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In vitro flow dynamics of four prosthetic aortic valves: A comparative analysis
CO21-929+3/89 53.00+.00
J. Biomechanics Vol. 22. No. 617, pp. 597-607, 1989. 0
Printed in Great Britain
1989 Pergamon Press plc
IN VITRO FLOW DYNAMICS OF FOUR PROSTHETIC
AORTIC VALVES: A COMPARATIVE ANALYSIS D. D. HANLE*, E. C. HARRISON~, A. P. YOGANATHAN~, D. T. ALLEN& and W. H. CORCORAN I/ *Chemical Engineering Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A; tcardiology Section, Los Angeles County-University of Southern California Medical Center, Los Angeles, CA 90033, U.S.A.; $ Biofluid Dynamics Laboratory, School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.; §De.partment of Chemical Engineering, University of California, Los Angeles, CA 90024, U.S.A.; 1)Chemical Engineering Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A. (deceased) Abstract-The velocity fields downstream of four prosthetic heart valves were mapped in oitro over the entire cross-section of a model aortic root using laser Doppler anemometry. The BjGrk-Shiley 60” convexoconcave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis were examined under both steady and pulsatile flows. Velocity profiles under steady flow conditions were a good approximation for pulsatile profiles only during midsystole. The pulsatile flow characteristics of the four valves showed variation in large scale flow structures. Comparison of the valves according to pressure drop, shear stress and maximum velocities are also provided.
INTRODUCTION
a)
Over 75,000 prosthetic heart valves are implanted annually throughout the world and most of these implants fall into one of four basic design classifications. Tilting disc valves, caged ball valves, bileaflet valves and bioprosthetics are all in common use. Representative valves from each of these four major classifications are shown in Fig. 1 and from their designs it is apparent that each will yield a significantly different flow field. A knowledge of the flow fields generated by the valves is useful in a variety of situations. For example, flow stagnation regions are more prone to thrombus formation than areas of high shear, thus velocity profiles can be useful in analyzing patterns of tissue overgrowth (Yoganathan et al., 1978, 1981). Shear stress distributions can be calculated from the velocity profiles and these stresses can be used to indicate the propensity of a valve to damage formed elements of blood (Chandran et al., 1983; Hanle, 1984; Hanle et al., 1987; Tiederman et al., 1986; Yoganathan et al., 1986; Woo and Yoganathan, 1986a, b). Finally, the presence of jets and other macroscopic flow structures can influence valve positioning and the interpretation of echo-Doppler measurements (Teirstein et al., 1985; Valdes-Cruz et al., 1986). This work presents velocity profiles and shear stress distributions for each of the four basic valve types shown in Fig. 1. The BjBrk-Shiley, Smeloff-Cutter, St. Jude Medical and Ionescu-Shiley valves were used
Received infinalform April 1988. *Current address: DuPont Glasgow Site, Medical Products Department, Wilmington, DE 19898, U.S.A.
b)
BJ&K-SHILEY
SMELOFF-CUTTER Double-Caged Prosthesis
Ball Valve
c)
ST JUDE
d)
IONESCU-St-KEY Rrlcordlol
Bwprosthess
Fig. 1. Valves studied; (a) Bjiirk-Shiley tilting disc; (b) SmelotT-Cutter caged ball; (c) St. Jude bileaflet; (d) Ionescu-Shiley bioprosthesis.
as representative examples of tilting disc, caged ball, bileaflet and bioprosthetic valves, respectively. These valves have been studied extensively and previous quantitative work on the flow characteristics of these and other prosthetic valves has been summarized by Rashtian et al. (1986). Qualitative descriptions of large scale flow structures under steady flow have been
597
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HANLE et al.
given by Schramm et al. (1980, 1982). The novel and significant features of the results presented here are (i) the velocity measurements were made at a sufficient number of locations to map large-scale flow structures, (ii) flow field measurements were made in both the axial and non-axial directions and (iii) both steady flow and pulsatile flows were considered. EXPERIMENTAL
The data were collected in oitro using the pulse duplicator system shown in Fig. 2. The velocity profiles were obtained using a one-dimensional laser Doppler anemometry (LDA) system; the method and apparatus are described in detail elsewhere (Hanle, 1984; Hanle et al., 1986). Briefly, the pulse duplicator generated physiologically realistic pressure drops and cardiac outputs across the prosthetic valves. Velocity profiles were measured at 317 grid points in a plane 31.8 mm downstream of the valve ring. The points formed-a grid as shown in Fig. 2 and were separated by 1.27 mm. At all grid points, velocity data were taken in each of the three mutually orthogonal coordinate directions. The three-dimensional velocity data were not, however, taken simultaneously. A single velocity component was measured for all grid
points on the data plane and then the experimental rig was set up for the next coordinate direction and the procedure repeated. The pumping fluid was a saline solution with a bulk viscosity of 0.01 kg m s- ‘, which is lower than the bulk viscosity of whole blood. Results by Schramm et al. (1980, 1982) indicate that at least the general characteristics of the velocity profiles downstream of aortic valves are relatively independent of whether water or blood analog solution is used. Pulsatile flow velocity measurements were made over many cardiac cycles and the results were averaged to yield mean velocities for each point in the cycle. Shear stresses were evaluated using these mean velocity data; since the flow was turbulent, the shear stresses consist of a laminar and a turbulent component, as shown in equation (1) ~zx =
.pminar) zx
+ prbulenl) IX
where rz and rX are the root mean square velocities in the z and x directions, r,, is one component of the shear tensor, p is the density of the fluid and p is the
Flow Resistors
1 Liquid Reservoir
Pressure Transducers:
Ventricular -Pump / Section
/
/
’ ,’ m
\ AorticYalve Flow Section
Laser- Doppler Anemometer
Data Plane 31.8 mm downstream of valve mount
A = 1.27mm
Fig. 2. Schematic diagram of the pulse duplicator; the data plane configuration is shown in the inset.
Flow dynamics of prosthetic aortic valves
viscosity of the fluid. The laminar component was assumed to be Newtonian and the turbulent component (or Reynolds shear stress) was based on an expression developed for tube flow (Tennekes and Lumely, 1972). Pressure measurements were made using pressure transducers located 150 mm downstream and 20 mm upstream of the valve base ring. Unless otherwise indicated, volumetric flow measurements were made using a Statham electromagnetic flow meter rigidly attached 200 mm downstream of the valve base ring.
599 RESULTS
Velocity profiles and shear stress distributions were measured downstream of the Bjiirk-Shiley 60” convexo-concave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis. The pressure drop and flow parameters used in the experiments were intended to be representative of arotic valves in patients at rest and are given in Tables 1 and 2.
Table 1. Parameters computed from the volumetric-flow data* Flow (1min-‘)
‘Me
Relative time? of occurrence in the cycle (ms)
3jjark-Shiley tilting disc valve Pulsatile pow: Pulse rate &IT Net forward flow
Q,.,.,.
70 min-’ 31.2kO.6 -8.2kO.7 5.08+0.13 20.6 + 0.1
191+8 326k5
27 (rotameter)
-
Steady pow: Q
-
Smeloff-Cutter caged ball valve P&wile
pow:
Pulse rate Lk Net forward flow
Q,.,.,.
70 min- ’ 30.4 f 0.4 -7.8kl.O 5.01+0.09 21.0+0.1
186&8 326+5
S_teady pow:
Q
27 (rotameter)
St. Jude Medical bileaflet valve Pulsatilejow:
Pulse rate k Net forward flow
Q,.,.,. Steadypow:
70 min-’ 29.6kO.4 -7.4kO.8 4.98 f 0.09 19.7+0.1
210+6 34015 -
27 (rotameter)
(z
Ionescu-Shiley bioprosthesis PulsatileJow:
Pulse rate Q”z
Net forward flow
Q,.,,,. S_teady jlow: Q
70 min-’ 29.8 k 0.3 - 10.5kO.4
5.22kO.13 19.5+0.1 27 (rotameter)
* Results given as mean f standard deviation. t Relative to the beginning of systolic ejection.
190+23 328+4
-
600
D. D. HANLE Table 2. Pressure data*
Type
Pressure (kPa)
Relative time? of occurrence in the cycle (ms)
Bjiirk-Shiley tilting disc valve, EOA = 2.71 cm* AP,,, VP,,, AP PS,S AP (steady flow)
2.6kO.2 19.7&0.5$ 12.1kO.41 0.8 io.2 1.4kO.l
169k27 195+13
Smeloff-Cutter caged ball valve, EOA = 2.17 cm* 3.3 +0.3 16Ok 12 182klO 18.4+0.5$ VP,,, AP 10.3kO.4$ 1.3kO.2 AP,,, G (steady flow) 2.2kO.l
AP,,,
St. Jude Medical bileaflet valve, EOA = 3.00 cm* AP,,, VP,,, Ap AP,,, AP (steady flow)
2.1 kO.2 19.3&0.4$ 12.5*0.3$ 0.6kO.l l.OkO.1
159+36 197+5
Ionescu-Shiley bioprosthesis, EOA = 2.57 cm* AP,,, VP,,,
2.9 k 0.2 20.5+0.5$
AP
13.3kO.61
APs,, p (steady flow)
173*17 194k 16
0.8kO.l
1.3+0.1
*Results given as mean + standard deviation.
tRe1ative to the beginning of systolic ejection. $Gauge pressure. Z-mean aortic pressure. VP,,,-maximum ventricular pressure. EOA--effective orifice area. APSYpmean systolic pressure gradient. AP-mean pressure drop during steady flow. AP,,X-maximum pressure gradient.
Bjiirk-Shiley tilting disc valve
The Bjiirk-Shiley convexo-concave valve is of the tilting disc variety. It consists of a satellite base ring with a pyrolytic carbon, tilting disc occluder. The occluder tilts open to 60” during systole; it has a convex downstream face and a concave upstream face which results in a more streamlined flow during systole than a flat disc. The valve has two regions of unequal area available for forward flow called the major and minor flow orifices. Due to the major and minor flow structure of this valve, the flow field, which is shown in Fig. 3(a), is very acentric. It consists of a large, lenticularly shaped jet near the tube wall and a second, smaller jet near the centerline of the tube. These two jets emerge from the major and minor outflow regions, respectively. A region of low vel-
et al.
ocities is also seen to correspond with the minor outflow region and may indicate that the flow in that region is nearly stagnant and hence thrombogenic. Particularly well-defined secondary-flow structures may play a role in generating the lenticularly shaped large jet and the central location of the smaller jet. The largely acentric flow contains a large wake generated behind the disc occluder which adversely affects the valve’s pressure drop characteristics. The pressure drop results for this prosthesis are somewhat higher than those found by other investigators (Aberg and Henze, 1979; Yoganathan et al., 1984). The acentricity of the flow caused high mean velocity gradients at the wall around much of the tube circumference. The high velocity gradients imply high shear stresses as shown in Fig. 4a. These high shear stresses are of such a magnitude that they may cause elevated levels of hemolysis to be associated with this valve in viva The shear stresses in the bulk flow are only moderate in magnitude and less than the level generally considered hemolytic (Nevaril et al., 1968; Sutera and Mehrjardi, 1975; Sallam and Hwang, 1983). The shear stresses are sufficient, however, to activate platelets and cause a chemical release reaction (Brown et al., 1975). The presence in the flow of activated platelets increases the possibility of thrombus formation and hence valve thrombosis and thromboembolic events. Smeloff-Cutter caged ball valve
The Smeloff-Cutter valve is a caged ball design. Three pronged cages are located both upstream and downstream of the valve orifice. When the valve is closed, the occluding ball sits in the upstream cage with the centerline of the ball positioned at the minimum cross-section of the valve base ring. The flow fields generated by the Smeloff-Cutter valve are shown in Fig. 3b. The profiles show evidence of wakes caused by the ball occluder and by the downstream cage struts. The ball occluder of the valve generates a large wake centered about the centerline of the tube. The three struts of the downstream cage generate three notable wakes near the tube wall. Only small secondary-flow structures are found, indicating that the lateral-flow occluding mechanism of this valve imparts very little azimuthal sense to the forward flow. The extent of the flow disturbance associated with the wakes generated by the struts and occluder suggest that these wakes may play a significant role in producing the relatively large pressure drop found for the Smeloff-Cutter valve. The pressure drop results for this prosthesis are higher than those found for the other three valves studied in the present investigation. The mean velocity gradients and associated shear stresses are reported in Fig. 4b. As with the Bjiirk-Shiley valve, the velocity gradients are elevated at the tube wall which implies high shear stresses in the flow near the wall. These high shear stresses are probably not of such a magnitude as to cause significant hemolysis in vivo. Shear stresses in the bulk flow
Flow dynamics of prosthetic aortic valves
a) BJbRK-SHILEY
CC (25mm)
Window Key
Window 8 18 Lhln
b) SMELOFF-CUTTER Window Key
601 (24mm) Window 8
Window I3
Window 20
c) ST JUDE (25mm) Wlrdw
Key
d) IONESCU-SHILEY Wlndow 8
Window Kay
(24 mm) Window 8
Window 17 16 L/min
Fig. 3. (a) Velocity profiles for the BjGrk-Shileyvalve at four points in the cardiac cycle. (b) Velocity profiles for the Smeloff-Cutter valve at four points in the cardiac cycle. (c) Velocity profiles for the St. Jude valve at four points in the cardiac cycle. (d) Velocity profiles for the Ionescu-Shiley bioprosthetic valve at four points in the cardiac cycle.
are only moderate in magnitude and are less than the level generally considered hemolytic. The shear stresses are, however, sufficient to activate platelets and cause a chemical release reaction. St. Jude Medical bileajlet valve
The St. Jude Medical prosthesis is a relatively new, low profile bileaflet valve covered entirely with pyroly-
tic carbon. When the valve opens, the two semicircular leaflets rotate out to an angle of 85”, leaving an orifice that is 85-9O%‘free from obstruction to flow. The mean axial velocities for the St. Jude Medical valve, as a function of time in the cardiac cycle, are given Fig. 3c. The flow structure downstream from the St. Jude Medical valve appears to have been affected by the two semicircular leaflet occluders. These two
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HANLE et al.
a) BJ6RK-SHILEY CC (25mm)
b) SMELOFF-CUTTER (24 mm) Window 0 hin
Window
Kev
0
WV0 Orientation
VOIVS Orientation
cl ST JUDE (25 mm) Wlndow Key
d) IONESCU-3HILEY (25 mm) Window 0
Window
Key
ONAl?’
VJIVO Orientation
/
Window
8
Window
14
Valve Orientation
Jg
Fig. 4. (a) Shear stress profiles for the Bjork-Shiley valve at three points in the cardiac cycle. (b) Shear stress profiles for the Smeloff-Cutter valve at three points in the cardiac cycle. (c) Shear stress profiles for the St. Jude valve at three points in the cardiac cycle. (d) Shear stress profiles for the Ionescu-Shiley bioprosthetic valve at three points in the cardiac cycle.
leaflets divide the base ring orifice of the valve into three regions. Two of these regions are themselves roughly semicircular in shape. The third region is rectangular in shape with a height-to-width ratio of about five. The areas of the two semicircularly shaped regions are equal, and together represent about 78%
of the area available for forward flow. The three major flow areas generate two parallel ridges in the axial velocities which can be seen during much of the systolic ejection interval. The flow also shows evidence of wakes generated by the two leaflets. The extent of the flow disturbance associated with these wakes
Flow dynamics
of prosthetic
suggests that they may play a significant role in generating the pressure drop found for this valve. Only small secondary-flow structures are found, indicating that the occluding mechanism of this valve imparts very little lateral or azimuthal sense to the forward flow. The mean velocity gradients and associated shear stresses are given in Fig. 4c. The shear stresses are elevated at the tube wall, but these shear stresses are probably not of a magnitude that would be associated with significant hemolysis in uiuo. Shear stresses in the bulk flow are only moderate in magnitude and are less than the level generally considered hemolytic. The shear stresses are sufficient to activate platelets and cause a chemical release reaction. The St. Jude Medical valve is found to have a relatively small pressure drop in both steady and pulsatile flow. A more detailed discussion of the hemodynamics of this valve is given by Hanle et al. (1988). lonescu-Shiley bioprosthetic valve
The standard Ionescu-Shiley valve is a bioprosthetic valve made of tissue from the bovine pericardial sack. The occluding mechanism consists of three equal-sized leaflets of roughly semi-lunar shape. The leaflets are fixed with glutaraldehyde at high pressure and mounted on a titanium stent. During forward flow the leaflets billow out to provide a circular orifice; upon deceleration and reverse flow the leaflets are pushed back to the closed position where their free edges seal the orifice by resting against one another. The velocity measurements, which are presented in Fig. 3d, suggest that the occluding mechanism of the Ionescu-Shiley valve causes flow structure markedly
a) BJORK-SHILEY
CC (25 mm)
Steady Flow
Pulsatile Flow
c) ST JUDE (25 mm)
aortic
603
valves
different from that downstream from a natural aortic valve. Somewhat surprisingly, the nearly circular flow orifice generated by the valve leaflets does not result in a circular flow structure downstream from the valve. The Ionescu-Shiley valve results have been discussed elsewhere (Hanle, 1984; Hanle et al., 1986, 1987); the more significant findings are noted here. A pronounced, jet-like structure emerges from this valve with a shape and orientation which suggests that the valve leaflets greatly constrict the forward flow. Significant, well-defined secondary-flow structures are also found and may play a role in generating the noncircular, jet-like flow. This flow constriction gives rise to large regions of flow separation, and flow separation is implicated as the cause of the moderate pressure drop found for this valve. Results from farther downstream in steady flow provide insight into the structure of the forward flow. The large axial jet is found to have diverged trisymmetrically, thus generating three individual regions of separated flow. High mean velocity gradients and elevated velocities in the bulk flow are shown to contribute to significant shear stresses in the bulk flow. The estimated shear stresses are on the order of those considered to be potentially hemolytic. These shear stresses, in turn, seem to correlate with the findings of others that this valve may generate notable intravascular hemolysis (Febres-Roman et al., 1980). The shear stresses are also sufficient to activate platelets and cause a chemical release reaction. Steady us pulsatilejow
A comparison of steady flow and pulsatile flow results (see Figs 5 and 6) shows that for the four valves
b) SMELOFF-CUTTER
(24 mm)
Steody Flow
Pulsatile Flow
d) IONESCU-SHILEY
(24 mm)
.Steody Flow
Fig. 5. A comparison
Pulsotile Flow
Steady Flow
Pulsatile Flow
of pulsatile and steady flow velocity profiles at similar total flow rates. The pulsatile velocity profiles shown are for mid-systole.
604
D. D.
HANLE et 01.
Valve Orientation
a) BJi)RK-SHILEY
CC
Valve Orientation
l4
b) SMELOFF-CUTTER
1”
0 cm/set Valve Orientation
c) ST JUDE
Valve Orientation
c
0 cmhc
d) IONESCU-SHILEY
0 cm/set
Steady Flow
Pulsatile Flow
Steody Flow
Pulsatile Flow
Fig. 6. A comparison of pulsatile and steady flow r.m.s velocity profiles at similar total flow rates. The pulsatile data shown are for mid-systole.
studied: (i) the steady flow results approximate those for pulsatile flow only during the middle of systole and (ii) the r.m.s. velocities for steady flow provide an upper bound to those found in pulsatile flow. The results indicate the flow structure of mid-systole is most similar to that for steady flow. The steady flow results are quite different from those for pulsatile flow during early and late systole. It is found that early systole can be estimated a priori as plug flow. Late systole would be difficult to predict a priori. Pulsatile flow results in diastole are found to have structures which would not be predictable from the steady flow results. A complete discussion of these findings is presented elsewhere (Hanle et al., 1986; Hanle, 1984).
DISCUSSION
Comparison of valves
Pressure drops, velocity profiles and shear stress distributions have been measured for each of the four valves considered in this work. During these measurements, the net forward flow was held relatively constant. Other parameters, such as mean aortic pressure, varied as a consequence of holding flow output constant. The comparisons made between valves are thus strictly valid only at equal forward flow rates. Pressure drop. A comparison of the pressure drop results reported in Table 2 shows that of the valves studied the St. Jude Medical valve generated the lowest pressure in both steady and pulsatile flow,
while the Smeloff-Cutter valve generated the highest. Accordingly, the St. Jude Medical valve had the largest effective orifice area (Gorlin and Gorlin, 1951) in both steady and pulsatile flow and the Smeloff valve had the smallest. The BjBrk-Shiley convexo-concave and Ionescu-Shiley valves each had intermediate pressure drop characteristics which were similar in magnitude. The mean aortic pressure also varied. It was highest for the Ionescu-Shiley valve and lowest for the Smeloff-Cutter, with a maximum difference of the order of 25%. Velocity projles. The four valves can also be compared with respect to velocity results obtained 31.8 mm downstream from the valve orifices during forward flow. The Ionescu-Shiley generated the highest mean axial velocities of the four valves. The Ionescu-Shiley valve also generated the highest mean velocity gradients and r.m.s. axial velocities in the bulk flow away from the tube wall and was the only valve to generate regions of separated flow which could be measured at the data plane. The Bjark-Shiley convexo-concave valve generated the highest mean velocity gradients near the tube wall and also generated the largest mean non-axial velocity components. The high gradients at the wall and the large non-axial velocities resulted from the considerable acentricity of the forward flow generated by the tilting disc. The Smeloff-Cutter and St. Jude Medical valves generated relatively low values for the maximum mean axial velocity across the data plane. The mean non-axial and axial velocities for these two valves were also relatively small.
Flow dynamics of prosthetic aortic valves
605
26 L/min
Empty Flow Section
lonescu-Shiley
Valve
Fig. 7. A comparison of the velocity profile generated by the pulse duplicator with no valve in the flow
section with that for the Ionescu-Shiley bioprosthesis.
None of the valves studied generated a flow field like that expected for the natural aortic valve. Schramm et al. (1980, 1982) have studied the natural valve in oitro and have shown an almost unconstricted forward flow which reattaches very quickly downstream. These observations correlate well with findings in our laboratory with an empty flow section, i.e. where during an experimental run no valve is mounted in the flow. Figure 7 shows a relatively undisturbed forward flow as compared with that for the Ionescu-Shiley valve. For the empty flow section, little flow constriction is evident at the data plane with no notable flow separation. The valve designs all generated axial flow structures unique to their design. The Ionescu-Shiley valve generated a triangularly shaped, centrally located jet of fluid containing very high mean axial velocities of greater than 240 cm s- i. These high mean axial velocities dropped off quickly away from the tube centerline giving rise to relatively large mean velocity gradients in the bulk flow. The Ionescu-Shiley valve also generated significant regions of flow separation. The Bjlirk-Shiley convexo-concave valve generated a large, lenticularly shaped jet of fluid which was located very close to approximately half of the circumference of the tube wall. This large, acentric jet contained high mean axial velocities rising beyond 160 cm s- ‘. A second, much smaller jet was generated by the Bjiirk-Shiley convexo-concave valve near the centerline of the tube. This centrally located jet, which was much smaller in cross-sectional area than the first and contained mean axial velocities only about half those of the first, was apparently generated by the flow through the minor outflow region. Regions of very low velocity were also generated by the Bjiirk-Shiley convexo-concave valve, as observed at the data plane. The largest of these low velocity regions was located near the tube wall on the opposite side of the tube from the maximum of the large, lenticularly shaped jet. The Smeloff valve generated three maxima in the mean axial velocities interspersed with three regions oflower velocities. A ‘saddle point’ in the mean axial velocities was thus observed near the center of the tube. The maxima for the Smeloff valve were not very distinct
and rose to only about 120 ems-‘. The St. Jude Medical valve generated two maxima in the mean axial velocities interspersed with two regions of lower velocities. Thus a ‘saddle point’ in the mean axial velocities was also observed for the St. Jude Medical valve as was observed for the Smeloff valve. The maxima for the St. Jude Medical valve rose to about 140 cm s- 1 and were a little more distinct than those for the Smeloff valve. Only very small mean velocity gradients were generated by the St. Jude Medical valve in the bulk flow. Shear stress distributions. The four valves were also compared with regard to the shear stresses generated at the time of maximum forward flow. For this comparison all velocity results were obtained at the data plane located 31.8 mm downstream from the valve orifice. The shear stresses were estimated from the mean axial velocities and the r.m.s. axial and r.m.s. non-axial velocities using equation (1). The relative magnitude and extent of the shear stresses generated downstream from the four valves can be compared by estimating the maximum total shear stress which was present over at least 10% of the tube cross-section. These maximum shear stresses are given in Table 3 for each of the four valves in steady and pusatile flow and for the empty flow section. The maximum shear stresses for the empty flow section were the lowest of those in Table 3. The St. Jude Medical valve overall had the smallest maximum total shear stresses; the Ionescu-Shiley valve had the largest. The relative values of the shear stresses are in qualitative agreement with the two dimensional LDA results of Woo et al. (1985, 1986a, b) and Yoganathan et al. (1986). Only relative values can be compared due to the differences in blood analog solutions used and pulse duplicator geometries. The major discrepancy in the relative shear stress values appears to be for the Ionescu-Shiley valve, where the total shear stresses reported in this work are higher than those for the Bjiirk-Shiley valve, in contrast to Yoganathan’s result (1986). The discrepancy is due to the fact that Yoganathan measured only centerline shear stresses for the Ionescu-Shiley valve and this work indicates that maximum shear stresses are off centerline.
606
D. D. HANLEet
al.
Table 3. Comparison of the magnitude of the estimated total shear stresses, r,,, generated downstream from the four prosthetic aortic heart valves studied in the present investigation* Maximum estimated total shear stresst (N mT2) Steady flow $ Pulsatile flows
Type
Empty flow section (25.4 mm) St. Jude (25 mm) Smeloff-Cutter (24 mm) Bjdrk-Shiley convexo-concave (25 mm) Ionescu-Shiley (25 mm)
Maximum turbulent shear stress (N m-‘) Yoganathan (1986) Woo (1985)
43 76 78
74 89
200 -
160 -
112 128
106 112
340 250
240 -
*Results for empty flow section also included as base-line. YThe maximum total shear stress is given which was estimated to be present over at least 10% of the tube cross-section. Experimental uncertainty: + 5 Nmw2. 1At steady flow rate of 27 lmin-i. $At a maximum pulsatile-flow rate of 30 1min -’ for Ionescu-Shiley, BjBrk-Shiley convexo-concave, and Smeloff-Cutter valves and 29 1min-’ for St. Jude valve. No pulsatile flow results were obtained for the empty flow section.
CONCLUSIONS
In vitro laser Doppler anemometry has been used to compare the flow dynamics of four aortic prosthetic heart valves. Detailed comparisons of valve hemodynamics are given in the previous section. The major points are summarized below. (1) All of the valves generated flow structures very unlike that expected for the natural aortic valve in vivo.
(2) A comparative analysis of steady and pulsatile flow showed that steady flow results only approximated the pulsatile flow results obtained during the middle of the systolic ejection phase of the pulse cycle. Early in systole, the flow was determined to be of a plug flow type. Late in systole, the flow was highly disturbed and would have been difficult to predict a priori. In addition it was found that the magnitude of the flow disturbance measured for steady flow was an upper bound on that measured for pulsatile flow. (3) Of the valves studied, the Ionescu-Shiley valve gave the highest shear stresses. These shear stresses were only slightly higher than the levels expected to generate hemolysis. (4) Large scale flow jets are created by the Bjiirk-Shiley and Ionescu-Shiley valves. The jets may be capable of sublethal damage to red blood cells or the endothelial lining of tissue. Strong jet-like flows were not observed for the St. Jude Medical or Smeloff-Cutter valves. Finally, it must be noted that this study concentrated solely on in oitro flow characteristics of moderately sized aortic valves at resting conditions. The effect of small valve sizes and flow rates simulating exercise needs to be examined to provide a full picture of valve hemodynamics. Further, the best flow performance in vitro does not necessarily imply the best performance on implantation. Valve durability, for example, has the potential to overwhelm any differences in flow characteristics between prostheses.
Acknowledgements-Funding for this work was provided by the Donald E. Baxter Foundation, the Children’s Heart Foundation of Southern California, and the American Heart Association, Greater Los Angeles Affiliate.
REFERENCES
Aberg, B. and Henze, A. (1970) Comparison between the in oitro flow dynamics of the standard and convexo-concave Bjork-Shiley tilting disk valve prosthesis. Stand. .I. Thorac. Curdiovusc. Surg. 13, 177-l 89. Brown, C. H., Lemuth, R. F., Hellurns, J. D., Leverett, L. B. and Alfrey, C. P. (1975) Response of human platelets to shear stress. Trans. Am. Sot. arti$ Intern41 Organs 21, 35-39.
Chandran, K. B., Cabell, G. N., Khalighi, B. and Chen, C. J. (1983) Laser anemometry measurements of pulsatile flow past aortic valve prostheses. J. Biomechanics 16, 865-873. Febres-Roman, P. R., Bourg, W. C., Crone, R. A:, Davis, R. C. and Williams,T. H. (1980) Chronic intravascular hemolysis after aortic valve replacement with Ionescu-Shiley xenograft: comparative study with BjGrk-Shiley prosthesis. Am. J. Curdiol. 46, 735-738.
Gorlin, R. and Gorlin, S. G. (1951) Hydraulic formula for calculation of the area of the stenotic mitral valve, other cardiac valves andcentral circulatory shunts. Am. Heart J. 41, l-29. Hanle, D. D. (1984) Ph.D. Thesis, California Institute of Technology. Hanle, D. D., Harrison, E. C., Yoganathan, A. P. and Corcoran, W. H. (1986) In oitro velocity measurements downstream from the Ionescu-Shiley aortic bioprosthesis in steady and pulsatile flow. Med. Biol. Engng Comput. 24, 449-459.
Hanle, D. D., Harrison, E. C., Yoganathan, A. P. and Corcoran, W. H. (1987) In vitro turbulence downstream from the Ionescu-Shiley bioprosthesis in steady and pulsatile flow. Med. Biol. Engng Comput. 25, 645-649. Hanle, D. D., Harrison, E. C., Yoganathan, A. P. and Corcoran, W. H. (1988) In oitro velocity measurements and turbulence downstream from the St. Jude prosthesis in steady and pulsatile flow. Engng Med: 17(4), 181-187.’ Nevaril, C. G., Lynch. E. C., Alfrey, C. P. and Hellums, J. D. (1968) Erythrocyte damage and destruction induced by shearing stress. J. Lab. clin. Med. 71, 784-790.
Flow dynamics
of prosthetic
Rashtian, M. Y., Stevenson, D. M., Allen, D. T., Yoganathan, A. P., Harrison, E. C., Edmiston, W. A., Faughnan, P. and Rahimtoola, S. H. (1986) Flow characteristics of four commonly used mechanical heart valves. Am. J. Cardiol. 58, 743-752. Sallam, A. M. and Hwang, N. H. C. (1983) Red blood cell hemolysis in a turbulent jet. Biomechanics Symposium, Houston, pp. 83-86. Schramm, D., Baldauf, W. and Meisner, H. (1980) Flow pattern and velocity field distal to human arotic and artificial heart valves as measured simultaneously by ultramicroscope anemometry in cylindrical glass tubes. Thorac. cardiovasc. Suraeon 28. 1333140. Schramm, D., Miiller-Mohnssen, H., Baldauf, W. and Meisner. , H. (1982) Fresh human aortic and artificial heart valves studied in vitro using ultramicroscopic anemometry. Further results. Thorac. cardiouasc. Surgeon 30, 273-280. Sutera, S. P. and Mehrjardi, M. H. (1975) Deformation and fragmentation of human red blood cells in turbulent shear Row. Biophys. J. 15, I-IO. Teirstein, P. S., Yock, P. G. and Papp, R. L. (1985) The accuracy of Doppler ultrasound measurement of pressure gradients across irregular, dual and tunnellike obstructions to bloodflow. Circulation 72, 577-584. Tennekes, H. and Lumely, J. (1972) A first course in turbulence. MIT Press, Cambridge, MA. Tiederman, W. G., Steinle, M. J. and Phillips, W. M. (1986) Two component laser velocimeter measurements downstream of heart valve prostheses. in pulsatile flow. J. hiomech. Engng 108, 59-64. Valdes-Cruz, L. M., Yoganathan, A. P., Tamura, T., Tomizuka, F., Woo, Y. R. and Sahn, D. J. (1986) Studies in uirro of the relationship between ultrasound and laser Doppler velocimetry and applicability of the simplified
aortic
valves
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Bernoulli relationship. Circulation 73, 300-308. Woo, Y. R. and Yoganathan, A. P. (1985) In oitro pulsatile flow velocity and turbulent shear stress measurements in the vicinity of mechanical aortic heart valve prostheses. Life Support Systems 3,283-312. Woo, Y. R. and Yoganathan, A. P. (1986a) Pulsatile flow velocity and shear stress measurements on the St. Jude bileaflet valve prosthesis. Stand. J. thorac. cardiovasc. Surg. 20, 15-28. Woo, Y. R. and Yoganathan, A. P. (1986b) In vitro pulsatile flow velocity and shear stress measurements in the vicinity of mechanical mitral heart valve prostheses. J. Biomechanits 19, 39-51. Yoganathan, A. P., Corcoran, W. H., Harrison, E. C. and Carl. J. R. (19781 The Biiirk-Shilev aortic m’osthesis: flow characteristics, thrombus formation and tissue overgrowth, Circulation 58, 70-76. Yoganathan, A. P., Reamer, H. H., Corcoran, W. H., Harrison, E. C., Shulman, I. A. and Parnassus, W. (1981) The Starr-Edwards aortic ball valve: flow characteristics, thrombus formation and tissue overgrowth. Artif Organs 5, 617. Yoganathan, A. P., Chaux, G., Gray, R. J., Woo, Y. R., De Robertis, M., Williams, F. P. and Matloff, J. M. (1984) Bileaflet, tilting disc and porcine aortic valve substitutes: in vitro hydrodynamic characteristics. J. Am. Co/l. Cardiol. 3, 313-320. Yoganathan, A. P., Woo, Y. R., Williams, F. P.. Stevenson, D. M., Franch, R. H. and Harrison, E. C. (1983) In oitro fluid dynamic characteristics of Ionescu Shiley and Carpentier Edwards tissue bioprostheses. Artif: Organs 7, 459469. Yoganathan, A. P., Woo, Y. R. and Hsing, W. S. (1986) Turbulent shear stress measurements in the vicinity of aortic heart valve prostheses. J. Biomechanics 19.4333442.
J. Biomechanics Vol. 22. No. 617, pp. 597-607, 1989. 0
Printed in Great Britain
1989 Pergamon Press plc
IN VITRO FLOW DYNAMICS OF FOUR PROSTHETIC
AORTIC VALVES: A COMPARATIVE ANALYSIS D. D. HANLE*, E. C. HARRISON~, A. P. YOGANATHAN~, D. T. ALLEN& and W. H. CORCORAN I/ *Chemical Engineering Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A; tcardiology Section, Los Angeles County-University of Southern California Medical Center, Los Angeles, CA 90033, U.S.A.; $ Biofluid Dynamics Laboratory, School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.; §De.partment of Chemical Engineering, University of California, Los Angeles, CA 90024, U.S.A.; 1)Chemical Engineering Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A. (deceased) Abstract-The velocity fields downstream of four prosthetic heart valves were mapped in oitro over the entire cross-section of a model aortic root using laser Doppler anemometry. The BjGrk-Shiley 60” convexoconcave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis were examined under both steady and pulsatile flows. Velocity profiles under steady flow conditions were a good approximation for pulsatile profiles only during midsystole. The pulsatile flow characteristics of the four valves showed variation in large scale flow structures. Comparison of the valves according to pressure drop, shear stress and maximum velocities are also provided.
INTRODUCTION
a)
Over 75,000 prosthetic heart valves are implanted annually throughout the world and most of these implants fall into one of four basic design classifications. Tilting disc valves, caged ball valves, bileaflet valves and bioprosthetics are all in common use. Representative valves from each of these four major classifications are shown in Fig. 1 and from their designs it is apparent that each will yield a significantly different flow field. A knowledge of the flow fields generated by the valves is useful in a variety of situations. For example, flow stagnation regions are more prone to thrombus formation than areas of high shear, thus velocity profiles can be useful in analyzing patterns of tissue overgrowth (Yoganathan et al., 1978, 1981). Shear stress distributions can be calculated from the velocity profiles and these stresses can be used to indicate the propensity of a valve to damage formed elements of blood (Chandran et al., 1983; Hanle, 1984; Hanle et al., 1987; Tiederman et al., 1986; Yoganathan et al., 1986; Woo and Yoganathan, 1986a, b). Finally, the presence of jets and other macroscopic flow structures can influence valve positioning and the interpretation of echo-Doppler measurements (Teirstein et al., 1985; Valdes-Cruz et al., 1986). This work presents velocity profiles and shear stress distributions for each of the four basic valve types shown in Fig. 1. The BjBrk-Shiley, Smeloff-Cutter, St. Jude Medical and Ionescu-Shiley valves were used
Received infinalform April 1988. *Current address: DuPont Glasgow Site, Medical Products Department, Wilmington, DE 19898, U.S.A.
b)
BJ&K-SHILEY
SMELOFF-CUTTER Double-Caged Prosthesis
Ball Valve
c)
ST JUDE
d)
IONESCU-St-KEY Rrlcordlol
Bwprosthess
Fig. 1. Valves studied; (a) Bjiirk-Shiley tilting disc; (b) SmelotT-Cutter caged ball; (c) St. Jude bileaflet; (d) Ionescu-Shiley bioprosthesis.
as representative examples of tilting disc, caged ball, bileaflet and bioprosthetic valves, respectively. These valves have been studied extensively and previous quantitative work on the flow characteristics of these and other prosthetic valves has been summarized by Rashtian et al. (1986). Qualitative descriptions of large scale flow structures under steady flow have been
597
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given by Schramm et al. (1980, 1982). The novel and significant features of the results presented here are (i) the velocity measurements were made at a sufficient number of locations to map large-scale flow structures, (ii) flow field measurements were made in both the axial and non-axial directions and (iii) both steady flow and pulsatile flows were considered. EXPERIMENTAL
The data were collected in oitro using the pulse duplicator system shown in Fig. 2. The velocity profiles were obtained using a one-dimensional laser Doppler anemometry (LDA) system; the method and apparatus are described in detail elsewhere (Hanle, 1984; Hanle et al., 1986). Briefly, the pulse duplicator generated physiologically realistic pressure drops and cardiac outputs across the prosthetic valves. Velocity profiles were measured at 317 grid points in a plane 31.8 mm downstream of the valve ring. The points formed-a grid as shown in Fig. 2 and were separated by 1.27 mm. At all grid points, velocity data were taken in each of the three mutually orthogonal coordinate directions. The three-dimensional velocity data were not, however, taken simultaneously. A single velocity component was measured for all grid
points on the data plane and then the experimental rig was set up for the next coordinate direction and the procedure repeated. The pumping fluid was a saline solution with a bulk viscosity of 0.01 kg m s- ‘, which is lower than the bulk viscosity of whole blood. Results by Schramm et al. (1980, 1982) indicate that at least the general characteristics of the velocity profiles downstream of aortic valves are relatively independent of whether water or blood analog solution is used. Pulsatile flow velocity measurements were made over many cardiac cycles and the results were averaged to yield mean velocities for each point in the cycle. Shear stresses were evaluated using these mean velocity data; since the flow was turbulent, the shear stresses consist of a laminar and a turbulent component, as shown in equation (1) ~zx =
.pminar) zx
+ prbulenl) IX
where rz and rX are the root mean square velocities in the z and x directions, r,, is one component of the shear tensor, p is the density of the fluid and p is the
Flow Resistors
1 Liquid Reservoir
Pressure Transducers:
Ventricular -Pump / Section
/
/
’ ,’ m
\ AorticYalve Flow Section
Laser- Doppler Anemometer
Data Plane 31.8 mm downstream of valve mount
A = 1.27mm
Fig. 2. Schematic diagram of the pulse duplicator; the data plane configuration is shown in the inset.
Flow dynamics of prosthetic aortic valves
viscosity of the fluid. The laminar component was assumed to be Newtonian and the turbulent component (or Reynolds shear stress) was based on an expression developed for tube flow (Tennekes and Lumely, 1972). Pressure measurements were made using pressure transducers located 150 mm downstream and 20 mm upstream of the valve base ring. Unless otherwise indicated, volumetric flow measurements were made using a Statham electromagnetic flow meter rigidly attached 200 mm downstream of the valve base ring.
599 RESULTS
Velocity profiles and shear stress distributions were measured downstream of the Bjiirk-Shiley 60” convexo-concave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis. The pressure drop and flow parameters used in the experiments were intended to be representative of arotic valves in patients at rest and are given in Tables 1 and 2.
Table 1. Parameters computed from the volumetric-flow data* Flow (1min-‘)
‘Me
Relative time? of occurrence in the cycle (ms)
3jjark-Shiley tilting disc valve Pulsatile pow: Pulse rate &IT Net forward flow
Q,.,.,.
70 min-’ 31.2kO.6 -8.2kO.7 5.08+0.13 20.6 + 0.1
191+8 326k5
27 (rotameter)
-
Steady pow: Q
-
Smeloff-Cutter caged ball valve P&wile
pow:
Pulse rate Lk Net forward flow
Q,.,.,.
70 min- ’ 30.4 f 0.4 -7.8kl.O 5.01+0.09 21.0+0.1
186&8 326+5
S_teady pow:
Q
27 (rotameter)
St. Jude Medical bileaflet valve Pulsatilejow:
Pulse rate k Net forward flow
Q,.,.,. Steadypow:
70 min-’ 29.6kO.4 -7.4kO.8 4.98 f 0.09 19.7+0.1
210+6 34015 -
27 (rotameter)
(z
Ionescu-Shiley bioprosthesis PulsatileJow:
Pulse rate Q”z
Net forward flow
Q,.,,,. S_teady jlow: Q
70 min-’ 29.8 k 0.3 - 10.5kO.4
5.22kO.13 19.5+0.1 27 (rotameter)
* Results given as mean f standard deviation. t Relative to the beginning of systolic ejection.
190+23 328+4
-
600
D. D. HANLE Table 2. Pressure data*
Type
Pressure (kPa)
Relative time? of occurrence in the cycle (ms)
Bjiirk-Shiley tilting disc valve, EOA = 2.71 cm* AP,,, VP,,, AP PS,S AP (steady flow)
2.6kO.2 19.7&0.5$ 12.1kO.41 0.8 io.2 1.4kO.l
169k27 195+13
Smeloff-Cutter caged ball valve, EOA = 2.17 cm* 3.3 +0.3 16Ok 12 182klO 18.4+0.5$ VP,,, AP 10.3kO.4$ 1.3kO.2 AP,,, G (steady flow) 2.2kO.l
AP,,,
St. Jude Medical bileaflet valve, EOA = 3.00 cm* AP,,, VP,,, Ap AP,,, AP (steady flow)
2.1 kO.2 19.3&0.4$ 12.5*0.3$ 0.6kO.l l.OkO.1
159+36 197+5
Ionescu-Shiley bioprosthesis, EOA = 2.57 cm* AP,,, VP,,,
2.9 k 0.2 20.5+0.5$
AP
13.3kO.61
APs,, p (steady flow)
173*17 194k 16
0.8kO.l
1.3+0.1
*Results given as mean + standard deviation.
tRe1ative to the beginning of systolic ejection. $Gauge pressure. Z-mean aortic pressure. VP,,,-maximum ventricular pressure. EOA--effective orifice area. APSYpmean systolic pressure gradient. AP-mean pressure drop during steady flow. AP,,X-maximum pressure gradient.
Bjiirk-Shiley tilting disc valve
The Bjiirk-Shiley convexo-concave valve is of the tilting disc variety. It consists of a satellite base ring with a pyrolytic carbon, tilting disc occluder. The occluder tilts open to 60” during systole; it has a convex downstream face and a concave upstream face which results in a more streamlined flow during systole than a flat disc. The valve has two regions of unequal area available for forward flow called the major and minor flow orifices. Due to the major and minor flow structure of this valve, the flow field, which is shown in Fig. 3(a), is very acentric. It consists of a large, lenticularly shaped jet near the tube wall and a second, smaller jet near the centerline of the tube. These two jets emerge from the major and minor outflow regions, respectively. A region of low vel-
et al.
ocities is also seen to correspond with the minor outflow region and may indicate that the flow in that region is nearly stagnant and hence thrombogenic. Particularly well-defined secondary-flow structures may play a role in generating the lenticularly shaped large jet and the central location of the smaller jet. The largely acentric flow contains a large wake generated behind the disc occluder which adversely affects the valve’s pressure drop characteristics. The pressure drop results for this prosthesis are somewhat higher than those found by other investigators (Aberg and Henze, 1979; Yoganathan et al., 1984). The acentricity of the flow caused high mean velocity gradients at the wall around much of the tube circumference. The high velocity gradients imply high shear stresses as shown in Fig. 4a. These high shear stresses are of such a magnitude that they may cause elevated levels of hemolysis to be associated with this valve in viva The shear stresses in the bulk flow are only moderate in magnitude and less than the level generally considered hemolytic (Nevaril et al., 1968; Sutera and Mehrjardi, 1975; Sallam and Hwang, 1983). The shear stresses are sufficient, however, to activate platelets and cause a chemical release reaction (Brown et al., 1975). The presence in the flow of activated platelets increases the possibility of thrombus formation and hence valve thrombosis and thromboembolic events. Smeloff-Cutter caged ball valve
The Smeloff-Cutter valve is a caged ball design. Three pronged cages are located both upstream and downstream of the valve orifice. When the valve is closed, the occluding ball sits in the upstream cage with the centerline of the ball positioned at the minimum cross-section of the valve base ring. The flow fields generated by the Smeloff-Cutter valve are shown in Fig. 3b. The profiles show evidence of wakes caused by the ball occluder and by the downstream cage struts. The ball occluder of the valve generates a large wake centered about the centerline of the tube. The three struts of the downstream cage generate three notable wakes near the tube wall. Only small secondary-flow structures are found, indicating that the lateral-flow occluding mechanism of this valve imparts very little azimuthal sense to the forward flow. The extent of the flow disturbance associated with the wakes generated by the struts and occluder suggest that these wakes may play a significant role in producing the relatively large pressure drop found for the Smeloff-Cutter valve. The pressure drop results for this prosthesis are higher than those found for the other three valves studied in the present investigation. The mean velocity gradients and associated shear stresses are reported in Fig. 4b. As with the Bjiirk-Shiley valve, the velocity gradients are elevated at the tube wall which implies high shear stresses in the flow near the wall. These high shear stresses are probably not of such a magnitude as to cause significant hemolysis in vivo. Shear stresses in the bulk flow
Flow dynamics of prosthetic aortic valves
a) BJbRK-SHILEY
CC (25mm)
Window Key
Window 8 18 Lhln
b) SMELOFF-CUTTER Window Key
601 (24mm) Window 8
Window I3
Window 20
c) ST JUDE (25mm) Wlrdw
Key
d) IONESCU-SHILEY Wlndow 8
Window Kay
(24 mm) Window 8
Window 17 16 L/min
Fig. 3. (a) Velocity profiles for the BjGrk-Shileyvalve at four points in the cardiac cycle. (b) Velocity profiles for the Smeloff-Cutter valve at four points in the cardiac cycle. (c) Velocity profiles for the St. Jude valve at four points in the cardiac cycle. (d) Velocity profiles for the Ionescu-Shiley bioprosthetic valve at four points in the cardiac cycle.
are only moderate in magnitude and are less than the level generally considered hemolytic. The shear stresses are, however, sufficient to activate platelets and cause a chemical release reaction. St. Jude Medical bileajlet valve
The St. Jude Medical prosthesis is a relatively new, low profile bileaflet valve covered entirely with pyroly-
tic carbon. When the valve opens, the two semicircular leaflets rotate out to an angle of 85”, leaving an orifice that is 85-9O%‘free from obstruction to flow. The mean axial velocities for the St. Jude Medical valve, as a function of time in the cardiac cycle, are given Fig. 3c. The flow structure downstream from the St. Jude Medical valve appears to have been affected by the two semicircular leaflet occluders. These two
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a) BJ6RK-SHILEY CC (25mm)
b) SMELOFF-CUTTER (24 mm) Window 0 hin
Window
Kev
0
WV0 Orientation
VOIVS Orientation
cl ST JUDE (25 mm) Wlndow Key
d) IONESCU-3HILEY (25 mm) Window 0
Window
Key
ONAl?’
VJIVO Orientation
/
Window
8
Window
14
Valve Orientation
Jg
Fig. 4. (a) Shear stress profiles for the Bjork-Shiley valve at three points in the cardiac cycle. (b) Shear stress profiles for the Smeloff-Cutter valve at three points in the cardiac cycle. (c) Shear stress profiles for the St. Jude valve at three points in the cardiac cycle. (d) Shear stress profiles for the Ionescu-Shiley bioprosthetic valve at three points in the cardiac cycle.
leaflets divide the base ring orifice of the valve into three regions. Two of these regions are themselves roughly semicircular in shape. The third region is rectangular in shape with a height-to-width ratio of about five. The areas of the two semicircularly shaped regions are equal, and together represent about 78%
of the area available for forward flow. The three major flow areas generate two parallel ridges in the axial velocities which can be seen during much of the systolic ejection interval. The flow also shows evidence of wakes generated by the two leaflets. The extent of the flow disturbance associated with these wakes
Flow dynamics
of prosthetic
suggests that they may play a significant role in generating the pressure drop found for this valve. Only small secondary-flow structures are found, indicating that the occluding mechanism of this valve imparts very little lateral or azimuthal sense to the forward flow. The mean velocity gradients and associated shear stresses are given in Fig. 4c. The shear stresses are elevated at the tube wall, but these shear stresses are probably not of a magnitude that would be associated with significant hemolysis in uiuo. Shear stresses in the bulk flow are only moderate in magnitude and are less than the level generally considered hemolytic. The shear stresses are sufficient to activate platelets and cause a chemical release reaction. The St. Jude Medical valve is found to have a relatively small pressure drop in both steady and pulsatile flow. A more detailed discussion of the hemodynamics of this valve is given by Hanle et al. (1988). lonescu-Shiley bioprosthetic valve
The standard Ionescu-Shiley valve is a bioprosthetic valve made of tissue from the bovine pericardial sack. The occluding mechanism consists of three equal-sized leaflets of roughly semi-lunar shape. The leaflets are fixed with glutaraldehyde at high pressure and mounted on a titanium stent. During forward flow the leaflets billow out to provide a circular orifice; upon deceleration and reverse flow the leaflets are pushed back to the closed position where their free edges seal the orifice by resting against one another. The velocity measurements, which are presented in Fig. 3d, suggest that the occluding mechanism of the Ionescu-Shiley valve causes flow structure markedly
a) BJORK-SHILEY
CC (25 mm)
Steady Flow
Pulsatile Flow
c) ST JUDE (25 mm)
aortic
603
valves
different from that downstream from a natural aortic valve. Somewhat surprisingly, the nearly circular flow orifice generated by the valve leaflets does not result in a circular flow structure downstream from the valve. The Ionescu-Shiley valve results have been discussed elsewhere (Hanle, 1984; Hanle et al., 1986, 1987); the more significant findings are noted here. A pronounced, jet-like structure emerges from this valve with a shape and orientation which suggests that the valve leaflets greatly constrict the forward flow. Significant, well-defined secondary-flow structures are also found and may play a role in generating the noncircular, jet-like flow. This flow constriction gives rise to large regions of flow separation, and flow separation is implicated as the cause of the moderate pressure drop found for this valve. Results from farther downstream in steady flow provide insight into the structure of the forward flow. The large axial jet is found to have diverged trisymmetrically, thus generating three individual regions of separated flow. High mean velocity gradients and elevated velocities in the bulk flow are shown to contribute to significant shear stresses in the bulk flow. The estimated shear stresses are on the order of those considered to be potentially hemolytic. These shear stresses, in turn, seem to correlate with the findings of others that this valve may generate notable intravascular hemolysis (Febres-Roman et al., 1980). The shear stresses are also sufficient to activate platelets and cause a chemical release reaction. Steady us pulsatilejow
A comparison of steady flow and pulsatile flow results (see Figs 5 and 6) shows that for the four valves
b) SMELOFF-CUTTER
(24 mm)
Steody Flow
Pulsatile Flow
d) IONESCU-SHILEY
(24 mm)
.Steody Flow
Fig. 5. A comparison
Pulsotile Flow
Steady Flow
Pulsatile Flow
of pulsatile and steady flow velocity profiles at similar total flow rates. The pulsatile velocity profiles shown are for mid-systole.
604
D. D.
HANLE et 01.
Valve Orientation
a) BJi)RK-SHILEY
CC
Valve Orientation
l4
b) SMELOFF-CUTTER
1”
0 cm/set Valve Orientation
c) ST JUDE
Valve Orientation
c
0 cmhc
d) IONESCU-SHILEY
0 cm/set
Steady Flow
Pulsatile Flow
Steody Flow
Pulsatile Flow
Fig. 6. A comparison of pulsatile and steady flow r.m.s velocity profiles at similar total flow rates. The pulsatile data shown are for mid-systole.
studied: (i) the steady flow results approximate those for pulsatile flow only during the middle of systole and (ii) the r.m.s. velocities for steady flow provide an upper bound to those found in pulsatile flow. The results indicate the flow structure of mid-systole is most similar to that for steady flow. The steady flow results are quite different from those for pulsatile flow during early and late systole. It is found that early systole can be estimated a priori as plug flow. Late systole would be difficult to predict a priori. Pulsatile flow results in diastole are found to have structures which would not be predictable from the steady flow results. A complete discussion of these findings is presented elsewhere (Hanle et al., 1986; Hanle, 1984).
DISCUSSION
Comparison of valves
Pressure drops, velocity profiles and shear stress distributions have been measured for each of the four valves considered in this work. During these measurements, the net forward flow was held relatively constant. Other parameters, such as mean aortic pressure, varied as a consequence of holding flow output constant. The comparisons made between valves are thus strictly valid only at equal forward flow rates. Pressure drop. A comparison of the pressure drop results reported in Table 2 shows that of the valves studied the St. Jude Medical valve generated the lowest pressure in both steady and pulsatile flow,
while the Smeloff-Cutter valve generated the highest. Accordingly, the St. Jude Medical valve had the largest effective orifice area (Gorlin and Gorlin, 1951) in both steady and pulsatile flow and the Smeloff valve had the smallest. The BjBrk-Shiley convexo-concave and Ionescu-Shiley valves each had intermediate pressure drop characteristics which were similar in magnitude. The mean aortic pressure also varied. It was highest for the Ionescu-Shiley valve and lowest for the Smeloff-Cutter, with a maximum difference of the order of 25%. Velocity projles. The four valves can also be compared with respect to velocity results obtained 31.8 mm downstream from the valve orifices during forward flow. The Ionescu-Shiley generated the highest mean axial velocities of the four valves. The Ionescu-Shiley valve also generated the highest mean velocity gradients and r.m.s. axial velocities in the bulk flow away from the tube wall and was the only valve to generate regions of separated flow which could be measured at the data plane. The Bjark-Shiley convexo-concave valve generated the highest mean velocity gradients near the tube wall and also generated the largest mean non-axial velocity components. The high gradients at the wall and the large non-axial velocities resulted from the considerable acentricity of the forward flow generated by the tilting disc. The Smeloff-Cutter and St. Jude Medical valves generated relatively low values for the maximum mean axial velocity across the data plane. The mean non-axial and axial velocities for these two valves were also relatively small.
Flow dynamics of prosthetic aortic valves
605
26 L/min
Empty Flow Section
lonescu-Shiley
Valve
Fig. 7. A comparison of the velocity profile generated by the pulse duplicator with no valve in the flow
section with that for the Ionescu-Shiley bioprosthesis.
None of the valves studied generated a flow field like that expected for the natural aortic valve. Schramm et al. (1980, 1982) have studied the natural valve in oitro and have shown an almost unconstricted forward flow which reattaches very quickly downstream. These observations correlate well with findings in our laboratory with an empty flow section, i.e. where during an experimental run no valve is mounted in the flow. Figure 7 shows a relatively undisturbed forward flow as compared with that for the Ionescu-Shiley valve. For the empty flow section, little flow constriction is evident at the data plane with no notable flow separation. The valve designs all generated axial flow structures unique to their design. The Ionescu-Shiley valve generated a triangularly shaped, centrally located jet of fluid containing very high mean axial velocities of greater than 240 cm s- i. These high mean axial velocities dropped off quickly away from the tube centerline giving rise to relatively large mean velocity gradients in the bulk flow. The Ionescu-Shiley valve also generated significant regions of flow separation. The Bjlirk-Shiley convexo-concave valve generated a large, lenticularly shaped jet of fluid which was located very close to approximately half of the circumference of the tube wall. This large, acentric jet contained high mean axial velocities rising beyond 160 cm s- ‘. A second, much smaller jet was generated by the Bjiirk-Shiley convexo-concave valve near the centerline of the tube. This centrally located jet, which was much smaller in cross-sectional area than the first and contained mean axial velocities only about half those of the first, was apparently generated by the flow through the minor outflow region. Regions of very low velocity were also generated by the Bjiirk-Shiley convexo-concave valve, as observed at the data plane. The largest of these low velocity regions was located near the tube wall on the opposite side of the tube from the maximum of the large, lenticularly shaped jet. The Smeloff valve generated three maxima in the mean axial velocities interspersed with three regions oflower velocities. A ‘saddle point’ in the mean axial velocities was thus observed near the center of the tube. The maxima for the Smeloff valve were not very distinct
and rose to only about 120 ems-‘. The St. Jude Medical valve generated two maxima in the mean axial velocities interspersed with two regions of lower velocities. Thus a ‘saddle point’ in the mean axial velocities was also observed for the St. Jude Medical valve as was observed for the Smeloff valve. The maxima for the St. Jude Medical valve rose to about 140 cm s- 1 and were a little more distinct than those for the Smeloff valve. Only very small mean velocity gradients were generated by the St. Jude Medical valve in the bulk flow. Shear stress distributions. The four valves were also compared with regard to the shear stresses generated at the time of maximum forward flow. For this comparison all velocity results were obtained at the data plane located 31.8 mm downstream from the valve orifice. The shear stresses were estimated from the mean axial velocities and the r.m.s. axial and r.m.s. non-axial velocities using equation (1). The relative magnitude and extent of the shear stresses generated downstream from the four valves can be compared by estimating the maximum total shear stress which was present over at least 10% of the tube cross-section. These maximum shear stresses are given in Table 3 for each of the four valves in steady and pusatile flow and for the empty flow section. The maximum shear stresses for the empty flow section were the lowest of those in Table 3. The St. Jude Medical valve overall had the smallest maximum total shear stresses; the Ionescu-Shiley valve had the largest. The relative values of the shear stresses are in qualitative agreement with the two dimensional LDA results of Woo et al. (1985, 1986a, b) and Yoganathan et al. (1986). Only relative values can be compared due to the differences in blood analog solutions used and pulse duplicator geometries. The major discrepancy in the relative shear stress values appears to be for the Ionescu-Shiley valve, where the total shear stresses reported in this work are higher than those for the Bjiirk-Shiley valve, in contrast to Yoganathan’s result (1986). The discrepancy is due to the fact that Yoganathan measured only centerline shear stresses for the Ionescu-Shiley valve and this work indicates that maximum shear stresses are off centerline.
606
D. D. HANLEet
al.
Table 3. Comparison of the magnitude of the estimated total shear stresses, r,,, generated downstream from the four prosthetic aortic heart valves studied in the present investigation* Maximum estimated total shear stresst (N mT2) Steady flow $ Pulsatile flows
Type
Empty flow section (25.4 mm) St. Jude (25 mm) Smeloff-Cutter (24 mm) Bjdrk-Shiley convexo-concave (25 mm) Ionescu-Shiley (25 mm)
Maximum turbulent shear stress (N m-‘) Yoganathan (1986) Woo (1985)
43 76 78
74 89
200 -
160 -
112 128
106 112
340 250
240 -
*Results for empty flow section also included as base-line. YThe maximum total shear stress is given which was estimated to be present over at least 10% of the tube cross-section. Experimental uncertainty: + 5 Nmw2. 1At steady flow rate of 27 lmin-i. $At a maximum pulsatile-flow rate of 30 1min -’ for Ionescu-Shiley, BjBrk-Shiley convexo-concave, and Smeloff-Cutter valves and 29 1min-’ for St. Jude valve. No pulsatile flow results were obtained for the empty flow section.
CONCLUSIONS
In vitro laser Doppler anemometry has been used to compare the flow dynamics of four aortic prosthetic heart valves. Detailed comparisons of valve hemodynamics are given in the previous section. The major points are summarized below. (1) All of the valves generated flow structures very unlike that expected for the natural aortic valve in vivo.
(2) A comparative analysis of steady and pulsatile flow showed that steady flow results only approximated the pulsatile flow results obtained during the middle of the systolic ejection phase of the pulse cycle. Early in systole, the flow was determined to be of a plug flow type. Late in systole, the flow was highly disturbed and would have been difficult to predict a priori. In addition it was found that the magnitude of the flow disturbance measured for steady flow was an upper bound on that measured for pulsatile flow. (3) Of the valves studied, the Ionescu-Shiley valve gave the highest shear stresses. These shear stresses were only slightly higher than the levels expected to generate hemolysis. (4) Large scale flow jets are created by the Bjiirk-Shiley and Ionescu-Shiley valves. The jets may be capable of sublethal damage to red blood cells or the endothelial lining of tissue. Strong jet-like flows were not observed for the St. Jude Medical or Smeloff-Cutter valves. Finally, it must be noted that this study concentrated solely on in oitro flow characteristics of moderately sized aortic valves at resting conditions. The effect of small valve sizes and flow rates simulating exercise needs to be examined to provide a full picture of valve hemodynamics. Further, the best flow performance in vitro does not necessarily imply the best performance on implantation. Valve durability, for example, has the potential to overwhelm any differences in flow characteristics between prostheses.
Acknowledgements-Funding for this work was provided by the Donald E. Baxter Foundation, the Children’s Heart Foundation of Southern California, and the American Heart Association, Greater Los Angeles Affiliate.
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