the flow through prosthetic heart valves - iimas ::...
TRANSCRIPT
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The flow through prosthetic
heart valves
Roberto ZenitRoberto ZenitInstituto de Investigaciones en Materiales
Universidad Nacional Autónoma de México
In collaboration with:
Adriana López (UG), René Ledesma (G), Prof.Guillermo Pulos (IIM-UNAM)
A. Juárez (Instituto Mexicano de Cardiología)
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OUTLINE
1. General Introduction: Heart valve research
2. Fluid Mechanics of Heart Valves
3. Our Experimental Investigation3. Our Experimental Investigation
– Visualization of flow structures and
interpretation
– Some Ideas for Blood Trauma
4. Foundations for valve design
5. Conclusions
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BIOLOGICAL FLOWS
Flow through heart valves•Flow structure and damage
Fish swimming
Shell selection by crabs
HEAT TRA$SFER
Vortex HT enhancement•Ring-wall collisions
TWO-PHASE FLOWS
Granular flows•Dry stuff
•Flow around objects
•Avalanches
•Wet stuff•liq. fluidized beds
•collisions
•shear flow
Bubbly flows•Newtonian stuff
•Single bubbles
•Pseudo turbulence
•Non newtonian stuff•Single bubbles
Emulsions•Formation
• Turbulent-capillary
break-up of threads
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Heart Research
• 1 million heart surgeries per year
worldwide
• 25% are related to failure of valves
• Valve replacements are readily • Valve replacements are readily
available
• Many unresolved problems…
• Instituto Nacional de Cardiología
Large scale collaboration project
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ATRIUM
Triscuspid ValveMitral Valve
VENTRICULE
Pulmonary Valve
Aortic Valve
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Heart Valves
Four valves:
Mitral and Tricuspid (inside)
Aortic and Lung (outside)
Two or four leaflets
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Prosthetic heart valves
• Mechanical
– caged-ball
(Starr-Edwards)
– tilting-disc
(Medtronic Hall)(Medtronic Hall)
– bileaflet mechanical (Saint Jude)
Rigid
Flexible
or rigid
• Biological- Porcine
- Bovine
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Prosthetic heart valves
• After 40 years ‘these devices are less than ideal and lead to many complications.’
• ‘Many of these complications/problems are directly related to the fluid mechanics
• ‘Many of these complications/problems are directly related to the fluid mechanics associated with the various mechanical and bioprosthetic valve designs.’
Yoganathan et al. (2004)
Ann. Rev. Biomed. Eng.
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Fluid mechanics issues for
heart valve replacements
• HEMOLYSIS (Destruction of Blood Cells)
– Cells ‘break’ as a result of the applied shear and turbulence
– Stress level and time dependence
– Not well understood
• THROMBOSIS (Formation of blood clots)
– (Low) wall shear rate
– Residence time
– Chemical reaction (properties of contacting surfaces, coagulability and other factors)
– Less well understood
• CAVITATIO$ (Formation and collapse of vapor bubbles)
– Low pressure zone appear during valve closing (only for mechanical valves)
– Implosion of bubbles causes cell damage
– Well understood but poorly studied
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Coagulation and Thrombosis
• Complex physico-
chemical process
• Hemostasis
• Main elements:
– Platelets
– Tissular factor (F
III)
– Coagulation factors
(FI –FXII,12
proteins)
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Shear induced platelet activation
•Cell lisis is not necessary
for platlet activation
•Dependence of strength
of ‘shear’ but ALSO
on exposure time105
Pa105
104
100
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‘Activation’ of von Willebrand factor (vWF)
Schneider et al. (2007)
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‘Local’ effort
• Design and production of bio-mechanical
valves
•Bovine pericardium (cheap, good properties)
•Good performance
•But… Mechanical Properties? Durability? Good Performance?
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Associated Research Projects
in our ‘local’ effort
• Mechanical properties
• Stenosis (rigidization)
• New materials (for leaflets and and structure)• New materials (for leaflets and and structure)
• Mechanical Design and Testing
• FLUID MECHANICS
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Part 1.
Study of the flow through
replacement heart valves
• Quantify the performance of bio-mechanical heart valves.
• Study the complex flow fields that result from the • Study the complex flow fields that result from the flow-valve interaction.
• Explain why bio- valves are ‘better’ than mechanical valves.
• Contribute to the
understanding of
blood trauma mechanisms
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Experimental setupWindkessel model
• Pulsatile Pump
• Compliance chamber
• Resistance valve
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Several planes (espacial resolutiuon)
and phase locking (temporal resolution)
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Pressure traces and phase locking times
upstream
downstream
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‘First’ set of 3D results
• Test valves
– Mechanical bileaflet and monoleaflet
– Biological tricuspid-type
• Set of conditions
– Working fluid water
– Frequency = 24 cycles/min=0.4 cycles/s
– Volume displaced = 45 cm3 / cycle
– 35% systole, 65% diastole– 35% systole, 65% diastole
• Phase Locking
– Five measuring planes downstream from the valve
– 30 measurements /cycle
– average over 200 cycles for each field
– 5x30x200 = 30,000 images per each set of conditions
• Visualization
– Velocity fields
– Q-criteria for vortex identification
(positive second invariant of velocity gradient tensor)
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Biological valve (Shear Rate)
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Mechanical valve (Shear Rate)
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Some thoughts about the flow field
• Unsteady, fully three-dimensional, inertial (Re~2500)
→ visualization and interpretation: not easy
• Measurements → access to all flow quantities• Measurements → access to all flow quantities
• Phase locking → not ‘proper’ measure of turbulence (approximation)
• Blood cells, nearly neutrally bouyant and small
→ fluid tracers (very small Stokes number)
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Results
Shear Rate
-1
mecγ 200 s≈&
-1
bioγ 100 s≈&
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Q Criteria
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Viscous Stresses
Biological Mechanical
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Turbulent Stresses
Biological Mechanical
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Interpretation
• Magnitude of viscous and turbulent
stresses, much lower than platelet
activation thresholdactivation threshold
• Magnitude shear rate is close to unfolding
threshold of the von Willebrand factor.
• It is expected that the shear threshold be
smaller for a non-simple shear flow
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Part 2.
A new generation of heart valves
• synthetic materials?
• physiological-like flow
• long-life good performance• long-life good performance
• limited understanding: physical mechanisms
which lead to a ‘proper performance’
– Material properties
– Leaflet dimensions
– Fluid-structure interaction
• Research in Progress
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Simpler geometry
What is the optimal performance? • minimize fluid stresses
• unidirectional flow
What are the optimal geometric dimensions L, d, h ?
What is the optimal material?
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Test different valves: geometries and materials
Test different flow conditions: frequency and disp. volume
20 pulses/min < f < 110 pulses/min
20 cm3 < V < 100 cm3
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Velocimetry
Ordinary 2D PIV, with phase locking
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Velocity measurements
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Valve performance
GOOD NOT SO GOOD
•Uni-directional flow
•‘Check’ valve
•Buckling of leaflets
•Flow reversal
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Normalization
Charasteristic elastic time scale:
ρ/E
dlfE =
Chracteristic volume:
ρ/E
( )wd
c hwlV =
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t/T=0.3
f=20 pulses/min V=45cm3
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$ormalized strain rate
Scaling?Scaling?
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In progress…
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Conclusions
• Simple experimental setup
• Found a relation for proper valve
performanceperformance
• Strain rate : flow conditions
• Basis for valve design!
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General Conclusions
• Experimental investigation
• Complex flow field past replacement heart valvesvalves
• Can determine ‘all’ fluid mechanics characteristics of flow
• Mechanisms for blood trauma
• Propose new set of design parameters
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GraciasGracias
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Coagulation cascade:
1) Damaged blood vessel wall
2) Exposed subendothelium
proteins (collagen)
3) Platelets bind collagen with
surface collagen
coagulation factors I and II
(glycoprotein)(glycoprotein)
4) Adhesion strengthened
further by von Willebrand
factor (vWF),
5) Links formed, platelets
glycoprotein (Ib/IX/V) and
the collagen fibrils. Platelet
activation.
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• Biol
• Da=0.018m
• Umax=1.2512 m/s• Umax=1.2512 m/s
• Mec
• Da=0.017m
• Umax=0.3262 m/s
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Some ideas for hemolysis
Velocity gradients, turbulenceDeformation and eventual breakup
Red Blood Cell
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Turbulence or shear ?
• Viscous shear stresses
• Turbulent shear stresses
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Mechanical valve
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Ideas for hemolysis
(borrowed from two-phase flows)
• Forces keeping the cell
together
• Viscous forces
• Turbulent forces
where
cell diameter
d = 10 µm.
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Dimensionless numbers
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Bubbles in turbulence
Risso and Fabre (1998)
Bubbles break beyond a certain critical turbulent Weber number.
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Droplets in extensional/shear flows
Extensional flow Simple shear flow
Ha and Leal (2001)
Marks (1998)Droplets break beyond a certain
critical Capillary number.
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Elastic forces: measurement of κDao et.al (2003)
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Dao et.al (2003)
Shear modulus = 13.3 µN/m
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Preliminary results
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Some ideas for thrombosis
Velocity gradients, turbulence Deformation, estimulation and activation
Platelet
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Falla por fatiga
Determinar Eo para inferir No
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Probeta de PB
Celda de carga
Mordazas
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Ensayo de Fatiga
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SPEED
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SPEED
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SPEED
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Streamwise
vorticity
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Streamwise
vorticity
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Streamwise
vorticity
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‘Turbulent’ intensity
• Variance of velocity in each direction
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‘Turbulence’222 ''' wvu ++
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‘Turbulence’222 ''' wvu ++
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‘Turbulence’222 ''' wvu ++
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Biological valve (Shear Rate)
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Mechanical valve (Shear Rate)
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Hemodynamics