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Effect of wedging on the flow characteristics past tilting disc aortic valve prosthesis
J. Biomerhan~s Voi
Prinrcd in
Great
19. So
3. pp 181-186. 1986.
axI
Bnun
-9290 86 I? 00 + al Pergamon Press Ltd.
EFFECT OF WEDGING ON THE FLOW CHARACTERISTICS PAST TILTING DISC AORTIC VALVE PROSTHESIS K. B. CHANDRAN,
REZA FATEMI,
L. F. H~RATZKA and C. HARRIS
Departments of Biomedical Engineering and Surgery, University of Iowa, Iowa City, IA 52242, U.S.A. Abstract-To evaluate the efficacy of implanting a tilting disc aortic valve prosthesis in an angulated (wedged) supra-annular position, an in oirro experimental study was performed. The aortic valve prosthesis was mounted in an axi-symmetric valvechamber in a wedged position and incorporated in a mock circulatory system. Measurements wereobtained on the transvalvular pressure gradient. percent regurgitation as well as velocity profiles and turbulent normal stressesdistal lo the valve. Our results showed that there was no significant reduction in the pressure gradient in mounting a larger sized valve in the wedged supra-annular position. On the other hand, the percent regurgitation increased with increase in heart rate and wedge angle. The valve failed to function properly above 110 beats min- ’ at any wedge angle with the normal flow rate. The velocity profiles also showed significant changes with an increase in the turbulent normal stress with increase in wedge anale. Hence our studv sunneststhat implanting the tilting disc prosthesis in a wedged supra-annular &sit& in the aorta is not a&&able
INTRODUCr’ION Replacement of diseased heart valves with a variety of prosthetic valves has become a common practice. Extensive hemodynamic studies, both in uiuo and in vitro, have been reported in the literature (Yellin et al., 1979; Gabbay and Frater, 1982) which show that the prosthetic valves of larger tissue annulus diameter (TAD) induce a correspondingly smaller transvalvular pressure gradient. In aortic valve replacement, the objective of the surgeon is to implant as large a size prosthetic valve as possible so as to minimize the pressure gradient. Frequently, surgeons encounter a patient with small aortic root so that prostheses of 21 mm TAD or smaller size have to be implanted. The size of the prostheses should be proportional to the size of the patients and smaller size prostheses fall below the ideal for most adult patients (Jones et al., 1978; Schaff et al., 1981; Wortham et ul., 1981). A method to implant prostheses with one or two sizes larger than the aortic annulus in patients with small aortic roots has been reported recently (David and Uden, 1983; Olin et al., 1983) with the expectation that the larger size valve will reduce the transvalvular pressure gradient. The method consists of placing the prosthesis sewing ring on top of the annulus of the non-coronary sinus, thereby angulating the vaIve in the outflow tract with the major flow orifice towards the annulus of the non-coronary sinus. David and Uden (1983) report pressure gradients ranging from 0 to 22 mmHg (7.2 f 5.8 mmHg) in 23 patients with the valves in an angulated supra-annular position compared to a range of O-l 8 mmHg (9.2 + 3.9 mmHg) in 32 patients with patch enlargement. Olin et al. (1983)as well as David and Uden (1983) report satisfactory results in placing larger sized valves in the supra-
Receiced
in the noncoronary sinus in patients with aortic valve replacement. Tilting disc valves normally open to an angle of 60-75” from the closed position in the normal position of the implant. In angulating such valves in the supraannular position in the noncoronary sinus, the angle of opening with respect to the aortic annulus increases close to 90”. The normal closing of the disc is due to reverse pressure gradient (larger pressure in the aortic side) acting on the projected area of the disc along the axis of the aorta. With larger opening angles, the projected area will become less and hence the closing characteristics of the valve will be impaired. If the disc opens to an angle of 90”, the projected area will be negligible and the disc may not close at all. Hence, with the tilting disc valve implanted at an angle to the aortic root, it can be expected that the valve function may be impaired. The percent regurgitation may also increase due to the larger traverse of the disc to close and the flow characteristics may also change significantly. This paper reports on an in t&o evaluation of the hemodynamics past tilting disc valves wedged at various angles to the aortic root in a mock circulatory system. The measurements under physiological pulsatile flow at various heart rates included the transvalvular pressure gradient, percent regurgitation as well as the velocity profiles and turbulent stresses distal to the valve. Our results indicate that, in angulating the tilting disc valve prosthesis in the supra-annular position, there was no beneficial effect on the pressure gradient while the percent regurgitation and turbulence distal to the valve increased.
annular position
EXPERIAMENTAL METHODS Medtronic-Hall tilting disc valves were used in this study to evaluate the efficacy of wedging the valve with respect to the aortic root. The valve was sutured into a plexiglass@ va I-.,e ring and mounted in an axi-
11 June 1985; in recked form 4 September 1985. I81
M 19:3-*
K. B. CHANDRAN,R.
182
FATEMI, L. F. HIRATZKA
symmetric valve chamber (Chandran ef al., 1983). Measurements were obtained for a 23 mm TAD valve in the normal mounting position followed by a 25 mm valve mounted in the normal position as well as at an angle of tilt (0) of 5’. IO’ and 20’ into the aortic chamber. The valve chamber was mounted in a mock circulatory system (Chandran and Yearwood, 1981) simulating physiological pulsatile flow across the valve. The blood analog fluid used was an aqueous glycerol solution with a viscosity coefficient of 0.035 P and a density of 1.13 gem-‘. An electromagnetic flow probe (in oico Metric Model K) connected to a flow meter (Carolina Medical Model sOID), mounted downstream from the valve chamber was used to measure the instantaneous flow rate in the cardiac cycle. Using an ensemble average of 100 cardiac cycles, the cardiac output (time averaged flow rate) as well as percent regurgitation was computed for each experiment. Experiments were conducted at pulsatile flow cycle rates of 50, 72, 100 and 140 beatsmin-‘. The mock circulatory system was adjusted such that a time averaged flow rate of 6.0 + 0.05 I. min- I was achieved across the valves. A cardiac output of 6 Lmin-’ is within the range expected clinically for adult patients and we maintained the same flow rate in all the experiments for comparison of the results. Pressure gradient was measured using a Validyne differential pressure transducer (Model 301D) and the peak systolic pressure gradient was computed by averaging over lOOcycles. Laser Doppler velocimetry was used to measure mean axial velocity profiles and turbulent normal stresses during various times in the cardiac cycle across a diametrical traverse downstream to the valve. Figure I shows a schematic of the tilting disc valve in the supra-annular position. The measuring volume of the laser was moved to various radial positions in the flow chamber by mounting the laser on a milling machine traverse. The details of the mock circulatory system, the velocity measurement techniques and the accuracy of the measurements have been previously reported (Chandran et al., 1983) and will not be repeated here in the interest of brevity. RESULTS
Tables 1 and 2 show thk percent regurgitation and peak systolic pressure gradient as a function of the Table 1. Percent regurgitation (*SD.)
Heart rate (beats min- ‘) 50 72 100 140 l
and C.
HARRIS
Ll
(4 8 /I
(b)
Fig. 1. Schematic of the tilting disc valve mounted at a wedge angle 0. Note that when 0 = 20”, the disc will tilt close lo 90” and will be parallel to the axis of the aorta in the fully open position.
wedge angle and heart rate. It can be observed from Table I that the regurgitation between the two valves in the normal implant position (0’ wedge angle)are not significantly different. However, as the wedge angle of the 25 mm valve is increased, the percent regurgitation increases. At 100 beats min- ‘, the amount of regurgitation has increased by more than 200% at the 20” wedge angle. More importantly, at even 5” wedge, the valve failed to function normally at higher heart rates when the time averaged flow rate was maintained at 6 I.min-‘. The valve functioned normally at 140 beats min- ’ only when the cardiac output was reduced to about 2 I. min- I at the 20’ wedge angle. One of the reasons for implanting a larger valve is to reduce the transvalvular pressure gradient. The peak systolic gradient values in Table 2 at various heart rates and wedge angles do not show any advantage in implanting a larger valve at the supra-annular position. As the wedge angle is increased, there is no significant drop in the gradient for the same heart rate. The values
for various heart rates and wedge angles at a mean flow rate of 6.0 -+0.05 I. min - ’
23 0
0
7.41 (0.59) 9.26 (0.83) 11.00 (1.01) 14.36 (1.41)
8.62 (0.91) 8.41 (1.08) 8.66 (0.61) 8.57 (1.12)
Valve size (mm) 25 Wedge angle (degrees) 5 10 8.12 (0.73) 9.38 (0.96) 11.77 (1.24) l
Marked variation in the regurgitation at this wedge angle and heart rate.
13.24 (1.03) 13.26 (0.69) 16.72 (0.21) *
20 17.89 (2.16) 17.62 (3.24) 26.06 (3.19) *
Effect of wedging on flow characteristics
183
Table 2. Peak systolic pressure gradient (mmHg i S.D.) for various wedge angles and heart rates at a mean flow rate of 6.0 t 0.05 I. min- ’ Valve size (mm) 25 Wedge angle (degrees) 5 10
23 Heart rate (beats min- ‘) 50 72 100 140
0
0
10.25 (1.23) 11.34 (2.45) 14.02 (1.67) 26.20 (4.90)
10.07 (1.78) 8.49 (2.39) 10.49 (2.53) 26.08 (4.77)
12.21 (2.19) 10.18 (2.17) 13.01 (3.52)
20
9.29 (1.25) 9.76 (1.94) 19.05 (6.15) *
l
11.15 (1.54) 13.23 (2.88) 30.68 (1.40) l
* Marked variation in the pressure gradient at the wedge angle and heart rate.
are also not significantly smaller than the corresponding values of the 23 mm valve. In the 25 mm valve, as the wedge angle is increased, a significant increase in the peak systolic pressure gradient is observed at lOObeatsmin-r. The results from the velocity profiles and turbulent normal stresses are presented below. For ease of comparison, the results are presented in a nondimensionalized form. The axial velocity component w is non-dimensionalized as w/i? where
10.0
6.0
-
6.0
-
Q is the time averaged flow rate and D is the tissue annulus diameter of the valve. The -- turbulent normal stress is non-dimensionalized as w’w’/W’ where w’ is the fluctuating component of the axial velocity. In all the velocity profiles presented below, the disc is tilting to the right of the figure. Hence the major flow orifice is to the right of the figure and the minor flow orifice is to the left. The non-dimensionalized peak systolic axial velocity as a function of radius at various heart rates for the 23 and 25 mm valves are presented in Figs 2 and 3, respectively. The velocity profiles downstream to the valve at the normal position (0=0”) are similar to those reported earlier by Bruss et al. (1983) and Chandran et 01. (1983). The two peaks in the velocity profile correspond to the minor and major orifices and a flow reversal is observed near the wall of the minor flow orifice. The shapes of the peak systolic velocity profiles are similar at various heart rates for both the valves. Figure 4 compares the peak systolic velocity profiles for the 23 and 25 mm valves at 72 beats min- I. Figure 5 shows the development of the velocity profiles in systole downstream to the 25 mm valve. In early systole (40 ms after valve opening), a uniform fluid motion in the forward direction is observed in the entire cross-section. In peak systole (220 ms), two jetlike flows corresponding to the orifices are evident with a valley corresponding to the disc in the tilted position. The flow reversal observed near the wall of the minor flow orifice is present for the rest of the systole. In late systole (320ms), the fluid is decelerating with corresponding reduction in the velocity magnitudes. These results can now be compared with the flow development past the valves in the wedged position.
-
50
bpm
---
72
bpm
-----100bpm -
-140bpm
I
*
,
Fig. 2. Velocity profiles distal to 23 mm valve in peak systole.
6.0
6.0
2 W
4.0
2.0
--------
-2.0
-
-
50 72 100
bpm bpm bpm
140
bpm
1
- 4.0 -1.0
-0.5
0.0
0.5
1.0
“a Fig. 3. Velocity profiles distal to 25 mm valve in peak systolc.
Figures 6, 7 and 8 show the velocity profiles at 72 beats min- ’ past the 25 mm valve at wedge angles of So, 10” and 20”, respectively. At S”, the two peaked
K. B. CHANDRAN. R. FATEMI.L. F. HIRATZKAand C. HARRIS
184 10.0
‘O’O 1 8.0
t 6.0
6.0
-2.0 -4.0
1 -1.0
-
23mm
---
25mm
-2.0
-
-
-
40 - -
------
I
I
I
I
-0.5
0.0
0.5
1.0
-4.0
I
. .l.O
0.0
-0.5
200
msec msec
320
m*c
I
1 0.5
1.0
‘10
“a
Fig. 4. Peak systolic velocity profiles for 23 and 25 mm valves at 72 beatsmin-‘.
Fig. 6. Velocity profiles in systolc distal to the 25 mm valve: e=5*; 7.2beatsmin-‘.
10.0
‘O’O I-----8.0
8.0
6.0
6.0
z4.0
4.0
W
E!
W
2.0
2.0
0.0
_;i
.
\ 40 ---
-2.0
- 4.0 -1.0
-0.5
0.0
0.5
).O
‘10
-----
ms
200 mr 320 ms
\
1
-1.0
-0.5
‘\.
l-. \ .I
/ I
A
0.0
0.5
1.0
r'O
Fig. 5. Velocity profiles in systole distal to the 25 mm valve: 0=0”; 72lxatsmin-‘.
Fig. 7. Velocity profiles in systole distal lo the 25 mm valve: 0= IO”; 72 beatsmin-‘.
velocity is still observed in peak systole even though the differences between the two peaks have reduced as more flow is directed through the minor flow orifice. A small region of flow reversal is still observed near the wall of the minor flow orifice. However, a tendency for flow reversal near the wall of the major flow orifice is indicated in late systole. For the 10” wedge (Fig. 7), no flow reversal is observed near the minor flow orifice but one is present in the major flow orifice. As the disc is tilting more towards the axis of the chamber, two centralized jets are observed on both sides of the disc in the open position. As the valve is tilted further to an angle of 20” (Fig. 8), most of the flow is through a central jet in the side of the major flow orifice. A large
region of flow reversal is also observed near the major Row orifice. A comparison of peak systolic velocity profiles at o”, lo” and 20” wedges at heart rates of 50 and IOObeatsmin-’ are shown in Figs 9 and 10, respectively. Similar to those results discussed at 72 beats min- ‘, a significant difference in the velocity profiles can be observed as the wedge angle is increased. As the wedge angle is increased, the flow is through the centralized jet near the disc with a region of flow reversal occupying a relatively large portion Of the cross-section. The velocity profiles for the 20” angle indicate a large velocity gradient near the disc indicating relatively large viscous shear stresses in that region.
Effect of wedging on flow characteristics
6.0
4.0 w =
W 2.0
0.0
\ -
-2.0
--
-----
40 200
In5 ms
320
ms
\
‘\
\ .-____
‘i-1 .,_I
-4.0 -1.0
-0.5
0.5
0.0
1.0
“a Fig. 8. Velocity profiles in systole distal to the 25 mm valve: 0=20”: 72beatsmin-‘.
185
A comparison of the non-dimensionaltied turbulent normal stresses at various wedge angles and heart rates is shown in Table 3. As can be observed, the turbulent stresses are comparable between the two valves in the normal position. However, as the wedge angle is increased, an increase in the stresses are observed. The increase is significant at the 20’ wedge where the central jet with large velocity gradients is present. A 200 % increase in turbulent stress is observed between the normal position and the 20’ wedge at IOObeatsmin-‘. With increasing wedge angles, the occluder will tend to be parallel to the flow direction in the fully open position. However the annulus of the valve will be at an angle to the root of the aorta and hence the fluid will be deflected towards the two orifices. As is evident from the velocity profiles described above, there is marked variation in the flow characteristics with increasing wedge angles and hence there is also an increase in the turbulent stresses. DLSCUSSION
In this paper, an in citro experimental evaluation of the efficacy of implanting a larger size tilting disc aortic
8.0
-
6.0
-
6.0
4.0 w = W
A!i
W
2.0
0.0
-2.0
-
-4.0 I -1.0
---
0” 10’
WEDGE WEDGE
-
-----
20’
WEDGE
----
I -0.5
- -
O” loo
WEDGE WEDGE
20’
WEDGE
1 0.0
-4.0 1 0.5
0.0
1.0
-1.0
-0.5
“a
‘\
1 0.5
Fig. 9. Peak systolic profiles distal to the 25 mm valve at various wedge angles: heart rate = 50 beats min- ‘.
Fig. 10. Peak systolic profiles distal to the 25 mm valve at various wedge angles: heart rate = 100 beats mine I.
Table 3. Nondimensional turbulent normal stress for various wedge angles and heart rates at the mean flow rate of 6.0 + 0.05 I. min - ’ 23 Heart rate (beats min- ‘) 50
72 100 140 l
1.0
‘/a
0 3.8 2.72 3.39 1.40
Valve size (mm) 25 Wedge angle (degrees) 0 5 IO 3.69 3.44 3.31 1.79
20
2.72 2.69 4.66
2.97 3.71 3.43
4.55 5.61 6.43
l
l
l
Marked variation in values at the wedge angle and heart rate.
186
K. B. CHANDRAN,R. FATEMI,L. F. HIRATZKAand C. HARRIS
prosthesis in a supra-annular position above the noncoronary sinus is discussed. From our results on the pressure gradient, percent regurgitation, velocity profiles and turbulent normal stresses distal to the valve, the following conclusions can be drawn. 1. By wedging a valve at the aortic root, there was no significant advantage gained in the transvalvular pressure gradient compared to implanting a valve of one size smaller in the normal position at the aortic root. 2. The percent regurgitation was found to increase with increase in wedge angle and heart rate. An increase of 200 7; was observed at a wedge angle of 20 and a heart rate of 140 beats min-‘. 3. Even at smaller wedge angles (SO), the valve function was impaired at the heart rate of 140 beatsmin-’ if a flow rate of 6 1.min-’ was maintained. Generally, the valve failed to close above 110 beatsmin-’ at any wedge angle. 4. The flow characteristics past the valve were markedly altered with increasing wedge angle. At a wedge angle of 20”, there was a central jet with a large region of flow reversal near the wall in the side of the major flow orifice. Large velocity gradients observed in this position also induced large turbulent stresses. Blackshear (1972) has shown that red blood cells which adhere to foreign surfaces may be damaged by shear stresses of the order of IO-100 dyncmm2. Sutera and Mehrjardi (1975) have demonstrated that red blood cells lose their biconcavity when subjected to bulk turbulent stresses of 500 dyn cm- ’ for a duration
of 4 min. The dimensional turbulent normal stresses in our study ranged from 740 dyn cm- ’ (0” wedge angle; 140 beatsmin-‘) to 2670dyncm-’ (20’ wedge angle; 100 beats min- ‘1. The turbulent shear stresses are generally a fraction of the turbulent normal stresses (Yoganathan et ul., 1979). Assuming that the turbulent shear stresses are one half the normal stresses measured, the magnitudes of turbulent shear stresses will still be relatively high to induce lethal or sub-lethal damage to the red blood cells in the vicinity of foreign surfaces. Clinically significant levels of hemolysis as well as thrombo-embolic complications are problems implanted prosthetic valves associated with (Yoganathan et al., 1979). With the corresponding increase in shear stresses and bulk turbulent stresses when the valves are implanted in the wedged position, these problems will be further aggravated. In conclusion, this study suggests that there is no advantage gained with respect to the transvalvular pressure gradient by implanting the valve in the angulated position. On the other hand, wedging the valve may
significantly increase the percent regurgitation and alter the flow characteristics past the valve prosthesis. Hence it is not advisable to angulate a tilting disc prosthesis in the supra-annular position.
REFERENCES Blackshear, P. L. (1972) Hemolysis and prosthetic surfaces. Chemistry of Biosurfaces (Edited by Hair, M. L.), Vol. 2, pp. 523-561. Marcel Dekker. New York. Bruss, K.-H., Reul, H., Van Gilse, J. and Knott, E. (1983) Pressure drop and velocity fields at four mechanical heart valve prostheses: Bjork-Shiley standard, Bjork-Shiley concave-convex, Hall-Kaster and St Jude Medical. f.ifi Support Systems 1, 3-22. 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. Chandran, K. B. and Yearwood, T. L. (1981) Experimental study of physiological pulsatile Row in a curved tube. J. Fluid Mech. 85, 497-518. David, T. E. and Uden, D. E. (1983) Aortic valve replacement in adult Patients with small aortic annuli. Ann. thorac. Surg. 36, 577-583.
Gabbay, S. and Frater, R. W. M. (1982) In vitro comparison of the newer heart valve bioprostheses in the mitral and aortic positions. Cardiac Bioprostheses (Edited by Cohn, L. H. and Gallucci, V.),pp. 456-468. Yorke Medical, New York. Jones, E. L.. Craves, J. M.. Morris, D. C., King, S. B., Douglas, J. S., Franch. R. H., Hatcher, C. R. and Morgan, E. A. (1978) Hemodynamic and clinical evaluation of Hancock xenograft bioprosthesis for aortic valve replacement with emphasis on management of the small aortic root. J. thorac. cardiocasc.
Surg. 75, 30&308.
Olin, C. L., Bomlim, V., Halvazulis, V.. Holmgren, A. G. and Lamke, B. J. (1983) Optimal insertion technique for the Bjork-Shiley valve in the narrow aortic ostium. Ann. thorac. Surg. 36, 567-576.
Schaff, H. V., Borkon, A. M., Hughes, C., Achuff, S., Donahoo, J. S.. Gardner, T. J., Watkins, L., Gott, V. L., Morrow, A. G. and Brawley, R. K. (1981) Clinical and hemodynamicevaluation of the I9 mm Bjork-Shiley aortic valve prosthesis. Ann. thorac. Surg. 32, 50-57. Sutera, S. P. and Mehrjardi, N. H. (1975) Deformation and fragmentation of human RBC in turbulent shear flow. Biophys. J. 15, I-IO. Wortham, D. C., Tri, T. B. and Bowen, T. E. (1981) Hemodynamic evaluation of the St. Jude medical valve prosthesis in the small aortic annulus. J. thorac. cardiowsc. Surg. 81, 615-620. Yellin, E. L., Frater, R. W. M.. McQueen, D. M. and Gabbay, S. (1979) In vitro hemodynamic analysis of prosthetic mitral valves using the Gorlin equation. Proceedings ofthe 11th Annual Instrumentation
Harrison, Yoganathan, (1979) In prosthesis.
American Association for Medical Meeting, (Edited by Yoganathan, A. P.,
E. C. and Corcoran. W. H.), pp. 1542. A. P., Corcoran, W. H. and Harrison, E. C. citro measurements in the vicinity of aortic J. Biomechonics 12, 135-152.
Prinrcd in
Great
19. So
3. pp 181-186. 1986.
axI
Bnun
-9290 86 I? 00 + al Pergamon Press Ltd.
EFFECT OF WEDGING ON THE FLOW CHARACTERISTICS PAST TILTING DISC AORTIC VALVE PROSTHESIS K. B. CHANDRAN,
REZA FATEMI,
L. F. H~RATZKA and C. HARRIS
Departments of Biomedical Engineering and Surgery, University of Iowa, Iowa City, IA 52242, U.S.A. Abstract-To evaluate the efficacy of implanting a tilting disc aortic valve prosthesis in an angulated (wedged) supra-annular position, an in oirro experimental study was performed. The aortic valve prosthesis was mounted in an axi-symmetric valvechamber in a wedged position and incorporated in a mock circulatory system. Measurements wereobtained on the transvalvular pressure gradient. percent regurgitation as well as velocity profiles and turbulent normal stressesdistal lo the valve. Our results showed that there was no significant reduction in the pressure gradient in mounting a larger sized valve in the wedged supra-annular position. On the other hand, the percent regurgitation increased with increase in heart rate and wedge angle. The valve failed to function properly above 110 beats min- ’ at any wedge angle with the normal flow rate. The velocity profiles also showed significant changes with an increase in the turbulent normal stress with increase in wedge anale. Hence our studv sunneststhat implanting the tilting disc prosthesis in a wedged supra-annular &sit& in the aorta is not a&&able
INTRODUCr’ION Replacement of diseased heart valves with a variety of prosthetic valves has become a common practice. Extensive hemodynamic studies, both in uiuo and in vitro, have been reported in the literature (Yellin et al., 1979; Gabbay and Frater, 1982) which show that the prosthetic valves of larger tissue annulus diameter (TAD) induce a correspondingly smaller transvalvular pressure gradient. In aortic valve replacement, the objective of the surgeon is to implant as large a size prosthetic valve as possible so as to minimize the pressure gradient. Frequently, surgeons encounter a patient with small aortic root so that prostheses of 21 mm TAD or smaller size have to be implanted. The size of the prostheses should be proportional to the size of the patients and smaller size prostheses fall below the ideal for most adult patients (Jones et al., 1978; Schaff et al., 1981; Wortham et ul., 1981). A method to implant prostheses with one or two sizes larger than the aortic annulus in patients with small aortic roots has been reported recently (David and Uden, 1983; Olin et al., 1983) with the expectation that the larger size valve will reduce the transvalvular pressure gradient. The method consists of placing the prosthesis sewing ring on top of the annulus of the non-coronary sinus, thereby angulating the vaIve in the outflow tract with the major flow orifice towards the annulus of the non-coronary sinus. David and Uden (1983) report pressure gradients ranging from 0 to 22 mmHg (7.2 f 5.8 mmHg) in 23 patients with the valves in an angulated supra-annular position compared to a range of O-l 8 mmHg (9.2 + 3.9 mmHg) in 32 patients with patch enlargement. Olin et al. (1983)as well as David and Uden (1983) report satisfactory results in placing larger sized valves in the supra-
Receiced
in the noncoronary sinus in patients with aortic valve replacement. Tilting disc valves normally open to an angle of 60-75” from the closed position in the normal position of the implant. In angulating such valves in the supraannular position in the noncoronary sinus, the angle of opening with respect to the aortic annulus increases close to 90”. The normal closing of the disc is due to reverse pressure gradient (larger pressure in the aortic side) acting on the projected area of the disc along the axis of the aorta. With larger opening angles, the projected area will become less and hence the closing characteristics of the valve will be impaired. If the disc opens to an angle of 90”, the projected area will be negligible and the disc may not close at all. Hence, with the tilting disc valve implanted at an angle to the aortic root, it can be expected that the valve function may be impaired. The percent regurgitation may also increase due to the larger traverse of the disc to close and the flow characteristics may also change significantly. This paper reports on an in t&o evaluation of the hemodynamics past tilting disc valves wedged at various angles to the aortic root in a mock circulatory system. The measurements under physiological pulsatile flow at various heart rates included the transvalvular pressure gradient, percent regurgitation as well as the velocity profiles and turbulent stresses distal to the valve. Our results indicate that, in angulating the tilting disc valve prosthesis in the supra-annular position, there was no beneficial effect on the pressure gradient while the percent regurgitation and turbulence distal to the valve increased.
annular position
EXPERIAMENTAL METHODS Medtronic-Hall tilting disc valves were used in this study to evaluate the efficacy of wedging the valve with respect to the aortic root. The valve was sutured into a plexiglass@ va I-.,e ring and mounted in an axi-
11 June 1985; in recked form 4 September 1985. I81
M 19:3-*
K. B. CHANDRAN,R.
182
FATEMI, L. F. HIRATZKA
symmetric valve chamber (Chandran ef al., 1983). Measurements were obtained for a 23 mm TAD valve in the normal mounting position followed by a 25 mm valve mounted in the normal position as well as at an angle of tilt (0) of 5’. IO’ and 20’ into the aortic chamber. The valve chamber was mounted in a mock circulatory system (Chandran and Yearwood, 1981) simulating physiological pulsatile flow across the valve. The blood analog fluid used was an aqueous glycerol solution with a viscosity coefficient of 0.035 P and a density of 1.13 gem-‘. An electromagnetic flow probe (in oico Metric Model K) connected to a flow meter (Carolina Medical Model sOID), mounted downstream from the valve chamber was used to measure the instantaneous flow rate in the cardiac cycle. Using an ensemble average of 100 cardiac cycles, the cardiac output (time averaged flow rate) as well as percent regurgitation was computed for each experiment. Experiments were conducted at pulsatile flow cycle rates of 50, 72, 100 and 140 beatsmin-‘. The mock circulatory system was adjusted such that a time averaged flow rate of 6.0 + 0.05 I. min- I was achieved across the valves. A cardiac output of 6 Lmin-’ is within the range expected clinically for adult patients and we maintained the same flow rate in all the experiments for comparison of the results. Pressure gradient was measured using a Validyne differential pressure transducer (Model 301D) and the peak systolic pressure gradient was computed by averaging over lOOcycles. Laser Doppler velocimetry was used to measure mean axial velocity profiles and turbulent normal stresses during various times in the cardiac cycle across a diametrical traverse downstream to the valve. Figure I shows a schematic of the tilting disc valve in the supra-annular position. The measuring volume of the laser was moved to various radial positions in the flow chamber by mounting the laser on a milling machine traverse. The details of the mock circulatory system, the velocity measurement techniques and the accuracy of the measurements have been previously reported (Chandran et al., 1983) and will not be repeated here in the interest of brevity. RESULTS
Tables 1 and 2 show thk percent regurgitation and peak systolic pressure gradient as a function of the Table 1. Percent regurgitation (*SD.)
Heart rate (beats min- ‘) 50 72 100 140 l
and C.
HARRIS
Ll
(4 8 /I
(b)
Fig. 1. Schematic of the tilting disc valve mounted at a wedge angle 0. Note that when 0 = 20”, the disc will tilt close lo 90” and will be parallel to the axis of the aorta in the fully open position.
wedge angle and heart rate. It can be observed from Table I that the regurgitation between the two valves in the normal implant position (0’ wedge angle)are not significantly different. However, as the wedge angle of the 25 mm valve is increased, the percent regurgitation increases. At 100 beats min- ‘, the amount of regurgitation has increased by more than 200% at the 20” wedge angle. More importantly, at even 5” wedge, the valve failed to function normally at higher heart rates when the time averaged flow rate was maintained at 6 I.min-‘. The valve functioned normally at 140 beats min- ’ only when the cardiac output was reduced to about 2 I. min- I at the 20’ wedge angle. One of the reasons for implanting a larger valve is to reduce the transvalvular pressure gradient. The peak systolic gradient values in Table 2 at various heart rates and wedge angles do not show any advantage in implanting a larger valve at the supra-annular position. As the wedge angle is increased, there is no significant drop in the gradient for the same heart rate. The values
for various heart rates and wedge angles at a mean flow rate of 6.0 -+0.05 I. min - ’
23 0
0
7.41 (0.59) 9.26 (0.83) 11.00 (1.01) 14.36 (1.41)
8.62 (0.91) 8.41 (1.08) 8.66 (0.61) 8.57 (1.12)
Valve size (mm) 25 Wedge angle (degrees) 5 10 8.12 (0.73) 9.38 (0.96) 11.77 (1.24) l
Marked variation in the regurgitation at this wedge angle and heart rate.
13.24 (1.03) 13.26 (0.69) 16.72 (0.21) *
20 17.89 (2.16) 17.62 (3.24) 26.06 (3.19) *
Effect of wedging on flow characteristics
183
Table 2. Peak systolic pressure gradient (mmHg i S.D.) for various wedge angles and heart rates at a mean flow rate of 6.0 t 0.05 I. min- ’ Valve size (mm) 25 Wedge angle (degrees) 5 10
23 Heart rate (beats min- ‘) 50 72 100 140
0
0
10.25 (1.23) 11.34 (2.45) 14.02 (1.67) 26.20 (4.90)
10.07 (1.78) 8.49 (2.39) 10.49 (2.53) 26.08 (4.77)
12.21 (2.19) 10.18 (2.17) 13.01 (3.52)
20
9.29 (1.25) 9.76 (1.94) 19.05 (6.15) *
l
11.15 (1.54) 13.23 (2.88) 30.68 (1.40) l
* Marked variation in the pressure gradient at the wedge angle and heart rate.
are also not significantly smaller than the corresponding values of the 23 mm valve. In the 25 mm valve, as the wedge angle is increased, a significant increase in the peak systolic pressure gradient is observed at lOObeatsmin-r. The results from the velocity profiles and turbulent normal stresses are presented below. For ease of comparison, the results are presented in a nondimensionalized form. The axial velocity component w is non-dimensionalized as w/i? where
10.0
6.0
-
6.0
-
Q is the time averaged flow rate and D is the tissue annulus diameter of the valve. The -- turbulent normal stress is non-dimensionalized as w’w’/W’ where w’ is the fluctuating component of the axial velocity. In all the velocity profiles presented below, the disc is tilting to the right of the figure. Hence the major flow orifice is to the right of the figure and the minor flow orifice is to the left. The non-dimensionalized peak systolic axial velocity as a function of radius at various heart rates for the 23 and 25 mm valves are presented in Figs 2 and 3, respectively. The velocity profiles downstream to the valve at the normal position (0=0”) are similar to those reported earlier by Bruss et al. (1983) and Chandran et 01. (1983). The two peaks in the velocity profile correspond to the minor and major orifices and a flow reversal is observed near the wall of the minor flow orifice. The shapes of the peak systolic velocity profiles are similar at various heart rates for both the valves. Figure 4 compares the peak systolic velocity profiles for the 23 and 25 mm valves at 72 beats min- I. Figure 5 shows the development of the velocity profiles in systole downstream to the 25 mm valve. In early systole (40 ms after valve opening), a uniform fluid motion in the forward direction is observed in the entire cross-section. In peak systole (220 ms), two jetlike flows corresponding to the orifices are evident with a valley corresponding to the disc in the tilted position. The flow reversal observed near the wall of the minor flow orifice is present for the rest of the systole. In late systole (320ms), the fluid is decelerating with corresponding reduction in the velocity magnitudes. These results can now be compared with the flow development past the valves in the wedged position.
-
50
bpm
---
72
bpm
-----100bpm -
-140bpm
I
*
,
Fig. 2. Velocity profiles distal to 23 mm valve in peak systole.
6.0
6.0
2 W
4.0
2.0
--------
-2.0
-
-
50 72 100
bpm bpm bpm
140
bpm
1
- 4.0 -1.0
-0.5
0.0
0.5
1.0
“a Fig. 3. Velocity profiles distal to 25 mm valve in peak systolc.
Figures 6, 7 and 8 show the velocity profiles at 72 beats min- ’ past the 25 mm valve at wedge angles of So, 10” and 20”, respectively. At S”, the two peaked
K. B. CHANDRAN. R. FATEMI.L. F. HIRATZKAand C. HARRIS
184 10.0
‘O’O 1 8.0
t 6.0
6.0
-2.0 -4.0
1 -1.0
-
23mm
---
25mm
-2.0
-
-
-
40 - -
------
I
I
I
I
-0.5
0.0
0.5
1.0
-4.0
I
. .l.O
0.0
-0.5
200
msec msec
320
m*c
I
1 0.5
1.0
‘10
“a
Fig. 4. Peak systolic velocity profiles for 23 and 25 mm valves at 72 beatsmin-‘.
Fig. 6. Velocity profiles in systolc distal to the 25 mm valve: e=5*; 7.2beatsmin-‘.
10.0
‘O’O I-----8.0
8.0
6.0
6.0
z4.0
4.0
W
E!
W
2.0
2.0
0.0
_;i
.
\ 40 ---
-2.0
- 4.0 -1.0
-0.5
0.0
0.5
).O
‘10
-----
ms
200 mr 320 ms
\
1
-1.0
-0.5
‘\.
l-. \ .I
/ I
A
0.0
0.5
1.0
r'O
Fig. 5. Velocity profiles in systole distal to the 25 mm valve: 0=0”; 72lxatsmin-‘.
Fig. 7. Velocity profiles in systole distal lo the 25 mm valve: 0= IO”; 72 beatsmin-‘.
velocity is still observed in peak systole even though the differences between the two peaks have reduced as more flow is directed through the minor flow orifice. A small region of flow reversal is still observed near the wall of the minor flow orifice. However, a tendency for flow reversal near the wall of the major flow orifice is indicated in late systole. For the 10” wedge (Fig. 7), no flow reversal is observed near the minor flow orifice but one is present in the major flow orifice. As the disc is tilting more towards the axis of the chamber, two centralized jets are observed on both sides of the disc in the open position. As the valve is tilted further to an angle of 20” (Fig. 8), most of the flow is through a central jet in the side of the major flow orifice. A large
region of flow reversal is also observed near the major Row orifice. A comparison of peak systolic velocity profiles at o”, lo” and 20” wedges at heart rates of 50 and IOObeatsmin-’ are shown in Figs 9 and 10, respectively. Similar to those results discussed at 72 beats min- ‘, a significant difference in the velocity profiles can be observed as the wedge angle is increased. As the wedge angle is increased, the flow is through the centralized jet near the disc with a region of flow reversal occupying a relatively large portion Of the cross-section. The velocity profiles for the 20” angle indicate a large velocity gradient near the disc indicating relatively large viscous shear stresses in that region.
Effect of wedging on flow characteristics
6.0
4.0 w =
W 2.0
0.0
\ -
-2.0
--
-----
40 200
In5 ms
320
ms
\
‘\
\ .-____
‘i-1 .,_I
-4.0 -1.0
-0.5
0.5
0.0
1.0
“a Fig. 8. Velocity profiles in systole distal to the 25 mm valve: 0=20”: 72beatsmin-‘.
185
A comparison of the non-dimensionaltied turbulent normal stresses at various wedge angles and heart rates is shown in Table 3. As can be observed, the turbulent stresses are comparable between the two valves in the normal position. However, as the wedge angle is increased, an increase in the stresses are observed. The increase is significant at the 20’ wedge where the central jet with large velocity gradients is present. A 200 % increase in turbulent stress is observed between the normal position and the 20’ wedge at IOObeatsmin-‘. With increasing wedge angles, the occluder will tend to be parallel to the flow direction in the fully open position. However the annulus of the valve will be at an angle to the root of the aorta and hence the fluid will be deflected towards the two orifices. As is evident from the velocity profiles described above, there is marked variation in the flow characteristics with increasing wedge angles and hence there is also an increase in the turbulent stresses. DLSCUSSION
In this paper, an in citro experimental evaluation of the efficacy of implanting a larger size tilting disc aortic
8.0
-
6.0
-
6.0
4.0 w = W
A!i
W
2.0
0.0
-2.0
-
-4.0 I -1.0
---
0” 10’
WEDGE WEDGE
-
-----
20’
WEDGE
----
I -0.5
- -
O” loo
WEDGE WEDGE
20’
WEDGE
1 0.0
-4.0 1 0.5
0.0
1.0
-1.0
-0.5
“a
‘\
1 0.5
Fig. 9. Peak systolic profiles distal to the 25 mm valve at various wedge angles: heart rate = 50 beats min- ‘.
Fig. 10. Peak systolic profiles distal to the 25 mm valve at various wedge angles: heart rate = 100 beats mine I.
Table 3. Nondimensional turbulent normal stress for various wedge angles and heart rates at the mean flow rate of 6.0 + 0.05 I. min - ’ 23 Heart rate (beats min- ‘) 50
72 100 140 l
1.0
‘/a
0 3.8 2.72 3.39 1.40
Valve size (mm) 25 Wedge angle (degrees) 0 5 IO 3.69 3.44 3.31 1.79
20
2.72 2.69 4.66
2.97 3.71 3.43
4.55 5.61 6.43
l
l
l
Marked variation in values at the wedge angle and heart rate.
186
K. B. CHANDRAN,R. FATEMI,L. F. HIRATZKAand C. HARRIS
prosthesis in a supra-annular position above the noncoronary sinus is discussed. From our results on the pressure gradient, percent regurgitation, velocity profiles and turbulent normal stresses distal to the valve, the following conclusions can be drawn. 1. By wedging a valve at the aortic root, there was no significant advantage gained in the transvalvular pressure gradient compared to implanting a valve of one size smaller in the normal position at the aortic root. 2. The percent regurgitation was found to increase with increase in wedge angle and heart rate. An increase of 200 7; was observed at a wedge angle of 20 and a heart rate of 140 beats min-‘. 3. Even at smaller wedge angles (SO), the valve function was impaired at the heart rate of 140 beatsmin-’ if a flow rate of 6 1.min-’ was maintained. Generally, the valve failed to close above 110 beatsmin-’ at any wedge angle. 4. The flow characteristics past the valve were markedly altered with increasing wedge angle. At a wedge angle of 20”, there was a central jet with a large region of flow reversal near the wall in the side of the major flow orifice. Large velocity gradients observed in this position also induced large turbulent stresses. Blackshear (1972) has shown that red blood cells which adhere to foreign surfaces may be damaged by shear stresses of the order of IO-100 dyncmm2. Sutera and Mehrjardi (1975) have demonstrated that red blood cells lose their biconcavity when subjected to bulk turbulent stresses of 500 dyn cm- ’ for a duration
of 4 min. The dimensional turbulent normal stresses in our study ranged from 740 dyn cm- ’ (0” wedge angle; 140 beatsmin-‘) to 2670dyncm-’ (20’ wedge angle; 100 beats min- ‘1. The turbulent shear stresses are generally a fraction of the turbulent normal stresses (Yoganathan et ul., 1979). Assuming that the turbulent shear stresses are one half the normal stresses measured, the magnitudes of turbulent shear stresses will still be relatively high to induce lethal or sub-lethal damage to the red blood cells in the vicinity of foreign surfaces. Clinically significant levels of hemolysis as well as thrombo-embolic complications are problems implanted prosthetic valves associated with (Yoganathan et al., 1979). With the corresponding increase in shear stresses and bulk turbulent stresses when the valves are implanted in the wedged position, these problems will be further aggravated. In conclusion, this study suggests that there is no advantage gained with respect to the transvalvular pressure gradient by implanting the valve in the angulated position. On the other hand, wedging the valve may
significantly increase the percent regurgitation and alter the flow characteristics past the valve prosthesis. Hence it is not advisable to angulate a tilting disc prosthesis in the supra-annular position.
REFERENCES Blackshear, P. L. (1972) Hemolysis and prosthetic surfaces. Chemistry of Biosurfaces (Edited by Hair, M. L.), Vol. 2, pp. 523-561. Marcel Dekker. New York. Bruss, K.-H., Reul, H., Van Gilse, J. and Knott, E. (1983) Pressure drop and velocity fields at four mechanical heart valve prostheses: Bjork-Shiley standard, Bjork-Shiley concave-convex, Hall-Kaster and St Jude Medical. f.ifi Support Systems 1, 3-22. 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. Chandran, K. B. and Yearwood, T. L. (1981) Experimental study of physiological pulsatile Row in a curved tube. J. Fluid Mech. 85, 497-518. David, T. E. and Uden, D. E. (1983) Aortic valve replacement in adult Patients with small aortic annuli. Ann. thorac. Surg. 36, 577-583.
Gabbay, S. and Frater, R. W. M. (1982) In vitro comparison of the newer heart valve bioprostheses in the mitral and aortic positions. Cardiac Bioprostheses (Edited by Cohn, L. H. and Gallucci, V.),pp. 456-468. Yorke Medical, New York. Jones, E. L.. Craves, J. M.. Morris, D. C., King, S. B., Douglas, J. S., Franch. R. H., Hatcher, C. R. and Morgan, E. A. (1978) Hemodynamic and clinical evaluation of Hancock xenograft bioprosthesis for aortic valve replacement with emphasis on management of the small aortic root. J. thorac. cardiocasc.
Surg. 75, 30&308.
Olin, C. L., Bomlim, V., Halvazulis, V.. Holmgren, A. G. and Lamke, B. J. (1983) Optimal insertion technique for the Bjork-Shiley valve in the narrow aortic ostium. Ann. thorac. Surg. 36, 567-576.
Schaff, H. V., Borkon, A. M., Hughes, C., Achuff, S., Donahoo, J. S.. Gardner, T. J., Watkins, L., Gott, V. L., Morrow, A. G. and Brawley, R. K. (1981) Clinical and hemodynamicevaluation of the I9 mm Bjork-Shiley aortic valve prosthesis. Ann. thorac. Surg. 32, 50-57. Sutera, S. P. and Mehrjardi, N. H. (1975) Deformation and fragmentation of human RBC in turbulent shear flow. Biophys. J. 15, I-IO. Wortham, D. C., Tri, T. B. and Bowen, T. E. (1981) Hemodynamic evaluation of the St. Jude medical valve prosthesis in the small aortic annulus. J. thorac. cardiowsc. Surg. 81, 615-620. Yellin, E. L., Frater, R. W. M.. McQueen, D. M. and Gabbay, S. (1979) In vitro hemodynamic analysis of prosthetic mitral valves using the Gorlin equation. Proceedings ofthe 11th Annual Instrumentation
Harrison, Yoganathan, (1979) In prosthesis.
American Association for Medical Meeting, (Edited by Yoganathan, A. P.,
E. C. and Corcoran. W. H.), pp. 1542. A. P., Corcoran, W. H. and Harrison, E. C. citro measurements in the vicinity of aortic J. Biomechonics 12, 135-152.