- Email: [email protected]
Hemodynamic evaluation of the Monostrut and spherical disc Björk-Shiley aortic valve prosthesis with Doppler echocardiography
Hemodynamic evaluation of the Monostrut and spherical disc Bjork-Shiley aortic valve prosthesis with Doppler echocardiography A Doppler echocardiographic study was performed to assess whether the Monostrut model of the Bjork-Shiley valve (Shiley, Inc., Irvine, Calif.) had an improved hemodynamic performance in comparison with the spherical disc model in the aortic position. Twenty retrospectively randomly selected patients were studied, 10 with each valve type. Within each valve type two sizes of valve were studied, 21 and 23 mm. The two groups were comparable with respect to age, postoperative time, fractional shortening, New York Heart Association functional class preoperatively, and body surface area. Pulsed and continuous wave Doppler measurements were recorded at rest. Continuous wave Doppler recordings were performed every 2 minutes after exercise with supine bicycle ergometry until 10 minutes after exercise. Peak and mean gradients across the aortic valve prostheses were estimated. Both groups achieved a significant and comparable rise in heart rate with exercise. The mean gradients ± standard error of the mean at rest and 2 minutes after exercise were 19.7 ± 1.9 mm Hg and 30.9 ± 2.2 mm Hg, respectively in the spherical disc group compared with 14.9 ± 1.1 mm Hg and 23.6 ± 1.7 mm Hg in the Monostrut group (p < 0.05 and p < 0.025, respectively). Peak transvalvuIar gradient at rest was 30.7 ± 2.7 mm Hg in the spherical group compared with 23.9 ± 1.9 rom Hg in the Monostrut group (p < 0.05). We conclude that the Monostrut Bjork-Shiley valve prosthesis has better hemodynamic performance than the spherical disc model in the aortic position. (J THORAe CARDIOVASC SURG 1992; 104:1025-8)
A. Kenny, MRCPI, MRCP(u.K.),a J. Woods, MRcp,a C. A. Fuller,a L. Sharples, PhD,b D. L. Stone, MD, MRcp,a F. C. Wells, MS, BSc, FRCS,a and L. M. Shapiro, MD, MRcp,a Cambridge, England
L e Monostrut model Bjork-Shiley aortic valve prosthesis (Shiley, Inc., Irvine, Calif.) incorporates design changes that should improve hemodynamic performance compared with the spherical model. I We performed a Doppler echocardiographic study to assess whether the Monostrut model has improved hemodynamics compared with the spherical disc model. The reliability and accuracy of Doppler echocardiography in assessing transvalvular pressure gradients in prosthetic aortic valves has been validated."
From the Regional Cardiothoracic Unit, Papworth Hospital," and MRC Biostatistical Unit,b Cambridge, England. Received for publication May 31,1991. Accepted for publication Dec. 13, 1991. Address for reprints: Antoinette Kenny, MRCPI, MRCP(U.K.), Papworth Hospital, Papworth Everard, Cambridge, U.K. CB3 8RE.
12/1/36360
Patients and methods The study population comprised 20 patients less than 70 years of age, 10 with the spherical disc model and 10 with the Monostrut model. Within each group five patients had a 21 mm and five had a 23 mm valve prosthesis. The patients were chosen by retrospective random selection from concurrent groups of patients who underwent aortic valve replacement with either the spherical disc or Monostrut prosthesis at our institution since 1985. During this time, when the use of both types of prostheses overlapped, patients received either a Monostrut or spherical disc model according to availability. There was no conscious bias as to which patient received which model. We elected to study patients in the 21 to 23 mm range because these were the size of prostheses most commonly inserted in this institution. Five patients from four groups (Monostrut 21 and 23 mm and spherical disc 21 and 23 mm) were retrospectively randomly selected. The age limit of 70 years was designated because participants were required to perform strenuous exercise. Medical ethical committee approval and informed patient consent was obtained. Preoperative status was ascertained from the patients' medical records. Patient characteristics were similar in both the spherical disc and Monostrut group (Table 1).
1025
10 2 6
The Journal of Thoracic and Cardiovascular Surgery
Kenny et al.
Table I. Patient characteristics
Monostrut
21 mm prostheses (n) 5 23 mm prostheses (n) 5 Age (yr) 53.1 ± Postoperative time (mo) 22.4 ± 32.1 ± Fractional shortening Body surface area (m-) 1.7 ± Heart rate (beats/min) Resting 76.8 ± 130.7 ± Peak exercise 95.7 ± 2 min after exercise NYHA preoperative class (n) I I II 2 III 4 IV 3 *p < 0.001 by paired [Fisher's exact test.
I
2.3 3.2 3.1 0.05
Spherical
5 5
55.8 ± 26.2 ± 33.6 ± 1.8 ±
Unpaired t test (p value)
2.8 2.5 3.9 0.07
0.49 0.39 0.78 0.28
3.5 72.5 ± 5.2 4.2* 129.4 ± 4.2* 4.2* 94.7 ± 5.1 *
0.52 0.84 0.89
It 3t 3t 3t
0.65
test; increase in heart rate over resting values.
Echocardiography. Echocardiography was performed with a Hewlett-Packard model 77020 AV-C26 echocardiograph, with a 2.5 MHz imaging transducer with facilities for pulsed wave Doppler and a dedicated (Pedoff) transducer for continuous wave Doppler echocardiography. Left ventricular cavity dimensions were assessed according to the leading-edge method, and fractional shortening was determined from left ventricular dimensions. Doppler studies were performed after 15 minutes of rest and repeated after exercise with supine bicycle ergometry. Continuous wave Doppler was used to determine the flow velocity across the aortic prosthesis at rest. We examined the prosthetic valve from three views: apical, suprasternal, and right parasternal, to determine the highest transvalvular velocities. The body position and ultrasound approach from which the highest velocities were obtained at rest were marked for quick reference after exercise. The peak velocity (V2) was calculated by averaging three to five consecutive complexes for patients in sinus rhythm and 10 for those patients in atrial fibrillation. The peak instantaneous pressure gradient across the prosthesis was calculated with the modified Bernoulli equation P = 4 (V2P. If the velocity in the left ventricular outflow tract exceeded l rn/sec (Vl), it was considered in the gradient calculation by inclusion in the equation P = 4 (V2 2 - V12) . The mean pressure gradient was calculated by averaging the gradient at 40 msec intervals throughout the velocity envelope. Three to five consecutive envelopes were averaged for patients in sinus rhythm and 10 for those in atrial fibrillation. Patients exercised with the use of supine bicycle ergometry; the end points of exercise were achievement of 75% of predicted maximal heart rate or fatigue, whichever occurred sooner. The initial workload was 25 Wand this was increased by 25 W at 3-rninute intervals. Continuous wave Doppler echocardiography was performed after exercise in the same manner as at rest, to determine the velocity across the aortic prosthesis. Readings were recorded at 2-minute intervals after exercise until lO minutes after exercise. The first Doppler recording was made 2 minutes after exer-
cise. This allowed time to realign accurately the Doppler beam with flow. A recent study also demonstrated that peak velocity occurred 2 minutes after exercise in normal subjects and not at peak exercise.' Statistical analysis. Continuous data are expressed as the mean and standard error of the mean. To determine whether differences in patient characteristics between groups of patients were statistically significant, the unpaired t test was used. The paired t test was used to study within-patient changes in heart rate. Differences between valve types and sizes of valves were analyzed with two-way analysis of variance. This technique allows us to compare valve types while adjusting for differences in valve sizes, and vice versa. In addition, the presence of valve type by size interactions were assessed. Differences with less than a 5% probability of being zero were considered statistically significant.
Results Both groups achieved a similar significant increase in heart rate with exercise (Table I). Two minutes after exercise heart rates remained significantly higher than resting rates for both groups. Echocardiography confirmed prosthetic valve integrity in all patients. There was no significant difference in mean fractional shortening between the two valve types (Table I). A satisfactory continuous wave recording at rest and 2 minutes after exercise was obtained in all patients. The mean gradient and peak instantaneous gradient at rest and 2 minutes after exercise for the four groups are tabulated (Table 11). By 10 minutes after exercise the values for these measurements had returned to resting values. The principal effect on mean and peak instantaneous gradients was attributable to valve type; there were no significant valve type by size interactions. The mean gradient was significantly higher in the spherical disc group compared with the Monostrut group at rest (FI ,16 = 5.21, P < 0,05 and at 2 minutes after exercise (F l ,16 = 6.45, p < 0.025; Table 11). The peak gradient at rest was significantly higher in the spherical disc than in the Monostrut group (FI,16 = 4.59,p < 0.05; Table 11). The mean and peak gradients across the spherical disc prosthesis were invariably higher than the 23 mm prosthesis, although these differences did not reach statistical significance, possibly because of the small numbers involved in each group. Only trivial transvalvular aortic regurgitation, not of clinical significance, was detected in both patient groups. Discussion The only data concerning the hemodynamic performance of the Monostrut aortic valve has come from transseptal left heart catheterization studies, and comparison has then been made with the hemodynamic data on other Bjork-Shiley valves from other studies. I, 4 This
Volume 104
Hemodynamics of Bjork-Shiley valve prosthesis
Number 4 October 1992
I 027
Table II. Mean and peak gradients at rest and after exercise Monostrut
Mean gradient (mm Hg) At rest 2 min after exercise Peak gradient at rest (mm Hg)
Spherical
2/ mm
23mm
2/ mm
23mm
Probability*
17.2 ± 1.7 24.9 ± 2.8 27.5 ± 3.1
12.7 ± 0.6 22.3 ± 2.1 20.3 ± 0.7
21.8 ± 3.4 31.9 ± 4.0 32.7 ± 4.8
17.7 ± 1.6 29.8 ± 2.1 28.7 ± 2.7
p < 0.05 p < 0.025 P < 0.05
'Values are for differences between valve type adjusted for size.
Table III. Comparison of hemodynamic data on Bjork-Shiley monostrut and spherical disc 21 and 23 mm aortic valve prostheses . Monostrut Study Present study* No. of valves Postoperative time (mo) Peak gradient (mm Hg) Mean gradient (mm Hg) Aris et al." No. of valves Postoperative time (mo) Peak-to-peak gradient (mm Hg) Mean gradient (mm Hg) Bjork and Lindblomlf No. of valves Postoperative time (mo) Mean gradient (mm Hg) Woods (personal communication)* No. of valves Postoperative time (yr) Peak gradient (mm Hg) Bjork et aI.7t ,:j: No. of valves Postoperative time (rno) Peak-to-peak gradient (rnm Hg) Mean gradient (mm Hg)
2/ mm
5
Spherical
23mm
2/ mm
5
5
22.4 ± 3.1 27.4 ± 3.1 17.2 ± 1.7
5 26.2 (2.5)
20.3 ± 0.7 12.7 ± 0.6
5
23mm
28.7 ± 2.7 17.7 ± 1.6
32.7 ± 4.8 21.8 ± 3.4
6 6.5 ± 1.1
13.1 ± 1.9 15.5 ± 8.1
9.3 ± 6.3 16.6 ± 4.2 5
12 13 11 ± 3.5
9 ± 2.3 10
12 11.4 36.5 ± 12.0
24.7 ± 5.8 13
5
7 22 ± 10 25 ± 8
13.8 14.5 ± 10
'Doppler study. tCatheter study. :j:Prosthesisstudied was Delrin disc.
is obviously not ideal, because differences in methods, techniques, and patient populations in different studies confound any true comparison of hemodynamic data. In this study we compared the hemodynamic function of the Monostrut and spherical disc aortic valves with noninvasive Doppler echocardiography and similar patient populations. The values for peak and mean gradients in the Monostrut 21 to 23 mm range are higher than those reported at transseptalleft heart catheterization by Aris and colleagues" and by Bjork and Lindblom I (Table III). However, this may reflect the longer postoperative follow-up period in our study and the sedation and fasting of patients for cardiac catheterization studies.
We demonstrated significantly higher values for mean and peak gradients at rest and mean gradient after exercise across the spherical disc valve. The differences in gradient demonstrated between the two models are small. However, because all prosthetic valves are moderately stenotic, the improved flow characteristics of a new prosthesis are clinically relevant. The mean and peak-to-peak transprosthetic gradients for the Monostrut valve reported by Aris et aJ.,5 although lower than those for previous Bjork-Shiley models, were not significantly different. This may reflect problems in making true comparisons between different studies as alluded to earlier in this discussion.
I028
Kenny et al.
We set out in this study to ascertain whether the design changes in the Monostrut valve produce improved hemodynamics. We conclude that the Monostrut valve is hemodynamically superior both at rest and after exercise compared with the spherical disc model. This satisfactory hemodynamic performance, together with the reported low rate of complications and improved durability,2,6 should justify the continued use of the Bjork-Shiley Monostrut aortic valve prosthesis. We are grateful to Sir T. English for allowing us to include his patients in this study. REFERENCES 1. Bjork VO, Lindblom D. The monostrut Bjork-Shiley heart valve. J Am Coli CardioI1985;6:1142-8. 2. Burstow DJ, Nishimura RA, Bailey KR, et a1. Continuous wave Doppler echocardiographic measurement of prosthetic valve gradients: a simultaneous Doppler-catheter correlative study. Circulation 1989;80:504-14.
The Journal of Thoracic and Cardiovascular Surgery
3. Gardin J, Kozlowski J, Debestani A, et a1. Studies of aortic flow velocity during supine bicycle exercise. Am J Cardiol 1986;57:327-32. 4. Aris A, Crexells C, Auge J, Oriol A, Caralps J. Hemodynamic evaluation of the integral Monostrut Bjork-Shiley prosthesis in the aortic position. Ann Thorac Surg 1985; 40:234-40. 5. Karp RB. In discussion: Aris A, Crexells C, Auge J, Oriol A, Caralps J. Hemodynamic evaluation of the integral Monostrut Bjork-Shiley prosthesis in the aortic position.Ann Thorac Surg 1985;40:234-40. 6. Aris A, Padro JM, Camara ML, Crexells C, Auge JM, Caralps JM. Clinical and hemodynamic results of cardiac valve replacement with the Monostrut Bjork-Shiley prosthesis. J THORAC CARDIOVASC SURG 1988;95:423-31. 7. Bjork V, Holmgren A, Olin C, Ovenfors C-O. Clinical and haemodynamic results of aortic valve replacement with the Bjork-Shiley tilting disc valve prosthesis. Scand J Thorae Cardiovasc Surg 1971;5:177-91.
A. Kenny, MRCPI, MRCP(u.K.),a J. Woods, MRcp,a C. A. Fuller,a L. Sharples, PhD,b D. L. Stone, MD, MRcp,a F. C. Wells, MS, BSc, FRCS,a and L. M. Shapiro, MD, MRcp,a Cambridge, England
L e Monostrut model Bjork-Shiley aortic valve prosthesis (Shiley, Inc., Irvine, Calif.) incorporates design changes that should improve hemodynamic performance compared with the spherical model. I We performed a Doppler echocardiographic study to assess whether the Monostrut model has improved hemodynamics compared with the spherical disc model. The reliability and accuracy of Doppler echocardiography in assessing transvalvular pressure gradients in prosthetic aortic valves has been validated."
From the Regional Cardiothoracic Unit, Papworth Hospital," and MRC Biostatistical Unit,b Cambridge, England. Received for publication May 31,1991. Accepted for publication Dec. 13, 1991. Address for reprints: Antoinette Kenny, MRCPI, MRCP(U.K.), Papworth Hospital, Papworth Everard, Cambridge, U.K. CB3 8RE.
12/1/36360
Patients and methods The study population comprised 20 patients less than 70 years of age, 10 with the spherical disc model and 10 with the Monostrut model. Within each group five patients had a 21 mm and five had a 23 mm valve prosthesis. The patients were chosen by retrospective random selection from concurrent groups of patients who underwent aortic valve replacement with either the spherical disc or Monostrut prosthesis at our institution since 1985. During this time, when the use of both types of prostheses overlapped, patients received either a Monostrut or spherical disc model according to availability. There was no conscious bias as to which patient received which model. We elected to study patients in the 21 to 23 mm range because these were the size of prostheses most commonly inserted in this institution. Five patients from four groups (Monostrut 21 and 23 mm and spherical disc 21 and 23 mm) were retrospectively randomly selected. The age limit of 70 years was designated because participants were required to perform strenuous exercise. Medical ethical committee approval and informed patient consent was obtained. Preoperative status was ascertained from the patients' medical records. Patient characteristics were similar in both the spherical disc and Monostrut group (Table 1).
1025
10 2 6
The Journal of Thoracic and Cardiovascular Surgery
Kenny et al.
Table I. Patient characteristics
Monostrut
21 mm prostheses (n) 5 23 mm prostheses (n) 5 Age (yr) 53.1 ± Postoperative time (mo) 22.4 ± 32.1 ± Fractional shortening Body surface area (m-) 1.7 ± Heart rate (beats/min) Resting 76.8 ± 130.7 ± Peak exercise 95.7 ± 2 min after exercise NYHA preoperative class (n) I I II 2 III 4 IV 3 *p < 0.001 by paired [Fisher's exact test.
I
2.3 3.2 3.1 0.05
Spherical
5 5
55.8 ± 26.2 ± 33.6 ± 1.8 ±
Unpaired t test (p value)
2.8 2.5 3.9 0.07
0.49 0.39 0.78 0.28
3.5 72.5 ± 5.2 4.2* 129.4 ± 4.2* 4.2* 94.7 ± 5.1 *
0.52 0.84 0.89
It 3t 3t 3t
0.65
test; increase in heart rate over resting values.
Echocardiography. Echocardiography was performed with a Hewlett-Packard model 77020 AV-C26 echocardiograph, with a 2.5 MHz imaging transducer with facilities for pulsed wave Doppler and a dedicated (Pedoff) transducer for continuous wave Doppler echocardiography. Left ventricular cavity dimensions were assessed according to the leading-edge method, and fractional shortening was determined from left ventricular dimensions. Doppler studies were performed after 15 minutes of rest and repeated after exercise with supine bicycle ergometry. Continuous wave Doppler was used to determine the flow velocity across the aortic prosthesis at rest. We examined the prosthetic valve from three views: apical, suprasternal, and right parasternal, to determine the highest transvalvular velocities. The body position and ultrasound approach from which the highest velocities were obtained at rest were marked for quick reference after exercise. The peak velocity (V2) was calculated by averaging three to five consecutive complexes for patients in sinus rhythm and 10 for those patients in atrial fibrillation. The peak instantaneous pressure gradient across the prosthesis was calculated with the modified Bernoulli equation P = 4 (V2P. If the velocity in the left ventricular outflow tract exceeded l rn/sec (Vl), it was considered in the gradient calculation by inclusion in the equation P = 4 (V2 2 - V12) . The mean pressure gradient was calculated by averaging the gradient at 40 msec intervals throughout the velocity envelope. Three to five consecutive envelopes were averaged for patients in sinus rhythm and 10 for those in atrial fibrillation. Patients exercised with the use of supine bicycle ergometry; the end points of exercise were achievement of 75% of predicted maximal heart rate or fatigue, whichever occurred sooner. The initial workload was 25 Wand this was increased by 25 W at 3-rninute intervals. Continuous wave Doppler echocardiography was performed after exercise in the same manner as at rest, to determine the velocity across the aortic prosthesis. Readings were recorded at 2-minute intervals after exercise until lO minutes after exercise. The first Doppler recording was made 2 minutes after exer-
cise. This allowed time to realign accurately the Doppler beam with flow. A recent study also demonstrated that peak velocity occurred 2 minutes after exercise in normal subjects and not at peak exercise.' Statistical analysis. Continuous data are expressed as the mean and standard error of the mean. To determine whether differences in patient characteristics between groups of patients were statistically significant, the unpaired t test was used. The paired t test was used to study within-patient changes in heart rate. Differences between valve types and sizes of valves were analyzed with two-way analysis of variance. This technique allows us to compare valve types while adjusting for differences in valve sizes, and vice versa. In addition, the presence of valve type by size interactions were assessed. Differences with less than a 5% probability of being zero were considered statistically significant.
Results Both groups achieved a similar significant increase in heart rate with exercise (Table I). Two minutes after exercise heart rates remained significantly higher than resting rates for both groups. Echocardiography confirmed prosthetic valve integrity in all patients. There was no significant difference in mean fractional shortening between the two valve types (Table I). A satisfactory continuous wave recording at rest and 2 minutes after exercise was obtained in all patients. The mean gradient and peak instantaneous gradient at rest and 2 minutes after exercise for the four groups are tabulated (Table 11). By 10 minutes after exercise the values for these measurements had returned to resting values. The principal effect on mean and peak instantaneous gradients was attributable to valve type; there were no significant valve type by size interactions. The mean gradient was significantly higher in the spherical disc group compared with the Monostrut group at rest (FI ,16 = 5.21, P < 0,05 and at 2 minutes after exercise (F l ,16 = 6.45, p < 0.025; Table 11). The peak gradient at rest was significantly higher in the spherical disc than in the Monostrut group (FI,16 = 4.59,p < 0.05; Table 11). The mean and peak gradients across the spherical disc prosthesis were invariably higher than the 23 mm prosthesis, although these differences did not reach statistical significance, possibly because of the small numbers involved in each group. Only trivial transvalvular aortic regurgitation, not of clinical significance, was detected in both patient groups. Discussion The only data concerning the hemodynamic performance of the Monostrut aortic valve has come from transseptal left heart catheterization studies, and comparison has then been made with the hemodynamic data on other Bjork-Shiley valves from other studies. I, 4 This
Volume 104
Hemodynamics of Bjork-Shiley valve prosthesis
Number 4 October 1992
I 027
Table II. Mean and peak gradients at rest and after exercise Monostrut
Mean gradient (mm Hg) At rest 2 min after exercise Peak gradient at rest (mm Hg)
Spherical
2/ mm
23mm
2/ mm
23mm
Probability*
17.2 ± 1.7 24.9 ± 2.8 27.5 ± 3.1
12.7 ± 0.6 22.3 ± 2.1 20.3 ± 0.7
21.8 ± 3.4 31.9 ± 4.0 32.7 ± 4.8
17.7 ± 1.6 29.8 ± 2.1 28.7 ± 2.7
p < 0.05 p < 0.025 P < 0.05
'Values are for differences between valve type adjusted for size.
Table III. Comparison of hemodynamic data on Bjork-Shiley monostrut and spherical disc 21 and 23 mm aortic valve prostheses . Monostrut Study Present study* No. of valves Postoperative time (mo) Peak gradient (mm Hg) Mean gradient (mm Hg) Aris et al." No. of valves Postoperative time (mo) Peak-to-peak gradient (mm Hg) Mean gradient (mm Hg) Bjork and Lindblomlf No. of valves Postoperative time (mo) Mean gradient (mm Hg) Woods (personal communication)* No. of valves Postoperative time (yr) Peak gradient (mm Hg) Bjork et aI.7t ,:j: No. of valves Postoperative time (rno) Peak-to-peak gradient (rnm Hg) Mean gradient (mm Hg)
2/ mm
5
Spherical
23mm
2/ mm
5
5
22.4 ± 3.1 27.4 ± 3.1 17.2 ± 1.7
5 26.2 (2.5)
20.3 ± 0.7 12.7 ± 0.6
5
23mm
28.7 ± 2.7 17.7 ± 1.6
32.7 ± 4.8 21.8 ± 3.4
6 6.5 ± 1.1
13.1 ± 1.9 15.5 ± 8.1
9.3 ± 6.3 16.6 ± 4.2 5
12 13 11 ± 3.5
9 ± 2.3 10
12 11.4 36.5 ± 12.0
24.7 ± 5.8 13
5
7 22 ± 10 25 ± 8
13.8 14.5 ± 10
'Doppler study. tCatheter study. :j:Prosthesisstudied was Delrin disc.
is obviously not ideal, because differences in methods, techniques, and patient populations in different studies confound any true comparison of hemodynamic data. In this study we compared the hemodynamic function of the Monostrut and spherical disc aortic valves with noninvasive Doppler echocardiography and similar patient populations. The values for peak and mean gradients in the Monostrut 21 to 23 mm range are higher than those reported at transseptalleft heart catheterization by Aris and colleagues" and by Bjork and Lindblom I (Table III). However, this may reflect the longer postoperative follow-up period in our study and the sedation and fasting of patients for cardiac catheterization studies.
We demonstrated significantly higher values for mean and peak gradients at rest and mean gradient after exercise across the spherical disc valve. The differences in gradient demonstrated between the two models are small. However, because all prosthetic valves are moderately stenotic, the improved flow characteristics of a new prosthesis are clinically relevant. The mean and peak-to-peak transprosthetic gradients for the Monostrut valve reported by Aris et aJ.,5 although lower than those for previous Bjork-Shiley models, were not significantly different. This may reflect problems in making true comparisons between different studies as alluded to earlier in this discussion.
I028
Kenny et al.
We set out in this study to ascertain whether the design changes in the Monostrut valve produce improved hemodynamics. We conclude that the Monostrut valve is hemodynamically superior both at rest and after exercise compared with the spherical disc model. This satisfactory hemodynamic performance, together with the reported low rate of complications and improved durability,2,6 should justify the continued use of the Bjork-Shiley Monostrut aortic valve prosthesis. We are grateful to Sir T. English for allowing us to include his patients in this study. REFERENCES 1. Bjork VO, Lindblom D. The monostrut Bjork-Shiley heart valve. J Am Coli CardioI1985;6:1142-8. 2. Burstow DJ, Nishimura RA, Bailey KR, et a1. Continuous wave Doppler echocardiographic measurement of prosthetic valve gradients: a simultaneous Doppler-catheter correlative study. Circulation 1989;80:504-14.
The Journal of Thoracic and Cardiovascular Surgery
3. Gardin J, Kozlowski J, Debestani A, et a1. Studies of aortic flow velocity during supine bicycle exercise. Am J Cardiol 1986;57:327-32. 4. Aris A, Crexells C, Auge J, Oriol A, Caralps J. Hemodynamic evaluation of the integral Monostrut Bjork-Shiley prosthesis in the aortic position. Ann Thorac Surg 1985; 40:234-40. 5. Karp RB. In discussion: Aris A, Crexells C, Auge J, Oriol A, Caralps J. Hemodynamic evaluation of the integral Monostrut Bjork-Shiley prosthesis in the aortic position.Ann Thorac Surg 1985;40:234-40. 6. Aris A, Padro JM, Camara ML, Crexells C, Auge JM, Caralps JM. Clinical and hemodynamic results of cardiac valve replacement with the Monostrut Bjork-Shiley prosthesis. J THORAC CARDIOVASC SURG 1988;95:423-31. 7. Bjork V, Holmgren A, Olin C, Ovenfors C-O. Clinical and haemodynamic results of aortic valve replacement with the Bjork-Shiley tilting disc valve prosthesis. Scand J Thorae Cardiovasc Surg 1971;5:177-91.