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Research Papers

A Detailed Fluid Mechanics Study of Tilting Disk Mechanical Heart Valve Closure and the Implications to Blood Damage

[+] Author and Article Information
Keefe B. Manning1

Department of Bioengineering, The Pennsylvania State University, 205 Hallowell Building, University Park, PA 16802kbm10@psu.edu

Luke H. Herbertson, Arnold A. Fontaine, Steven Deutsch

Department of Bioengineering, The Pennsylvania State University, 205 Hallowell Building, University Park, PA 16802

1

Corresponding author.

J Biomech Eng 130(4), 041001 (May 16, 2008) (8 pages) doi:10.1115/1.2927356 History: Received April 10, 2007; Revised October 02, 2007; Published May 16, 2008

Hemolysis and thrombosis are among the most detrimental effects associated with mechanical heart valves. The strength and structure of the flows generated by the closure of mechanical heart valves can be correlated with the extent of blood damage. In this in vitro study, a tilting disk mechanical heart valve has been modified to measure the flow created within the valve housing during the closing phase. This is the first study to focus on the region just upstream of the mitral valve occluder during this part of the cardiac cycle, where cavitation is known to occur and blood damage is most severe. Closure of the tilting disk valve was studied in a “single shot” chamber driven by a pneumatic pump. Laser Doppler velocimetry was used to measure all three velocity components over a 30ms period encompassing the initial valve impact and rebound. An acrylic window placed in the housing enabled us to make flow measurements as close as 200μm away from the closed occluder. Velocity profiles reveal the development of an atrial vortex on the major orifice side of the valve shed off the tip of the leaflet. The vortex strength makes this region susceptible to cavitation. Mean and maximum axial velocities as high as 7ms and 20ms were recorded, respectively. At closure, peak wall shear rates of 80,000s1 were calculated close to the valve tip. The region of the flow examined here has been identified as a likely location of hemolysis and thrombosis in tilting disk valves. The results of this first comprehensive study measuring the flow within the housing of a tilting disk valve may be helpful in minimizing the extent of blood damage through the combined efforts of experimental and computational fluid dynamics to improve mechanical heart valve designs.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) The single shot chamber mimics the closure dynamics of the Bjork-Shiley Monostrut valve. (b) On the left-hand side is a view of the intact Bjork-Shiley Monostrut MHV. To the right, the modification to the valve housing is displayed. The window was later filled in with acrylic to maintain similar fluid dynamic patterns and rigidity.

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Figure 2

The LDV measurement planes and locations in relation to the MHV are depicted. The coordinate axes are indicated for the three velocity components. The four planes (each 500μm apart) are located within the housing starting with Plane 1, approximately 200μm from the valve occluder.

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Figure 3

These schematics depict side and front views of the overall flow structure generated by the closing occluder for four successive times

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Figure 4

Three dimensional flow structures are constructed with the vectors indicating direction and the contour signifying axial velocity strength. The valve closes right to left, with x=0 representing the centerline of the leaflet. The four plots show the flow (a) 1ms before impact, (b) at impact, (c) 1ms following closure, and (d) 2ms after closure.

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Figure 5

The measurement plane closest to the valve (200μm away) is shown (a) 1ms before impact, (b) at impact, (c) 1ms following closure, and (d) 2ms after closure. Axial flow (W) is represented by the contour, and the crossflow velocities (U and V) are represented by the vectors. This is the only plane before impact in which flow is moving up toward the tip of the valve. High velocity jets form near the valve tip.

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Figure 6

The flow profiles captured 1.2mm away from the valve (Plane 3) are shown for 1ms time bins ranging from (a) 1ms before impact until (d) 2ms after closure.

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Figure 7

Mean and max shear rates are shown for Plane 3 at times (a) 1ms prior to impact, (b) at impact, and (c) 1ms after impact. The shear rate profiles are weaker, but more structured further upstream from the leaflet. A rapid acceleration/deceleration at impact causes shear rates to increase drastically at the instant of closure. Peak shear rates as high as 80,000s−1 are still calculated at impact near the valve tip. At measurement locations closest to the MHV housing, shear rates of 100,000s−1 were seen.

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Figure 8

A representative histogram within a 1ms time window shows the vertical movement (through the axial velocity) of the vortex in Plane 4 at the time of impact. The bimodal distribution indicates that two distinct flows are being measured within the measurement volume.

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