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[Theoretical analysis of the efficacy of G suits during exposure to continuously increasing gravitational overloads].

A mathematical model of human circulation was employed to examine circulation responses to +GZ acceleration the value of which increased linearly at the rate 0.1 G/sec, using subjects having an anti-G suit on and sitting in a relaxed posture. It has been calculated that the anti-G suit can compensate as much as 83% of the increment of hydrostatic pressure in leg vessels and as much as 57% in abdominal vessels. The suit effect on resistance and capacity vessel properties makes an approximately equal contribution to an increase of the acceleration tolerance threshold. However the occlusion effect of the anti-G suit causes a significant increase of afterload.

Acceleration↗
PubMed1986

Description of flow phenomena in magnetic resonance imaging.

The magnetic resonance (MR) signal from the hydrogen nuclei of blood is not only determined by the MR parameters T1, T2 and proton density, but is strongly dependent on the movement of the protons. Magnetic resonance imaging (MRI) offers therefore the possibility to visualize the distribution of moving spins, especially blood flow, noninvasively and without contrast agents; moreover, the velocity of the moving spins can be quantified. Flow phenomena in MRI are a pretentious field because the complicated hydrodynamics of the living system is coded in the MR signal; therefore, an understanding of the underlying physical principle of the MR signal is required to interpret and extract flow information from the image. However, rather simple theoretical and experimental models of fluid transport in vessels allow to explain the main features of various effects observed in images.

Blood Circulation↗
PubMed1987

The effect of the gradient of vascular permeability on fluid and plasma protein exchange in the mesenteric microcirculation.

The present work is a theoretical analysis of the intramural and transmural flows of fluid and plasma proteins. The transvascular fluxes are treated on the basis of a phenomenological approach resulting from the irreversible thermodynamics. The concept leads to a generalization of the well known Starling equation for the fluid exchange and to an additional equation which accounts for the exchange of plasma proteins. The key element in the theory is the incorporation of axial variations of the vascular permeability in the model. Numerical results are obtained for the cat mesentery taking into account the particular angioarchitecture of that tissue. The strength of the present model lies in the precise localization of the transvascular fluxes which results from the differential treatment of the fluid dynamic and mass transfer problem. The results confirm vital microscopic findings of various authors that fluid filtration and the plasma exchange occur predominantly on the arteriolar and venular side of the microcirculatory bed. In contrast, the exchange in the capillaries is relatively small regardless of the actual value of the gradient of permeability. In the organ specific situation studied, the transvascular fluxes are directed outward everywhere to the tissue. The general effect of the vascular gradient of permeability is to enhance the local transvascular fluxes on the venous side.

Blood Proteins↗
PubMed1987

Signal processing options for detecting conduction abnormalities in ischemic ventricles.

Sources of error in averaging for late ventricular potentials include variations in the magnitude and timing of late potentials as well as variability in the temporal alignment of successive cardiac cycles during averaging. Since these errors can both attenuate high frequency signals and add artifactual components to the ECG, we developed methods to estimate and minimize them. Studies on patients indicated that small misalignments can obliterate high frequency components of the QRS, while broadening the low amplitude tail of the QRS, possibly leading to erroneous interpretations. The digital cross-correlation method of alignment was evaluated using simulated signals, and was shown to be relatively insensitive to gross variations in waveshape, and appeared to be less accurate than real-time pattern recognition schemes. A real-time alignment method based on a mathematical model was developed that could measure its accuracy, and was implemented in hardware. Our method improves alignment and average fidelity. Direct recordings from human and canine ventricles revealed that beat-to-beat variability in activation patterns of ischemic regions often followed a regular pattern such as 2:1 block. Spectral analysis of ECG recordings showed that a sub-harmonic, indicative of a 2:1 pattern, could be detected in the ECG during periods when ischemic regions of the ventricle were experiencing 2:1 block. These results suggest the possible utility of sub-harmonic analysis as a tool for detection of abnormal electrophysiological conduction.

Animals↗
PubMed1980

[Angioarchitecture of gliomas. 1: Models of cerebral vessels, casting materials, preparation methods, changes in tumor vessels].

A report is given on the study of cerebrovascular systems in experimental animals and corpses and the demonstration of cerebral tumours be means of models of vascular system. After a survey of the development of model representation, the technique of the vessel injection, the various injection materials and methods of preparation are described in detail. Advantages and disadvantages of the individual methods and certain peculiarities in model representations of transplantation tumours and various human brain tumours are presented. Besides, a multitude of pictures which document the results of the experiments, general laws of the tumour development, angioarchitectufe of the cerebral tumours and the changes in pre-existing and newly formed vessels in and around the tumour are briefly dealth with. The individual results of systematic investigations into the vascular system of cerebral tumours--of human brain vessel models, of transplantation tumours and special angioarchitecture of gliomatous brain tumours--will be published in individual articles appearing later.

Animals↗
PubMed1982

Interpretation of multigated Fourier functional images.

First-harmonic Fourier analysis is currently used to aid in the interpretation of multigated cardiac studies. Its intrinsic inaccuracies are not generally appreciated. This study investigates the characteristics and magnitudes of the errors of this technique. The study analyzes computer-generated phantoms that isolate the various motions of the ventricles (contraction, translation, and rotation) with the first-harmonic approach. The first-harmonic output is compared with a more accurate fitting scheme using multiple terms of the Fourier expansion. Significant artifacts of the inaccuracy of the first harmonic appear in the phantom studies and are observed in patient examples. We conclude that caution is needed in interpreting first-harmonic phase and amplitude images, and particularly in associating them with parameters like the onset of contraction and the stroke volume.

Computers↗
PubMed1980

[Dynamics of pO2 changes in brain tissue during physiologic variations in capillary blood flow rate].

The effect of the rate of the blood flow on pO2 dynamics in capillaries, nerve cells, and surrounding tissue was analyzed on the mathematical model. A two-fold variation in the blood flow rate through capillaries from the average level of 500 micro/sec induced equal changes of pO2 of about 12 torr. A two-fold change in the intensity of oxygen consumption by a cell from the level of 400 microliter O2/g.min shifted the average pO2 in the cell body and surrounding tissue approximately by 2 times and 1.2--1.3 times, resp. Under all these conditions, when the rate of blood flow or the oxygen consumption by a cell changed sharply, a new stable state of pO2 was reached in 2--3 sec. The data obtained suggest that the rates of blood flow in cerebral capillaries are determined by physiological changes in oxygen demand by nerve cells and glia. The diffusion as a mechanism of oxygen transport in the brain, seems both to supply nerve cells with a sufficient amount of O2 under stationary conditions and induce rapid changes in the rate of O2 transport in response to changes in demand for it.

Blood Flow Velocity↗
PubMed1995

A new experimental model and formula for flow determination and possible application in sizing of interatrial communication.

The concept of an artificial interatrial communication in application of the Fontan procedure in high risk patients has raised the problem to exactly determine the shunt amount. This experimental study shows that in vitro flow studies of interatrial communications can be performed without animal blood and PTFE membranes respectively. Furthermore the simulation of an pulmonary vascular venous flow is not necessary. Additionally a new formula with acceptable accuracy for flow estimation is derived from the presented data obtained by this study.

Animals↗