Retrograde Flow Bileaflet Mechanical Heart Valves · Abstract: In vitro experiments were conducted...
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Abstract: In vitro experiments were conducted to investigate the time-varying retrograde flow characteristics of 27 mm bileaflet aortic valves. The retrograde flow fields of bileaflet valves range ffom uniform squeeze flow and regurgitant jet flow fields to flow fields involving complex secondary flows. Stress levels are sufficiently high to cause cell damage or activation in both the squeeze and jet flows. The squeeze occur over a shorter time span but encompass a larger area of flow and potential exposure to more cells. It is speculated that valves exhibiting complex flow patterns have lower incidence of thrombus due to increased washing of the hinge region and valve perimeter by these flow patterns. These results, and the test protocol developed in this study, may improve future capabilities for predicting thrombus potential in new valve designs. Introduction: Fluid stresses occurring in retrograde flow fields during valve closure may play a significant role in thrombogenesis. The retrograde flow occurs as the blood flows back through the valve due to an adverse pressure gradient while the leaflets are either closing or are already closed. As the leaflets close, blood continues to "squeeze" through the decreasing flow area causing potentially harmful levels of fluid viscous shear stresses imposed on the formed elements. This process continues until the leaflets are fully closed. Once the leaflets are fully closed, a second source of retrograde flow is the regurgitant jets, which may be important in blood element damage. These jets issue forth fiom the small openings in the hinge regions and flow with a relatively high velocity on the order of 5 m/s at the orifice of the jet. The high velocity and intrinsically turbulent nature of these jets may impose large shear stresses on the formed blood elements []. The retrograde flow fields of bileaflet valves and their clinical implications have not been studied in great detail. In vitro experiments were conducted to investigate the time-varying retrograde flow characteristics of 27 mm bileaflet aortic valves. Three-component, coincident Laser Doppler Anemometry velocity measurements were obtained facilitating the determination of the 3-D principal stresses in the valve flow fields. Since both the squeeze and regurgitant jet flows are strongly time-varying flows, lucid presentation of the variation of the shear stresses as a function of time was imperative. Animation of the data was essential for extracting the needed information quickly. Methodology: The experiments were performed in the Georgia Tech aortic valve in vitro model under physiologic pulsatile flow conditions of 5 l/min mean cardiac output and 30 L/min peak systolic flow rate. Three-component coincident LDA measurements were obtained over a region approximately 90 mm2, I mm upstream of the valve housings around the 1.2.6.03 hinge region. The mapping was performed with an incremental resolution of 0.127-0.254 mm. A resettable clock was employed to gate pulsatile data acquisition throughout end systole and diastole. Phase window averaging was conducted over several hundred cycles for the generation of mean velocity and turbulence statistics in 20 ms intervals. All data were transit time weighted averaged for velocity bias correction, and low-pass digital filtering was employed to remove high frequency LDA noise if present. A 3-D principal stress analysis was applied to the measured Reynolds stress tensor to identifu peak stresses in the flow [2]. A blood analog fluid was used providing a physiologic kinematic viscosity of approximately 3.5 cSt and a refractive index of 1.49. Animation of the data allowed the investigation of the full temporal characteristics of the flow. Animation was accomplished by importing the experimental data base into CFD post processing graphical data reduction algorithms (Fieldview, Intelligent Light lnc. NJ, USA). Two bileaflet valves were studied in the course of this investigation: St. Jude Medical, and a prototype design. In addition to the retrograde flow stress levels, the investigation also assessed washout capacity around the valve housings and the potential for thrombus formation around the valve perimeters. Results and Discussion: The St. Jude bileaflet valve retrograde flow fields within the hinge regions are characterized by squeeze flow during valve closure, and regurgitant jets after closure. The squeeze flow occurs through the central gap between the two leaflets, and persists for a period starting from the onset of valve closure to the complete closure of the valve. This lasts approximately 40 to 60 ms of end systole. This region of flow encompasses a large cross sectional area of the valve and involves a significant volume of blood. The squeeze flow profile between the leaflets is nearly uniform and resembles a two-dimensional jet with peak velocities reaching 0.6 m/s. The corresponding peak principal normal stresses and maximum shear stresses are 3800 and 800 dynes/cm2, respectively. Stress levels are sufficient to potentially cause damage to blood elements considering the close proximity to the valve leaflets. Significant secondary flow patterns are observed around the perimeter of the valve in the region investigated in this study. These flow patterns are observed around and behind the pivot guards of the St. Jude valve, indicating that substantial washing of this area may occur during both the forward and closing flow phases of the cycle. This may have clinical significance in reducing or slowing the rate of thrombus growth within the hinge regions of this valve. These flow features are illustrated in the vector plots shown in figure l. For the St. Jude valve tested, the regurgitant jet flow field is characterized by the presence of three jets. The Retrograde Flow through Bileaflet Mechanical Heart Valves Aiit P. Yoganathan, Arnold A. Fontaine, Jeffrey T. Ellis, and Timothy M. Healy School of Chemic"al and Mechanical Engineering, Institute for Bioengineeririg and Bioscience, Georgia Institute of Technology, Atlanta, GA 30332 USA Medical & Biological Engineering & Computing Vol. 34, Supplement 1, Part 1, 1996 The 1Oth Nordic-Baltic Conference on Biomedical Engineering, June 9-13, 1996, Tampere, Finland 211
Transcript of Retrograde Flow Bileaflet Mechanical Heart Valves · Abstract: In vitro experiments were conducted...
Abstract: In vitro experiments were conducted to investigatethe time-varying retrograde flow characteristics of 27 mmbileaflet aortic valves. The retrograde flow fields ofbileaflet valves range ffom uniform squeeze flow andregurgitant jet flow fields to flow fields involving complexsecondary flows. Stress levels are sufficiently high to cause
cell damage or activation in both the squeeze and jet flows.The squeeze occur over a shorter time span but encompass alarger area of flow and potential exposure to more cells. It isspeculated that valves exhibiting complex flow patterns havelower incidence of thrombus due to increased washing of thehinge region and valve perimeter by these flow patterns.These results, and the test protocol developed in this study,may improve future capabilities for predicting thrombuspotential in new valve designs.
Introduction:
Fluid stresses occurring in retrograde flow fields duringvalve closure may play a significant role in thrombogenesis.The retrograde flow occurs as the blood flows back throughthe valve due to an adverse pressure gradient while theleaflets are either closing or are already closed. As theleaflets close, blood continues to "squeeze" through thedecreasing flow area causing potentially harmful levels offluid viscous shear stresses imposed on the formedelements. This process continues until the leaflets are fullyclosed. Once the leaflets are fully closed, a second sourceof retrograde flow is the regurgitant jets, which may be
important in blood element damage. These jets issue forthfiom the small openings in the hinge regions and flow witha relatively high velocity on the order of 5 m/s at the orificeof the jet. The high velocity and intrinsically turbulentnature of these jets may impose large shear stresses on theformed blood elements []. The retrograde flow fields ofbileaflet valves and their clinical implications have not beenstudied in great detail.
In vitro experiments were conducted to investigate thetime-varying retrograde flow characteristics of 27 mmbileaflet aortic valves. Three-component, coincident LaserDoppler Anemometry velocity measurements wereobtained facilitating the determination of the 3-D principalstresses in the valve flow fields. Since both the squeezeand regurgitant jet flows are strongly time-varying flows,lucid presentation of the variation of the shear stresses as afunction of time was imperative. Animation of the data wasessential for extracting the needed information quickly.
Methodology:
The experiments were performed in the Georgia Techaortic valve in vitro model under physiologic pulsatile flowconditions of 5 l/min mean cardiac output and 30 L/minpeak systolic flow rate. Three-component coincident LDAmeasurements were obtained over a region approximately90 mm2, I mm upstream of the valve housings around the
1.2.6.03
hinge region. The mapping was performed with an
incremental resolution of 0.127-0.254 mm. A resettableclock was employed to gate pulsatile data acquisitionthroughout end systole and diastole. Phase windowaveraging was conducted over several hundred cycles forthe generation of mean velocity and turbulence statistics in20 ms intervals. All data were transit time weightedaveraged for velocity bias correction, and low-pass digitalfiltering was employed to remove high frequency LDAnoise if present. A 3-D principal stress analysis wasapplied to the measured Reynolds stress tensor to identifupeak stresses in the flow [2]. A blood analog fluid was
used providing a physiologic kinematic viscosity ofapproximately 3.5 cSt and a refractive index of 1.49.
Animation of the data allowed the investigation of thefull temporal characteristics of the flow. Animation wasaccomplished by importing the experimental data base intoCFD post processing graphical data reduction algorithms(Fieldview, Intelligent Light lnc. NJ, USA).
Two bileaflet valves were studied in the course of thisinvestigation: St. Jude Medical, and a prototype design. Inaddition to the retrograde flow stress levels, theinvestigation also assessed washout capacity around thevalve housings and the potential for thrombus formationaround the valve perimeters.
Results and Discussion:
The St. Jude bileaflet valve retrograde flow fields withinthe hinge regions are characterized by squeeze flow duringvalve closure, and regurgitant jets after closure. Thesqueeze flow occurs through the central gap between the twoleaflets, and persists for a period starting from the onset ofvalve closure to the complete closure of the valve. Thislasts approximately 40 to 60 ms of end systole. This regionof flow encompasses a large cross sectional area of the valveand involves a significant volume of blood. The squeezeflow profile between the leaflets is nearly uniform andresembles a two-dimensional jet with peak velocitiesreaching 0.6 m/s. The corresponding peak principal normalstresses and maximum shear stresses are 3800 and 800dynes/cm2, respectively. Stress levels are sufficient topotentially cause damage to blood elements considering theclose proximity to the valve leaflets.
Significant secondary flow patterns are observed aroundthe perimeter of the valve in the region investigated in thisstudy. These flow patterns are observed around and behindthe pivot guards of the St. Jude valve, indicating thatsubstantial washing of this area may occur during both theforward and closing flow phases of the cycle. This mayhave clinical significance in reducing or slowing the rate ofthrombus growth within the hinge regions of this valve.These flow features are illustrated in the vector plots shownin figure l.
For the St. Jude valve tested, the regurgitant jet flowfield is characterized by the presence of three jets. The
Retrograde Flow through Bileaflet Mechanical Heart Valves
Aiit P. Yoganathan, Arnold A. Fontaine, Jeffrey T. Ellis, and Timothy M. HealySchool of Chemic"al and Mechanical Engineering, Institute for Bioengineeririg and Bioscience,
Georgia Institute of Technology, Atlanta, GA 30332 USA
Medical & Biological Engineering & Computing Vol. 34, Supplement 1, Part 1, 1996The 1Oth Nordic-Baltic Conference on Biomedical Engineering, June 9-13, 1996, Tampere, Finland 211
major jet originates from the central plane between theleaflets through a gap in the seam between the leaflets andthe intersection with the housing. This jet persiststhroughout diastole and is approximately 1.5 mm indiameter. The peak velocity is roughly 0.8 m/s, with peakturbulent normal and shear stress levels of 4300 and 1800dynes/cm2, respectively.
The other two regurgitant jets are considered to be minorjets as they persist for only 40 ms at early diastole withlower(<50%) velocity and stress levels than in the primaryjet. One jet appears to originate from the hinge socket of thevalve while the other jet emanates from a gap between theleaflet and housing at roughly 45 degrees from the centralseam between the leaflets.
Contrary to the flowfield of the St. Jude valve, theprototype valve is characterized by a uniforrn squeeze flowpattern occurring within the gap between the two leaflets.The flow field behind the leaflet or adjacent to the housing isrelatively stagnant with little or no secondary flow patterns.The squeeze flow occurs over a smaller cross sectional areaof the valve than in the St. Jude, and exhibits peak velocityand stress levels which are slightly higher than in the St.Jude valve. The peak velocity is roughly 0.7 m/s with peakturbulent normal and shear stress levels of 5000- and 1800dynes/cm2 respectively.
The prototype valve exhibits a regurgitant jet througheach of its hinges. These jets are nearly 2 mm in diameterand persist throughout the retrograde flow phase (late systoleand diastole). Figure 2 illustrates the retrograde flow fieldof the prototype valve in late systole. The uniforrn squeezeflow bptween the leaflets and the regurgitant jet issuing fromthe hinge socket are visible. The regurgitant jet of theprototype valve has velocity and stress levels which aretwice that of the St. Jude valve, 1.8 m/s with normal andshear stresses of 10,000 and 3800 dynes/cm2.
The lack of significant flow around the periphery of thevalve coupled with the higher turbulent stress levels may beimportant factors in the high rate of thrombus formationobserved with the prototype valve design.
Conclusions:
The retrograde flow fields within the hinge region andimmediately adjacent to the housing of the St. Jude Medicaland a prototype bileaflet valve have been studied. Localturbulent stresses encountered in the squeeze flowregurgitant jet regions of both valves encompass a largevolume of blood and may be sufficient to cause bloodelement damage. The regurgitant jets generally persistthroughout diastole.
The St. Jude valve exhibits strong secondary flowpatterns around the perimeter and behind the valve leafletsindicating good washout particularly around the pivotguards. These secondary flow patterns may significantlyimpact the relatively low thrombogenecity of the St. Judedesign. In contrast, the prototype valve exhibits higherstress levels and greatly reduced secondary flow fieldfeatures. Near stagnant flow is observed behind the leafletsand adjacent to the housing throughout late systole anddiastole.
References:
Hinge
Housing
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field at late systole.
Saint Jude MedicalLate Systole
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Medical & Biological Engineering & Computing Vol. 34, Supplement 1, Part 1, 1996The 1Oth Nordic-Baltic Conference on Biomedical Engineering, June 9-13, 1996, Tampere, Finland
tl] Baldwin J.T., Deutsch S., et.al., Determination ofprincipal Reynolds stresses in pulsatile flows afterelliptical filtering of discrete velocity measurements, J.Biomech. Eng. ll5: 396-403, 1993.
[2] Fontaine A.A., Ellis, J.T., et. al., Identification of peakstress in cardiac prostheses: A comparison of 2-D vs 3-Dprincipal stress analyses, (in press) J. ASAIO, May 1996.
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