Velocity Analysis of Bileaflet Artificial Heart Valve: from Opening to... Position

Journal of A pplied Sciences Res earch, 5(10): 1455-1462, 2009
© 2009, INSInet Publication
Velocity Analysis of Bileaflet Artificial Heart Valve: from Opening to near Close
Position
Nebras Hussein Ghaeb, Taha Yaseen Khalaf, Ali Al-Timemy
Dept. of Biomedical Engineering, Al-Khaw arizimi College of Eng., Baghdad University,
Baghdad, Iraq,
A bs tr ac t: The aim of this s tudy was to inves tigate the velocity dis tribution for the blood flow us ing t h e
b ile a fle t me c hanical heart valves . Seven kinds of bileaflets valves with 55o , 60o , 65o , 70o , 75o , 80o and 85 o
were mounted in the A ortic pos ition on inclin e d angles from 10o (near open pos ition) to an angle near
clos e. The velocity dis tributions have been calculated for 44 ca s e s u s in g A NSYS v10, bas ed on finite
elements technique. M odels of the mechanical heart valves are pres e n t e d in this paper des cribing s teady
and laminar flow characteris tics in a bileaflet aortic hea rt valves models that are inves tigated over a range
of s ys tolic flo w rates (depending upon the clos ing – opening angle). The purpos e of this s tudy is to
evaluate the velocity s tream of the bileaflet heart valve numerically. Velocity dis tributions and obt a ined
from finite element method were us ed to give the behavior through pros thetic heart valves which repres ent
the firs t-line information about valve performance.
Key words : Bileaflet valve, A rtificial Heart Valve, Pros thes es Heart valve, hymod y n a mic o f Blood flow
INTRODUCTION
The human heart is the biological pump that
moves blood thro u g h o ut the circulatory s ys tem. It is
es s ential to live and therefore is important that it
functio n s correctly throughout a pers on’s lifetime.
Components o f the heart include the four main
compartments (two atrium and two ventricles ) and the
heart valves . For various reas ons s uch as heart valve
dis eas es includin g valvular s tenos is and ins ufficiency,
valv e fa ilure can occur leading to the need for valve
replacement. This mo s t often occurs in the aortic and
mitral valves , making up 95% of replaced valves [1].
There are two types of heart valves : mec h anical
pros thes es , with rigid, manufactured occluders , and
biological or tis s ue va lv es , with flexible leaflet
occluders of animal or human o rigin. Pros thetic valves
differ from one another w it h regard to s everal
characteris tics , in c lu d in g d u ra b ilit y (longevity),
thrombogenicity, and hemodynamic profile [2].
a. Mechanical Valves : M echanical heart valves are
made from materials of s ynthetic origin like me t a ls ,
ceramics a n d polymers , whereas the biological valves
may employ in addition to s ynthetic mate rials .
Biological tis s ue valves are made from porcine aortic
valves or fabricated us ing bovine pericardial tis s ue and
s u itably treated with gluteraldehyde to pres erve the m
a n d t o remove antigenic proteins .. Therefore tis s ue
Corresponding Author:
v a lv e s are rarely us ed in children and young adults at
pres ent. On the other hand, mecha n ical valves made
with high s trength biocompatible material are durable
and have long–term functional capability. However,
mechanical valves are s ubject to thrombus d e pos ition
and s ubs equent complications res ulting from e mboli
an d s o patients with implanted mechanical valves need
to be on long-term anticoagulant thera p y . Currently,
mechanical v alves are preferred except in elderly
patients or thos e who cannot b e p u t u n d er
anticoagulant therapy, like wo me n who may s till wis h
to bear children, or hemolytic patients [3].
In 1952 Dr. Charle Hufnagel, clinically introduced
a ball valve into the des cending aorta for the treat ment
of aortic ins ufficiency. The outer cas ing and ball of his
aort ic valve was made of methyl methacrylate
(P ers pex), which was known to inhibit the coagula t io n
of blood (Fig. 1)[4]. In total over 200 patients received
this pros thes is with no anticoagulant therapy. Th es e
valves were recovered up to 30 years after implantation
with no obvious wear[5].
The Harken ball valve and the widely us e d StarrEdwards ball valve (Fig. 2) we re b o th introduced in
1960. Both valve des igns incorporated a ball of s ilicone
rubber he ld in place by a cage.. This type of valve
functions on the principle that w hen downs tream
pres s ure increas es , the ball is forced into t h e v alve
orifice thus s t o pping blood flow and preventing
regurgitation. The cage holds the ball in place d u ring
Nebras Hussein Ghaeb, Dep t . of Biomedical Engineering, Al-Khawarizimi College of Eng.,
Baghdad University, Baghdad, Iraq,
Phone: +9647702208352,
Email: [email protected]
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J. App. Sci. Res., 5(10): 1455-1462, 2009
forward flow when the ups tream pres s ure is higher.
The valve us ed in the mitral p o s ition s ince 1965 and in
the aortic pos ition s ince 1968[6].
Fig. 1: The Hufnagel Valve
Fig. 3: The Lillehei-Kas ter tilting dis c valve
Fig. 2: The Starr-Edwards caged-ball valve
H owever, the next big innovation in valve des ign
d id not occur until the introduction of the tilting dis c
valve in 1963 by Lillehei, Cruz, and Kas ter. T h e s e had
a free-floating d is c , retained by a s haped cage (Fig. 3).
The pivot point was moved and the cage was replaced
with lateral guides . Subs equent improvements , with the
guides being fu rt her reduced with abbreviated earlike
guards , produced the Omnis cien c e and Omnicarbon
valves , both of which are s till in us e [7].
The BjÄ ork-Shiley (BS) tilting dis c valve utilized
two U -s h aped wire s truts welded to a Satellite orifice
as the dis c retainer, together with a Pyrolyte dis c [8].
The only tilting dis c valve currently in us e in the
M edtronic-Hall valve (Fig. 5).
T h e bileaflet valve was introduced in 1978 w it h
the St. J u d e M e dical (SJM ) valve (Fig. 6) that is s till
in us e today [8].
They have two s emicircular leaflets retained within
the ring by hinges . The potential for impeded leaflet
movement due to interference with cardiac s tructures is
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Fig. 4(a): BjÄ o rk-S h iley Convex-Concave valve. (b)
BjÄ ork-Shiley monos trut valve.
J. App. Sci. Res., 5(10): 1455-1462, 2009
they are le s s prone to thromboembolic problems , do
no t caus e damage to blood cells , do not require long
term anticoagulants , and do not s uffer from any o f t he
s tructural problems experienced by mechanica l heart
valves [8].
Fig. 5: M edtronic-Hall tilting dis c valve
Fig. 6: St. Jude M edical bileaflet valve
s lim, as the open leaflet are pos itioned in the middle of
the blood s tre a m a n d enclos ed within the ring in the
clos ed pos ition. Bile a flet valves are the mos t protected
as the leaflets hardly protrude from the valv e ring,
even during maximum opening. T he large effective
orifice area of the bileaflet valves , contributes to create
a flat, ne a rly normal flow profile with far les s
obs truction and turb u le nce. Bileaflet valves typically
have a s mall amount (5 to 10 ml per b e a t ) o f normal
regurgitation [2].
b. Bio-pros thetic Valves : There are two types o f biopros thetic valves : animal tis s ue and human tis s ue
valves . The des ign of a bio-pros thetic valve is s imilar
to that of the natural heart valve, t herefore has better
flow characteris tics than mechanical valves . In addition
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2. Computational Fluid Dynamic: A ppro ximately
170,000 individuals worldwide rec e ive pros thetic heart
valves every year, and over half (55%–65%) Receive
mechanical heart valves (M HVs ). Recipients of M H Vs ,
however, mus t t ake anticoagulant medication becaus e
of the potential for thromboembolic complic ations .
Such complications are though t t o be caus ed by highblood s hear s tres s es , turbulence, and the overall
complexity of the hemodynamics in M HVs [9].
Flow problems in general can be s tu d ie d b y three
approaches , which a re theoretical method, computer
s imulation and experimental meas urement . A s the fluid
motion us ually in v o lves a very complicated nonlinear
b ehavior, which is not eas y to get an analytical
s olution, and that may limit the theoretical method only
applicable for s ome s imple flow cas es (e.g. collid ing
flow, Couette motion). In contras t, the comput e r
s imulation us ing numerical methods s uch as fin ite
element method, finite difference method and finite
volume method, can obtain d e tailed flow fields to
d e s cribe fluid motion in many engineering application s
once the boundary conditions are properly defined [1 0 ].
Computational Fluid Dynamics (CFD) modeling of
M HVs promis es to provide des igners wit h a tool to
refine exis ting M HV des ig n s ; however, the information
provided by CFD mus t be accurate and relia b le [9]. For
the mos t part CFD modeling of the h e mo dynamics in
M HVs has been limited to 2D s tudies or 3D s tudies
with limited s p atial res olution combined with the
as s umption of flow s ymmetry with re s p e c t to
geometrical planes of s ymmetry [1 1 ,1 2 ,1 3 ]. In a recent
numerical s tudy, Ge et al. carried out c a lculations for
a typical M HV geometry without ad opting s implifying
s y mmetry as s umptions on grids that are at leas t o n e
order of magnitude finer than thos e us ed in previou s ly
reported s tudies in the literature [9].
Recent s tudies have focus ed on the numerical
s imulation of the flow phenomen a involved in the
functioning of an M HV durin g a cardiac cycle. Several
different numerical techniques have been developed for
this purpos e. In particular, c o mp u t a t ional fluid
dynamics has been us ed to redes ig n valves and
minimize th e p roduction of turbulence, regions of high
s he ar s tres s es , pres s ure los s and s tagnation regions in
the vicinity of the leaflets [1 4 ].
CFD modeling of M HVs began 30 years ago with
the 2-D immers ed bo u ndary method of Pes kin [1 5 ] and
has continued to this day as CFD me t h ods have
J. App. Sci. Res., 5(10): 1455-1462, 2009
improved and computational power has increas ed.
N e v e rtheles s , no cons ens us has been reached in the
literature regarding the importance of 3D e ffects or the
le v e l of grid refinement that is neces s ary to accurately
repres ent the flow within M HVs . In fa c t , mos t of the
recently reported CFD s tudies o f M HV flows have
either as s umed 2D flow or re p o rted 3D computations
on grids with relatively limited s patial res olution. M any
of thes e 3D s imulations have further as s umed,
pres umab ly for computational expedience, the flow to
be s ymmetric with res pect to one or more of the
geometric planes of s ymmetry of the valve [9].
3. Modeling Heart Valve: T h e fluid dynamic analys is
of implantable me dical devices is of fundamental
importance, becaus e it could furn is h indications about
the funct io n a lit y and biocompatibility with the
phys iological environment: it is well known that the
rais e of t h ro mbogenic and haemolytic phenomena are
s tric t ly correlated to the modifications of the flow field
caus ed by the pres ence of thes e devices . To inves tigate
this potentially dangerous condition, phys icians can
adopt s everal experimental techniques havin g the
common aim to e v a lu a t e the velocity profiles
characteris tic for each pros thetic device [1 6 ].
Thus we us e the numerical techniq u es for this
analys is . The finite element technique s a re being us ed
for heart valve des ign. W ith recent a d v a n ces in
c o mp u t a t io n al fluid dynamics (CFD ) a n d t h e
de v e lopment of numerical algorithms that account for
the interaction betwe e n the fluid and the valve leaflets
(fluid-s tructure interaction; FSI), the tools have become
available to s ubject t hes e computer des igned valves to
‘virtual bench tes ts [1 7 ].
W e hav e p revious ly demons trated that we were
able to s imula t e the motion of the blood (dis tribution
velocity of the b lood) when the trans fer it through the
bileaflet heart valve at laminar flow conditions , and the
when the change in the angle leaflets .
a. Governing Equations of Motion: A s far as the
fluid dynamic model is concerned, blood was mo d e le d
as a homogeneous , incompres s ible Newtonian fluid,
with a s pecific mas s ñ of the value of 1.05 kg/m3 and
a cons tant dynamic vis c o s it y µ of the value of 0.004
Pa.s ; the gravitational effects were neglected. Flow has
be en as s umed to be laminar, time–dependent. The flow
mot io n through the fully open bileaflet mechanical
heart valve model up to p e a k s ys tole has been
des cribed by the principle of momentum cons erva t io n ,
expres s ed by the Navier-Stokes equations and by the
continuity equation, rep re s enting the principle of mas s
cons ervation. For a trans ient analys is the flow motion
governing equatio n s , in the Cartes ian coordinates
s ys tem des cribed in Fig 7, are given as [1 6 ,1 8 ,1 9 ]:
1458
Fig. 7: Sketch with dimens ions
(x - M omentum):
(1)
(y - M omentum)
(2)
(M as s )
(3)
W here u, v, and p repres ent, res pectively, the three
Cartes ian components of the velocity vector and the
pres s ure in each point of the flu id domain, and where:
(4)
The geometry of the mechanical heart valve and
the a rt e ry wall us ed in this calculation is s hown in
Fig. 7.
The hous ing of the valves cons is ts of an expanding
tube with a diameter ratio of 1.3. The inlet diameter is
22mm, which res ults in an outlet diameter of 28mm.
The inlet and outlet conduits are b o t h 50mm in
length [1 7 ].
The mecha n ical heart valve us ed for the analys is
is the St. Jude M edical b ileaflet mechanical heart valve
(Fig 8).
J. App. Sci. Res., 5(10): 1455-1462, 2009
The valve had a tis s ue annulus diameter of 19
mm, an o rifice internal diameter of 16 mm and was
geometrically identical to the pros thes es implanted into
patients [1 6 ].
The maximum o p e ning angle of the valve is
10°and the clos ing angle of the valve is 85° , s ee table
(1) fo r the detailed opening – clos ing angle that have
been examined with the number of runs .
impregnated with tu ngs ten s o that the valve can eas ily
be s een following implantation). Th e s ewing cuff, us ed
to attach the valve to the heart, is mad e o u t of double
velour polyes ter[2 1 ,2 2 ].
RES ULTS AND DIS CUS S ION
In the current work the procedure of collection the
res ults could be explained as :
a.
B.
C.
d.
E.
Calculate the geomet ry dimens ions for the
s pecified Total angle.
Build the geometry in A N SYS 10 bas ed on the
total angle and the s pecified opening angle.
Supplied the boundary conditions and the inp u t
velocity profile for the geometry (s ee table 2)
Find out the c o n tour dis tribution for the input
model with the velocity profile.
Repeat points (a ) t h rough to (d) for the other
geometry models .
Table 2: Boundary conditions and input velocity profile
walls
No slip conditions with velocity equal zero
Input
Mean velocity 10Cm/s
Pressure to support the ventricle during the closin g of
the mitral valve 80 mmHg
The build models have 2205 eleme n t s with 2414
nodes as s hown in Fig 9.
The velo c it y profiles of the three different total
angles for opening of 10 degree are s hown in Fig 10.
Fig. 11 s hows the M ean velocity after the valve
with res pect to t h e total angle for variable clos ing
angles .
The res ults s hows that the Bileaflet 60 valve is not
s t a b le in s teps with res pect to the other tes ted valves
where the dis tribution of t h e mean velocity comes to
be variable in res pons e within the increas e of the
clos ing an g le not like the other valves with different in
s moothing of velocity fa lls according to the clos ing
effect, and this me a ns that the more s mooth curve the
les s in turbulence an d eddies effect on the valve
clos ure.
Fig. 8: M odel of the bileaflet aortic valve prototype
Table 1: Open – close angle
No
1
2
3
4
5
6
7
T otal
angle
55
60
65
70
75
80
85
Open
angle
10
10
10
10
10
10
10
T otal number of runs
Close
angle
45
50
55
60
65
70
75
No of
Run
5
5
6
6
7
7
8
44
The Re y n olds number is bas ed on the inlet length
a n d t he mean velocity 10 cm/s [7] at the entrance of th e
channel. A ls o, the maximum pres s ure to s upport in t he
ventricle during the clos ing mitral valve 80 mmHg [2 0 ].
M o s t artificial valves are made of titanium alloy
(Ti 6A 14V), graphite, pyro ly tic carbon, and polyes ter.
T he titanium is us ed for the hous ing or oute r rin g ,
graphite coated with p y ro lytic carbon is us ed for the
bileaflets , and 100% pyrolytic carbon is us ed for the
inner ring. The pyrolytic c a rb o n is s ometimes
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Not highly difference s in the s tability of the mean
velocities for the other total angles where they are in
between the 0.03 m/s to 0.05 m/ s for the 10 degree
opening angle, while the Bileaflet with 85 total angle
have highes t value of the opening 10 degree in mean
velo city tends to reach the 0.1 m/s which repres ent the
input condition fo r o ur tes t cas es , and this is means
that t h e flow comes to be with the optimum condition
due to continuity and les s in los s in turbulence and
eddies .
J. App. Sci. Res., 5(10): 1455-1462, 2009
Fig. 9: The mes hed Geometry
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J. App. Sci. Res., 5(10): 1455-1462, 2009
Fig. 10: Velocity contour for three angles
Fig. 11: M ean Velocity vers us the clos ing s teps angle for the 7 total angles
4.
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