Journal of Mechanics in Medicine and Biology

The phase opposition of velocity waveforms between coronary arteries (predominantly diastolic) and veins (systolic) is the most prominent characteristic of coronary hemodynamics. The phase opposition indicates the importance of intramyocardial capacitance vessels, as a determinant of phasic coronary arterial and venous flows. To investigate the functional characteristics of the intramyocardial capacitance vessels and its physiological significance, we analyzed the change in venous flow following changes in coronary arterial inflow. It was shown that during diastole the intramyocardial capacitance vessels have two functional components, unstressed volume and ordinary capacitance. Unstressed volume is defined as the volume of blood in a vessel at zero transmural pressure, and it was ~5% of the volume of the myocardium. The systolic coronary venous outflow showed a significant, positive correlation with the total displaceable blood volume stored in the intramyocardial unstressed volume and ordinary capacitance. When the unstressed volume was saturated, the coronary inflow was decreased significantly, compared with that for the unsaturated condition. Thus, the increase in intramyocardial blood volume decreases the coronary arterial inflow, whereas it enhances coronary venous outflow. The latter is an interesting analogy to the Starling's law of the heart.

The effect of a vessel joint of different properties on the heart pulse propagation and anastomosis junction is investigated. The system under consideration is modelled by an elastic piecewise-constant vessel consisting of two perfectly jointed vessels of different thicknesses and radii. A mathematical model is developed, to perform numerical calculations and to obtain some qualitative and quantitative estimations. The effect of different thicknesses and radii of jointed blood vessels on the concentrations of pressure, radial vessel wall displacement, bending moments and shear forces are analyzed in detail.

Transmitral flow velocity pattern obtained by pulsed Doppler technique reflects left ventricular (LV)diastolic function, but an increase in left atrial pressure pseudo-normalizes the flow pattern and masks diastolic dysfunction. Flow propagation velocity (FPV) measured, using color M-mode Doppler and baseline-shift technique, represents an average velocity of early diastolic LV filling flow from mitral orifice to mid-ventricle. In patients with ischemic heart disease and dilated cardiomyopathy, including those with pseudonormalized transmitral flow pattern, FPV had good correlation with the time constant of early diastolic LV pressure decay (Tau), indicating that FPV is a useful noninvasive diastolic-function parameter which does not pseudo-normalize. Unlike the conventional Doppler parameters, FPV was well correlated with Tau in patients with hypertrophic cardiomyopathy, and was distinctly decreased even in hypertensive patents without LV hypertrophy. Thus, FPV is a unique noninvasive diastolic parameter, which can sensitively and accurately detect LV diastolic impairment.

The objectives of this paper are: (1) to analyze how the bypass diameter and the degree of the stenosis affect the flow-induced wall shear stresses in the stenotic and bypass vessels and (2) to propose a clinically usable method for a proper selection of the bypass diameter. The problem is solved in the following stages: First, the steady pressure and flow within the vessels are obtained using general laws of mass and energy conservation. Then, on the basis of modified Womersley's theory to take into account the nonlinear mechanical properties of the vessels, the oscillating pressure and flow within the vessels are calculated. They are used to derive expressions for the reflection coefficients at the distal (converging) bifurcation and to calculate wall shear stresses.

We hypothesize that the minimal differences in the wall shear stresses of the normal artery, stenotic artery and the bypass graft mitigate the development of anastomotic neointimal hyperplasia and graft pseudo intima. Using cost functions, based on steady and mean wall shear stresses, we calculated the optimal value of the undeformed bypass diameter for various degrees of stenosis. We found that the sum of cross-sectional areas of the optimal graft and the stenotic artery (normalized with respect to the cross-sectional area of the normal artery) varies between 1.14 and 1.28, an interval, which is consistent with the findings of McDonald (1974) for the area of the major branches in the body. Thus we conclude that the results obtained are consistent with the stated hypothesis.

We have aimed to assess total Ca²⁺ handling in excitation-contraction coupling in a beating left ventricle (LV). Our newly developed integrative analysis method utilizes the internal Ca²⁺ recirculation fraction (RF), O₂ consumption (Vo₂) for Ca²⁺ handling, and O₂ cost of Emax (contractility index) of the LV. We have obtained the O₂ cost of Emax from Vo₂ measured at different contractility levels, and have combined the cost with RF calculated from the beat-constant of the exponential decay component of the postextrasystolic potentiation. Our method calculates the unknown total Ca²⁺ handling from the RF and the "Ca²⁺ handling Vo₂". The calculated total Ca²⁺ handling fell between 30 and 110 μmol/kg, depending on contractility and pathological conditions. The present method also enable's reasonable assessment of futile Ca²⁺ cycling and of the Ca²⁺ reactivity of Emax. Our method seems useful to better understanding of the pathophysiology of total Ca²⁺ handling in a beating heart.

The models of viscoelasticity are considered from the point of view of their applicability to describe the behavior of blood vessel wall material. First, some convenient and popular models are considered briefly. These models are widely used, particularly in engineering applications, because of their simplicity and clear physical treatment. At the same time, these models are not representative of many real (and particularly anatomical) materials. As a result, new nonlinear models have been developed by Fung and other researchers for biomaterials. However before them, one of the generalized model of viscoelasticity has been developed by Knopoff and MacDonald. Here this model is considered and applied for evaluation of blood vessel wall characteristics. Unlike the convenient models, this model is based on fundamental thermodynamic concepts, and takes into account some more realistic features of viscoelastic solids. Application of this model to determine the characteristics of blood vessel material is presented.

An athlete may be near exhaustion because his muscles have depleted their supply of ATP, and oxidation and glycolysis have not yet been able to replenish the muscles with their primary source of energy.

In order that the time relations of these processes offer some practical hints, a simplified "pay-off" theory for the "best" possible way to reach a desired velocity is examined, in terms of its physical appropriateness and mathematical foundation.

The derivation of the basic equations under quite general assumptions is provided. A discussion of the physical interpretation of the state and control parameters is given, and a solution of typical simplified problems is obtained.

Flow visualization in reciprocating flow inside branching tubes was performed to investigate the mechanism of axial gas exchange in the human lung system. A bronchial tube model is employed which is geometrically similar to the average human lung system. Water is used as the working fluid. The ranges of Reynolds and Womersley numbers in the present study correspond to those of normal human respiration and HFV (high frequency ventilation), respectively. It is revealed that the axial gas exchange phenomenon occurring in the human lung system is governed by a "trap-and-release" mechanism caused by the formation-and-destruction of the separation regions, which are formed on the wall of the parent tube and or daughter tubes, depending on the direction of the reciprocating flow.

Diabetes mellitus is a heterogeneous clinical syndrome characterized by hyperglycemia and long-term specific complications: retinopathy, neuropathy, nephropathy, and cardiomyopathy. Automatic neuropathy leads to visceral denervation producing a variety of clinical abnormalities: cardiac and respiratory dysrythaemias, gastrointestinal motility disorders, urinary bladder dysfunction and impotence. Diabetes mellitus is a leading cause of blindness; renal failure and limb amputation all over the world. The need to detect diabetic risk factors and treat organ disorders and complications associated with diabetes provides the impetus for us to develop the technology for assessment of diabetes, its etiology and severity, as well as for assessing the efficacy of pharmacological therapy.

This paper concerns:

(i) modelling of blood-glucose regulation and tolerance-testing,

(ii) demonstrating patient-simulation of the blood-glucose regulatory models, by means of which the model parameters can be evaluated and related to physiological parameters, and

(iii) elucidating how the glucose-regulatory system model's pole-zero representation and the blood glucose-insulin transfer-function can explain the blood glucose response data in intravenous and oral glucose tolerance tests.

An easy-to-implement simple clinical-application method is developed to simulate the response of the blood-glucose regulatory model in diabetic patients during intravenous glucose tolerance test and to estimate the model parameters, which can then enable differential diagnosis of diabetes and its severity as well as in early detection of risk-to-diabetes.

In the oral glucose-tolerance test, the role of the gut is to facilitate transport of glucose across the intestinal wall. The Michaelis-Menten equation, describing this enzyme-catalyzed reaction rate, can be employed to conclude that the intestinal glucose absorption rate into the blood-compartment from the gut during the oral glucose-tolerance test is constant, almost resembling a rectangular pulse Nevertheless, we have formulated a new rate-control model to simulate the oral glucose-tolerance test data, by means of the response-function of a second-order system of a single-compartment (consisting of the gut and the blood-glucose pool), with the oral glucose-bolus as the impulse-input.

We have also demonstrated application of this rate-control model to patients undergoing oral glucose-tolerance test, to evaluate the model parameters. By categorizing the ranges of these parameters for normals and diabetics (varying from mild to severe), we can reliably apply this model and procedure clinically.