Hypercholesterolemia is a risk factor for bioprosthetic valve calcification and explantation

Article Outline

• Abstract
• Methods
  • Historical cohort
  • Case-control study
  • Statistical analyses
• Results
  • Cohort analysis
  • Case-control analysis
  • Discussion
• Acknowledgements
• References
• Biography
• Copyright

Abstract 

Objective

There are pathophysiologic similarities between calcification and atherosclerosis. We wished to determine whether risk factors for atherosclerosis were linked to bioprosthetic valve calcification and dysfunction.

Methods

We performed a retrospective cohort study on 144 patients at a single institution who had bioprosthetic aortic or mitral valves removed, serum cholesterol levels recorded, and valve calcification assessed on the basis of hematoxylin and eosin staining and radiography of the valve. We also performed case-control analysis of a group of 66 patients whose tissue valves were explanted and compared them with an age- and position-matched group of 66 patients with similar duration of implantation. We also compared mean serum cholesterol levels.

Results

In the retrospective cohort study cholesterol (P = .035), younger age at implantation (P = .014), and coronary artery disease (P = .017) were linked to calcification of the valve by means of univariate analysis. In stepwise multiple regression analysis only the mean serum cholesterol level was linked to calcification (P = .02). Sex, hypertension, smoking, diabetes, and implant position were not linked to calcification. In the case-control analysis the mean serum cholesterol level of the explanted valve group was significantly higher (189 vs 163 mg/dL, P < .0001) than that of the group whose valves did not require explantation. For those whose serum cholesterol levels were greater than 200 mg/dL, the odds ratio was 3.9 (95% confidence interval, 1.7-8.9) for valve explantation.

Conclusions

Increased serum cholesterol level may be a risk factor for bioprosthetic valve calcification requiring explantation.

Keywords:  16, 35

Bioprosthetic valve replacement has improved the outcomes of patients with valvular heart disease. Tissue valves have central flow and a high freedom from thrombus formation, usually without the need for anticoagulation. However, most tissue valves are altered biologic materials that undergo degenerative processes.¹, ²

A study of the causes of valve failure necessitating reoperation at our institution has reported that sterile degeneration, or primary tissue failure, accounted for 74% of bioprosthetic valve failures.³ Primary tissue failure was most often related to calcification of the tissue cusps. Calcification was present in 86% of patients, and cuspal defects in 79% of bioprosthetic valves were removed for primary tissue failure. Seventy-five percent of valves with degenerative failure had both calcification and tears.

No medical therapy, as of yet, has been demonstrated to be effective in the prevention of the progression of bioprosthetic calcification. Recently, a compelling argument has been espoused demonstrating the similarities between calcific aortic sclerosis in the native valve and atherosclerosis.⁴, ⁵ Chronic inflammatory changes in the leaflet, including cellular infiltration of macrophages and T lymphocytes and lipoprotein deposition in apposition to areas of calcification, has prompted investigation as to whether medical therapy can be applied to slow the progression of valvular failure.

Several studies have documented a correlation between lipids and calcification. Pohle and colleagues⁶ have documented that both coronary calcification and aortic valve calcification progress more rapidly in subjects with low-density lipoprotein (LDL) levels of greater than 130 mg/dL. Callister and associates⁷ demonstrated that patients who reduced their cholesterol levels with lipid-lowering agents significantly reduced the progression of coronary calcification. In another study the risk factor that most significantly correlated with coronary calcification was LDL levels, and this was stronger than age, male sex, blood pressure, fasting insulin level, or smoking.⁸ Native aortic valve calcium accumulation, as assessed by means of electron beam computed tomography, is diminished by statin use.⁹

We hypothesized that serum cholesterol levels might be associated with increasing bioprosthetic valve calcification, as assessed by means of a retrospective cohort study. We also questioned whether increased serum cholesterol levels are associated with a higher incidence of valve explantation, as assessed by means of a case-control study.

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Methods 

For the cohort study, as well as for the case-control analysis, the Brigham & Women's Hospital Institutional Review Board approved the study. Exclusion criteria included the following: (1) lack of serum cholesterol measurement; (2) a bioprosthetic valve in the tricuspid position; or (3) rapid valvular failure (<1 year, deemed to be due to a technical cause). No patients were known to have renal failure or secondary hyperparathyroidism. Total serum cholesterol levels were available in all study patients, and our laboratory's normal range is 140 to 230 mg/dL. Because patients had multiple cholesterol measurements while the valve was in place, the mean serum cholesterol level was obtained by determining the average in any single year and then averaging together all years during which the valve remained implanted. Cholesterol values assayed during a period up to 1 month after cardiac surgery were discarded. Coronary artery disease was defined by a history of myocardial infarction, angioplasty, or cardiac catheterization identifying a greater than 50% stenosis. Diabetes mellitus was defined by a need for either oral or parenteral hypoglycemic agents. Hypertension was defined as medical treatment with more than one antihypertensive agent. Smoking was self-reported, and patients were divided into former smokers and smokers at the time of valve implantation.

Historical cohort 

We performed a retrospective cohort study of all patients referred to our single center over 10 years (1992-2002) for individuals who had their bioprosthetic aortic or mitral valves removed and who had serum cholesterol levels (excluding up to 1 month after valve surgery) measured during the period when the valve was in use. One hundred forty-four patients met the inclusion and exclusion criteria. We reviewed the medical records for the pathologic assessment of calcification at the time of explantation. Calcification was assessed by means of a combination of histologic and radiographic methods. Valves were radiographed for calcium and stained with hematoxylin and eosin after decalcification of the tissue for sectioning. A grading scheme of 0 to 4+ was reported, with heavy calcification given a rating of 4+ and no calcification given a rating of 0.

Case-control study 

We performed a case-control study of patients referred to our single center over 10 years (1992-2002) for individuals who had their bioprosthetic aortic or mitral valves removed and who had serum cholesterol levels measured while the valve was in place. Control subjects were matched to patients for (1) duration of valve survival, (2) age of the patient at the time of implantation (within 1-5 years of patient age), and (3) valve position (aortic vs mitral) by using the Brigham Cardiac Surgery Database. We matched patients against our computerized prospective database from 1992 onward, using a computerized sorting algorithm that first sorted by duration of survival (for control subjects) or removal (for patients), then by age at implantation, and finally by position. Fewer patients are enrolled in the case-control analysis than in the cohort analysis because of adequate statistical power with approximately 70 patients in each group and difficulty in finding adequate control subjects for all patients. It should be noted that the same patients in the cohort analysis were used as patients here. Matched control subjects, however, are a different group of patients and are not represented in the group of patients in the cohort analysis.

Patients and control subjects were temporally mismatched, on average, by 6 years because the mean ± SD year of birth for the patients was 1930 ± 1.8 versus 1936 ± 1.7 for control subjects (P < .001). Thus the age of the patient at implantation is matched, but patients represent an earlier surgical population. We performed 2 additional analyses to investigate whether this had any effect on mean cholesterol levels between patients and control subjects. First, we removed all pairs in which one or both members were taking statins to remove the confounding effects of these drugs (data not shown). Second, we reanalyzed patients by matching for year of operation (thus valves were implanted during the same year in both patients and control subjects), age of patient at implantation, and position. This effectively removed any possible confounding effects of changing medical-surgical practices, although it reduced our analyses to 22 patients.

Statistical analyses 

All analyses were carried out with the Statistica software package (version 5.5, Tulsa, Okla), and P values are reported. For cohort analyses, we used the nonparametric test of correlation because calcification was not distributed in a normal manner (Table 1 for the distribution of calcification). We used the Spearman rank order coefficient to compare ordered categoric data (eg, calcification) versus continuous data (eg, serum cholesterol level). To compare ordered categorical versus dichotomous data (eg, sex), we used the Wilcoxon rank sum test. All risk factors that had a P value of less than .10 in the univariate analyses were also analyzed by means of stepwise multiple regression.

TABLE 1. Cohort analysis

For case-control analysis, groups were examined by using a 2-tailed paired Student t test, and all P values are reported. The odds ratio for explantation was calculated for those whose serum cholesterol levels were greater than 200 mg/dL.

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Results 

Cohort analysis 

Of the 144 patients who met study inclusion and exclusion criteria, 82 (57%) were men (Table 1). The mean ± SD age at implantation was 55 ± 12.9 years. Because the valves in this group were implanted in aggregate from 1984.8 ± 4.4 and because these patients went on to have their valves explanted, they represent a younger population than those who are currently receiving tissue valves. The youngest patient in this series to receive a bioprosthetic valve was 23 years of age, and the oldest was 79 years of age (age at the time of implantation). The mean valve survival was 10.9 ± 4.4 years, ranging from 1 year to 21 years. Ninety-seven (67%) patients had valves placed in the aortic position, and 47 (33%) had valves placed in the mitral position. Coronary artery disease was present in 69 (48%) patients, diabetes mellitus was present in 26 (18%) patients, and hypertension was present in 87 (60%) patients. The mean cholesterol level of the group (n = 144) was 191 ± 42 mg/dL. Eighteen (13%) patients had their valves removed without any calcification. The distribution of the calcification is shown in Table 1, and the majority of patients had 1 to 2+ calcification. This non-Gaussian distribution of calcification led us to use nonparametric analyses of correlation. Causes of failure included leaflet tear (91 [63%] patients), degeneration (40 [28%] patients), bacterial endocarditis (5 [3.5%] patients), and others, including perforation, thrombosis, or paravalvular leak (8 [5.55] patients combined). Removing patients with bacterial endocarditis and other causes still resulted in a significant correlation between calcification and cholesterol level (r = 0.169, P = .04). These causes might have coexisted with valvular calcification, and this is in agreement with previous reports from our institution.³

Stepwise multiple regression was performed on all dependent variables with a significance of less than .10 determined by means of univariate analysis. Thus implant age, coronary artery disease, average serum cholesterol level, and diabetes mellitus were used as dependent variables in predicting calcification. Of the 4 dependent variables, only average cholesterol level achieved significance (P = .02) in the multiple regression (Table 2).

TABLE 2. Significance and correlations of cohort analysis

Case-control analysis 

Sixty-six patients met study inclusion and exclusion criteria as patients in the explanted valve group, and 32 (48\%) were male (Table 3). The mean ± SE age at implantation was 57.6 ± 1.7 years. The youngest patient in this series to receive a bioprosthetic valve was 24 years of age, and the oldest was 79 years of age (age at the time of implantation). The mean valve survival was 7.3 ± 0.3 years, ranging from 1 year to 9 years. Patient age, duration of valve survival (or until explantation), and position were all matched. Forty-three (65\%) patients had valves placed in the aortic position, and 23 (35\%) had valves placed in the mitral position. Coronary artery disease was present in 33 (50\%) patients, diabetes mellitus was present in 17 (26\%) patients, hypertension was present in 42 (64\%) patients, and 15 (23\%) patients smoked at some point while their valvular prostheses were in place. The mean ± SD cholesterol level of the explanted patients (n = 66) was 189 ± 40 mg/dL, which was significantly higher than the value of 163 ± 39 mg/dL in the control group (P < .0001, Figure 1). The mean serum cholesterol level is still in the high normal range for North Americans, implying that calcification might still occur in the high normal range, which is consistent with vascular atherosclerosis. Four (6.1\%) patients were taking statins during the period of valve function versus 12 (18\%) patients in the control group (P = .018). Coronary artery disease, presence of hypertension, presence of diabetes mellitus, and smoking status were not significantly different between patients and control subjects (Table 3).

TABLE 3. Case-control analysis

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• Figure 1. 

  Plot of mean serum cholesterol levels in the group of patients whose valves remained intact (control group) versus those undergoing reoperation (explanted). Groups were matched for age of patient at implantation, sex, and duration of valve survival.

Because patients represent an earlier cohort on average than control subjects by 6 years, we performed 2 additional analyses to rule out potential sources of bias. In one subgroup analysis all patient pairs in which one or both patients were taking statins were removed from analysis (data not shown). In another analysis matching was performed by comparing valves matched and implanted during the same year (including matched at position and age of patient, Table 4 ). The results of both these analyses demonstrate that cholesterol levels are significantly higher in patients (explanted valve patients) than in control subjects.

TABLE 4. Analysis of case-control matching for year of implantation, age at implantation, and valve position

We removed patient pairings in which one or both of the patients were taking statins because these drugs might influence calcification of the valve. A total of 16 paired groups (12 control subjects and 4 patients, 32 in total) were removed, and the results were reanalyzed to exclude possible confounding effects of statins on the results. Patients were still matched for position of valve and age of implantation, as well as duration of survival. The mean cholesterol difference between groups was still highly significant (P < .00001; patients, 190.8 mg/dL; control subjects, 158.5 mg/dL). There was no significant difference between groups in terms of diabetes, hypertension, sex, smoking, or coronary artery disease.

We also matched patients for the same year of implantation surgery, age at implantation, and valve position (aortic vs mitral). This effectively removed any influence of possible changing medical-surgical practices through time, although it limited our analysis to 22 patients (Table 4). There was still a significant difference in the mean serum cholesterol levels between patients and control subjects (187 mg/dL in patients and 155 mg/dL in control subjects; P = .047, paired t test). Diabetes (P = .059), hypertension (P = .67), smoking (P = .37), coronary artery disease (P = .34), and statin use (P = .96) were not significantly different between the 2 subgroups.

For the complete case-control analysis, the odds ratio for valvular re-replacement in patients whose serum cholesterol levels were greater than 200 mg/dL was 3.9× (95% confidence interval, 1.7-8.9; Table 3).

Discussion 

We have demonstrated for the first time that increasing serum cholesterol levels are associated with increased bioprosthetic valvular calcification. Furthermore, patients who had their valves explanted had higher serum cholesterol levels than an age-, duration of survival-, and position-matched group with unexplanted valves (189 vs 163 mg/dL, P < .0001). Finally, the odds ratio for valve explantation and re-replacement was 3.9-fold higher in patients whose serum cholesterol levels were greater than 200 mg/dL.

It is controversial whether increased serum lipid levels correlate with the progression of peripheral atherosclerosis, coronary calcification, and native aortic valve sclerosis.⁴, ¹⁰, ¹¹ Pohle and colleagues⁶ recently demonstrated that the rate of change of both coronary and native aortic valve calcification progresses more rapidly in individuals whose serum LDL levels are greater than 130 mg/dL. Palta and coworkers¹² also demonstrated that those with a serum cholesterol level of greater than 200 mg/dL had a rate of native aortic valve area reduction roughly twice that of those with a lower cholesterol level (P = .04).

In this cohort study we demonstrate that blood pressure, sex, and smoking are not linked to tissue valve calcification. We also demonstrate that coronary calcification is linked to tissue valve calcification; thus what is harmful to the coronary arteries might be harmful for tissue valves, and what is protective of the coronary arteries might be protective of the valves. Prior reports have concluded that tissue valves are appropriate for patients undergoing mitral valve replacement with coronary artery disease who are more than 60 years of age and for aortic valve replacement in patients who are more than 70 years of age, irrespective of coronary artery disease.¹³ Patients with subcritical coronary disease who undergo tissue valve replacement might be candidates for statin therapy.

Younger age has previously been demonstrated as a risk factor for leaflet failure.¹, ¹⁴, ¹⁵, ¹⁶ In our study younger age was linked to increasing calcification. Interestingly, the absolute magnitude of the linear association (ρ = 0.205, P = .014) was similar for younger patients as for cholesterol (ρ = 0.176, P = .035), implying that the contribution of cholesterol to calcification of the valve is of a similar importance as younger age. In our study diabetes, a well-documented risk factor for atherosclerosis, demonstrated a trend (P = .069) toward significance in univariate analysis. Patients were considered diabetic if they were being actively medically treated, and thus our study might have missed some patients with untreated diabetes. Also, our limited numbers might not have had adequate power to elicit the contributions of these other risk factors for atherosclerosis on valvular calcification.

Bellamy and associates,¹⁷ however, recently reported that there was no correlation between the rate of progression of aortic stenosis with cholesterol levels in 157 patients with aortic stenosis. Statin therapy, however, was associated with a decreased rate of progression of stenosis-sclerosis of the aortic valve. In that study the authors confirmed that aortic stenosis progresses with a reduction in orifice size of approximately 0.1 cm²/y in patients who are not receiving statin therapy. For those receiving medical therapy, the decrease in mean aortic valve area measured was only 0.04 cm²/y, which is approximately half the native rate. Palta and coworkers¹² also documented a near doubling of the rate of aortic valve area reduction in those whose serum cholesterol levels were greater than 200 mg/dL. After adjustment for sex, age, and baseline aortic valve area, statin therapy remained protective, although cholesterol levels during statin therapy showed no correlation with the progression of aortic stenosis.¹⁷ Both Novaro and colleagues¹⁸ and Aronow and associates¹⁹ also reported similar results that statin-treated groups had a slower rate of aortic stenosis progression. Shavelle and coworkers⁹ have documented that patients receiving statins have a diminished rate of accumulation of aortic valve calcium, as assessed by means of electron beam computed tomography, although serum cholesterol levels were not available. It is an intriguing finding that the effects of the statins are not solely attributable to lipid-lowering effects. This is not without precedent because the statins have been shown to stabilize plaques as part of the beneficial effect in preventing cerebrovascular occlusion and in reducing inflammation. Currently, there is no information on the rate of progression of bioprosthetic valve sclerosis with statin therapy. In our case-control study patients whose valves were not explanted had a higher incidence of statin use than those whose valves were explanted (18% vs 6%, P = .018). Some patients might have been started on statins in the midst of their valve use. This prompted reanalysis with all statin pair groups removed. The mean serum cholesterol level was still higher (190.8 vs 158.5 mg/dL, P < .0001) when the confounding effects of statins were removed. We also matched patients so that operations occurred in the same year while keeping the age of the patient at implantation and position matched and found the explanted valve group to have a higher serum cholesterol level than the control subjects (187 vs 155 mg/dL, P = .047).

The correlation of increased cholesterol levels with bioprosthetic valvular calcification invites mechanistic speculation.⁵ Eliminating lipids from tissue valves by means of ethanol extraction prevents in vivo calcification.²⁰ It is possible that glutaraldehyde-fixed valves have the appropriate physical characteristics to serve as a nidus for crystallization. Another potential mechanism is the migration of cells into the matrix of the valve. Blood-borne progenitor cells that are capable of differentiating into various lineages from the marrow might have the ability to deposit matrix, which can calcify. Additionally, oxidized LDL, which colocalizes with T lymphocytes and calcium, has been demonstrated in nonrheumatic stenotic aortic valves.²¹ Older literature had documented that silicone rubber prostheses in the Starr-Edwards valve (Edwards Lifesciences, Irvine, Calif) absorb lipids.²² Ferrans and associates²³ had documented that cholesterol ester crystals were found in porcine aortic valves that had been implanted in the mitral position for 8 years.

This study has its limitations. It is retrospective, and the population for the cohort study is selected for patients who required reoperation. Patients who did not undergo reoperation were not included in the cohort study, although we attempt to address this in the case-control study. One could attempt to address this by using electron beam computed tomography to measure the progression of calcification over time in tissue valves. Serum cholesterol values were used rather than LDL levels. Smoking was self-reported.

In summary, we have shown in a retrospective cohort and case-control study that cholesterol might contribute to bioprosthetic valve calcification, which might lead to valve re-replacement. A randomized prospective clinical trial to determine whether statins prevent bioprosthetic valve calcium accumulation is warranted.

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Acknowledgements 

We thank Dr Fred Schoen of the Department of Pathology at the Brigham and Women's Hospital for his review of this manuscript. We also thank Dr Farzad Mostashari for his statistical review of this manuscript.

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References 

1. Cohn LH, Collins JJ, Rizzo RJ, Adams DH, Couper GS, Aranki SF. Twenty-year follow-up of the Hancock modified orifice porcine aortic valve. Ann Thorac Surg. 1998;66(suppl):S30–34
   • View In Article
   • MEDLINE
   • CrossRef
2. Jamieson W, Burr L, Munro A, Miyagishima R. Carpentier-Edwards standard porcine bioprosthesis (a 21-year experience). Ann Thorac Surg. 1998;66(suppl):S40–S43
   • View In Article
   • MEDLINE
   • CrossRef
3. Schoen FJ. Cardiac valve prostheses (review of clinical status and contemporary biomaterials issues). J Biomed Mater Res. 1987;21:91–117
   • View In Article
   • MEDLINE
4. Mohler ER. Are atherosclerotic processes involved in aortic-valve calcification?. Lancet. 2000;356:524–525
   • View In Article
   • Full Text
   • Full-Text PDF (58 KB)
   • CrossRef
5. Demer LL. Cholesterol in vascular and valvular calcification. Circulation. 2001;104:1881–1883
   • View In Article
6. Pohle K, Maffert R, Ropers D, et al.  Progression of aortic valve calcification (association with coronary atherosclerosis and cardiovascular risk factors). Circulation. 2001;104:1927–1932
   • View In Article
   • CrossRef
7. Callister TQ, Raggi P, Cooil B, Lippolis NJ, Russo DJ. Effect of HMG-CoA reductase inhibitors on coronary artery disease as assessed by electron-beam computed tomography. N Engl J Med. 1998;339:1972–1978
   • View In Article
   • MEDLINE
   • CrossRef
8. Bild D, Folsom A, Lowe L, et al.  Prevalence and correlates of coronary calcification in black and white young adults (the coronary artery risk development in young adults (CARDIA) study). Arterioscler Thromb Vasc Biol. 2001;21:421–426
   • View In Article
   • CrossRef
9. Shavelle D, Takasu J, Budoff M, Mao S, Zhao X, O'Brien KD. HMG CoA reductase inhibitor (statin) and aortic valve calcium. Lancet. 2002;359:1125–1126
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (333 KB)
   • CrossRef
10. Pearlman A. Medical treatment of aortic stenosis (promising or wishful thinking). J Am Coll Cardiol. 2002;40:1731–1734
    • View In Article
    • Full Text
    • Full-Text PDF (75 KB)
    • CrossRef
11. O'Brien MF. What is the progression of aortic valve stenosis (bicuspid, congenital, acquired, degenerative)? Can this progression be modified by statin therapy? Is “prophylactic” AVR for mild AS indicated during CABG? Presented as an abstract at the American College of Cardiology, Hawaii, February 2003. American College of Cardiology; 2003
    • View In Article
12. Palta S, Pai AM, Gill KS, Pai RG. New insights into the progression of aortic stenosis (implications for secondary prevention). Circulation. 2000;101:2497–2502
    • View In Article
13. Jones EL, Weintraub WS, Craver JM, Guyton RM, Shen Y. Interaction of age and coronary disease after valve replacement (implication for valve selection). Ann Thorac Surg. 1994;58:378–385
    • View In Article
    • MEDLINE
    • CrossRef
14. Kirklin JK, Smith D, Novick W, et al.  Long-term function of cryopreserved aortic homografts (a ten-year study). J Thorac Cardiovasc Surg. 1993;106:154–166
    • View In Article
    • MEDLINE
15. Cleveland D, Williams W, Razzouk A, et al.  Failure of cryopreserved homograft valved conduits in the pulmonary circulation. Circulation. 1992;86(suppl):II150–153
    • View In Article
    • MEDLINE
16. Miller D, Stinson E, Oyer P, et al.  The durability of porcine xenograft valves and conduits in children. Circulation. 1982;66(suppl):I172–185
    • View In Article
    • MEDLINE
17. Bellamy M, Pellika P, Klarish K, et al.  Association of cholesterol levels, hydroxymethylglutaryl coenzyme-A reductase inhibitor treatment, and progression of aortic stenosis in the community. J Am Coll Cardiol. 2002;40:1723–1730
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (139 KB)
    • CrossRef
18. Novaro GM, Tiong IY, Pearce GL, Lauer MS, Sprecher DL, Griffin BP. Effect of hydroxymethylglutaryl coenzyme A reductase inhibitors on the progression of calcific aortic stenosis. Circulation. 2001;104:2205–2209
    • View In Article
    • CrossRef
19. Aronow W, Ahn C, Kronzon I, Goldman M. Association of coronary risk factors and use of statins with progression of mild valvular aortic stenosis in older persons. Am J Cardiol. 2001;88:693–695
    • View In Article
    • Full Text
    • Full-Text PDF (77 KB)
    • CrossRef
20. Vyavahare NR, Jones PL, Hirsch D, Schoen FJ, Levy RJ. Prevention of glutaraldehyde-fixed bioprosthetic heart valve calcification by alcohol pretreatment (further mechanistic studies). J Heart Valve Dis. 2000;9:561–566
    • View In Article
    • MEDLINE
21. Olsson M, Thyberg J, Nilsson J. Presence of oxidized low density lipoprotein in nonrheumatic stenotic aortic valves. Arterioscler Thromb Vasc Biol. 1999;19:1218–1222
    • View In Article
    • MEDLINE
    • CrossRef
22. Chin H, Harrison E, Blankenhorn D, Moacanin J. Lipids in silicone rubber valve prostheses after human implantation. Circulation. 1971;43/44(suppl):I51–56
    • View In Article
23. Ferrans VJ, McManus B, Roberts WC. Cholesteryl ester crystals in a porcine aortic valvular bioprosthesis implanted for eight years. Chest. 1983;83:698–701
    • View In Article
    • MEDLINE
    • CrossRef

Drs Cohn and Farivar