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Twenty-Five
作者：Twenty-Five
【关键词】& valves,;,prosthesis,;,risk,factors,;,epidemiology,;,surgery
&&& the International Epidemiology Institute (W.J.B.), Rockville, Md&&& Johns Hopkins University (M.A.I., R.B.), Baltimore, Md&&& the Cardiac Surgery Institute (T.D.I.), Cincinnati, Ohio&&& London, England (D.E.A.)&&& Massachusetts General Hospital (A.W.), Boston, Mass&&& Saint Antonius Hospital (J.D.), Nieuwegein, the Netherlands&&& Trustees Bowling-Pfizer Settlement Funds (J.K.S.), Cincinnati, Ohio&&& the University of Cincinnati (D.H.), Cincinnati, Ohio.
&&& Abstract
&&& Background― The first Bj;rk-Shiley convexoconcave (BSCC) prosthetic heart valves were implanted in 1978. The 25th anniversary provided a stimulus to summarize the research data relevant to BSCC valve fracture, patient management, and current clinical options.
&&& Methods and Results― Published and unpublished data on the risks of BSCC valve fracture and replacement were compiled, and strategies for identifying candidates for prophylactic valve reoperation were summarized. By December 2003, outlet strut fractures (OSFs), often with fatal outcomes, had been reported in 633 BSCC valves (0.7% of 86 000 valves implanted). Fractures still continue to occur, but average rates of OSFs in 60° valves are now <0.1% per year. OSF risk varies markedly by valve characteristics, especially valve angle and size, with weaker effects associated with other manufacturing variables. OSF risks are mildly lower among women than men but decline sharply with advancing age. The risks of valve replacement typically greatly exceed those of OSF. By comparing individualized estimated risks of OSF versus valve replacement, guidelines have been developed to identify the small percentage of BSCC patients (mostly younger men) who would be expected to have a gain in life expectancy should reoperative surgery be performed.
&&& Conclusions― Twenty-five years after the initial BSCC valve implants, fractures continue to occur. Continued monitoring of BSCC patients is needed to track and quantify risks and enable periodic updating of guidelines for patients and their physicians.
&&& Key Words: surgery
&&& Introduction
&&& The Bj;rk-Shiley convexoconcave (BSCC) prosthetic heart valve was developed to provide effective relief, with minimal complications, for patients with diseased native valves.1 Two primary forms of the valve were produced. One had a 60° opening angle that was approved for use by the United States Food and Drug Administration in 1979. The second, introduced somewhat more than a year later but never approved for use in the United States (although the manufacturer was granted an export license), had a 70° opening angle. Figure 1 shows the BSCC valve with its tilting disc held in place by a large, fixed, inner strut and a smaller, welded, outlet strut. Fractures of the BSCC prosthetic heart valve’s outlet struts and escape of the disc, resulting in embolization, massive regurgitation, and often death, began to be reported shortly after the valve’s introduction, leading to withdrawal from the market of 70° valves in 1983 and of 60° valves in 1986. Epidemiological data to quantify the annual incidence and cumulative probabilities of outlet strut fracture (OSF) first arose in the 1980s and have been updated periodically thereafter, although some of the key relevant data have appeared only in unpublished technical reports. Herein we review the available data, including information not previously published in the open scientific literature, on valve fractures among BSCC patients, noting strengths and limitations. We report on the 25-year history of OSFs among BSCC patients, the risks involved in reoperations to replace the BSCC valves, and the attempts to develop guidelines to determine eligibility for monetary compensation and to assist in objective decisions about valve replacement surgery. The review is still timely, because options for the management and care of BSCC patients remain a current issue in clinical medicine as the second quarter-century of follow-up of the patients begins.
&&& See p 2717
&&& Methods
&&& We reviewed the scientific literature and available data pertinent to the incidence and risks of fracture and replacement of BSCC heart valves. The Trustees of the Bowling-Pfizer Heart Valve Settlement Funds (hereafter referred to as Trustees), a group established by court settlement to conduct research to help benefit the class of BSCC patients,2 provided access to their extensive files of all research involving BSCC patients. The settlement led to the formation of a Supervisory Panel of knowledgeable physicians charged with assembling existing data, commissioning and reviewing new research, estimating future risks of valve fracture among BSCC patients, and developing criteria and guidelines under which patients would receive monetary compensation for prophylactic reoperations to replace their BSCC valves. We also conducted computerized PubMed searches to ensure completeness of information on the risks of BSCC valve fractures and on recent studies of prophylactic reoperations, searching on the phrases Bjork-Shiley convexoconcave (or convexo-concave) heart valve, mechanical (or prosthetic) heart valve redo operation, mechanical heart valve redo operation, mechanical heart valve reoperation, and prosthetic heart valve reoperation/epidemiology.
&&& Data on the risks of valve fracture have arisen primarily from 2 sources: cohort studies involving individual follow-up of patients with BSCC valves and passive-surveillance registry of OSFs reported to the manufacturer (Shiley and then Pfizer).3,4 For this review, we did not conduct independent pooled or meta-analyses of the fracture data from the individual cohort studies but focused on the passive-surveillance system because it represents the single source of worldwide data on OSFs, and the results from the cohort and passive-surveillance studies were generally similar.
&&& Periodically, updated passive-surveillance OSF data are provided to the Trustees for calculation of rates of OSFs, with reported OSFs as numerators and estimated valve-years of follow-up as denominators. The strength of the surveillance system is its international coverage. Even though most of the patients are not known, rates of OSF can be estimated by assuming an overall age and sex distribution similar to that in the known sample and assuming that the total patient population experiences underlying mortality similar to that of other groups of heart valve patients or to the observed long-term experience of the large cohorts of BSCC patients in the United Kingdom, the Netherlands, and the United States.5,6 A weakness in the data is the incomplete identification of fractures. Because it is believed that not all OSFs have been reported to Pfizer, upward adjustments of the estimated rates have been made to account for underreporting. The adjustments are based on the assumption that the higher observed rates of OSFs in the Netherlands and the United Kingdom, where surveillance of BSCC patients is active and thought to be more complete, are attributable to more complete detection of OSFs. Thus, the worldwide baseline incidence of OSF is assumed to equal the higher rate reported in the Netherlands and the United Kingdom. An additional 10% upward adjustment of the estimated OSF rate has been made because even in the Netherlands or United Kingdom, some OSFs may have been missed.7,8
&&& Information quantifying postoperative risks for BSCC patients who elected to have their valves replaced is very limited. The data arise primarily from a US/UK x-ray imaging survey for the detection of fracture of 1 of the 2 legs of the outlet strut9,10 and from the records of other prophylactic reoperations in the Netherlands and the United Kingdom. To overcome this shortcoming, published literature on redo non-BSCC prosthetic valve replacement operations were reviewed, as were unpublished data on reoperative risks among functional class I and II patients provided to the Trustees. The latter was necessitated because of the limited data for persons who had non-BSCC prosthetic valves replaced for prophylaxis or under circumstances that would be similar to those faced by BSCC patients considering elective replacement of a valve believed to be at high risk of failure. We did not conduct pooled or meta-analyses of the reoperative data but report results from the large studies that assessed prophylactic replacement of prosthetic heart valves.
&&& Results
&&& The manufacturer has estimated that 86 000 BSCC valves were shipped, not returned, and are believed to have been implanted in patients around the world between 1978 and 1986. The large majority (nearly 82 000) were 60° valves. Approximately 4000 70° valves were exported by the manufacturer and implanted in patients outside the United States during 1981 to 1983.
&&& Rates of OSF
&&& Annual rates (numbers of OSFs per person-years of follow-up) have been reported in the individual cohort studies. Herein we present the worldwide data from the reporting system maintained by the manufacturer, because the surveillance covers all BSCC valves and the widest time period, extending through 2003. As of the end of 2003, a total of 633 OSFs (cumulative 0.7% of all valves), with 479 in 60° valves (0.6% of the implanted 60° valves) and 154 in 70° valves (3.9% of the 70° valves) had been reported to Pfizer.4 On an annualized basis, taking into account predicted reductions in the size of the population due to mortality, the overall average rate of reported OSF during 1979 to 2003 was 0.05% per year for 60° valves and 0.32% per year for 70° valves.4 Adjusting for suspected underreporting raised the estimated annual rates to 0.14% and 0.72% per year, respectively.
&&& Risk Factors for OSF
&&& Recent analyses of manufacturing records describing the production of individual valves suggest that some additional characteristics of the manufacturing process may predict the risk of OSF.15,25,26 The most prominent among such variables was having >1 hook deflection test (a quality-control process to test for flexibility of the valve’s outlet strut) performed. Having >1 of these tests tends to be highly correlated with the rework status of the valves (an indicator of whether the valve was known to have received additional manipulation, such as smoothing of cracks or rewelds), a modest but significant predictor of OSF.6
&&& The major patient characteristic thus far identified as a risk factor for OSF is the age of the patient (in some studies, age at implantation was the variable assessed).5,6,13,22 The risk of OSF decreases by 5% with each increasing year of age. The trend in risk with the current age of the patient is shown in Table 3 with data from the Dutch/UK/US combined cohorts, for which <3% of the fractures occurred among persons aged 65 and older at the time of the OSF. Two studies8,22 suggest that risk rises with increasing body size and surface area. Such a relation would indicate higher risks of OSF among men than women, and the cohort studies provide evidence that the risk of OSF among women is only about 60% of that among men.5,12
&&& The population of BSCC patients is aging. By 2004, more than two thirds of the original BSCC valve implant population is estimated to have died.4 The high mortality arises in part because of natural forces of mortality and in part because of the patients’ underlying heart disease. At the time of BSCC heart valve implantation, the median patient age was 57 years, but among current survivors, the median age is estimated to be slightly >70 years. The aging of the BSCC heart valve population alone will result in a substantial reduction in the overall (crude) incidence of OSF in the coming years compared with what was experienced in the 1980s and 1990s. Analyses of age-adjusted temporal trends in OSF incidence from the combined Dutch/UK/US cohort data, however, suggest that there has been only a little decrease in risk over time beyond that associated with aging.5
&&& It should be noted that an OSF does not always result in fatality. Indeed, it has been estimated that perhaps 35% to 45% of patients with fractured mitral valves survive because they can receive emergency surgery, although the large majority (>90%) of aortic valve fractures result in death.2
&&& Reoperations of Non-BSCC Valves
&&& In our review of published articles describing risks from reoperations to replace artificial (biological or mechanical non-BSCC) valve prostheses, a wide range of early (usually defined as in-hospital or within 30 days of surgery) mortality after reoperative procedures to replace heart valve prostheses was reported. Table 4 shows the largest of these studies,2837 namely, those reporting outcomes from >300 (and up to 200031) redo operations. After excluding emergency operations when indicated, the median overall in-hospital mortality was 10%, a figure nearly double the 5% in-hospital mortality commonly reported to be associated with the initial operation to implant an artificial valve.38 Few of the surveys reported 90-day mortality, so deaths occurring within 1 to 2 months after hospital discharge may have been missed. Furthermore, most studies included only in-hospital deaths, so some in-home deaths within 30 days of surgery also may have been missed. Some of the studies reported temporal trends, generally but not always36 showing somewhat smaller mortality in more recent years.
&&& Non-BSCC Valve Reoperative Risks by Functional Class
&&& Besides the results of Piehler et al,31 published data on reoperative mortality among patients in otherwise excellent health are scant. Recent unpublished data especially compiled for and reported to the Trustees from the Cleveland Clinic39 and from a consortium of US surgery centers,40 however, show an in-hospital mortality of 2% to 3% after prosthetic valve replacement surgery among subgroups of 204 and 709 patients, respectively, predominantly those in NYHA functional class I and II with no comorbidity. Reoperative mortalities of 7% to 9% are commonly reported in the studies listed in Table 4 among bioprosthetic valve patients seeking replacement because of deterioration of the biological valve, although patients with such replacements tend to be older, and nearly every study has shown reoperative risk rising with age.
&&& In addition to mortality, other adverse outcomes of reoperations to replace heart valve prostheses include myocardial infarction, stroke, renal failure, and other morbidity. Information to quantify risk factors for these outcomes is sparse, although the prevalences of several types of morbidity have been reported in several studies. The prevalences of myocardial infarction, stroke, and renal failure have each ranged from 1% to 6% in the few studies reporting such rates. The large US study31 did not list prevalences of these conditions but noted persistent neurological deficit in 1% of patients. In total, the likelihood of serious morbidity is believed to at least equal that of mortality after replacement surgery. As indicated later, this concept of equal mortality plus serious morbidity was used in developing guidelines about compensation for elective BSCC valve replacement. Published data show that the reoperative surgeries required a median hospital stay of 10 days. Furthermore, after replacement of the BSCC valve, the patients still had a prosthetic valve, with the accompanying risks of embolic stroke, endocarditis, and perivalvular leaks.
&&& Elective Reoperations of BSCC Valves
&&& Published information on prophylactic reoperations among BSCC patients is limited. In the Netherlands, the 30-day mortality after the initial implantation of 2500 BSCC prosthetic valves was 7%, with an 8% mortality among 124 patients who underwent nonemergency elective redo operations to replace their BSCC valve.41,42
&&& In the early to mid-1990s, a cineradiographic imaging survey was conducted among persons believed to be at elevated risk for OSF, predominantly patients with large (29+ mm) mitral valves.9 The radiographic images sought evidence of fracture of 1 of the 2 legs of the outlet strut, under the assumption that such single-leg separations (SLSs) were precursors to OSF. Nearly all persons with SLSs detected opted to have the valve replaced. The patients selected for enrollment in the survey were thought to be generally healthy individuals, although a subsequent follow-up revealed that more than half had been recently hospitalized and half had difficulty walking up >1 flight of stairs.43 Nevertheless, the patients form a group that may be representative of persons considering prophylactic replacement of their BSCC valve, and thus, the reoperative mortality experience may have predictive value.
&&& Unpublished data on postsurgery mortality from the imaging study9,10 are available for 33 patients with SLSs and 24 others who had elective explantation. The information was collected by the Trustees as part of the initial follow-up of the study participants. The average ages at explant among the 2 groups of patients were, respectively, 61 and 52 years. In addition, the cohort studies of the BSCC patients identified 53 Dutch patients42,44 and 19 British patients (unpublished data) undergoing prophylactic reoperations at average ages of 49 and 55 years, respectively. In combination, among these 129 BSCC redo patients, 10 deaths (8%) occurred within 90 days of the surgery (Table 6). The reoperative mortality varied with age, being significantly lower among younger patients.
&&& Comparison of Risks of OSF Versus Reoperation for BSCC Patients
&&& Several published reports have attempted to systematically compare the estimated risks of OSF versus those of reoperations to replace BSCC valves.27,42,4450 Blackstone and Kirklin45 estimated predicted survival probabilities for those with versus those without reoperations. Mortality after reoperation was assumed to equal mortality from the original valve implantation, so the risks of reoperation may have been underestimated. Nevertheless, the authors recommended that BSCC valves in general should not be recalled for prophylactic explantation. Other reports used future life expectancy as the index on which to compare effects of reoperation versus nonreoperation.27,4648 These estimations used data from the European cohort studies27,4549 or Shiley passive-surveillance studies32 to estimate the risks of OSF and data on mortality after the initial operations4749 or reoperations27,46 on non-BSCC heart valves to estimate the risks of explanting BSCC valves. Each showed that age was the key factor, with reoperations to replace BSCC valves predicted to result in longer life expectancies only for younger patients in groups at high risk for OSF. A recent review in the Netherlands indicated that such guidelines for BSCC valve replacement have been widely implemented in clinical practice.50
&&& Bowling-Pfizer Guidelines
&&& In 1997, new guidelines were promulgated by the Bowling-Pfizer Trustees and Supervisory Panel to provide a means for authorizing monetary compensation for the costs of BSCC valve replacement surgery and secondarily to provide background information that might assist patients and their physicians in deciding whether BSCC heart valves should be replaced. The guidelines were amended in 2000 and again in 2003.6 The considerations were based on comparisons of risks of valve fracture versus risks from reoperations to replace the valves and life expectancy under each scenario. The settlement language required that monetary compensation for reoperation be provided only if a meaningful gain in life expectancy would be predicted to result from BSCC valve replacement, under the assumption that the patients were otherwise in optimal health. In the 1997 guidelines, persons with a gain in life expectancy of 2 or more years were considered eligible for reoperative benefits from the Trust funds. The 2-year period was adopted to account for recovery time and for expected operative morbidity. In the subsequent guidelines, persons with any positive expected gain in life expectancy were considered eligible.
&&& The guidelines6 estimated the risk of OSF on the basis of the passive-surveillance data available to the manufacturer and supplied to the Trustees, together with data from the cohort studies, and calculated predicted life expectancies with versus without reoperation as a function of age, sex, and valve position. The estimations of fracture risk (and subsequent life expectancy) can be made for individual BSCC patients whose valve characteristics are known (these characteristics can be readily ascertained if the valve serial number is known). Reference can then be made to a table of critical values corresponding to rates of OSF above which there is an expected gain in years of life after reoperation to replace the BSCC valve. These OSF threshold values were determined by taking into account the predicted underlying age-specific overall mortality of the heart valve patients, together with the predicted rates of mortality and serious morbidity from prosthetic valve replacement surgery. The overall mortality predictions were based on mortality observed thus far among BSCC patients, and the reoperative risk predictions were based on the available data among persons considered to be in excellent health.5,6 The reoperative risks were assumed to vary with patient age according to a quadratic relation (as shown most clearly in the large American study of redo operations31), ranging from 3.6% at age 35 to 6% at age 58 and 12.2% at age 80 years. The figures, eg, 6% at age 58 years, were the sum of equal average postoperative mortality (3%) plus serious morbidity (3%) for patients in otherwise optimal health. The determination of whether an individual patient qualifies for reoperative benefits from settlement funds can be made by calling the Claims Administrator toll-free at 1-800-977-0779 and providing information about the patient’s date of birth, sex, implant position, and valve serial number (or numbers, if there are both mitral and aortic BSCC implants).
&&& Discussion
&&& The BSCC heart valve is associated with more known serious adverse outcomes than any other implanted medical device. BSCC valves began to be adopted for general use in large numbers in 1979, with nearly 86 000 valves implanted before their withdrawal from the market in 1986. In the ensuing 25 years, >600 patients are known to have had valve fractures, often with catastrophic outcomes. Natural forces of mortality among heart valve disease patients have decreased the current worldwide BSCC patient population to near 22 000 survivors, still a large number requiring clinical management and care in the coming decades.
&&& Research to identify persons at high risk of valve fracture has spanned >2 decades. The hope has been to ascertain and delineate valve and/or patient characteristics that might lead to an increased risk of OSF. Then, those found to be at sufficiently elevated risk of OSF become candidates for valve replacement (and, since the settlement, for monetary compensation from the settlement funds), with surgical options evaluated by the patients and their physicians and taking into account individual patient circumstances. The research to date has identified several valve-related factors and a smaller number of patient characteristics that contribute to elevated fracture risk. It is now clear that rates of OSF decrease with advancing age. This in part may be caused by underdetection of OSF in the elderly, but predominantly it seems to reflect reduced fracture risks from lower stresses on the valves’ outlet struts as patients age. Because reoperative risks also rise with age, only younger persons (mainly those now <65 years) are viable candidates for replacement surgery. Indeed, on the basis of the current guidelines,6 few patients older than 60 years were eligible for monetary compensation from settlement funds for valve replacement surgery.
&&& On the basis of the cumulative evidence to date, the large majority of patients with BSCC valves will never experience a valve fracture. The small percentage believed to be at high risk because of their individual valve characteristics and who are known because they are members of the settlement class have been notified for consideration of surgical options. Any patient (or his/her physician) can obtain his/her estimated annual probability of fracture by calling the Claims Administrator at the number given earlier. Unfortunately, valve fractures are likely to continue to occur in some patients who are not at high risk, simply because the pool of such patients is so large. Even with a low annual OSF risk of 1 to 5 in 10 000, some fractures are likely to occur. However, the considerably higher risks of serious adverse outcomes from reoperation, which even for good surgical candidates may be 1 to 5 per 100, tend to rule out surgical intervention. Hence, cardiologists and cardiothoracic surgeons need to maintain awareness of the BSCC problem.
&&& Although rates of OSF have abated as the cohort of BSCC patients ages, with a median age now exceeding 70 years, some patients who have a high projected fracture risk and who have good cardiovascular function are likely to benefit from prophylactic replacement. To provide the necessary information for informed decisions by these and other patients, the cohort of BSCC prosthetic heart valve patients requires continual follow-up to monitor trends in valve fracture and potential improvements in surgical outcomes. The 25-year history of the BSCC heart valve provides a basis for prediction of future risks and a means to develop and refine approaches for the benefit of this group of patients worldwide.
&&& Acknowledgments
&&& Funding for this research was provided by the Trustees of the Bowling-Pfizer Heart Valve Settlement Funds (Cincinnati, Ohio), a group appointed by the US Federal Court to oversee and arrange for research for the benefit of patients with BSCC heart valves. We dedicate this manuscript to the memory of one of our coauthors, Mr J. Kermit Smith, the Chairman of the Trustees’ Supervisory Panel from its inception until his recent death.
&&& Disclosure
&&& All authors served as members of or consultants to the Supervisory Panel of the BSCC Heart Valve Settlement.
&&& Footnotes
&&& Deceased.
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