The closing behaviour of the aortic valve : a hydrodynamical
analysis to improve heart valve prostheses
Citation for published version (APA):
Steenhoven, van, A. A. (1979). The closing behaviour of the aortic valve : a hydrodynamical analysis to improve heart valve prostheses. Technische Hogeschool Eindhoven. https://doi.org/10.6100/IR25270
DOI:
10.6100/IR25270
Document status and date: Published: 01/01/1979
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. )THE CLOSING BEHAVIOUR OF THE AORTIC VALVE
A HYDRODYNAMICAL ANAL YSIS TO IMPROVE HEART
VAL VE PROSTHESES
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE 'v\IETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL EINDHOVEN, OP GEZAG VAN DE RECTOR MAGNIFICUS, PROF. IR. J. ERKELENS, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN DEKANEN IN HET OPENBAAR TE VERDEDIGEN OP
DINSDAG 16 OKTOBER 1979 TE 16.00 UUR
DOOR
ANTONIUS ADRIANUS VAI\l STEENHOVEN
DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOREN:
Prof.Dr. P.C. Veenstra en
Aan Wilhelmien, WijMk.e. en He.dde.
De dierexperimenten werden uitgevoerd in het Laboratorium van de Capaciteitsgroep Fysiologie, Rijksuniversiteit Limburg.
Het verschijnen van dit proefschrift werd mede moge! ijk gemaakt door steun van de Nederlandse Hartstichting.
CONTENTS
Abstract List of symbols
1. General introduetion
1.1 Brief historica] review of investigations into heart valve replacements
1 .1.1 Introduetion 1.1.2 Biologica] valves 1.1.3 Mechanica] prestheses 1.1.4 Discussion
1.2 The purpose and scope of the Eindhoven heart valve research project
1.3 Object of the present investigation 2. The heart and the aortic valve
2.1 The anatomy and physiology of the heart 2.1.1 Introduetion
2.1.2 The anatomy 2.1.3 The physiology
2.2 The anatomy and histology of the aortic valve 2.2.1 Introduetion
2.2.2 The leaflets 2.2.3 The aortic ring
2.2.4 The sinuses of Valsalva 2.2.5 Discussion
3. Two-dimensional model studies of the closing behaviour of the aortic valve
3.1 Introduetion
3.2 The experimental set-up
3.2.1 Dimensions of the analogue 3.2.2 Velocity measurements
3.2.3 Recording of the cusp displacements
3.3 Qualitative experimental observatlons In relation to the Strouhal number
3.4 Physical models 3.4.1 Introduetion
3.4.2 The high Strouhal-number model 3.4.3 The low Strouhal-number model
page 11 13 17 17 17 17 20 22 25 28 39 39 39 39 IlO 42 42 113 45 45 46 49 49 51 51 53 56 57 60 60 60 62
page
3.5 Experimental verification of theory 65
3.5.1 Introduetion 65
3.5.2 Valve closure at different Strouhal numbers 66
3.5.3 The high Strouhal-number model 67
3.5.4 The low Strouhal-number model 68
3.5.5 The stationary sinus vortex 70
3.6 The influence of an unsteady vortex on valve etosure 71 3.7 The influence of sinus shape on valve closure 74
3.8 Concluding discussion 75
4. Adaption of the model to axial symmetry 79
4. 1 Phys i ca 1 mode 1 79
4.2 Application of the model to the simulation
experimentsof Belthouse and Talbot 81
4.3 Concluding discussion 81
5.
Animal experiments on the behaviour of the aartic valve 83 5.1 Historica! review of recording methods of val vu larmovements 83
5.1.1 Introduetion 83
5.1.2 1n-v~o cinematography 83
5.1.3 In-vivo cinematography 84
5.1.4 Indirect methods 84
5.1.5 The methad used in the present investigation 85
5.2 Haterials and experimental methods 85
5.3 Data processing 88
5.3.1 Hemadynamie data 88
5.3.2 Calculation of the mean curves 91 5.3.3 The reliability of the experimental results 92
5.4 General results 97
5.5 Hemadynamie factors in valve beh~viour 103
5.5.1 Introduetion
5.5.2 The curves of valve behaviour 5.5.3 The shape of the valve orifice 5.6 General discussion 5.7 Conclusions 103 105 114 115 119
page 6. Applicatlon of results to the design of heart valve
prestheses 123
6.1 Introduetion 123
6.2 Appl ication of the natural valvular closing mechanism
to the design of a heart valve presthesis 124
6.2. 1 Design specificatlons 124
6.2.2 Construction of the prototype 125
6.3 Valve testing procedure 6. 3. 1 Int roduc ti on
6.3.2 Materials and methods 6.3.3 Data processing 6.4 Results
6.5 Concluding discussion 7. Summary and conclusions Samenvatting Nawoord Levensbericht 127 127 127 130 131 133 137 141 143 144
ABSTRACT
In order to obtain specifications to improve the design of heart valve prostheses, the closing behaviour of the aortic valve has been
investigated in model studies as well as in ~n-v~vo experiments. Two-dimensional studies on the aortlc valvular ciosure were performed by means of an analogue. The interaction between the contents of the cavity behind the leaflet (the sinus of Valsalva) and decelerating aortic flow was studied. lt appears that the flow phenomena and the cusp motion are strongly dependent on the Strouhal number. Simp I ified quasi-one-dimensional theoretica\ models based on the phenomena observed prove to give a reasonable qual itative picture of the
experimentally observed valve behaviour. Experimental results indicate that the strength of the vertex trapped in the sinus does not affect the mechanism of valvular closure. The geometry of the cavity is not very critica!, though there is very clearly a lower limit to its depth.
In open-ehest dogs the hemodynamics of the aortic valve was studied. Direct cinematographic high-speed recordings of the aortic valve movement have been made. Simultaneously, the ECG, ascending aortic volume flow and the pressures In the aorta, left ventricle and left atrium were recorded. Valve behaviour in relation to the aortic flow can broadly bedescribed as fellows. The opening of the aortic valve at the onset of systole proceeds very fast. Valvular ciosure already starts during the acceleration phase of the aortic fluid. At the onset of the deceleration phase about 8% of the ciosure is
accomplished. During fluid deceleration valve ciosure continues; at the moment of zero aortic flow 80% of the ciosure is achieved. The back flow in the valve completes the closure. Comparison of the closing behaviour as calculated from theory and as measured under different hemodynamic conditions, shows a reasonable agreement. The natura! valvular closing mechanism is applied toa disc valve, Behind the disc a long shallow cavity is provided. The·test results reveal that applicatlon of the natura! valvular closing mechanism may improve heart valve prostheses.
LIST OF SYMBOLS A 0 A A[O] A ma x a a ma x a[t=oO] a[qao=O] b e eff. g H[t] HR h 0 h h . m1n J L p ao ps ao óP ao Plv [PI -P ]s v ao pla Pla p
aortic cross-sectional area instantaneous valve orifice area
valve orifice area at the onset of aortic flow deceleration
maximum value valve orifice area valve orifice area fraction: A/Amax' chapter 5
A/A
0 , chapter 6
maximum value valve orifice area fraction mid-systolic valve orifice area fraction end-systolic valve orifice area fraction sinus depth
eccentricity of disc suspension efficiency of a valve
gravitational constant unit step function
2 m 2 m 2 m 2 m m m 2 m/s
heart rate beats/min
channel height of the analogue
instantaneous and local height of the aorta in the region of the sinus
instantaneous minimum value of h moment of inertia of the contents of
4 the sinus In the analogue: ~ pnR sinus height
cusp length aortic pressure
maximum systolic aortic pressure systolic aortlc pressure drop: Ps
ao m m m kg.m m m kPa kPa kPa
left ventricular pressure kPa
maximum systolic pressure drop over the valve kPa
left atrium pressure kPa
mean left atrium pressure kPa
instantaneous and local pressure in the region kPa of the sinus
pressure just upstream of the entrance plane of the valve
qao . mtn q [a= 1 ] ao r r r 0 St
sv
t[a=1 ]-t [a=O] t[q ao . mtn llt u u u s V V xmean pressure difference across the cusp aortic blood flow
peak f 1ow
back flow ratio
relative opening flow
sinus radius
hydrogen bubble radius Reynolds number:
Uh /v for the analogue 0
Ur /v fora physiological system 0
correlation coefficient
instantaneous and local radius of the cone as formed by the cusps
aortic radius Strouhal number:
R/Ur : for the analogue
r /Ur: fora physiological system 0
stroke volume ti me
opening time
moment of maximum back flow closing time: t[a=1] - t[a=O] maximum velocity of the mainstream
kPa ml/s ml/s m m m m ml s s s s m/s instantaneous and local velocity in the region m/s of the sinus
mainstream velocity just upstream of the entrance plane _of the valve
sinus fluid velocity
velocity transverse to the mainstream/ rising velocity sectien 3.2.2.1
voltage of 1 inear output anemometer system space co-ordinate m/s m/s m/s V m
a
e:
n
a
V
opening angle of valve eccentricity of valve: e/r
0
closing parameter in the analogue: 95% confidence interval of À
closing rate
h . /h m1 n o
closlng parameter in a physiological system: A/A , chapters 4 and 6
0
A/A[O] , chapter 5 fluid viscosity
rotatien angle, defined in fig. 3.6a fluid density density of hydrogen deceleration time stream function kinematic viscoslty vorticity of core rad 1/s Ns/m2 rad kg/m3 kg/m3
1. GENERAL INTRODUCTION
1.1 Brief historica! review of investigations into heart valve replacements
1. 1.1 lQ![Q~~~!lQQ
Wel 1-functioning valves are necessary for the pumping action of the heart. However, valvular disease is common and in advanced forms causes severe disability and ultimately death. Marked impravement in the quality and lengthof life of patlents suffering from cardiac-valvular disease has resulted from surglcal treatment, which usually
involves the replacement of the diseased valve. Since heart-lung machines became cl inically appl icable [Melrose, 1953; Li llehei and DeWall, 1958] , many types of surgical repair and valvular replacements have been used. The design and implantation of suitable valves have been the object of cardiac research in the past decades. Because direct
inspeetion of the aortic and mitral valves is possible during
surgery, the design of new valves was generally carried out by surgeons. Basically two different types of heart valves can be distinguished: (i) the biologica! valves and (ii) the mechanica! prostheses. In this
chapter the history of development and the characteristics of both types wil! be briefly reviewed.
1.1.2 êl9!99l~~!-~~!Y~~
In biologica! valves materials of biologica! origin are used. In the present section three main types of biologica! valves wil! be described: a homograft valve (= allograft), a heterograft valve
(= xenograft) and the fascia lata valve as an example of autologous tissue valves.
Homografts·
Total replacement of the mltral valve was first described by Murray et al. [1956], who substituted a homograft valve (an aortic valve removed from a corpse) into the mitral area. In the past decades this technique has been refined and used both in the mitral and in the aortic position [Barratt-Boyes, 1965; Bigelow et al., 1967; Gianel ly et al., 1968; McDonald et al., 1968; Braunwald et al., 1968; Ross, 1968; Ross and Yacoub, 1969].
Although the short-term results of these replacements were satisfactory, the long-term results were dlsappointing. Valvular
insufficiency and stenosis occurred due to rupture and calcification of the leaflets,while occasional rigidlty of the leaflets was found as a result of tissue ingrowth. These complications were especlally seen
if the homografts were preserved by freeze-drying. [Smith, 1967; Gianelly et al., 1968; Ross and Yacoub, 1969; Barratt-Boyes, 1971] When fresh homografts were used, degenerative changes occurred as a result of host rejection [Buch et al., 1971].
This has ledtoa reassessment of the methods of sterllizatlon and preservation. Aparicio et al. [1975] claimed that the morphologlcal changes are fewer and less severe if the valve is sterilized and preserved by gamma-radlation at 4°C than if preserved by freeze-drying. In his study, however, the maximum duration of
graft-implantation was 17 months. Van der Kamp [1976] developed a method for storage of the valves, basedon freeze-drying, which was claimed to be even better than gamma-radiation. On the other handThompsonet al. [1977] and Barratt-Boyes et a1.[1977J reported promising results from an antiblotic sterillzatlon and preservatten method. Wlth this technlque good results were obtained in aortic valve replacements, even after an
implantation period of six years. Furthermore Wheatly and McGregor [1977], also using homografts treated with antibiotics, discussed the viabilt"ty of the homograft valves. They concluded that lmplantatlon of a viabie valve results in gross valve leaflet distortien and shrinkage with consequent lossof function. Contrary to this, non-v(able valves showed only sl ight afteration in valve dlmensions, while valvular functlon was maintained.
Beterografts
"As a modiflcation of the homograft transplantation a preserved heterograft aortlc valve (an aortic valve removed from a dead animali usually a pig) mounted on a rigid cloth-covered frame can be used [lonescu et al., 1968 ]. The major advantages of heterograft valves over homografts for human valve repfacement are the availabllity and the wider range in si ze of heterografts [Harris et al., 1969] .
lnitially these valves were not durable, especially when the valve was freeze-dried or stored in formaldehyde [Buch et al., 1970; lonescu et al., 1972; Yarbrough et al., 1973; Duran, 1975]. Carpentier [1969]
introduced the use of glutaraldehyde as a method of producing improved cross-linking between the collagen fibres to extend durablllty.
available (the Hancock valve, see fig. 1,1, and the Angell-Shlley valve). In the Hancock valve a porcine aortic valve preserved in glutaralde-hyde, which is carefully mounted on a flexible stent, is applied [Reis et al., 1971]. The early results with this valve are eneauraging
[Zuhdi et al., 1974; Albert et al., 1977; Sauvage, 1977], although the systolic pressure drop across the first type of valve Is rather high,desplte lts natura! geometry [Morrls et al., 1977]. The pressure drop is slgnlficantly less In the modlfled valve lntroduced in 1975, where the muscular leaflet of the porclne valve Is excised and replaced by the non•muscular leaflet of another valve [Wright, 1977; Levine et al., 1978]. However, the durablllty of these valvès is still open to discusslon, because no long-term results are avallable yet [Sauvage, 1977; Spencer and Wallace, 1977]. On the one hand Christie et al. [1978] showed that the flow characteristics of the Hancock valve are vlrtually the same as those of normal human valves. Thls observatlon could be of lmportance wlth respect to the long-term performance of thls valve. On the other hand the studles of Carpentier et al. [1974] and Ferrans et al. [1978] showed that the collageneaus frameworkof these valves deterlorates prog•ressively In course of time, because the valves do not contaln vlabie cells and are not able to reptace the collagen that has been broken down. Thls could be the crltlcal factor
In the durabillty of these valves [Wallace, 1975].
Fascia ~ata vaZves
Another approach for using a biologica! valve is to take a strip of fascia from the pattent's thlgh and to shape lt toa valve during the operatlon [Sennlng, 1967; lonescu and Ross, 1969]. This method Is technically difficult. Besldes, Llncoln et al. [1971] stated that fascia lata valves develop thlckenlng and clcatriclal contraction and become insufficlent In the long term. Welch et al. [1971] found that a few months after remaval the fascia lata shows necrosis, degeneration and perforatlons. Yarbrough et al. [1973] reported that the lmplanted valve becomes rigid, the orifice stenotic and the tissue necrotic. Taylor et al. [1975] stated that apart from durablllty failures, thls valve In the mltral position offers a significant obstructlon to the left ventricular filling, partlcularly durlng exerclse. In 1975 Senning and Rothlln stated that further use of this method of heart valve reptacement Is not to be advised.
More positive results are given by Dubiel et al. [1973] whobelieve that fascia lata cells are able to remain viable. More recently Bailey et al. [1976] stated that, despite an early increment of shrinkage of about 30%, late studies of fascia lata indicate no lossof cellularity, no measurable lossof tissuestrengthor flexibility, while no late calcification was observed.
1. 1,3 ~~fbêQlfêl-~rQ~~b~~~~
Whi I st some surgeons use homografts or heterograftsas substitutes for the aortic and mitral valves, ethers, pointing to the difficulties experienced with the natura! valves, concentrate on prosthetlc
replacements. Most of these investigators abandoned the concept that the prosthetic valve should resembie the normal human anatomy,
entertaining the idea that the wheel does net resembie the human leg, but it has proved to be a goed substitute [Cooley, 1977].
The first implantation of an artificial valve in the circulatory system in men was achieved by Hufnagel et al. [1954], Without the use of a heart-lung machine he inserted a ball valve, contained in a rigid tube, into the deseending aorta; however, with limited benefit to the patient. Since that time many different prosthetic heart valves have been developed. The four basic types will be briefly reviewed, according to Wright [1972].
The prosthetic tricuspid leaflet valve
This valve, shaped after the natura! aortic valve, was used as an aortic prosthesis. The first type of this kind was the Roe valve [Roe and Moore, 1958]. Recently the Oxford valve [Bellhouse et al., 1972] and the Aachener Taschenklappe [Reul and Häussinger, 1973] were developed and tested, but they are net yet introduced into clinical practice. The main problems with this type of valve in the past were leaflet rupture, thickening and stiffening due to progresslve scar formation [Oavi la et al., 1967] and thrombus formation [Wrlght, 1972].
The hall valve
The Starr-Edwards ball valve is being used since 1960[Starr, 1960], During this period the valve has been redesigned (for example the Smeloff-Cutter presthes is; Cartwright et al., 1964 ) • The valve consists of a spherical bal 1 and an annular orifice, surrounded by a suturing ring.
The ball Is retalned by either a three- (for the aortlc posltion) or four- (in the mitral area} legged cage. Tlll the early seventles the Starr-Edwards valves and especially the models 1200 (aortic.position) and 6120 (mitral posltion) were the most successful valve prostheses
In clinlcäl us~ [Lennox, 1973]. The main problem associated with bal I valves is thrombo-embol ism [Macmanus et al., 1977; Dale, 1976; Lee et al,, 1975].
The low-profile or disc valve
Complications arlslng from the dimensions of the ball valves led to a search fora !ow-profile valve, which requires less space In the left ventricle. Early types are the University of Cape Town valve [Barnard et al., 1962] and the valves deslgned by Melrose et a1,[196~],
all for the mltral positlon. Although the results were disappointing, in the mlddie of the slxties many types were designed. In most of these valves the movable part Is disc-shaped and is retained wlthin a cage structure. Disc valves still used·;are the Beall valve [Beall et al., 1968], the Cooley-Cutter valve [Cooley et al., 1973] and the Kay-Shiley valve [Kay et al., 1966], The main disadvantage of the disc valves as compared toother prostheses, Is the wear of the dlsc or cage struts [Paton, 1969; Sllver and Wilson, 1977].
The pivoting disc valve
The search to produce a prosthesls w1th relatlvely good flow characteristlcs resulted In the free floatlng and rotatlng dlsc valve. The best known examples are the Björk-Shlley valve [Björk, 1969], see fig.1.1, and the Li llehei-Kaster valve [ Kaster and Llllehel, 1967], which were clinically lntroduced In 1969 and 1971, respectively. Both valves have a tlltlng disc. In the flrst valve the discopens and closes between two eccentrlcally sltuated support legs. The second one has two lateral disc gulde shlelds, whlch restraln the free floating disc toa speclfic excursional are. The main advantage of both valves is that the flow is virtually lamlnar, which results In a much better hemodynamic behavlour than of other mechanica! prostheses. In
partlcular the pressure drop over the opened valves proves to be relatlvely low [Brawley et al., 1975; Wright, 1976]. Yoganathan et al. [1978b], however, reported that the Björk-Shlley valve still causes thrombus formation and tissue overgrowth. Dale and Myhre [1978] reported that some lntravascular hemolysls Is due to lmplantation of
bath types cf disc valves. In order to imprave the overall performance
of this kind of valves, the Edinburgh group applled an aerofoil
obturator [Macleod et al,, 1976; Knight et al., 1977) , whereas in the St.Jude Medica\ presthesis the disc is divided in two halves [Emery
et al., 1978). However, neither valve has yet been introduced lnto clinical practice.
1.1,4 Discussion
---Biological vaZves
Tissue valves of different types have been employed for many years. By a variety of methods they are prepared from human valves obtained at autopsy or from porcine aartic valves, Tissue leaflets prepared from fascia lata, dura rnater [Zerbini, 1975) or pericardlal tissue
[Tanden et al., 1978] have also been used, The main advantages of this type of valves are the almest complete absence of thrombus formation, the relatively physiological hemadynamie characteristics as far as central flow and low pressure drop are considered,and a low incidence
of hemolysis [Austen and Hutter, 1977]. However, the problems associated with tissue valves are the following [Gonzalez-Lavin and Ross, 1971; Wright, 1972; Lennox, 1973; Wallace, 1975; Austen and
Hutter, 1977; Mullerand Nolan, 1977].
(i) lt is impossible to obtaln fresh aseptie valves of the correct size in the required numbers at the right time.
(i i) Preserved valves, whether of animal or human crigin are
subject to complications, even after an implantation period
of six years. The complîcations are: insufficiency, cusp
rupture, bacterial endocarditis, calcium deposition and
stenosis.
(i i i ) An additional drawback of the autologous valves, like these
made from fascia lata, is that the construction of these valves is technically difficult.
The failures partly result from the methods of sterilization and preservation. The introduetion of glutaraldehyde as a cross-linking agent topreserve the integri,ty of collagen molecules in the porcine valve appears to be promising. A considerable number of valves
preserved in this way has al ready been used, Although the durabi lity of these valves seems to befair so far, long-term evaluation is
required to appreciate the advantages of thls preservation technique. Most probably the non-viabie tissue wi 11 alsofall in course of time [Wallace, 1975].
Meahaniaa~ prostheses
Although different devices for aortlc valve replacement are now wldely used, none of them Is !deal. The mechanlcal devices are usually composed of a metal frame contalnlng a moving obturator. The latter may be a hinged flap, a sphere or a disc. The maln advantage of these valves Is that they are mechanically strong and durable. Despite the fact that the prestheses have been much improved in the past decades, they still have some serlous dlsadvantages:
(i) Prosthetic valves, especially if they are relatlvely smal i, may induce a conslderable pressure difference across the fully opened valve. Thls amounts from 1.3 to 2.6 kPa for the Björk-Shlley valve in aortic posltlon [Llotta et al., 1970; Brawley et al., 1975]. Moreover, during exerclse when cardiac output lncreases, the pressure drop lncreases progresslvely [Kaster et al., 1970]. Thls disadvantage Is even more pronounced when the pumplng capaclty of the heart Is not optlmal, for Instanee when valvular disease caused myocardlal damage [Messmer et al., 1972; Muller and Nolan, 1977]. (11) All prosthetic devices carry some risk of thrombus formatlon,
due to thematerlal used, the flow behavlour and the valvular geometry [Mueller et al., 1975; Dale, 1977; Figl iola and Mueller, 1977]. An embolus In the carebralor coronary arteries has serlous or even fatal consequences for the patlent. Antlcoagulants are generally prescribed to reduce embollzatlon. All mechanlcal prestheses require postoperatlve treatment with antlcoagulants for the rest of the pattent's life [Brawley et al., 1975; Cooley, 1977]. Application of anticoagulants in turn has resulted In specific lncidence of morbldlty and mortality [Lennox, 1973]. Cloth-covered
prestheses were developed to avoid the use of anticoagulants. However,these prestheses introduced new problems; cloth wear, tissue lngrowth [Austen and Hutter, 1977] and chronlc
hemolysls [Lefemine et al., 1974; Shah et al., 1978] were observed. Further modlflcatlons llke aporous metal coating,
have been introduced [MacGregor et al., 1976], but no long-term clinical results are yet available.
(lil) Because of the high shear stresses In the viclnlty of a heart valve presthesis [Figliola and Mueller, 1977; Yoganathan et al., 1978a] and the mechanica! Interaction between occluder and frame [Lefemine et al., 1974], darnaging of red blood cells (hemolysls) is inevitable [Eyster et al., 1971; Wllllams et al., 1971]. This can leadtoa shortenlng of the erythrocyte survival time and cause significant anaemla and an lncrease In plasma hemoglobin levels.
All prosthetic valves show more or less the disadvantages mentioned. Among the mechanica! prestheses the plvoting dlsc valves seem now to be the most preferabie ones [Liotta et al., 1970; Björk and 01 in, 1970; Kaster et al., 1970; Olin, 1971; Lillehei et al., 1972; Salam et al., 1976; Wright, 1976].
ConcZusion
The choice between a mechanica! and a tissue valve is difficult. Generally the durabil ity of biological valves is limited, whereas in mechanica! prestheses the hemodynamica! characteristics are usually
non-physiological and embolization can hardly be avoided [Lennox, 1973]. Today the choice is mainly determined by the patient's
situation, I.e. age, localizatlon of the disease and cardlac functlon. Out of the types mentloned, the tiltlng dlsc mechanlcal presthesis and the glutaraldehyde-treated heterograft have a slight preferenee [Editorial Lancet, 1976; Roberts, 1976]; examples of it are shown in fig. 1.1.
a.
F.{.gi.ute J .1. Photog.ir.a.ph-6
o6
a, .the Ha.nc.oc.k va.f.ve arui. b • .the BjlJJtk-Shil.ey va.f.ve.In table l.l a survey of the n~r of v&lves used ti 11 l977 Is given. Presthesis Bali valves Dise valves Pivoting-disc valves Glutaraldehyde treated heterografts Yea" introdueed 1961 1965 1969 1971
Tab.te 1. 1. SUJTimaJtlf
oó
pJtoóthuu óo.td c.ommVtc.-i..alllf[Me.heman artd Runelwv, 1978].
-Number
156.000 37.800 164.000 38.500
With an eye to future research on heart valves, it can be stated
·that the development of leaflet valves is preferable. A main problem which therefore needs to be solved, is the preservation of the valve for long periods without changlng the essential characteristlcs of the normal living valve. Moreover, it must be investigated whether the hemodynamica! and mechanica! behavlour of the valve can guarantee a sufficiently long llfe-time. However, so long as the leaflet valves are not durable, it is worthwhile to improve the mechanica! prostheses.
1.2 The purpose and scope of the Eindhoven heart valve research project The present study was prompted by the fact that biologica! triple leaflet valve prestheses show a limited I ife-span. This is especially true for fascia lata valves and porcine aortic heterografts.
Apart from possible tissue degeneration, abnormal hydrodynamica! and mechanica! condltions were recognized as the possible major
contrlbuting factors to valve failure. lt is supposed that this fililure is due to higher local stresses ht the teaflets of the presthesis as compared to the natura! valve.
In
the present study we wi l1 restriet ourselves to the aortlc valve and its reptacement for the reasen that the natura! valve Is non-muscular and therefore relatively easy to model. Moreover the aortic valve or its reptacement has to withstand a high diastollc pressure drop, re5ulting In high stresses within theteaflets.
Three basic hypotheses for the preserree of high local stresses in the leaflets of aortlc valve prestheses were formulated, which up to
now form the basis of our research [van Steenhoven et al., 1975a; Spaan et al., 1975]:
(i) Retarded valve closure.
Bellhouse and Talbot [1969] stressed the lmportance of the sinus of Valsalva behlnd each leaflet for gradual valvular closure, by demonstrating in model experlments that valves without these cavitles close later than normal valves. According totheir observations, this results in an increase of the total rev~rsed flow (back flow) through the valve up to approximately 25% as compared to 2% for the natura! valve. The reversed flow must be stopped by the valve, which results in a pressure peak over the valve and stress peaks within the leaflets at the moment of valve closure.
The first hypothesis states that an artlficial triple leaflet valve presthesis is subject to retarded valve closure. It Is
supposed that thls is due to an lnefficient use of the sinuses of Valsalva. The strenger reversed flow as a result of
retarded valve ciosure causes high pressure peaks across the leaflets and high stresses within them.
(ii) Leaflet flapplng.
The natura! aortic valve opens completely during systole. During mld-systole a trapped vertex exlsts within the sinus of Valsalva in such a way, that it reduces the vèlocity difference of the blood flow across each leaflet [Keele, 1952; Bellhouse and Talbot, 1969]. In the opened natural aortic valve some flutter is observed [Plnto et al., 1978].
lnstability of leaflets may be induced, either by turbulent eddies downstreamof the leaflets because of a stenotic behaviour of the valve, or by a velocity difference across the leaflets (the so-called Kelvin-Helmholtz instability)
[Benjamin, 1963].
The secend hypothesis says, that prosthetic leaflets are more unstable than natura! aortic valve leaflets, because of a more stenotic nature and a poor vertex formation behind the leaflets. This results in higher stress peaks withln the leaflets during mld-systole.
(i i i) Valve material properties.
The natural aortic valve leaflets are attached toa deformable and elastic aortlc wall. Besides, the leaflets have a very specific anisotroplc structure [Clark, 1973; Mlssirlls, 1973]. The third hypothesis Is, that in a closed prosthetic device the load bearing mechanisms are not as optima! as in the natural valve, due toa different leaflet attachment and different characterlstlcs of the leaflet materlal. This
results In higher stresses and stress gradients withln the leaflets during diastole.
The ultimate goal of the present research on basis of these hypotheses is twofold:
(i) To obtaln the parameters which determine the stress in valve teaflets by means of theoretical and experimental modell!ng. (i!) To formulate techn!cal spec!f!cations for the design and
lmplantation of artiflcial triple leaflet valve prostheses, whlch show a more optima! function!ng and have a longer llfe span.
The research phllosophy Is basedon the oplnlon that observation of the behavlour of the natura! aort!c valve gives ins!ght into the
relevant parameters. This research can be performed in
experiments In an analogue as well as in-vivo experiments. Then !t may be hoped that from thls understandlng, spec!fications can be given for artlficlal valves. As a consequence the project has four maln topics:
(i) The hydrodynamica! behaviour of the aortlc valve.
The alm Is to descrlbe the Interaction between flow pattern and cusp mot!on. The main subject has been the investlgation of the closlng mechanism of the aortic valve. To thls end experiments in an analogue were performed to find a physlcal model [van Steenhoven et al., 1975 b,c, 1976, 1977; van Steenhoven and van Dongen, 1979]. Furthermore the valve ciosure Is studled in animals [van Steenhoven et al., 1978 a, b, 1979a]. In the future the maln attention will be pald to leaflet lnstabilities.
(ii) The dynamica! behaviour of the aortic valve.
(i i I)
in the valve, the movements of the valve components and the pressures in the ~alve and over the valve leaflets. Till now, the investigations are directed towards the pressure-volume relationshlp of the aortic valve in termsof compl iance.[van Renterghem et al., 1979].
The ~echanical behaviour of the aortic valve.
The aim is here to correlate the pressures wlthin the valve to the local stresses within the valve leaflets. To achleve thls, attention is being paid to valve histology and the material properties of the valvetissue [Sauren et al., 1979 a,b]. The results from this and the pressure-load will betheinput data for the theoretica! modelling of valve mechanics and kinematlcs.
(iv) The application to valve prostheses.
In order to apply the basic !nformation obtained to a better design of heart valve prostheses, the stress reducing
mechanisms as found to exist in the natura! aortlc valve must be translated into design specifications [van Steenhoven et al., 1979b].
1.3 Object of the present investigation
The work presented is mainly devoted to the hydrodynamic~l
behaviour of the aortic valve, and in particular to its clo&ure. As dlscussed in the previous sectlon, a proper understanding of natura! aortlc vaive ciosure might be essential for the design of artiflciai
triple-leaflet valve prostheses. As Bellhouse and Talbot [1969] showed, the aortic valvestarts to close during the deceleration phase of systollc aortic flow. This prevents high stress within the leaflets at ciosure.
Two-dimensional experlments on aortic valve ciosure were performed In an analogue (Chapter 3). The Interaction between the sinus contents and the aortic flow, when the latter is decelerating, was studled at different deceleration rates of the mainstream. Theoretica] models based on the phenomena observed are proposed. An acceptable agreement between theory and experiment is found. Furthermore the influences of the sinus vertex and the shape of the sinus on valve ciosure are discussed.
Some extension of the model toa three-dimensional geometry is necessary, since it has to be applicable to the natural aortic valve
(Chapter 4).
The physlcal model was extensively tested in animal experiments (Chapter 5). In experimental open-ehest dogs direct cinematographic high-speed recordings of the aortic valve movement were made.
Simultaneously ECG, ascending aortic volume and the pressures in the aorta, left ventricle and left atrium were recorded. Comparison of the film frames with the aortic volume flow signals under different hemodynamic circumstances, reveals a good confirmity between the theoretically predicted and the in-vivo determined closing behaviour.
The second aim of the present investigation is to apply the findings on aortic -valve ciosure to the construction of an improved aortic heart valve. Therefore in Chapter 6, a prototype and the test
results are described.
A summary and conclusions wiJl complete the present dissertation (Chapter 7).
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Cardiovasc. Res •
.!..l.
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l
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Zerbini, E.J.: Results of replacement of cardiac valves by homologous dura mater valves.
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ll
(1974), pp. 479-491.2, THE HEART AND THE AORTIC VALVE
2.1 Thé ~n~tómy ~nd physlology of the heart
2.1.1 !Q~r22~~~l2Q
The heart is a rather complicated pump; the present chapter gives a brief review of some of lts anatomical and physlologlcal aspects. The information presented is taken from textbooks [Guyton, 1971] and survey articles [Wright, 1972; Arts, 1978]; it is basic and meant for
readerswhoare not famil lar wlth the anatomy and the physiology of the heart,
2.1.2 Ib~-~Q~~Q~Y
Fig. 2.1a shows diagrammatlcally a saggital cross-section of the heart. The heart consists of four cavities, the right (RA) and left
a.
~
_A __
APEX
F.<.gWte 2.1. Vi.a.gJta.m o6 the. heaJr.;t [Aiz.U, 1978]: a.. Se.c.tion o6 the heaJr.;t
.<.n
the. ~a.ggilalp!a.ne..
RA = !Ught WUwn, LA = .te.6t
wu..wn,
b.RV
=
!Ught ve.nVU.c..e.e., LV=
.te.6t ve.nt!Uc..te., TV = tlUC.!UiP-id va..tve., MV = m-U:M..e. va..tve., PV = pui.momvr.y va..tve., AV = a.o/t:Üc. va..tve., PT=
pui.momvr.y tnunk, AO=
a.o~, PPM = pa.p-i.U.M.y mlL6c..tU.b. C4o~~-~e.c.t.<.on o6 the. ve.nt!Uc..e.e. .<.n a. p!a.ne. wiU.c.h ~ .<.ncü.c.a.te.d M AA in 6-i.g • 2 • 1 a..
(LA) atria and the right (RV) and left (LV) ventricles. As indicated in fig. 2. lb the atria! and ventricular (VS) septa separate the two atria and ventricles. The free wal! of the right ventricle is
significantly thinner than that of the left ventricle, because a lower pressure is present in the right ventricle as compared to the left one. The valves of the heart are situated in the basal plane. Opposite to the basal plane is the apex of the heart; at this site the ventricular walls are relatively thin.
The systemic veins, which collect the blood from the systemic circulation, empty lnto the right atrium, whereas the pulmoriary veins, which conduct the reoxygenated blood from the lungs, empty into the
left atrium. The right atrium Is connected to the right ventricle through the tricuspid valve (TV), whereas the left atrium is connected to the left ventricle through the mitral valve (MV). A number of papillary muscles (PPM) originate from the ventricular walls and are connected by the chordae tendlneae to the leaflets of the tricuspid and mitral valves. The pulmonary valve (PV) is situated between the right ventricle and the pulmonary trunk (PT). Aftera short dlstance the pulmonary trunk bifurcates In the left and rlght pulmonary arteries, which supply the leftand right lung with desaturated blood,
respectively. The aortic valve (AV) connects the left ventr.cle to the aorta (AO). In the dog the first large si de branches of the ·aorta are the brachlocephallc artery and the left subclavlan artery, which supply the head and the upper extremlties with blood. In humans three branches of the aorta are present to thls purpose.
In total the heart has four valves, two in the rlght heart and two in the left heart. The inlet valves (the tricuspid and mitral) are also called the atrio-ventricular (A-V) valves, whereas the outlet valves (the pulmonary and aortic) are called the semilunar valves. As stated earlier, In the present study we wil! restriet ourselves to the aortlc valve.
2.1.3 I~~-~~~~12129~
Under normal conditions the human heart contracts and relaxesabout once a second.The period from the beginning of one heart contraction to the beginning of the next one Is called the cardlac cycle. lt consistsof a period of relaxation, diastole, followed by a perled of contraction, systole. Each contraction Is lnitlated by the generation
