Results With Mechanical Cardiac Valvular Prostheses

CURRENT REVEWS Results With Mechanical Cardiac Valvular Prostheses Cary W. Akins, MD Cardiac Surgical Unit, Massachusetts General Hospital, Boston, Massachusetts Mechanical cardiac valvular prostheses continue to be more popular than bioprostheses for heart valve replacement operations. Five different brands of mechanical heart valves are now approved for implantation in the United States: models 1260 and 6120,,,, and. Each model of mechanical valve has certain positive and negative attributes, but none is functionally mechanically perfect. A review of the published long- term results with these valves favors the and valves. A new method of assessing the thrombogenic potential and requirement for anticoagulation of the different mechanical valves, namely the composite thromboembolism and bleeding index, is proposed. Evaluation of the new index demonstrates a modest advantage for the valve, particularly in the aortic position. (Ann Thorac Surg 1995;60:1836-44) D espite the increased aging of the valve replacement population in the United States and a trend toward insertion of more bioprostheses in these patients, mechanical heart valves continue to predominate over bioprostheses in the United States with an approximate 65 % to 35% market share advantage. Although the division of the valve market between mechanical and tissue valves varies widely among countries (for example, mechanical valves constitute approximately 90% of the market in Japan but only 10% of the market in Brazil), mechanical valves make up about 60% of the heart valves implanted worldwide. A report in 1991 reviewed the previous 15 years' experience from the English-language literature for the four mechanical cardiac valvular prostheses then approved by the Food and Drug Administration (FDA) for implantation in the United States [1]. Since that review, one more mechanical valve, the bileaflet prosthesis, was approved for market release by the FDA in 1993. This review updates the reported experience with the five mechanical valves currently approved by the FDA. The review is divided into three parts: first, an assessment of the functional mechanical characteristics of the prostheses; second, a review of the published complications; and third, a suggestion for an index that views in a new way the primary problem with mechanical valves, their inherent thrombogenicity, and requirement for anticoagulation. The Study Valves Table lists the five mechanical cardiac valvular prostheses currently approved by the FDA for implantation in the United States, their dates of introduction, the estimated number of each prosthesis inserted according to Address reprint requests to Dr Akins, Cardiac Surgical Unit, Massachusetts General Hospital White 503, 32 Fruit St, Boston, MA 02114. the individual manufacturer, and their current list prices in the United States. Photographs and a full description of the design characteristics of the models 1260 and 6120,,, and prostheses can be found in the review from 1991 [1]. The valve is a bileaflet prosthesis whose pyrolytic carbon-coated discs pivot inside a pyrolytic carbon housing. The Dacron sewing ring is carbon coated (Fig 1). The two leaflets open to 78 degrees from the horizontal axis, and the housing is rotatable within the sewing ring. Functional Mechanical Characteristics Table 2 lists a very subjective assessment of the functional mechanical performance of the five prostheses. The scale of numbers chosen is only intended to confer relative differences among the various valves and is not meant to have any other significance. The loss of structural integrity has only been reported for 2 model 6120 mitral valves in which silicone ball variance developed, and for approximately 15 to 20 valves. The Start-Edwards 1260 aortic prosthesis, the standard valve (the only model available in the United States), the valve, and the valve have not been reported to suffer loss of structural integrity in the clinical setting. The lowest profile is presented by the valve, followed closely by the prosthesis, whose pivot guards make it somewhat taller. Although the two single-disc valves have a low profile in the closed position, they do have a higher profile in the open position than do the bileaflet designs. The caged-ball design of the valves gives them the highest profile. Rotatability is not of any consequence for the Starr- 1995 by The Society of Thoracic Surgeons 0003-4975/95/$9.50 SSD 0003-4975(95)00766-0

Ann Thorac Surg REVEW AKNS 1837 1995;60:1836-44 MECHANCAL VALVES Table 1. Mechanical Cardiac Valvular Prostheses Approved by the Food and Drug Administration Year Number List Valve Approved Design nserted Price 1965 Ball-cage 150,000 $4,300 (1260, 6120) 1977 Single tilting 170,000 $4,000 disc 1977 Bileaflet 600,000 $4,010 1978 Single tilting 45,000 $3,800 disc 1993 Bileaflet 110,000 $4,250 Edwards caged-ball design. Of the four disc valves, only the prosthesis cannot be rotated within its sewing ring, which could be disadvantageous in some circumstances. A rotatable version of that prosthesis is reportedly being currently tested. Freedom from occluder impingement is poorest for the valve, because the occluder seats horizontally at the equator of the housing and thus can be immobilized by retained valve remnants or sutures that are left too long. Complete closure of the single disc and the discs of the two bileaflet valves can be hindered by subvalve structures, sutures, or native valve tissue as well, but the leaflets are uncommonly totally immobilized. Like the single disc of the valve, the discs of the valve seat closer to the edge of the housing and are, therefore, not as protected as those of the valve. mpingement of the ball of a valve, although uncommon, can occur if sutures or valve fragments fall across the metal housing and keep the ball from completely seating. The and valves have acceptably low and essentially equal transvalve gradients, even in small sizes. The gradient across a Carbo- Medics valve is minimally higher, probably due to the fact that the leaflets are designed not to open quite as far as those of the valve. n smaller sizes the valves have almost unacceptable gradients, and the has gradient relief characteristics that place it between the valves and the other disc prostheses. Complete opening of a mechanical valve can be viewed in two ways: first, the completeness of rotation or translation of the occluder from the closed to the open position; and second, the length of time that the occluder stays open during that portion of the cardiac cycle when it is intended to be open. Complete opening of the single disc of the valve is virtually always achieved both in terms of rotation and translation and also in terms of maintenance of the open position. n the aortic position both discs of the and the valves rotate to the open position uniformly and remain open during systole. However, one could argue that a perfect complete opening score is not justified for these two bileaflet valves because increasing evidence suggests that in the mitral position, when a bileaflet valve is implanted in the anatomic orientation, an important percent of those valves will demonstrate biphasic partial closure of both leaflets during diastole ("diastolic fluttering") in patients with atrial fibrillation [2, 3]. n an important number of cases the disc of the valve does not rotate to the open position completely [4, 5]. Although complete opening of the ball in a valve is the norm, ball rebound off the distal cage can affect the secondary orifice of the cagedball design, and also the inertia of the ball may keep it from completely opening or closing at high heart rates. Dynamic regurgitant fraction, that part of valvular regurgitation that occurs before the occluder becomes seated in the housing, is lowest in the single-disc prostheses, the and, followed closely by the bileaflet designs, the and. The minimally lower score for the bileaflet valves relates to the reported clinical frequency of asynchronous closure of the two discs in bileaflet valves [2, 3]. The inertia of a ball delays its closure. Static leak rate, that part of valvular regurgitation that occurs once the occluder is seated, is essentially nonexistent for the valves. The and valves have a moderate built-in leak rate to flush the disc and housing. The increased length of the lines of closure between the discs and the housing in conjunction with the tolerances needed to fulfill the engineered requirements for washing the valve components give the and valves somewhat higher static leak rates. Thus, each prosthetic design has some very strong mechanical advantages but also some important limitations. Review of Long-Term Complications To the reports used to prepare the previous review on this topic have been added those studies published Fig 1. cardiac valvular prosthesis.

1838 REVEW AKNS Ann Thorac Surg MECHANCAL VALVES 1995;60:1836-44 Table 2. Mechanical Valve Functional Characteristics" Characteristic Structural integrity 4+ 5+ 3+ 5+ 5+ Profile 1+ 3+ 4+ 3+ 5+ Ability to rotate 0 5+ 0 5+ 5+ nterference with occluder 4+ 2+ 4+ 3+ 3+ Transvalve gradient 1 + 5+ 5+ 4+ 4+ Complete opening 4+ 5+ 4+ 3+ 4+ Dynamic regurgitant fraction 3 + 5 + 4+ 5 + 4 + Static leak rate 5+ 4+ 3+ 4+ 3+ -~ A grade of 5+ is the best and 0, the worst. through January 1995. The reports chosen had to use relatively standard definitions of valve-related complications [6], provide separate results for aortic and mitral prostheses, and contain enough information that the actual number of adverse events and the corresponding patient-years of follow-up could be accurately measured or closely approximated. As with the previous review, the statistical methods used in the available literature impose the use of linearized rates of complications; acknowledge the limitations of that method. There have been no new reports on the Start-Edwards valves. New sources of data were available for the [7-11 and my unpublished data], St. Jude Medical [12-20], and valves [21]. Data on the valve came from one large cumulative experience [22]. However, that multiinstitutional series reports only the linearized rates for "late" events. The author's contention was that because the FDA currently requires linearized rates for only those complications that occur more than 30 days after implantation and does not count any events that occur if the patient is never discharged postoperatively, reporting only "late" events is appropriate. Obviously the hazard function for most complications is highest in the early period after mechanical valve implantation and for prostheses with shorter periods of follow-up, "early" events will have a proportionally greater impact on the calculated linearized rates of complications. However, most reports about other prostheses include the "early" events, as suggested by the guidelines, and, therefore, the "early" events for the valve, which can be found in their company's published clinical update [231, have been included. The combined study populations for each type of prosthesis and the cumulative patient-years of follow-up are recorded in Table 3. The percentages in Table 3 demonstrate that we continue to have good long-term information on only a small sample of each valve type. As in the previous review [1], data are presented as a composite linearized rate for each complication, that is, the total number of all events from all applicable studies divided by the total patient-years of follow-up. The calculated rates from the previous study and the current report are both listed. Following each composite rate is the range of linearized rates reported, not the standard error of the mean. Each table also lists the number of patient-years of follow-up available to generate that particular composite linearized rate in addition to the range of 5- and 10-year actuarial event-free rates. Aortic Valves Nonstructural dysfunction, which comprises largely paravalvular leak and to a lesser extent hemolysis, is lowest in the aortic position (Table 4) for the Starr- Edwards valve, followed closely by the and valves. t is a little higher for the valve and further elevated for the valve. The incidence of thromboembolism in the aortic position (Table 5) is lowest for the valve, for which it has actually dropped between reports. Thromboembolism rate is a little higher and essentially currently equal for the,, and St. Jude Medical valves, while rising a bit between reports for the last prosthesis. The highest rate of thromboembolism is reported for the valve. n the case of the thrombosis subset of thromboembolism in the aortic position (Table 6) the valve has yet to have a reported case, and very low rates occur with the,, and St. Jude Medical valves. The incidence for the is minimally increased. The incidence of anticoagulant-related complications for aortic valves (Table 7) is reported to be the lowest now for the valve, but the number of patientyears of follow-up is very small. The rate of bleeding complications remains very low for the Table 3. Study Populations Number of Patient-Years Valve Valves Studied a of Follow-up 7,704 (5.1%) 37,479 4,479 (2.6%) 18,004 9,514 (1.6%) 36,676 1,174 (2.6%) 2,612 1,079 (1.0%) 2,656 a Numbers in parentheses are valves studied as a percentage of all valves of that model implanted.

., Ann Thorac Surg REVEW AKNS 1839 1995;60:1836-44 MECHANCAL VALVES Table 4. Aortic Valve Replacement: Nonstn ctural Dysfunction Rate Follow-up Valve and Year (Range) (Pt-y) 1991 0.2 (0.1-1.4) 7,681... 1995 0.2 (0.1-1.4) 7,681... 1991 0.5 (0-0.9) 1,761 100 1995 0.5 (0.1-1.4) 3,276 95-100 1991 0.5 (0-3.4) 2,281... 1995 0.4 (0-3.4) 10,234... 1991 2.4 (0.9-6.7) 746 98 1995 2.1 (0-6.7) 843 98 1995 0.8 (0.8) 1,553 95 99 97 Table 6. Aortic Valve Replacement: Thrombosis Rate Follow-up Valve and Year (Range) (Pt-y) 1991 0.2 (0.1-0.2) 15,072 95 1995 0.2 (0.1-0.2) 15,070 95 1991 0.2 (0-1.1) 6,248 94-100 1995 0.2 (0-1.1) 8,263 94-100 1991 0.2 (0-0.7) 4,737 99 1995 0.2 (0-0.3) 13,710 99 1991 0.5 (0-0.8) 611 96-100 1995 0.4 (0-0.8) 708 96-100 1995 0 (0) 1,553 100 76-91 76-91 100 valve. The,, and Carbo- Medics have moderately elevated and very similar rates, with the rate for the valve having fallen between reports. The,, and valves have very similar low rates of prosthetic endocarditis in the aortic position (Table 8) with little change between reports. The rate reported for the valve is somewhat elevated. The rates of reoperation for aortic prostheses (Table 9) remain lowest for the valve and are minimally elevated for the,, and valves. The earlier higher rate for the valve has decreased during the interval between reports. Mitral Valves The incidence of nonstructural dysfunction in the mitral position (Table 10) is lowest for the valve and minimally elevated for the,, and prostheses. The rate for the valve is somewhat higher. Thromboembolism in the mitral position (Table 11) remains lowest and unchanged for the valve and somewhat higher for the valve. The rates for the and Table 5. Aortic Valve Replacement: Thromboembolism Rate Follow-up Valve and Year (Range) (Pt-y) 1991 2.1 (1.4-3.3) 19,324 89-95 1995 2.1 (1.4-3.3) 19,324 89-95 1991 1.8 (0.8-4.7) 6,411 82-96 1995 1.4 (0.7-4.7) 9,443 82-96 1991 1.6 (0.7-2.8) 6,351 88-98 1995 2.0 (0.9-2.8) 17,242 88-98 1991 3.0 (1.9-5.1) 766 84-93 1995 2.7 (0-5.1) 863 84-93 1995 1.9 (1.9) 1,553 94 76-91 76-91 87 67-90 Table 7. Aortic Valve Replacement: Anticoagulation Complications Rate Follow-up Valve and Year (Range) (Pt-y) 1991 1.9 (0.8-3.1) 19,324 1995 1.9 (0.8-3.1) 19,324 1991 0.8 (0.7-2.6) 5,490 1995 0.9 (0.7-1.7) 6,027 1991 2.5 (0.2-7.9) 5,107 1995 2.2 (0-7.9) 14,845 1991 2.3 (2.3) 42 1995 0.7 (0-2.3) 139 1995 2.3 (2,3) 1,553 Pby = patient-years. 87-95 87-95 91 91-94 89 82-92 97 97 92 74-93 74-93 8O 73-95

1840 REVEW AKNS Ann Thorac Surg MECHANCAL VALVES 1995;60:1836-44 Table 8. Aortic Valve Replacement: Prosthetic Valve Endocarditis Rate Follow-up Valve and Year (Range) (Pt-y) 1991 0.7 (0.4-1.1) 18,761 95 92-97 1995 0.7 (0.4-1.1) 18,761 95 92-97 1991 0.4 (0-1.2) 5,490 100... 1995 0.5 (0.3-1.2) 6,323 96-100 96 1991 0.5 (0.1-2.1) 4,110 99... 1995 0.4 (0.1-1.7) 12,084 99 94-99 1991 1.6 (0-1.9) 255 94-96... 1995 1.4 (0-1.9) 352 94-96... 1995 0.4 (0.4) 1,553 98... Table 10. Mitral Valve Replacement: Nonstructural Dysfunction Rate Follow-up Valve and Year (Range) (Pt-y) 1991 0.3 (0.1-0.8) 9,051... 1995 0.3 (0.1-0.8) 9,051... 1991 0.4 (0.3-0.6) 2,171 100... 1995 0.7 (0-2.1) 2,824 93-100 83 1991 1.0 (0.7-2.2) 1,344... 1995 0.6 (0-2.2) 12,341... 98 1991 1.0 (0.4-1.9) 673 99... 1995 0.8 (0-1.9) 933 99... 1995 1.4 (1.4) 1,101 95... valves are increased a bit further, with the highest rates seen for the valve, for which prosthesis the incidence fell during the period between reports. The rate for the thrombosis subset in mitral valves (Table 12) remains very low for the,, and valves. Thrombosis rate in mitral valves is somewhat elevated, whereas the rate remains the highest for valves, even though the incidence has improved between reports. The incidence of anticoagulation-related complications for mitral valves (Table 13) remains lowest and unchanged for the valve and a little bit higher for the and valves. The rate is still higher for the valve and highest for the valve, having risen for that valve in the period between reports. Prosthetic endocarditis in mitral prostheses (Table 14) remains low in the,, and valves. The incidence is minimally higher for the valve and remains substantially elevated for the valve, despite an improvement between reports. Reoperation for mitral valves (Table 15) remains lowest for the valve and a little higher for the valve. The rates for the and valves are slightly higher and still con- Table 9. Aortic Valve Replacement: Reoperation Follow-up Valve and Year Rate (Range) (Pt-y) 1991 0.7 (0.4-3.7) 19,324 95 1995 0.7 (0.4-3.7) 19,324 95 1991 1.8 (0.4-2.1) 1,021 100 1995 1.5 (0.6-3.0) 2,536 90-100 1991 0.3 (0-0.9) 2,914 98 1995 0.4 (0-1.4) 10,417 98-99 1991 2.4 (2.4) 42... 1995 0.7 (0-2.4) 139... 1995 0.6 (0.6) 1,553 97 90-98 90-98 97 92-99 Table 11. Mitral Valve Replacement: Thromboembolism Rate Follow-up Valve and Year (Range) (Pt-y) Start-Edwards 1991 3.6 (1.5-5.7) 18,155 62-81 1995 3.6 (1.5-5.7) 18,155 62-81 1991 1.8 (0.5-4.2) 6,63; 84-89 1995 1.8 (0.5-4.2) 8,183 84-91 1991 2.4 (0.4-4.0) 5,237 89-99 1995 2.5 (0.4-4.4) 17,696 89-99 1991 5.1 (1.7-12.8) 942 90-97 1995 4.4 (1.7-12.8) 1,202 90-97 1995 3.3 (3.3) 1,101 91 55-91 55-91 91 77-90

Ann Thorac Surg REVEW AKNS 1841 1995;60:1836-44 MECHANCAL VALVES Table 12. Mitral Valve Replacement: Thrombosis Valve and Year Rate (Range) Follow-up (Pt-y) 1991 0.4 (0-0.5) 9,726... 1995 0.4 (0-0.5) 9,726... 1991 0.3 (0-1.1) 4,762 96 1995 0.3 (0-1.1) 5,915 96-100 1991 0.5 (0-0.8) 3,231 96 1995 0.2 (0.1-0.9) 14,437 96 1991 2.9 (0.4-9.4) 942 95-100 1995 2.3 (0.4-9.4) 1,202 95-100 1995 0.8 (0.8) 1,101 97 96 96 99 Table 14. Mitra Valve Replacement: Prosthetic Valve Endocarditis Valve and Year Rate (Range) Follow-up (Pt-y) 1991 0.4 (0.3-0.8) 16,026 95-97 1995 0.4 (0.3-0.8) 16,026 95-97 1991 0.4 (0-1.7) 3,286 100 1995 0.5 (0-1.7) 3,939 95-100 1991 0.4 (0.1-2.2) 3,559 98 1995 0.2 (0-2.2) 14,903 98 1991 2.4 (0-5.4) 507 98 1995 1.8 (0-5.4) 767 98 1995 0.7 (0.7) 1,101 96 92-98 92-98 88 98-99 siderably higher for the valve, even with an improvement in the interval between reports. Thromboembolism and Bleeding ndex The principal concern about mechanical valves remains their thrombogenic potential and the need for anticoagulation, essentially two sides of the same problem. ndeed, for each patient one might ask the question, at what cost of bleeding is one willing to seek no valve thrombosis and a low thromboembolic rate? The basis for this question is demonstrated in Figure 2, where one hopes that each patient can be managed to achieve his or Table 13. Mitral Valve Replacement: Anticoagulation Complications Rate Follow-up Valve and Year (Range) (Pt-y) 1991 1.7 (1.0-3.7) 16,026 82-93 1995 1.7 (1.0-3.7) 16,026 82-93 1991 1.1 (0.5-4.8) 3,286... 1995 1.2 (0.5-4.8) 3,732 91 1991 1.8 (0.3-2.9) 4,466 91-97 1995 1.7 (0.2-6.4) 16,679 90-97 1991 2.7 (2.7) 222 94 1995 3.5 (2.7-4.2) 482 94 1995 2.2 (2.2) 1,101 95 67-90 67-90... 86 81-98 her anticoagulation at point X, namely, no thrombosis and the lowest level of thromboembolism for the lowest level of bleeding complications. Some reports in the literature, particularly before 1985, reported thromboembolic rates without reporting the corresponding rates of anticoagulant-related hemorrhage. Data from these reports would be counted in the composite rate for thromboembolism, but not for anticoagulant-related bleeding. Such reports might tend to distort the calculated rates. Thus, have generated a new measure of mechanical valve thrombogenicity, the "'composite thromboembolism and bleeding index," by selecting only those reports that provided accurate data for both thromboembolism and anticoagulant-related bleeding. The composite linearized rate in this index is calculated by adding together Table 15. Mitral Valve Replacement: Reoperation Valve and Year Rate (Range) Follow-up (Pt-y) 1991 1.0 (0.6-1.7) 13,410 1995 1.0 (0.6-1.7) 13,410 1991 1.5 (1.3-1.7) 1,614 1995 1.6 (1.3-1.9) 2,267 1991 0.9 (0.6-3.0) 2,871 1995 0.6 (0.2-3.0) 14,132 1991 5.4 (5.4) 222 1995 2.5 (0-5.4) 482 1995 1.4 (1.4) 1,101 93-94 93-94 94 92-94 97-99 92-99 93 84-95 84-95 88 94-98

1842 REVEW AKNS Ann Thorac Surg MECHANCAL VALVES 1995;60:1836-44 rr >. -" L n- O uj N n" 2 <: W Z i Embolism Thrombosis... Bleeding ~bolism, X~ss Bleeding s S # # Table 16. Aortic Valve Replacement: Thromboembolism and Bleeding ndex Valve Follow-up Patients (Pt-y) Rate (Range) References 4,024 19,324 4.05 (2.41-5.80) [24-30] 1,450 6,027 2.77 (1.98-5.67) [31-33p St. Jude Medical 3,861 14,845 4.28 (0.87-10.67) [3, 13, 15-19, 30, 34-38] 81 139 2.16 (0-7.14) [21, 39] 603 1,553 4.25 (4.25) [22, 23] '~ ncludes my own unpublished data.. rombosis o 6 o NTERNATONAL NORMALZED RATO (NR) Fig 2. Relationship of level of anticoagulation to the incidences of thrombosis, thromboembolism, and bleeding for a hypothetical patient with a mechanical heart valve. X marks the ideal point of no thrombosis and the fewest thromboembolic events for the lowest level of anticoagulant-related bleeding. all thromboembolic and bleeding events from those reports that provide both numbers and dividing by the cumulative patient-years of follow-up from those reports. Studies that report data for only one of the complications are excluded. Further, in keeping with the previous design, the results for aortic and mitral valves have been separated. The composite thromboembolism and bleeding indices for aortic prostheses are recorded in Table 16, along with the total number of patients and patient-years of follow-up available to generate that number for each model of valve. The lowest composite index for aortic valves is reported for the valve, but this number is generated from data available for only 81 patients and 139 patient-years of follow-up. For the other valves for which there is much better follow-up, the lowest index is reported for the valve at 2.77% per patient-year. The rate for the valve is considerably higher, followed by almost identical rates for the bileaflet and valves. Table 17 contains the composite thromboembolism and bleeding rates for mitral valves. The lowest incidence is reported for the valve, followed closely by the prosthesis. The and valves have moderately higher rates, with a much increased rate for the valve, albeit again with a limited number of patients. Whether this new index proves to be of value to surgeons in choosing among various valves remains to be seen. One would hope that at the very least all future reports on the long-term results with mechanical valves would clearly provide information about both thromboembolic and bleeding complications. Comment The findings in this review are generally similar to those reported in 1991. Although the models 1260 and 6120 valves have demonstrated noteworthy durability, their poorer gradient relief and somewhat elevated incidence of valve-related complications make them not truly competitive with current disc prostheses. Results reported with the valve, although widely disparate, do not favor its hemodynamic performance or its freedom from complications. The and, to a lesser extent, the valves continue to be the most popular mechanical prostheses in the United States because of their clinically documented excellent and comparable hemodynamic performance in addition to their acceptably low rates of valve-related complications. f there are any differences between the long-term performance of Table 17. Mitral Valve Replacement: Thromboembolism and Bleeding ndex Follow-up Rate Valve Patients (Pt-y) (Range) References 3,242 16,026 5.47 (4.10-9.37) [27, 28, 40, 41] 1,083 3,732 3.99 (2.04-7.66) [9, 31-33, 42] "~ St. Jude Medical 3,850 16,968 4.16 (0.35-10.81) [3, 9, 13-19, 35-38, 43] 198 482 8.09 (6.15-10.36) [21, 42] 476 1,103 4.99 (4.99) [22, 23] " ncludes my own unpublished data.

Ann Thorac Surg REVEW AKNS 1843 1995;60:1836-44 MECHANCAL VALVES these two prostheses, it may be that the valve may have a modest advantage in terms of diminished thrombogenicity, whereas the valve has a lower reported rate of reoperation. The major potential drawback of the valve remains its reported loss of structural integrity in occasional patients, albeit a very low percentage, whereas the primary shortcoming of the valve is still the attention required of the surgeon at the time of insertion to avoid occluder impingement. Finally the valve, the new addition to the list, probably fits into the ranking just behind the and valves on the basis of somewhat higher transvalve gradients and increased incidence of thromboembolism despite adequate anticoagulation, especially in the mitral position. Longer follow-up will be necessary to accurately determine whether the valve or the valve will become truly competitive with the Medtronic- Hall and valves. This report does not provide any determinations of statistical significance comparing the results with different prostheses. Any retrospective, cumulative study such as this is potentially subject to major problems with statistical reliability. The numbers generated for the reported complications for each prosthesis are clearly dependent on the validity of the individual studies, which were often performed according to different protocols. n addition the characteristics of the different patient populations that constitute the various study groups are obviously widely disparate. have, therefore, never felt comfortable applying univariate statistical methods to this multivariate problem, namely, comparing the differences between observed results with different prostheses from differing populations, collected by various methods. One particular difficulty uncovered in this review warrants some emphasis and demonstrates how results with the same valve used in different institutions can produce conflicting results. Recent reports of results with the St. Jude Medical valve from Japan and Germany, two highly developed countries with well-organized health care systems, are examples of this apparent anomaly. Beginning with aortic prostheses, Aoyagi and colleagues [18] in Kurume, Japan, report the linearized rates for thromboembolism, anticoagulantrelated bleeding, and the composite thromboembolism and bleeding index to be 1.0%, 0.4%, and 1.3% per patient-year, respectively. Similarly, Nakano and associates [17] from Tokyo, Japan, report the linearized rates for thromboembolism, anticoagulant-related bleeding, and the composite thromboembolism and bleeding index for aortic valves to be 1.4%, 0.1%, and 1.5% per patient-year, respectively. n contrast Horstkotte and co-workers [16] from Dusseldorf, Germany, report for their aortic prostheses the linearized rates of thromboembolism, anticoagulant-related bleeding, and the composite thromboembolism and bleeding index to be 2.7%, 4.1%, and 6.8% per patient-year, respectively. n addition the German data do not contain events that occurred within the first 3 months after valve implantation, nor do they contain more than the first event for patients who suffered multiple events! For mitral valves Aoyagi and colleagues [18] report the linearized rates of thromboembolism, anticoagulant-related bleeding, and the composite thromboembolism and bleeding index to be 1.0%, 0.3%, and 1.2% per patient-year, respectively. Also, Nakano and associates [17] report their linearized rates for thromboembolism, anticoagulant-related bleeding, and the composite thromboembolism and bleeding index for mitral valves to be 1.7%, 0.2%, and 1.9% per patient-year, respectively. However, the German report [16] yields greatly different results for mitral prostheses with the linearized rates for thromboembolism, anticoagulantrelated bleeding, and the composite thromboembolism and bleeding index being 4.4%, 6.4%, and 10.8% per patient-year, respectively, with early and multiple events again excluded. How is one to account for this extraordinary variability in results with the same prosthesis used in different high-quality health care systems? There is no clear answer. We have learned certain lessons about reporting results with mechanical heart valves over the years. First, the more frequently one interviews patients, the greater will be the number of incidents remembered by the patients, and thus the higher the incidence of complications. Second, the thromboembolic and bleeding complications recorded in any study may be important functions of the individual prosthesis, but they are probably also functions of other variables, for example, patient age, diet, inherent hematologic function, type of valvular disease, the capabilities of the person managing the patient's anticoagulation, or the intensity with which the information about complications is sought. One other issue raised by this review is the efficacy of the previously published guidelines for reporting morbidity and mortality after cardiac valvular procedures [6]. The impact of other variables, especially patient factors, on the complications attributed to mechanical prostheses is becoming increasingly clear. We may be at a time when the previous guidelines need to be revised to achieve a more thorough assessment of the impact of other risk factors on the complications ascribed to mechanical cardiac valvular prostheses. f a review such as this has any usefulness, it is that first, it brings together information from many studies to increase the sample size for each prosthesis; second, it may serve as a standard against which one may compare future results; third, it may serve as a stimulus for other cardiac surgical groups to investigate and report their results; and finally, it may encourage us to reassess the methods we use to evaluate mechanical valves. References 1. Akins CW. Mechanical cardiac valvular prostheses. Ann Thorac Surg 1991;52:161-72. 2. Feldman HJ, Gray RJ, Chaux A, et al. Noninvasive in vivo and in vitro study of the St. Jude mitral valve prosthesis. Am J Cardiol 1982;48:1101-9.

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