic speckle motions is also performed.

Algorithms↗
PubMed1999

An equation describing spread of membrane potential changes in a short segment of blood vessel.

The spread of membrane potential changes throughout certain cells and tissues plays an important role in their physiology. The attenuation of such changes in any tissue is usually characterized by the cable length constant lambda, which can be determined experimentally if the equations describing membrane potential spread in the tissue are known. Here we derive an equation describing spread of membrane potential changes in a short cable, which is an appropriate model for short segments of blood vessels. This equation is more general than those already published in that the positions of both the current source that gives rise to a potential change, and the point at which the change is measured, can be anywhere along the cable.

Animals↗
PubMed1999

Nonlinear analysis of blood flux in human vessels.

Laser Doppler fluxmetry (LDF) is frequently used in research on microcirculation of blood. Usually LDF time series are analysed by conventional linear methods, mainly Fourier analysis. These methods may not be optimal for the investigation of nonlinear effects of vasomotion, heartbeat or vessels. Nonlinear methods are based on a reconstruction of the system trajectory in an embedding space describing not only the measured time series but the behaviour of the whole system. The fill factor is a tool for displaying the main properties of this attractor in two dimensions and for determining diverse parameters for further analysis. A quantitative characterization of the system is possible by the distribution of correlation dimensions in the embedding space. The singular value decomposition (SVD) can be used to display and characterize individual degrees of freedom. These methods were applied to LDF time series from nine healthy controls and nine patients with Raynaud's phenomenon due to connective tissue disease. The fill factor and the SVD indicate qualitatively that in the controls vasomotion and heartbeat are the main influences on blood flow and act fairly independently of each other. In the patients there was a mixture of strong but irregular degrees of freedom. The mean and the maximal local correlation dimensions were significantly higher in the patient group. Nonlinear analysis of LDF time series provides additional information which cannot be detected using conventional approaches.

Adult↗