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Research Papers

Experimental Technique of Measuring Dynamic Fluid Shear Stress on the Aortic Surface of the Aortic Valve Leaflet

[+] Author and Article Information
Choon Hwai Yap

Wallace H. Coulter School of Biomedical Engineering,  Georgia Institute of Technology and Emory University, 315 Ferst Drive, Atlanta, GA 30332cyap3@mail.gatech.edu

Neelakantan Saikrishnan

Wallace H. Coulter School of Biomedical Engineering,  Georgia Institute of Technology and Emory University, 315 Ferst Drive, Atlanta, GA 30332nsaikrishnan3@mail.gatech.edu

Gowthami Tamilselvan

Wallace H. Coulter School of Biomedical Engineering,  Georgia Institute of Technology and Emory University, 315 Ferst Drive, Atlanta, GA 30332tami@gatech.edu

Ajit P. Yoganathan1

Wallace H. Coulter School of Biomedical Engineering,  Georgia Institute of Technology and Emory University, 315 Ferst Drive, Atlanta, GA 30332ajit.yoganathan@bme.gatech.edu

1

Corresponding author.

J Biomech Eng 133(6), 061007 (Jun 28, 2011) (15 pages) doi:10.1115/1.4004232 History: Received April 13, 2011; Revised May 09, 2011; Posted May 16, 2011; Published June 28, 2011; Online June 28, 2011

Aortic valve (AV) calcification is a highly prevalent disease with serious impact on mortality and morbidity. The exact cause and mechanism of the progression of AV calcification is unknown, although mechanical forces have been known to play a role. It is thus important to characterize the mechanical environment of the AV. In the current study, we establish a methodology of measuring shear stresses experienced by the aortic surface of the AV leaflets using an in vitro valve model and adapting the laser Doppler velocimetry (LDV) technique. The valve model was constructed from a fresh porcine aortic valve, which was trimmed and sutured onto a plastic stented ring, and inserted into an idealized three-lobed sinus acrylic chamber. Valve leaflet location was measured by obtaining the location of highest back-scattered LDV laser light intensity. The technique of performing LDV measurements near to biological surfaces as well as the leaflet locating technique was first validated in two phantom flow systems: (1) steady flow within a straight tube with AV leaflet adhered to the wall, and (2) steady flow within the actual valve model. Dynamic shear stresses were then obtained by applying the techniques on the valve model in a physiologic pulsatile flow loop. Results show that aortic surface shear stresses are low during early systole (<5dyn/cm2 ) but elevated to its peak during mid to late systole at about 18–20 dyn/cm2 . Low magnitude shear stress (<5dyn/cm2 ) was observed during early diastole and dissipated to zero over the diastolic duration. Systolic shear stress was observed to elevate only with the formation of sinus vortex flow. The presented technique can also be used on other in vitro valve models such as congenitally geometrically malformed valves, or to investigate effects of hemodynamics on valve shear stress. Shear stress data can be used for further experiments investigating effects of fluid shear stress on valve biology, for conditioning tissue engineered AV, and to validate numerical simulations.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Model of the native aortic valve constructed by trimming and suturing fresh porcine aortic valve to a plastic stented ring; and (b) the clear acrylic idealized sinus geometry aortic chamber to house the valve

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Figure 2

Schematic of the flow loops used. (a) Steady flow loop with a circular tube with a piece of aortic valve leaflet glued to the wall; (b) Steady flow loop with the native valve model; and (c) pulsatile flow loop with the native valve model mimicking physiologic pressures.

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Figure 4

The position of the piston pump head as measured by the backscattered light position tracking method, and as reported by the position sensor of the piston pump. A close match is observed between the two, validating the position tracking method under dynamic conditions.

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Figure 5

Near-wall velocity measurements in flow loop 1 and the associated standard deviation, at 3.5 L/min and 7.5 L/min steady flow rates. The parabolic curve fits these measured velocity profile well, demonstrating good coefficient of determination. Further, the no-slip velocity condition was observed at the wall.

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Figure 6

Velocity and back-scattered LDV laser light intensity measurements in flow loop 2 near to the aortic surface of the valve leaflet under (a) 7 L/min and (b) 14 L/min steady flow rates. A qualitative match was observed between location of peak back-scattered light and the location of no-slip velocity, suggesting that the leaflet surface is at this point.

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Figure 7

Flow and pressure curves simulated in the valve model over the cardiac cycle

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Figure 8

(a) Back-scattered light intensity along the line at which LDV measurements were done, over the cardiac cycle, and (b) the leaflet position along the line at which LDV measurements were done, over the cardiac cycle, segmented from the back-scattered light intensity map. (c) The out-of-plane velocity of the valve leaflet calculated from leaflet positions. The valve leaflet has low out-of-plane velocities during the time window of shear stress measurements (unshaded portions), but high velocities during the opening and closing time phases. (d) Orientation of the leaflet with respect to the LDV probe. The valve leaflet was misaligned with the probe by no more than 0.15 rad during the time window of shear stress measurements (unshaded portions).

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Figure 9

The required correction to measured velocity due to beam angle distortion in nonstream-wise direction measurements, as a function of radial distance from the sinus wall

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Figure 10

Velocity field measured by LDV in the (a) stream-wise and (b) nonstream-wise directions, depicted by the color contour map, and the location of the valve leaflet, depicted by the black line

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Figure 11

(a) Ensemble averaged velocity measurements at a measurement location 0.9mm away from leaflet systolic position over the cardiac cycle, and (b) the standard deviation of the ensemble average, for both the stream-wise and the nonstream-wise directions

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Figure 12

Magnitudes of the two terms composing the stream-wise shear stress on the valve leaflet over the cardiac cycle

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Figure 13

Fluid shear stress on the central portion of the valve leaflet, computed for the entire cardiac cycle except for time periods when the leaflet is rapidly opening or closing, in both the stream-wise and the nonstream-wise directions

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Figure 14

(a) Typical flow profile on the aortic surface of the valve leaflet; (b) the shear rate that would have been computed if the velocity measurement spatial resolution was poorer than 89 microns used in the current study; and (c) the errors in shear rate calculations at these resolutions

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Figure 15

Schematic of the strictly stream-wise pipe flow

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Figure 16

(a) schematic of the Couette flow. (b) Zoomed in view of vector field of Couette flow simulated with Matlab, under-sampled for plotting purposes. (c) The resulting velocity profile measured from the wall-normal line and from the 0.15 rad slanted line.

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Figure 17

(a) Schematic of the Hiemenz flow, adapted from White [36]. (b) Zoomed in view of vector field of Hiemenz flow simulated with Matlab, under-sampled for plotting purposes. Stagnation point is located at coordinate [0,0]. (c) Wall shear stress computed at various x-axis locations in the Hiemenz flow field. Velocity sampling were obtaine along a vertical line at various x-axis locations. (d) Percentage difference in wall shear stress computation when velocities are sampled along vertical lines versus when velocities are sampled along lines 0.15 rad from the vertical. (e) Absolute difference in wall shear stress computation when velocities are sampled along vertical lines versus when velocities are sampled along lines 0.15 rad from the vertical.

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Figure 3

(a) Schematic for the definition of the axes directions (stream-wise, nonstream-wise and radial). (b) Scheme for LDV measurements. Measurements were performed at the midpoint between the base of the leaflet and the free-edge. (c) Schematic for the shear stress computation. The measured velocity data was first mirror imaged to construct a periodic waveform, and a low pass filter was applied to interpolate the data points before velocity gradient at the leaflet surface was computed as the shear rate. Shear stress is computed as the shear rate multiplied by the kinematic viscosity.

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