Papafaklis Michail_Blood Flow Simulation in Coronary Arteries

8/8/2019 Papafaklis Michail_Blood Flow Simulation in Coronary Arteries

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Computational Fluid Dynamics

A new research tool for assessing

coronary physiology

Michail I. Papafaklis MD, PhD

Michailideion Cardiac Centre

Medical School, University of Ioannina


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Summary

Physiology and Hemodynamics

Computational analysis of blood flow and

blood flow simulation in coronary arteries

The role of hemodynamic forces incoronary artery disease


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Physiology

Cardiology Is FlowCardiology Is Flow

The primary aim of the cardiovascular system is to drive, control andmaintain blood flow to all parts of the body

Complexity of the circulatory system

The heart is an intermittent pump blood flow in the arterial

circulation is pulsatile

The blood vessels are branched, distensible tubes of various

dimensions The blood is a complex suspension

Richter and Edelman Circulation2006

Despite this complexity, the function of the circulatory

system can be explained by the principles of basic

fluid mechanics that apply to non-biologic systems,

such as household plumbing systems
http://upload.wikimedia.org/wikipedia/en/f/f9/Waterpipes.jpeg


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Hemodynamics

The term hemodynamicshemodynamics describes the physicalprinciples governing pressure, flow and resistance as

they relate to the cardiovascular system, and the flow ofblood in the vessels

Blood flow is determined by two factors: A pressure difference between the two ends of a vessel or group

of vessels

The resistance that blood must overcome as it moves throughthe vessel or vessels

Pressure (P) = Flow (Q) Resistance (R)1 peripheral resistance unit (PRU)=100mmHg/100mlsec-1


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Fluid Mechanics

Stress (Pa) = Force (Nt)/Surface (m2)

Shear stress (tangential component)

Normal stress (vertical component)

Angular deformation

(strain) of fluids is

caused by shear stresses

Newtonian fluidsShear stress = viscosity Strain rate

Non-Newtonian fluids

Viscosity is not constant, but depends on the strain rate


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Hemodynamics

Components of hemodynamic stress


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Hemodynamics

Rheological properties of blood Whole blood is a

concentrated suspension

of blood cells in plasma

(electrolytic solution of

inorganic and organic

molecules, and proteins)

Viscometric properties of

whole blood Depend on Hct

Depend on the strain rate

(non-Newtonian fluid)mainly in low strain rates


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Hemodynamics

Poiseuille flowQ=(/128)Pd4/L w

= 32Q/d3

Assumptions

Blood is considered to behave as a Newtonian fluid

The vessel is considered to be a straight tube withrigid walls and constant circular cross-section

Blood flow is considered

to be steady, laminar andfully-developed

Jean Louis Marie Poiseuille (1799-1869)Gotthilf Heinrich Ludwig Hagen (1797-1884)


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Coronary circulation

The coronary circulation net is the denser

capillary net in the human body The resting coronary blood flow is

about 225 ml/min or about 0.8 ml/g

of the heart muscle During exercise conditions the

coronary blood supply can increase

by 4 to 10 times the resting bloodflow rate


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Coronary circulation

Arteries Coronary arteries are tubes with a non-circular

cross-section, which is progressively reduced in size

going distally (tapering) Several bifurcations and branches are present

The arterial wall is distended in every heartbeat(pressure wave)

Coronary arteries are situated on the epicardium andfollow its curvature

The geometry of the system is dynamic because of the

constant movement of the heart during every cardiaccycle

Coronary blood flow is pulsatile

Most flow (mainly for the left coronary artery) occurs

during the diastolic phase of the cardiac cycle


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Hemodynamics

Differential analysis Detailed description of the flow field and accurate computation of

hemodynamic parameters in all flow situations

Cartesian coordinates

Continuity equation Navier-Stokes equation


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Computational techniques (use of computers) applied for

the numerical solution of the differential flow equations,since a direct analytic solution for complex flow fields isnot feasible

A practical and reliable tool for studying the time-varying,3D blood flow patterns in complex arterial geometries

CFD computations are based on a much more complexflow model compared to Poiseuille flow and provide

useful info on spatial and temporal changes ofhemodynamic parameters (e.g. flow velocity near thevessel wall and shear stress)

CFD allows the simulation of the hemodynamicenvironment under controlled conditions


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General Principles of the method

In CFD modelling a complex geometry is

discretized into a large number of smallerbut regular (typically, tetrahedral orhexahedral) elements computational

mesh Numerical solution of the differential equations

of motion at the nodes connecting these

elements after applying certain boundaryconditions

Computation of hemodynamic quantities

(pressure, velocity components, shear stress)


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Application in human vessel models

CFD was initially applied in theoretical idealized

computer models of various vessels (aorta, carotids,coronary arteries, aorto-coronary bypass grafts)

These idealized computer vessel models were based onmean measurements from cadaver specimens or

angiographic data However, normal anatomic differences from one person

to another cause great variations in terms ofhemodynamics and necessitate the incorporation of the

detailed arterial characteristics of every person in CFDmodelling

In order to achieve blood flow simulation for every vesseland every patient, the most important element is real 3D

reconstruction of the lumen vessel patientpatient--specificspecificarterial models


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3D coronary artery reconstruction Anatomic characteristics of coronary arteries

Small dimension (mean diameter 3mm)

3D complex (epicardial curvature) and time-dependent geometry

(heart movement)

Fusion of coronary angiography and intravascular

ultrasound (IVUS) data

Coronary angiography

Geometry of coronary artery lumen

Position and geometry of IVUS catheter in the arterial lumen

IVUS

Detailed geometry of the lumen and arterial wall (atheromatic

plaque) in every cross-section


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3D coronary artery reconstruction

+

Point cloud of the lumen

Point cloud of the

outer vessel wall

Bourantas et al. Comput Med Imaging Graph2005


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3D coronary artery reconstruction

Point clouds of the

lumen &&outer vessel wall

3D lumen && outer

vessel wall

reconstructionLAO view

3D lumen && outer

vessel wall

reconstructionRAO view

Papafaklis

MI. PhD Thesis 2008

Bourantas et al. Comput Med Imaging Graph2005


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Mesh Generation Discretization of the

complex arterial

lumen intohexahedrons, whosesize and distributionlargely govern theaccuracy of thecomputed velocityfield

Papafaklis MI. PhD Thesis 2008


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Fluid properties & boundary conditions Definition of fluid properties (blood)

Homogeneous Newtonian / non-Newtonian fluid Density (): 1050 kg/m3

Dynamic viscosity (): 0,0035 Pasec Boundary conditions

Laminar flow

Steady (e.g. Q=1ml/sec) /Pulsatile flow

Velocity profile (flat / parabolic) at the inlet of the

artery

Non-slip condition at the arterial wall

Pressure condition at the outlet of the artery


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Numerical Solution Discretization of the

equations for each

control volume

(hexahedron)

Initial approximate

solution at each node ofthe computational mesh

Continuous iterations of

the solution

Error minimization and

convergence to the

exact solution


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Blood flow simulation

3D lumen

reconstructionand

blood particle

tracking in aright coronary

artery



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Blood flow simulation Streamlines in a right coronary artery lumen



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Blood flow simulation Velocity profiles in lumen cross-sections



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Blood flow simulation 3D colour map of shear stress distribution


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Blood flow simulation Angioscopic view

Wahle et al. SPOIE Proceedings 2001


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CFD is a technological tool which in combinat ionw ith imaging modalities can accurately simulate the

hemodynamic environment in human arteries

Why do we need to

simulate blood

flow in the

arteries ?

Because it

is fancy ! ? !


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Hemodynamic forces &

Atherosclerosis

Chronic, inflammatory and fibroproliferative disease mainly of large-

and medium-sized arteries Remains the leading cause of death in the developed world

Despite the systemic nature of its associated risk factors (i.e.hypertension, smoking, hyperlipidemia, diabetes mellitus, social

stress), atherosclerosis is a site specific disease and itsmanifestations are focal and eccentric

Specific regions are affected: branch points (outer edges of bloodvessel bifurcations) and the inner walls of curved arteries

Within each patient, each coronary obstruction progresses,regresses, or remains quiescent in an independent manner

Local factors (hemodynamic forces) play a major role in the regionallocalization of atherosclerosis


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Hemodynamic forces &

Localization of atheromatic plaques

A i i f l h i h


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Association of low shear stress w ithatherosclerosis

in coronary arterial segments

Van Langenhove et al. Circulation2000

A i ti f l h t ith


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in coronary arterial segments

Stone et al. Eur Heart J2007

Serial intracoronary study (3D anatomy and CFD) inpatients with stable angina revealed plaque progression

in areas with low shear stress

A i ti f l h t ith


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in left main coronary artery bifurcation

In vivo study using 3Dreconstruction and CFDdemonstrated anassociation of disturbedflow and low shearstress with increased

plaque thickness Plaque thickness was

found to be inverselyrelated to shear stress in

five patients

Papafaklis

et al. Int

J Cardiol

2007

Papafaklis et al. (abstr) Heart2007


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Shear stress and

Vulnerable plaques Low SS has been lately found in a serial IVUS study (coupled with

CFDanalysis) in swines to be an independent predictor of plaquelocation, development and progression to a high-risk plaque with:

Intense lipid accumulation

Inflammation

Thin fibrous cap

Internal elastic lamina fragmentation

Media thinning

Excessive expansive remodelling

The magnitude of low SS at baseline was associated with theseverity of high-risk plaque characteristics

Low SS

IntenseinflammationExcessiveexpansiveremodelling

TCFA

Chatzizisis et al. Circulation2008

Role of shear stress in


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Role of shear stress inin-stent restenosis

Bare metal stents Neointima thickness is inversely related to shear stress

Low shear stress areas

must be considered as

areas with a higher

probability for neointimalhyperplasia to be present

and more prominent

Adjunctive brachytherapy was found to reduce the effect of shear

stress on neointimal hyperplasia

Areas with sufficient irradiation and thus, minimal neointimal thickness

minimal effect of SS on in-stent restenosis because of brachytherapyinduced cell death

Wentzel

et al. Circulation2001

Papafaklis et al. (abstr)J Am Coll Cardiol2006

Papafaklis et al. Int J Cardiol2008

Role of shear stress in


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Role of shear stress inneointima distribution

Drug-eluting stents Low shear stress is associated with neointimal hyperplasia

Shear stress does not seem to play a role in tissue regression and

lumen enlargement which occur after drug-eluting stent implantation

Gijsen

et al.Am J Cardiol 2003

Papafaklis et al. (abstr)

J Am Coll

Cardiol

2008

Papafaklis et al.

Heart Vessels 2007


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Clinical Implications

Detection of vulnerable plaques at an earlier stage by

adding CFD techniques to current imaging methods:

invasive (IVUS-based virtual histology and palpography,

thermography, intravascular MRI, angioscopy)

noninvasive (MSCT, MRI, PET)

Assess the vulnerability risk of a vessel or patient Selective invasive treatment (use of stents to stabilize

high-risk plaques ? ? ?)

Limitation: vulnerable plaques are not single localized lesionsrestricted to one coronary artery only

Prognosis of probable locations of restenosis after stent

implantation


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Conclusions

Low shear stress explains the focal susceptibility of thearterial system to atherosclerosis and neointima

distribution following stent implantation

Incorporation ofin vivoin vivohemodynamic measurements by

using CFD in current diagnostic methods may increaseour knowledge on the pathobiology of coronary arterydisease with beneficial implications for the vulnerablepatient

Future: Create a prognostic model for vulnerableplaques based on hemodynamic parameters


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Thank you

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Papafaklis Michail_Blood Flow Simulation in Coronary Arteries