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J Thorac Cardiovasc Surg 1994;108:1010-1020
© 1994 Mosby, Inc.
SURGERY FOR ACQUIRED HEART DISEASE |
Los Angeles, Calif.
From the Division of Cardiothoracic Surgery, Cedars-Sinai Medical Center, Los Angeles, Calif.
Address for reprints: Steven S. Khan, MD, Division of Cardiothoracic Surgery, Cedars-Sinai Medical Center, 8700 Beverly Blvd., Room 6215, Los Angeles, CA 90048.
Abstract
We analyzed the long-term results of valve replacement with the St. Jude Medical bileaflet valve (St. Jude Medical, Inc., St. Paul, Minn.) in our first 1000 implantations between 1978 and 1992. A total of 399 patients had mitral valve replacement, 471 aortic valve, and 130 double (mitral and aortic) valve replacement. The average patient age was 64 ± 15 years and the majority of patients (52%) had concomitant coronary disease. With 4328 patient-years of follow-up, 83% of the mitral group, 76% of the aortic group, and 77% of the double valve group were free of thromboembolism at 10 years after operation, and 87% of the mitral group, 82% of the aortic group, and 85% of the double valve group were free of valve-related hemorrhage. At 10 years, 91% of the mitral group, 84% of the aortic group, and 84% of the double valve group were free of valve-related death. However, overall survival at 10 years was only 42% ± 4% for the mitral group, 43% ± 4% for the aortic group, and 43% ± 6% for the double valve group. For all three groups, age was a highly significant factor stratifying survival (p < 0.001), as was the presence of coronary disease (all p < 0.001). The excellent freedom from valve-related death at 10 years of 84% to 91% is in striking contrast to the overall survivals of 42% to 43% at 10 years. This difference suggests that the primary factors limiting long-term survival after valve replacement with the St. Jude Medical valve are not valve-related factors, but other patient factors such as age and concomitant coronary disease. (J THORACCARDIOVASCSURG1994;108:1010-20)
Since its introduction in the United States, the St. Jude Medical valve (St. Jude Medical, Inc., St. Paul, Minn.) has become the most commonly implanted mechanical valve in this country. In vitro studies have consistently demonstrated low gradients across this valve both by catheterization techniques 
1-5 and by Dopplerechocardiography. 6-9 However, Doppler echocardiography may measure spuriously high gradients in the St. Jude Medical valve 10 and may underestimate valve areas. 11 Although a recent report has observed the frequent occurrence of fibrin strands on the St. Jude Medical valve when transesophageal echocardiography has been used, 12 actual clinicalthromboembolism has not been prevalent. 13-17 In contrast to results in the early era of valve replacement surgery, the excellent hemodynamics of the St. Jude Medical valve and low incidence of thromboembolism suggest that valve-related complications may no longer be the primary factors determining patient survival. We therefore sought to determine the outcomes after valve replacement with the St. Jude Medical valve in our first 1000 implantations and to compare the relative roles of valve-related complications with patient-related factors in determining survival.
METHODS
Data collection
All patients older than the age of 18 years undergoing cardiac operations at Cedars Sinai Medical Center, a university affiliated, 1000 bed private tertiary care hospital, have been entered prospectively into a computer database. Data are entered into the computer at the time of discharge from the hospital by trained chart reviewers and patients are monitored with annual mailed questionnaires. If a possible valve-related event is noted on the questionnaire, a trained nurse specialist calls the patient and the physician to determine whether the event was a valve-related complication.
Patient selection
Only patients undergoing St. Jude Medical valve replacement were included in this analysis. Patients with triple valve replacements or tricuspid or pulmonic valve replacement were excluded from the analysis because of their small numbers. Patients undergoing valve replacement with concomitant valve repair procedures were included in the study. The results of the first 1000 patients in the cardiac surgery database meeting the inclusion criteria were analyzed. This material constituted a total of 4328 patient-years of follow-up with an average duration of follow-up of 4.3 years per patient.
Operative technique
All operations were performed by one group of cardiac surgeons using systemic hypothermia with blood cardioplegia or St. Thomas' Hospital solution, or both. 17 Mitral valve replacement (MVR) was performed with the valve placed in the antianatomic position and aortic valve replacement (AVR) with one of the pivot guards positioned against the ventricular septum and the other between the left coronary and noncoronary cusps. Until 1992, the mitral subvalvular apparatus was resected in virtually all patients. Thereafter, it was left intact to the maximum extent possible. The smaller valve sizes predominated in the aortic position: 45% of aortic valves were either size 19 or 21 mm whereas 46% were 23 or 25 mm. In the mitral position, 53% of valves were 27 or 29 mm and 38% were 31 or 33mm.
Anticoagulation
All patients were given warfarin for anticoagulation unless contraindications existed or severe bleeding complications developed. The prothrombin time was maintained at 1.5 to 2.5 times control until October 1987, when the target prothrombin time was changed to 1.5 to 2.0 times control. Since 1992, we have recommended a target international normalized ratio (INR) of 2.5 to 3.5. The warfarin dose is regulated by the surgical team while the patient is in the hospital and by the referring physician after the patient is discharged.
Statistical analysis
All variables were defined in compliance with the guidelines established by The American Association for Thoracic Surgery and the Society of Thoracic Surgeons. 18 Variables analyzed included structural deterioration, valvular thromboembolism (including episodes of embolism, transient ischemic attacks, and valve thrombosis), anticoagulant-related hemorrhage (defined as episodes resulting in death, stroke, surgery, hospitalization, or transfusion), endocarditis, valve-related mortality, and permanent valve-related impairment (defined as any permanent functional deficit caused by structural valve deterioration, nonstructural dysfunction, thromboembolism, anticoagulant-related bleeding, endocarditis, or reoperation). Variables were compared between groups by unpaired t tests, the Fisher exact test, or 
2 analysis as appropriate. Life table methods were used to analyze actuarial freedom from death, thromboembolism, hemorrhage, and permanent valve-related impairment. Survival curves were compared between groups by the Wilcoxon statistic for censored survival data. For evaluation of factors affecting long-term survival, a stepwise Cox proportional hazards model was developed. Probability values of 0.05 or less were considered significant for all analyses.
RESULTS
Preoperative patient profile
A total of 1000 patients were included in the analysis: 399 MVR, 471 AVR, and 130 double valve replacements (DVR). Table I compares baseline characteristics of the three different valve groups. The average age was highest for patients undergoing isolated AVR (65 ± 15 years, p = 0.009). The most common cause of valvular disease for the MVR and DVR groups was rheumatic heart disease (35% and 48%, respectively), and the most common cause for patients with AVR was degenerative disease (41%).
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Preoperative hemodynamics
Information on preoperative hemodynamics and left ventricular function was available for a subset of patients. Left ventricular ejection fractions were available for 38% of the patients and are shown in 
Table I by valve type. The preoperative left ventricular ejection fraction was 59% ± 20% for all patients and did not differ significantly by valve location (p = 0.54). Left ventricular end-diastolic volumes were measured in 290 patients and are shown in Table I. There were no significant differences in left ventricular end-diastolic volume between the AVR, MVR, and DVR groups (p = 0.67).
Operative characteristics
In view of the advanced average age of the patients, it is not surprising that many required simultaneous coronary artery bypass operations. Overall, 52% had coronary artery disease defined by the presence of at least one vessel with 50% stenosis. Concomitant bypass grafting was performed in 48% of the AVR group, 41% of the MVR group, and 32% of the DVR group (p = 0.002). Although the patients undergoing MVR were most likely to require preoperative intraaortic balloon pump insertion (MVR 6.3%, AVR 1.7%, DVR 2.3%, p = 0.001), the MVR and DVR groups had a similar frequency of balloon pump insertion during or after the operation (MVR 7.0%, DVR 6.9%, AVR 2.8%, p = 0.009). The MVR and DVR groups also required more frequent tricuspid valve repairs (MVR 10%, DVR 11.5%, AVR 1.1%, p < 0.001). Although we currently routinely use chordal preservation procedures for MVR with the St. Jude Medical valve, only 0.5% (2/400) of patients undergoing MVR and 2.3% (3/130) of those undergoing DVR in this series had chordal preservation procedures.
Operative mortality
The overall operative (in-hospital/30 day) mortality was lowest for the patients undergoing an isolated AVR (4.9%) followed by those having DVR (11.5%) and highest for the MVR group (13.0%, p < 0.001). When mortality was analyzed by reoperation status, patients undergoing MVR reoperation had a slightly lower risk of in-hospital death than patients undergoing their first MVR (8.9% versus 13.6%, p = NS*).). However, in-hospital mortality was slightly higher for patients having AVR reoperation (7.4% versus 4.7%) and DVR reoperation (19.1% versus 10.1%) than for those having their first operation, although these differences were also not statistically significant. One-year survival was highest for patients who had AVR (92%) and was roughly equivalent for those having MVR (82%) and DVR (85%).
Valve-related complications
The thromboembolic rates for the St. Jude Medical valve were low. Fig. 1 demonstrates the actuarial freedom from thromboembolism for patients with MVR, AVR, and DVR. At 10 years 83% ± 4% of those with MVR, 76% ± 4% of those with AVR, and 77% ± 6% of those with DVR were free of thromboembolism. The linearized event rates of thromboembolism for MVR, AVR, and DVR are shown in Table II and were 2.4/100 patient-years, 2.5/100 patient-years, and 3.2/100 patient-years, respectively.
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Factors affecting long-term survival were analyzed by the Cox model multivariate regression. For all three valve groups, patient age, NYHA class, and the presence of coronary artery disease were strong independent predictors of survival (all p < 0.0001). Gender was not a predictor of long-term survival for any group. Use of a preoperative intraaortic balloon pump was a significant independent predictor of survival for the MVR and AVR groups but not for the DVR group.
Outcomes in patients with coronary artery disease
The presence of coronary artery disease was an important discriminating factor in the multivariate Cox model of long-term survival. We therefore analyzed survival data separately by the extent of coronary artery disease. Fig. 5 shows survival curves for patients without coronary disease, patients with one- or two-vessel disease, and patients with left main or three-vessel disease. The differences in survival are striking despite the fact that virtually all patients with significant coronary artery disease underwent revascularization. Survival at 10 years was 62% for patients with no coronary artery disease, 35% for patiets with one- or two-vessel disease, and 24% for patients with three-vessel or left main disease (p < 0.0001). Fig. 5 also suggests that the differences in survival between patients with and without coronary artery disease may become more marked as the time interval from surgery increases.
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Operative mortality rates were slightly higher for patients having valve reoperation (10.8% versus 8.8%), although the difference was not statistically significant. The DVR group had the highest reoperation mortality (19.1%). Although the number of events was small in long-term follow-up, the linearized thromboembolic rate for patients having valve reoperation (Table III) was 2.8% overall (MVR 3.2%, AVR 2.8%, and DVR 1.8%) and the rate of hemorrhage was 0.0%. The linearized rate of valve-related death was 1.1% (MVR 0.6%, AVR 0%, DVR 3.6%). Overall survivals at 1 and 5 years were 91% and 72% after MVR reoperation, 92% and 92% after AVR, and 66% and 43% after DVR. These differences in long-term survival were statistically significant (p = 0.03), with patients undergoing AVR having the best survival, those undergoing DVR the worst survival, and those undergoing MVR intermediate results. Thus the event rates for thromboembolism, hemorrhage, and valve-related death in patients undergoing valve reoperation are similar to those found in the overall group.
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This experience with 1000 consecutive implantations demonstrates that the St. Jude Medical valve provides excellent clinical results through 12 years of use. The rates of thromboembolism and of valve-related complications remain low. The linearized thromboembolic rate for the aortic valves of 2.5% is slightly higher than the 1.6% reported in the literature for the St. Jude Medical valve and the 1.8% for the Medtronic Hall valve (Medtronic, Inc., Minneapolis, Minn.) but comparable with the 2.1% for the Starr-Edwards valve (Baxter Healthcare Corporation, Edwards Div., Santa Ana, Calif.) as summarized in a recent review by Akins. 19 The mitral thromboembolism rate of 2.4% is also comparable with the composite value of 2.4% Akins reports for mechanical mitral valves. These thromboembolic rates are comparable with the 2.3% rate observed in patients with tissue valves by Jamieson and associates. 20 Our linearized mitral and aortic thromboembolism rates are also similar to the 2.9%/year rate of thromboembolism found by Kratz and coworkers 21 in a recent review of their 10-year experience with the St. Jude Medical valve.
The linearized rate of hemorrhage of 2.0% seen in our study is acceptably low, and our data suggest a time-dependent decrease in hemorrhage without an increase in thromboembolism rates, which coincided with a change in our target prothrombin time ratios from 2.0-2.5 to 1.5-2.0. The effect of a lower target prothrombin time is also evident when comparing the 92% freedom from hemorrhage at 5 years for our patients having AVR to the 67% freedom from hemorrhage at 5 years reported for aortic mechanical valves in the randomized Veterans Administration Cooperative Study. 22 However, it is comparable with the composite value of 89% freedom from hemorrhage reported by Akins 19 for St. Jude Medical and 91% for Medtronic Hall aortic valves.
The overall survivals of this cohort at 10 years (AVR 43%, MVR 42%, DVR 43%) are lower than those reported by Nitter-Hauge and Abdelnoor 23: AVR 72%, MVR 56%, and DVR 60% at 10 years. A major reason for this difference may be the more advanced age of our patients at the time of operation: 64 years versus 56 years. Thus our 10 year survivals are similar to the 48% reported by Jamieson and colleagues 19 in elderly patients after tissue valve replacement. Further, 43% of our patient population had concomitant coronary artery bypass procedures compared with only 25% in Nitter-Hauge's study. Fig. 5 clearly demonstrates the adverse impact of coronary artery disease on long-term outcomes.
One of the most striking findings in this study is the significant difference between the excellent freedom from valve-related death and the long-term survival of our patients. The multivariate Cox model analysis demonstrates that the major independent factors affecting long-term outcome in these patients currently are their advanced preoperative NYHA class, advanced age, and the presence of coronary artery disease. These findings suggest that valve-related complications are a less important cause of death in the current populaiton of patients undergoing mechanical valve replacement and play a minor role in determining the long-term prognosis of a patient compared with patient-related factors. This has previously been pointed out by Mitchell and colleagues, 24 who evaluated outcomes in 2719 patients and found that patient-related factors such as patient age and hypertension strongly influenced not only overall survival, but also the occurrence of valve-related events such as valve failure and thromboembolism rates. The importance of concomitant coronary artery disease in determining prognosis has clearly been shown for both MVR 25,26 and AVR, 27,28 although studies with shorter follow-up have been inconclusive. 29 It is likely that as the population coming to valve surgery ages, and therefore presumably becomes sicker, valve-related factors will continue to diminish in importance relative to patient-related factors.
Our results also highlight the increasing percentage of patients undergoing valve reoperations. Not surprisingly, the commonest prior valve implanted in both the aortic and mitral positions was a porcine valve. The results in the subset of patients undergoing valve reoperation illustrate the excellent outcomes after AVR reoperation with the St. Jude Medical valve, with a 92% overall survival at 5 years. However, the 1-year survival in the DVR group is 66% and 5-year survival only 43% with the MVR reoperation group having 5-year survivals falling between the AVR and DVR groups. Thus, although operative mortality in these patients is acceptably low, there are important differences in long-term outcomes. The outcomes after AVR reoperation are excellent, with poorer results in the DVR and MVR groups.
Limitations
Although our study demonstrates no structural valve failures in 1000 consecutive patients, it is possible that a valve failure occurred and was not diagnosed. For example, if a patient with structural valve failure died suddenly, valve failure would not be diagnosed if an autopsy was not performed. Another potential limitation with our study is that prothrombin time monitoring and regulation of warfarin dosages were left to the individual private attending physicians. Therefore, although our group can make recommendations about target prothrombin times, the accuracy of maintaining these goals depends heavily on the individual patient's physician. However, it has not been shown that decentralized management of warfarin increases the potential for either inadequate anticoagulation or over-anticoagulation, and this approach closely mirrors real-world anticoagulation at most medical centers. There is, in fact, evidence that home monitoring of prothrombin times can provide accurate measurements and achieve better anticoagulation control than standard outpatient care. 30
This experience with 1000 St. Jude Medical valve implantations demonstrates no episodes of structural failure and a low rate of thromboembolic complications. Outcomes at 5 and 10 years after valve implantation have become more dependent on patient characteristics than on the risk of valve-related complications.
Appendix: DISCUSSION
Dr. W. R. Eric Jamieson (Vancouver, B.C., Canada).
Dr. Khan, we have watched with interest the experience with the St. Jude Medical valve at Cedars-Sinai. You have identified a number of important factors—that survival is not related to valve-related variables but to patient-related variables and that better control of anticoagulation with less high-intensity anticoagulation has reduced hemorrhage but not furthered thromboembolism.
In our experience with the St. Jude Medical valve at the University of British Columbia, we have identified similar valve-related mortality in our two series. We have less hemorrhage, and perhaps we are following those guidelines that you are now following, but thromboembolism has become much more prevalent. I wonder whether the difference is related to the type of follow-up. All of our patients are personally interviewed, and our incidence of thromboembolism with the St. Jude Medical valve is 2 to 2.5 times greater—4.9%/patient-year. I would like you to comment on that.
Also, in relation to hemorrhage and thromboembolism, have you evaluated permanent impairment or the concept of residual morbidity and what influence it has on the patients in your group?
Dr. Khan.
Our follow-up relies on mailed questionnaires sent out to patients. Those are then reviewed by one of our data analysts, and patients who have any kind of suspicious event, such as dizzy spells, are called by a research nurse who has worked on the database since its inception. The nurse calls the patient, reviews symptoms, pulls charts, and, if necessary, calls the physician. All our events are classified by personal interview, but the initial screening for an event is done by a mailed questionnaire. This may represent one of the differences between your thromboembolism rates and ours.
Dr. Jamieson.
This may be a variable. As you noted in your manuscript, thromboembolism in our tissue valve population is more prevalent than in the literature as well.
My other question concerned permanent impairment and the influence on patients. Have you evaluated the freedom from permanent impairment or residual morbidity from thromboembolism and hemorrhage on your patient population?
Dr. Khan.
Although we have not evaluated disability specifically by cause, we did examine the late New York Heart Association class. The vast majority of these patients are in NYHA class I or II after the operation, but some patients are in class III or IV at 1 year. We have not tried to determine how much of that loss of functional status is related to thromboembolism or bleeding complications, however.
Dr. Jamieson.
This is a very important concept when the influence of mechanical valves in patients is being evaluated.
What has been the incidence of thrombosis of the prosthesis? What are your indications for thrombolytic therapy or surgery? And what is your overall experience?
Dr. Khan.
We published that information in Circulation (1993;87:30-7). We have had roughly 20 episodes of valve thrombosis in 16 different patients, and several of those were repetitive episodes out of this 1000 patient population. We are now comfortable using thrombolytic therapy and usually use urokinase initially. However, urokinase is very expensive and streptokinase would probably be a more cost-effective choice. We have an 84% success rate through lytic therapy. Our first line of treatment now for thrombosed valves is thrombolytic therapy.
Dr. Jamieson.
You stated that you are now attempting to preserve the chordal attachments and posterior leaflet and anterior leaflet. Can you describe for us whether there are any technical problems with the St. Jude Medical valve in the preservation of one or both leaflets?
Dr. Khan.
We are routinely using chordal preservation procedures now, and they represent 2% to 3% of this patient population. Perhaps Dr. Chaux can describe possible technical problems more specifically.
Dr. Chaux.
We have not seen any problems in the cases in which we have done chordal preservation, but we are always careful to tack those chordae as much as possible toward the anulus and away from the prosthesis. Impairment of the motion of the leaflet resulting from any part of the valvular structure has not been detected in this series. We have started to reattach the tip of the papillary muscles to the anulus, using polytetrafluoroethylene, to try to reconstruct the valvular structure in patients with rheumatic heart disease. We were not doing before. That is a very recent experience.
Dr. Jamieson.
Have you identified any difficulty with the nonrotatability of this prosthesis in implantation? Are any particular technical problems identified?
Dr. Chaux.
No, actually none. Right from the beginning of our experience, we have always determined ahead of time the best way to position the prosthesis, because we know it does not rotate. In so doing, we have been able to avoid any problems. We do not have rigid rules that we have to position this prosthesis in the anatomic position or in the vertical position. We just place it in such a way to minimize any possible impairment to the opening of the leaflet, and we have been successful with that strategy.
Dr. Khan.
Most patients undergo transesophageal echocardiography after the operation, and we have never seen a case of leaflet impingement.
Dr. James R. Pluth (Scottsdale, Ariz.).
I become distressed when I hear the statement made that there is a zero or low incidence of valve-related deaths and then find the authors attribute mortality to cardiac nonvalve-related causes. Personally, I am reluctant to use mechanical valves that may have an eightfold regurgitant fraction compared with tissue valves in a patient with a dilated left ventricle and poor contractility. In that situation, I think a heterograft valve with a low regurgitant fraction is a better solution and is kinder to the hemodynamics of the heart than would be a mechanical valve. I did not see that you tried to determine whether your patients had aortic regurgitation or aortic stenosis and what effect this might have had on long-term survival. I believe there are factors that cause death that are not necessarily failure-related, but perhaps valve-related because of an inherent physiology in their construction.
Dr. Khan.
You said an eightfold incidence?
Dr. Pluth.
Fivefold to eightfold anyway, from 2% to 3% in a heterograft valve to as much as 12% to 20% in a St. Jude Medical valve.
Dr. Khan.
We 1 have done in vitro studies of regurgitation through the St. Jude Medical valve, and we typically find about a 12% regurgitant fraction. We did look at whether stenotic or regurgitant lesions make a difference. The survivals are very different between patients with aortic stenosis and aortic regurgitant lesions. It is the same when comparing mitral stenosis and mitral regurgitation, with both the regurgitant lesions resulting in much poorer survival. The type of lesion is an important predictor.
Dr. Pluth.
We looked at about 1400 aortic valves some years ago. Although the study was not prospective, it did appear that heterograft valves resulted in a better survival for patients with aortic regurgitation than did mechanical valves. That is why I am asking whether you analyzed that.
Dr. Khan.
It is not clear that the amount of regurgitation through the valve affects long-term survival, because regurgitation is a small percentage of forward flow. In the catheterization laboratory, regurgitant fractions of 20% or less are graded as mild. 2
Dr. D. Craig Miller (Stanford, Calif).
The incidence of valve-related late death at 10 years, particularly for the MVR group, is exceptionally good. Having 90% of the MVR group free of valve-related death at 10 years is phenomenal. We would expect this figure for the AVR group but not for the MVR group.
I would like to follow up in that regard with what Dr. Pluth was just touching on. Today we define valve-related deaths as all of those that can be proved to be due to the valve or anticoagulation plus all of the sudden unexpected deaths. This figure is clearly an overestimate of true valve-related deaths, but makes comparison of different series a more intelligent exercise. The AATS/STS guidelines stipulate that they be included. You did not tell us how many patients died suddenly and unexpectedly when potentially their deaths could have been due to a valve-related cause.
Dr. Khan.
We have not broken down sudden unexpected deaths according to causes in this analysis. It may be that many of the patients in the MVR group are dying of ischemic disease. That was very close to the most common cause of death. I think 32% of the deaths in the MVR group were of ischemic origin. The patients may simply be dying of acute infarction or recurrent infarction before valve-related complications develop. A patient with coronary disease may not live long enough to have a valve-related event. We have not separated out valve-related sudden death from ischemic sudden death.
Dr. Matloff (Los Angeles, Calif.).
The bottom line is that the issue of regurgitation has been associated with the St. Jude Medical valve and other bileaflet valves from the very beginning. We have studied this extensively by a variety of techniques and have never identified any concomitant hemodynamic or other functional correlate of that regurgitation. In fact, some would say that the washing effect is what is responsible for the low incidence of thromboembolic phenomena that we have seen.
I will grant you that the incidence of thromboembolism in nonmechanical valves is generally taken to be less than that with mechanical valves. However, the fact remains, and I think this study begins to point it out, that the difference between mechanical and nonmechanical valves may have become very small. What this paper is really all about from our perspective is patients who have valvular heart disease, rather than patients who necessarily have a particular type of valve. That is not to say that this will be true for all valve substitutes. However, with the St. Jude Medical valve, and this retrospective, we are beginning to see the emergence of what valvular disease is truly all about.
If Dr. Pluth thinks back to his early experience at the Mayo Clinic, back in the Midwest, he will remember that valve type was a very powerful determinant early on and became less so as things changed in terms of patient selection. We are getting back there now to be able to do the valve surgery and to see what happens on a basis more of what the patient brings to us than the valve. The mental set that we are exchanging one disease, that is, a mechanical valve, for another valvular heart disease is no longer appropriate. There will be some other papers shortly that will I think support this point.
Footnotes
Read at the Nineteenth Annual Meeting of The Western Thoracic Surgical Association, Carlsbad, Calif., June 23-26, 1993. 
References
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W.R. E. Jamieson, R. T. Miyagishima, G. L. Grunkemeier, E. Germann, C. Henderson, G. J. Fradet, L. H. Burr, and S. V. Lichtenstein Bileaflet mechanical prostheses performance in mitral position Eur. J. Cardiothorac. Surg., June 1, 1999; 15(6): 786 - 794. [Abstract] [Full Text] [PDF]
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