Bicuspid-Associated Aortic Root Aneurysm: Mid to Long-Term Outcomes of David V Versus the Bio-Bentall Procedure

Incidence, risk factors, and long-term outcomes associated with permanent pacemaker implantation after aortic root replacement

OBJECTIVE

Permanent pacemaker implantation (PPI) after aortic valve replacement is associated with long-term mortality. However, data regarding PPI after aortic root replacement (ARR) is lacking. Herein we describe the incidence, risk factors, and long-term outcomes of PPI after ARR.

METHODS

Consecutive patients undergoing ARR from 2005 to 2020 were selected after excluding those with endocarditis, type A dissection, or preoperative PPI. Patients requiring PPI after ARR were identified, along with the indication and timing. Independent factors associated with PPI after ARR were identified and long-term survival was assessed.

RESULTS

The incidence of PPI was 3.8% (n = 85) among 2240 patients undergoing ARR. PPI was performed a median of 7 days (interquartile range, 5-12 days) after ARR most commonly for complete heart block (73%). Bicuspid aortic valve (odds ratio [OR], 1.89; P = .02), female sex (OR, 1.74; P = .04), preoperative heart block (OR, 2.70; P = .02), and prior aortic valve replacement (OR, 2.18; P = .01) were independently associated with PPI while preoperative aortic insufficiency (OR, 0.52; P = .01) and valve-sparing root replacement procedure compared with bio-Bentall (OR, 0.40; P = .01) were protective. Patients requiring PPI after ARR were not at increased risk of operative or long-term mortality compared with patients not requiring PPI (P = .26); however, those undergoing PPI experienced significantly longer hospital length of stay (13 vs 7 days; P < .001).

CONCLUSIONS

The incidence of PPI after ARR remains low, particularly after VSRR. Preoperative conduction disturbance, prior AVR, and bicuspid aortic valve are all associated with increased risk of PPI. Although PPI is associated with longer length of stay, it is not associated with early or late mortality.

Long-term outcomes after aortic root replacement for bicuspid aortic valve-associated aneurysm

OBJECTIVE

Patients with congenital bicuspid aortic valve often require root replacement. This study aims to describe their long-term rates of mortality and reoperation.

METHODS

This is a multicenter retrospective study of 747 patients with bicuspid aortic valve who underwent aortic root replacement for aortic aneurysm between 2004 and 2020. Cumulative incidence curves for aortic valve and aortic reoperations were graphed. A Kaplan-Meier survival curve for the patient cohort was created alongside an age- and sex-matched curve for the US population. Multivariable Cox regression was used to determine characteristics associated with long-term mortality.

RESULTS

The median age of our cohort was 54 [43-64] years old, and 101 (13.5%) patients were female. In patients with bicuspid aortic valve dysfunction, 274 (36.7%) had aortic insufficiency, 187 (25.0%) had aortic stenosis, and 142 (19.0%) had both. In-hospital mortality occurred in 10 (1.3%) patients. There were 56 aortic valve reoperations and 19 aortic reoperations, with a combined cumulative incidence of 35% (95% confidence interval [CI], 23%-46%) at 15 years. In addition, there was comparable survival between the patient cohort and the age- and sex-matched US population. Age (hazard ratio [HR], 1.04; 95% CI, 1.01-1.06), concomitant CABG (HR, 2.28; 95% CI, 1.29-4.04), and bypass time (HR, 1.01; 95% CI, 1.00-1.01) were associated with increased mortality.

CONCLUSIONS

Patients who undergo aortic root replacement with bicuspid aortic valve have an increased rate of aortic reoperation (35%; 95% CI, 23%-46%) while their survival appears to be comparable to the general US population (79%; 95% CI, 73%-87%) at 15 years.

Propensity score matching analysis of valve-sparing versus aortic root replacement in type A aortic dissection patients

This study compared long-term survival and reintervention rates between Valve-Sparing Root Replacement (VSRR, n = 244) and Aortic Root Replacement (ARR, n = 499) in 743 patients undergoing Type A acute aortic dissection (AAD), given the lack of prospective comparative data. Multivariable analysis is identifying advanced age, high Body Mass Index (BMI), Marfan syndrome, severe aortic regurgitation, bicuspid aortic valve, increased aortic root diameter, and reduced aortic cross-clamp time (ACC) as significant factors associated with ARR. After Propensity Score Matching (PSM), VSRR is showing significantly higher 5-year survival rates than ARR (80.2% vs. 64.1%, P = 0.001), validated by Inverse Probability of Treatment Weighting (IPTW) analysis. Reintervention rates are being found comparable, with endocarditis more prevalent in ARR and aortic regurgitation in VSRR. Subgroup analysis indicated that patients aged less than 60 years and those with a BMI greater than 24 in the VSRR group exhibited significantly improved survival probabilities compared to the ARR group. These findings support the wider utilization of valve-sparing root replacement (VSRR) in appropriately selected patients, highlighting its potential advantages for suitable candidates.

Long-term outcomes comparison of Bentall-De Bono-versus valve-sparing aortic root replacement: An updated systematic review and reconstructed time-to-event meta-analysis

BACKGROUND

For patients with aortic root dilatation and a structurally normal aortic valve (AV) undergoing composite aortic valve-graft (Bentall-De Bono) versus valve-sparing aortic root replacement (VSARR) procedures there are conflicting data regarding early and long-term benefits. We undertook a study-level meta-analysis to compare the results of both procedures.

METHODS

Three databases were assessed, and both randomized trials and observational studies were considered eligible. Kaplan-Meier curves of long-term survival and reoperation risk were reconstructed and compared with Cox linear regression and incidence rate ratios (IRR) with 95 % confidence intervals (CI). Landmark analysis and time-varying hazard ratio (HR) were analyzed. Odds ratios (OR) were calculated for early mortality, postoperative stroke, and re-exploration for postoperative bleeding. A random effects model was used. Sensitivity analyses included leave-one-out-analysis, meta-regression and subgroups analysis.

RESULTS

1456 articles were identified, including 39 observational studies, totaling 14,651 patients (Bentall-De Bono = 9557 and VSARR = 5094). Twelve studies were adjusted. The mean weighted follow-up was 5.05 ± 3.7 years. VSARR was associated with significantly greater survival (HR = 0.50; 95 % CI, 0.45-0.57; p < 0.0001) at 15-year follow-up. The reoperation risk was higher following VSARR (HR = 1.30; 95 % CI, 1.03-1.63; p = 0.02.), although time-varying HR model and landmark analysis reported an increased risk of reoperation within 5 years after VSARR (HR = 1.57; 95 % CI, 1.23-2.01; p < 0.001), after which the difference disappeared. Subgroups analysis of studies excluding aortic dissection showed a comparable rate of late reoperation.

CONCLUSIONS

VSARR is associated with improved long-term survival compared to Bentall-De Bono. The risk of late reoperation is higher within 5 years following VSARR, after which the two procedures are comparable.

Valve-sparing aortic root replacement in resternotomy settings

OBJECTIVES

Although valve-sparing aortic root replacement (VSRR) has demonstrated satisfactory outcomes, its utility in reoperative sternotomy settings remains uncertain. This study evaluates the perioperative safety and long-term durability of reoperative sternotomy VSRR.

METHODS

All consecutive VSRR procedures at 2 centers from 2005 to 2020 were included. Reoperative VSRR was defined as prior sternotomy for any reason. Time-dependent outcomes were compared between groups after balancing of covariates with inverse probability of treatment weighting. Multivariable Cox regression and the Fine-Gray subdistribution hazard model were used to identify factors associated with 10-year survival and aortic valve reintervention, respectively.

RESULTS

Of 778 VSRR procedures, 69 (8.9%) were reoperative sternotomies. Compared with primary, patients undergoing reoperative VSRR were younger (age 43 years [range, 34-54 years] vs age 51 years [range, 39-61 years]; P < .001) and had less aortic insufficiency (33.3% vs 48.5%; P = .02). Cardiopulmonary bypass times were longer in reoperative VSRR with similar rates of circulatory arrest. Reoperative VSRR had a higher incidence of postoperative acute kidney injury (10.1% vs 3.2%; P = .01) and prolonged ventilation (24.6% vs 9.9%; P < .001); operative mortality was higher in reoperative VSRR (2.9% [2 out of 69] vs 1.4% [10 out of 709]) although not significantly (P = .66). Ten-year survival was 82.0% (range, 70.0%-96.0%) and aortic valve reintervention rate was 11.8% (range, 2.3%-19.4%) after reoperative VSRR. After inverse probability of treatment weighting adjustment, survival and aortic valve reintervention rates were not different. Reoperative sternotomy was not associated with long-term mortality (P = .13) or aortic valve reintervention (P = .77) on multivariable analysis.

CONCLUSIONS

Although complex, reoperative sternotomy VSRR can be performed with low operative mortality in carefully selected patients. Ten-year survival and aortic valve reintervention rates were acceptable and comparable to primary VSRR.

Adverse Technical Events During Aortic Root Replacement

BACKGROUND

Although adverse technical events during aortic root replacement (ARR) are not uncommon and are extremely challenging, there is scant literature to help surgeons prepare for such situations. We describe our experience of outstanding technical events during ARR.

METHODS

This is a retrospective study of 830 consecutive ARRs at a single center from 2012 to 2022. Technical events were defined as intraoperative events that led to an unplanned cardiac procedure, need for mechanical circulatory support, or additional aortic cross-clamping. Logistic regression identified factors associated with operative mortality and technical events.

RESULTS

Technical events occurred in 90 patients (10.8%) and were attributed to bleeding (n = 26), nonischemic ventricular dysfunction (n = 23), residual valve disease (n = 20), myocardial ischemia (n = 19), and iatrogenic dissection (n = 2). Prior sternotomy (odds ratio [OR], 2.38; 95% CI, 1.36-4.19; P = .002) and complex aortic valve disease (OR, 3.09; 95% CI, 1.09-8.75; P = .03) were associated with technical events. Patients with technical events had higher rates of operative mortality (6.7% vs 2.3%, P = .03) and all major postoperative complications. Surgical indications of dissection (OR, 13.57; 95% CI, 4.95-37.23; P < .001) and complex aortic valve disease (OR, 14.09; 95% CI, 3.67-54.02; P < .001) but not adverse technical events (OR, 2.42; 95% CI, 0.81-7.26; P = .11) were associated with operative mortality.

CONCLUSIONS

Adverse technical events occurred in 10.8% of ARRs and were associated with reoperative sternotomies. Technical events are associated with increased postoperative complications.

Valve-sparing root replacement versus composite valve graft root replacement: Analysis of more than 1500 patients from 2 aortic centers

OBJECTIVES

The long-term outcomes comparing valve-sparing root replacement, composite valve graft with bioprosthesis, and mechanical prosthesis have yet to be explored. We investigated the long-term survival and reintervention rates after 1 of 3 major aortic root replacements in patients with tricuspid aortic valves and patients with bicuspid aortic valves.

METHODS

A total of 1507 patients underwent valve-sparing root replacement (n = 700), composite valve graft with bioprosthesis (n = 703), or composite valve graft with mechanical prosthesis (n = 104) between 2004 and 2021 in 2 aortic centers, excluding those with dissection, endocarditis, stenosis, or prior aortic valve surgery. End points included mortality over time and cumulative incidence of aortic valve/proximal aorta reintervention. Multivariable Cox regression compared adjusted 12-year survival. Fine and Gray competing risk regression compared the risk and cumulative incidence of reintervention. Propensity score-matched subgroup analysis balanced the 2 major groups (composite valve graft with bioprosthesis and valve-sparing root replacement), and landmark analysis isolated outcomes beginning 4 years postoperatively.

RESULTS

On multivariable analysis, both composite valve graft with bioprosthesis (hazard ratio, 1.91, P = .001) and composite valve graft with mechanical prosthesis (hazard ratio, 2.62, P = .005) showed increased 12-year mortality risk versus valve-sparing root replacement. After propensity score matching, valve-sparing root replacement displayed improved 12-year survival versus composite valve graft with bioprosthesis (87.9% vs 78.8%, P = .033). Adjusted 12-year reintervention risk in patients receiving composite valve graft with bioprosthesis or composite valve graft with mechanical prosthesis versus valve-sparing root replacement was similar (composite valve graft with bioprosthesis subdistribution hazard ratio, 1.49, P = .170) (composite valve graft with mechanical prosthesis subdistribution hazard ratio, 0.28, P = .110), with a cumulative incidence of 7% in valve-sparing root replacement, 17% in composite valve graft with bioprosthesis, and 2% in composite valve graft with mechanical prosthesis (P = .420). Landmark analysis at 4 years showed an increased incidence of late reintervention in composite valve graft with bioprosthesis versus valve-sparing root replacement (P = .008).

CONCLUSIONS

Valve-sparing root replacement, composite valve graft with mechanical prosthesis, and composite valve graft with bioprosthesis demonstrated excellent 12-year survival, with valve-sparing root replacement associated with better survival. All 3 groups have low incidence of reintervention, with valve-sparing root replacement showing decreased late postoperative need for reintervention compared with composite valve graft with bioprosthesis.

Bicuspid valve repair outcomes are improved with reduction and stabilization of sinotubular junction and annulus with external annuloplasty

OBJECTIVE

We investigated long-term outcomes of bicuspid aortic valve (BAV) repair, with external annuloplasty, according to aorta phenotype.

METHODS

Between 2003 and 2020, all patients with BAV operated on for aortic insufficiency (AI) and/or aneurysm were included. Repairs included isolated AI repair with subvalvular with or without sinotubular junction (STJ) (single or double) annuloplasty, supracoronary aorta replacement (with or without hemiroot remodeling), and root remodeling with external subvalvular ring annuloplasty.

RESULTS

Among 343 patients operated, reparability rate was 81.3% (n = 279; age 46 ± 13.3 years). At 10 years (median follow-up: 3.42 years; interquartile range, 1.1, 5.8), survival was 93.9% (n = 8 deaths, similar to general population), cumulative incidence of reoperation was 6.2% (n = 10), AI grade >2 was 5.8% (n = 9), and grade >1 was 23.0% (n = 30). BAV repair stabilizing both the annulus and STJ with annuloplasty, compared with nonstabilized STJ repair (single annuloplasty), had lower incidence of reoperation (2.6% vs 22.5%, P = .0018) and AI grade >2 (1.2% vs 23.6%, P < .001) at 9 years. Initial commissural angle <160° was not a risk factor for reoperation, compared with angle ≥160° if symmetrical repair was achieved (2.7% and 4.1%, respectively, at 6 years, P = .85). Multivariable model showed that absence of STJ stabilization (odds ratio, 6.7; 95% confidence interval, 2.1-20, P = .001) increased recurrent AI, but not initial commissural angle <160° (odds ratio, 1.01; 95% confidence interval, 0.39-2.63, P = .98). Commissures adjusted symmetrically led to lower transvalvular gradient, compared with nonsymmetrical repair (8.7 mm Hg vs 10.2 mm Hg, P = .029).

CONCLUSIONS

BAV repair, tailored to aorta phenotype, is associated with excellent durable outcomes if both annulus and STJ are reduced and stabilized with external ring annuloplasty. Commissural angle <160° is not associated with reoperation if symmetrical repair is achieved.

Reintervention after valve-sparing aortic root replacement: A comprehensive analysis of 781 David V procedures

OBJECTIVE

Studies of reintervention after valve-sparing aortic root replacement (VSRR) are limited by sample size and failure to evaluate all types of reinterventions, including distal aorta and transcatheter interventions. In this report, reintervention after VSRR using a large patient cohort was comprehensively analyzed.

METHODS

In a series involving 2 academic aortic centers, 781 consecutive patients from 2005 to 2020 undergoing David V VSRR for aortic aneurysm (91%) or dissection (9%) were included. Median age was 50 years, and 23% had a bicuspid aortic valve (AV). Median follow-up was 7.0 years. Open or transcatheter reintervention on the AV, proximal, or distal thoracic aorta was identified. Cumulative incidence was calculated, and subdistribution hazard models identified factors associated with reintervention. Time-dependent incidence of reintervention was plotted using risk-hazard functions.

RESULTS

Sixty-eight reinterventions (57 open, 11 transcatheter) were performed. Reinterventions were divided by indication into degenerative AV (n = 26, including 1 transcatheter aortic valve replacement), endocarditis (n = 11), proximal aorta (n = 8), and distal aorta (n = 23, including 10 thoracic endovascular aortic repairs). Risk of reintervention for endocarditis peaked 1 to 3 years after VSRR, whereas other indications had stable, low rates of occurrence throughout the follow-up period. The cumulative incidence of reintervention was 12.5% whereas the cumulative incidence of AV reintervention was 7.0% at 10 years and was associated with residual postoperative aortic insufficiency. In-hospital mortality after reintervention was 3%.

CONCLUSIONS

Reintervention rates after VSRR are relatively low in long-term follow-up and can be performed with acceptable operative risk. The majority of reinterventions are performed for indications other than AV degeneration, with the timing of reintervention varying by the specific clinical indication.

Is surgical risk of aortic arch aneurysm repair underestimated? A novel perspective based on 30-day versus 1-year mortality

OBJECTIVES

The decision to undergo aortic aneurysm repair balances the risk of operation with the risk of aortic complications. The surgical risk is typically represented by perioperative mortality, while the aneurysmal risk relates to the 1-year risk of aortic events. We investigate the difference in 30-day and 1-year mortality after total arch replacement for aortic aneurysm.

METHODS

This was an international two-centre study of 456 patients who underwent total aortic arch replacement for aneurysm between 2006 and 2020. Our primary end-point of interest was 1-year mortality. Our secondary analysis determined which variables were associated with 1-year mortality.

RESULTS

The median age of patients was 65.4 years (interquartile range 55.1-71.1) and 118 (25.9%) were female. Concomitantly, 91 (20.0%) patients had either an aortic root replacement or aortic valve procedure. There was a drop in 1-year (81%, 95% confidence interval (CI) 78-85%) survival probability compared to 30-day (92%, 95% CI 90-95%) survival probability. Risk hazards regression showed the greatest risk of mortality in the first 4 months after discharge. Stroke [hazard ratio (HR) 2.54, 95% CI (1.16-5.58)], renal failure [HR 3.59 (1.78-7.25)], respiratory failure [HR 3.65 (1.79-7.42)] and reoperation for bleeding [HR 2.97 (1.36-6.46)] were associated with 1-year mortality in patients who survived 30 days.

CONCLUSIONS

There is an increase in mortality up to 1 year after aortic arch replacement. This increase is prominent in the first 4 months and is associated with postoperative complications, implying the influence of surgical insult. Mortality beyond the short term may be considered in assessing surgical risk in patients who are undergoing total arch replacement.

Valve Sparing vs Composite Valve Graft Root Replacement: Propensity Score-Matched Analysis

BACKGROUND

Both valve-sparing root replacement and composite valve graft (CVG) are acceptable options in aortic root replacement. We compare outcomes of these 2 approaches and durability of the aortic valve.

METHODS

A consecutive 1635 patients without acute dissection underwent primary aortic root replacement from 1997 to 2022; 473 (29%) underwent valve-sparing root replacement, and 1162 (71%) received CVG. Propensity score matching was used to reduce baseline differences.

RESULTS

The CVG group was older (59 ± 14 years vs 49 ± 14 years; P < .001) with more comorbidities, such as hypertension (88.4% vs 66.4%; P < .001), diabetes (7% vs 1.7%; P < .001), ischemic heart disease (5.1% vs 1.3%; P = .001), pulmonary disease (6.6% vs 1.3%; P < .001), renal impairment (8.6% vs 1.3%; P < .001), class III-IV heart failure (35% vs 9.2%; P < .001), bicuspid aortic valves (44.8% vs 24.1%; P < .001), and severe aortic insufficiency (50.2% vs 13.2%; P < .001). Operative mortality was 0.4% (0% in valve sparing); incidence of major postoperative complications was 2.9% (3.6% vs 1.1%; P = .009). Ten-year survival was 93.1% (91.2% vs 97.7%; hazard ratio [HR], 1.7; 95% CI, 0.9-3.3; P = .120). Mean follow-up was 65 ± 60 months; aortic valve reoperations were similar (5.8% vs 5.7%; HR, 0.8; 95% CI, 0.4-1.4; P = .401). Recurrent moderate-severe aortic insufficiency was less prevalent in CVG (6.1% vs 11.1%; HR, 0.14; 95% CI, 0.07-0.27; P < .001). Propensity score matching identified 225 pairs. There was no difference in 10-year survival or reoperations. Recurrent moderate-severe aortic insufficiency was higher with valve sparing.

CONCLUSIONS

Both valve-sparing operations and CVG provide excellent early and late outcomes out to 10 years. Valve sparing is associated with a higher risk for development of aortic insufficiency but no difference in reoperations.

Central aortic versus axillary artery cannulation for aortic arch surgery

Objective

Central aortic cannulation for aortic arch surgery has become more popular over the last decade; however, evidence comparing it with axillary artery cannulation remains equivocal. This study compares outcomes of patients who underwent axillary artery and central aortic cannulation for cardiopulmonary bypass during arch surgery.

Methods

A retrospective review of 764 patients who underwent aortic arch surgery at our institution between 2005 and 2020 was performed. The primary outcome was failure to achieve uneventful recovery, defined as having experienced at least 1 of the following: in-hospital mortality, stroke, transient ischemic attack, bleeding requiring reoperation, prolonged ventilation, renal failure, mediastinitis, surgical site infection, and pacemaker or implantable cardiac defibrillator implantation. Propensity score matching was used to account for baseline differences across groups. A subgroup analysis of patients undergoing surgery for aneurysmal disease was performed.

Results

Before matching, the aorta group had more urgent or emergency operations (P = .039), fewer root replacements (P < .001), and more aortic valve replacements (P < .001). After successful matching, there was no difference between the axillary and aorta groups in failure to achieve uneventful recovery, 33% versus 35% (P = .766), in-hospital mortality, 5.3% versus 5.3% (P = 1), or stroke, 8.3% versus 5.3% (P = .264). There were more surgical site infections in the axillary group, 4.8% versus 0.4% (P = .008). Similar results were seen in the aneurysm cohort with no differences in postoperative outcomes between groups.

Conclusions

Aortic cannulation has a safety profile similar to that of axillary arterial cannulation in aortic arch surgery.

Extent of aortic replacement and operative outcome in open proximal thoracic aortic aneurysm repair

Objectives

There are few data to delineate the risk differences among open aortic procedures. We aimed to investigate the influence of the procedural types on the outcomes of proximal thoracic aortic aneurysm repair.

Methods

Among 1900 patients who underwent aortic replacement in our institution between 2005 and 2019, 1132 patients with aortic aneurysm who underwent a graft replacement of proximal thoracic aorta were retrospectively reviewed. Patients were divided into 4 groups based on the extent of the aortic replacement: isolated ascending aortic replacement (n = 52); ascending aortic replacement with distal extension with hemiarch, partial arch, or total arch replacement (n = 126); ascending aortic replacement with proximal extension with aortic valve or root replacement (n = 620); and ascending aortic replacement with distal and proximal extension (n = 334). "Eventful recovery," defined as occurrence of any key complications, was used as the primary end point. Odds ratios for inability to achieve uneventful recovery in each procedure were calculated using ascending aortic replacement as a reference.

Results

Overall, in-hospital mortality and stroke occurred in 16 patients (1.4%) and 24 patients (2.1%). Eventful recovery was observed in 19.7% of patients: 11.5% in those with ascending aortic replacement, 36.5% in those with partial arch or total arch replacement, 16.6% in those with proximal extension with aortic valve or root replacement, and 20.4% in those with distal and proximal extension (P < .001). With ascending aortic replacement as the reference, a multivariable logistic regression revealed partial arch or total arch replacement (odds ratio, 10.0; 95% confidence interval, 1.8-189.5) was an independent risk factor of inability to achieve uneventful recovery.

Conclusions

Open proximal aneurysm repair in the contemporary era resulted in satisfactory in-hospital outcomes. Distal extension was associated with a higher risk for postoperative complications.

Surgical options for aortic root aneurysm disease: which procedure, which patient

PURPOSE OF REVIEW

Several surgical repair strategies are available for patients with aortic root aneurysms. This review summarizes the indications for surgery, surgical treatment options, as well their associated outcomes.

RECENT FINDINGS

Despite the development and increasing adoption of valve sparing aortic root replacement, most patients with aortic root aneurysms still undergo placement of a composite valved graft. Valve sparing aortic root replacement may have a lower rate of bleeding and thrombotic complications during long-term follow-up with excellent long-term survival and low rates of aortic valve reintervention.

SUMMARY

Patients with aortic root aneurysms who are symptomatic or reach the recommended size criteria should undergo surgical repair. Most patients receive composite valve graft conduits with good outcomes. The presence of normal aortic leaflet tissue and minimal regurgitation, including those with connective tissue disorders, should be evaluated for valve sparing aortic root replacement. Valve sparing procedures are durable and obviate the need for lifelong anticoagulation and avoid the risk of structural degeneration of bioprosthetic valves.