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Biomedical subjects

D M McQueen

Publications and source records attributed to D M McQueen.

12 recordsLinked to original sources
PubMed1995

A general method for the computer simulation of biological systems interacting with fluids.

At this Symposium on Biological Fluid Dynamics, it is appropriate to ask whether there is any common theme that unites the diverse problems that arise in the study of living systems interacting with fluids. The answer that immediately comes to mind is this: biological fluid dynamics invariably involves the interaction of elastic flexible tissue with viscous incompressible fluid. (In many cases the tissue is not only elastic, it is also active, i.e. capable of doing work on the fluid). This paper describes the immersed boundary method, which is a general framework for the computer simulation of biofluid dynamic systems. This method has already been applied to blood flow in the heart (including the computer-assisted design of prosthetic cardiac valves), platelet aggregation during blood clotting, aquatic animal locomotion, wave propagation along the basilar membrane of the inner ear, and flow in collapsible tubes. In the immersed boundary method, the elastic (and possibly active) biological tissue is treated as a part of the fluid in which additional forces (derived from the tissue stresses) are applied. Because the tissue is represented in terms of its force field, the method remains straightforward, even when the geometry of the biological tissue is complicated, dynamic and not known in advance.

Animals↗
PubMed1994

Mechanical equilibrium determines the fractal fiber architecture of aortic heart valve leaflets.

In this work, the structure of the aortic valve is derived from its function, which (in the closed-valve configuration) is to support a uniform pressure load. It is assumed that this load is transferred to the aortic wall by a one-parameter family of fibers under tension. The equation of equilibrium for this fiber structure turns out to be equivalent to the equation of motion of vortex lines in the self-induction approximation. The method of Buttke (J. Comput. Phys. 76:301-326, 1988) is used to solve these equations and, hence, to determine the fiber architecture of the aortic leaflets. Because of a singularity at the center of the aortic valve, the computed fiber architecture has a fractal character with increasing complexity at progressively smaller scales. The computed fiber architecture resembles the branching braided structure of the collagen fibers that support the real aortic valve.

Aortic Valve↗
PubMed1992

Cardiac fluid dynamics.

The heart is modeled as a system of elastic and/or contractile fibers immersed in a viscous incompressible fluid. Simulated heart walls and valves are constructed by arranging the fibers according to an idealized version of the actual distribution of muscle fibers in the heart walls and collagen fibers in the valve leaflets. Then the combined motion of the fluid-fiber system is predicted through the numerical solution of its coupled equations of motion. Fluid equations are solved by a finite difference method on a fixed, regular computational lattice. Fiber points move freely through this lattice without being constrained to lie at the lattice intersections. Communication between fibers and fluid involves interpolation of the fluid velocity to the fiber points and the spreading of the fiber forces to the computational lattice of the fluid. Both of these operations make use of a smoothed approximation to the Dirac delta function. The entire method is suitable for implementation on vector, parallel, or parallel-vector hardware. Applications include the investigation of normal cardiac function, the simulation of disease processes affecting the mechanical function of the heart or its valves, and the computer-assisted design of prosthetic cardiac valves.

Aortic Valve↗
PubMed1988

Two-mass model of the vocal folds: negative differential resistance oscillation.

The vocal folds and glottis are analyzed as a single system rather than as two separate but interacting systems, i.e., an aerodynamic one (the glottis) and a mechanical one (the vocal folds). Simplified steady flow calculations based on the two-mass model, and similar to those of Ishizaka and Matsudaira [SCRL Monograph No. 8, Santa Barbara, CA (1972)], are made except that flexible walls are assumed for both dc and ac flows. A negative differential resistance is found for steady flow when the coupling spring is weak compared to that of the lower mass. Dynamic transverse motion of the masses is represented by two transverse series resonant circuits in parallel within the glottis. The vocal tract is represented by a lumped resistance and inertance in series. Sustained, self-excited, small-amplitude oscillations can be obtained when the magnitude of the negative differential resistance is equal to the real part of the impedance of the rest of the circuit. The oscillation frequency depends only on the elasticity and mass of the vocal folds. The present analysis differs from Ishizaka and Matsudaira's analysis because their oscillation frequency decreases as dc volume velocity increases.

Glottis↗
PubMed1985

Effects of timing of atrial systole on LV filling and mitral valve closure: computer and dog studies.

Atrioventricular (AV) delay that results in maximum ventricular filling and physiological mechanisms that govern dependence of filling on timing of atrial systole were studied by combining computer experiments with experiments in the anesthetized dog instrumented to measure phasic mitral flow. Ventricular filling volume is maximized at AV delay of 100 ms in the computer study and 80 ms in the dog study. At any time in diastole atrial contraction accelerates mitral flow, opening the mitral valve widely; atrial relaxation then decelerates mitral flow, moving the valve leaflets toward closure. The time the valve remains closed following atrial systole varies inversely with AV delay. When AV delay is optimal, the mitral valve is moving rapidly toward closure but is not yet closed at onset of ventricular systole. The decline in filling volume as AV delay decreases below its optimum value is primarily the result of premature termination of atrial ejection by ventricular systole. As AV delay increases above its optimal value, filling volume progressively decreases because of premature mitral valve closure that limits effective diastolic filling period. There is no significant retrograde mitral flow at any point in diastole for any AV delay.

Animals↗
PubMed1985

Computer-assisted design of butterfly bileaflet valves for the mitral position.

This paper describes the application of computer testing to a design study of butterfly bileaflet mitral prostheses having flat or curved leaflets. The curvature is in the plane normal to the pivot axes and is such that the convex sides of the leaflets face each other when the valve is open. The design parameters considered are the curvature of the leaflets and the location of the pivot points. In this study, stagnation is assessed by computing the smallest value (over the three openings of the valve) of the peak velocity, and hemodynamic performance is judged by a benefit/cost ratio: the net stroke volume divided by the mean transvalvular pressure difference. Unlike the case of a pivoting single-disc valve, the inclusion of a constraint on the maximum angle of opening of the leaflets is found to be essential for adequate, competent performance. Results are presented with both 85 degrees and 90 degrees constraints, since best performance is achieved with the opening-angle constraint in this range. Asymmetry of leaflet motion which is observed with flat leaflets in the mitral position is reduced with modest leaflet curvature. Leaflet curvature also ameliorates central orifice stagnation, which is observed with flat leaflets. Curvature of the valve produces the following improvements in comparison with the best flat valve when the opening-angle constraint is 85 degrees: a 38% increase in the minimum peak velocity and a 16% increase in the hemodynamic benefit/cost ratio. With a 90 degrees constraint the corresponding improvements are 34% and 20%, respectively.

Computers↗
PubMed1983

Computer-assisted design of pivoting disc prosthetic mitral valves.

This paper describes the use of the new technique of computer testing to study the simulated performance of flat and curved pivoting disc prosthetic mitral valves. The design parameters considered are the radius of curvature of the occluder and the position of the pivot point. The performance criteria are the net stroke volume, the mean forward pressure difference, and the peak anterior velocity. The last of these criteria may be important in the prevention of small-orifice valve thrombosis. The best overall valve in the study has a radius of curvature equal to 1.5 times the diameter of the occluder and a pivot point located 0.39 mitral-ring diameters from the anterior border of the mitral anulus. The maximum angle of opening of this optimal valve is limited to about 70 degrees by fluid-mechanical forces. Nevertheless, the inclusion of a redundant stop in the mechanical design of the valve is indicated, since the computer experiments also reveal excessive opening and failure to close in valves with nearby parameter values. These studies illustrate the usefulness of computer testing in prosthetic valve design.

Animals↗
PubMed1982

Fluid dynamics of the mitral valve: physiological aspects of a mathematical model.

This paper presents a computational method that can be used to study the fluid computational method that can be used to study the fluid dynamics of the natural or prosthetic mitral valve in a computer test chamber with contractile muscular walls that model the left side of the heart. Having solved the equations of motion and predicted the performance of the valve, the computer presents the results in terms of flow patterns (streamlines), pressure-contour plots, and graphs of velocity, pressure, flow, tension, and distance as functions of time. Predicted echocardiograms and phonocardiograms are also generated by the computer. These predictions are compared with the results of animal experiments, and the method is used to study the diastolic role of the chordae tendineae, the pathophysiology of mitral valve prolapse syndrome, and the fluid dynamics of pivoting disc valves. Our results support the hypothesis that the chordae have a functional role during diastole. The results also establish a computer model of mitral prolapse, which should be useful in functional studies of that condition, and show that the position of the pivot point and the curvature of the occluder can have a profound effect on the angle of opening of pivoting disc valves. These studies illustrate the applications of the method in physiology, pathophysiology, and prosthetic valve design.

Animals↗
PubMed1979

In vitro hydrodynamic comparison of mitral valve bioprostheses.

With the use of the pulse duplicator built in our laboratory, the hydrodynamic characteristics of three sizes of the four commercially available mitral bioprostheses, Hancock, Carpentier-Edwards, Angell-Shiley, and Ionescu-Shiley, were studied and compared. A wide range of performance was found: for example, during pulsatile testing, at peak flow of 15 1/min (corresonding to a normal resting cardiac output) transvalvular gradients varied from as high as 20 mm Hg (Angell-Shiley) to 5 mm Hg(Ionescu-Shiley) in the 25 mm mounting diameter size. Effective orifice areas (EOA) are significantly different in valves of the same mounting size, e.g., at peak flows of 20 1/min, the Ionescu-Shiley 25 provides an EOA of 1.7 cm2 while the Angell-Shiley provides only 1.17 cm2. The EOAs of all bioprostheses have been found to increase with increasing flow (e.g., from 10--30 1/min peak flow, the Hancock 25 changed from 1.25 cm2 to 1.50 cm2). The Gorlin formula, as constituted for calculating the area of stenotic mitral valves, is inappropriate for prosthetic valves. But the discharge coefficients of the bioprostheses have been found to be around 1 when the planimetered area of the open valve orifice is determined at a given flow. By using this discharge coefficient, the Gorlin formula will give an excellent estimate of the true orifice area of mitral bioprostheses.

Bioprosthesis↗
PubMed1978

In vitro hydrodynamic comparison of mitral valve prostheses at high flow rates.

A pulse duplicator system for evaluating the hemodynamic performance of mitral prostheses is described. Under conditions stimulating normal resting physiology, all valves tested had measurable but acceptably small pressure drops. Under conditions simulating exercise, all were moderately to severely stenotic. Valves with nearly equal mounting diameters were compared. The Hancock, Beall, and Starr-Edwards valves (Group A) were found to be significantly more stenotic than the Björk-Shiley, Cutter-Cooley, Ionescu-Shiley, and Lillehei-Kaster valves (Group B). In the 29 to 30 mm. mounting diameter size at cardiac outputs of 5 and 9 L. per minute, Group A had average pressure drops of 3.2 and 10.5 mm. Hg and Group B, pressure drops of 1.6 and 5.3 mm. Hg, respectively. In the 24 to 26 mm. mounting diameter size, at cardiac outputs of 9 L. per minute, all the valves had critically large pressure drops (9 to 17.6 mm. Hg). The standard Gorlin formula is inappropriate for computing the orifice area of prosthetic valves. The discharge coefficient for a valve (a measure of how well the valve uses its primary flow area) and a performance index (a measure of how well the valve uses its mounting area) have been computed from a knowledge of the orifice size, without the necessity of assuming a value for the discharge coefficient required by the Gorlin formula. The biological valves (Hancock and Ionescu-Shiley) provide an efficient orifice for fluid flow at the free leaflet margins and have large discharge coefficients. On the basis of the fluid dynamic equation of motion, steady flow, root mean square (RMS) flow, and peak flow, combined with the appropriate transvalvular gradients, were all shown to yield equally accurate characterizations of valvular hydrodynamic performance. Mean flow, unfortunately the only value obtainable clinically, yielded effective orifice areas 10 percent smaller than either of the other three flow values.

Bioprosthesis↗