Valve complications were generally poorly reported in the randomized trials. Valve regurgitation occurred in 4.4% and 6.4% of patients (OR 0.36, 95% CI 0.11–1.17), valvular dysfunction occurred in 1.4% and 2.6% of patients (OR 0.55, 95% CI 0.13–2.35), and reoperation for valvular complications occurred in 0.7% versus 1.9% of patients (OR 0.47, 95% CI 0.11–1.95) were not significantly different between stentless and stented valve groups, respectively.
Stroke, Thromboembolic Events
Stroke or neurologic complications did not differ between stentless (3.6%) and stented (4.0%) valve groups (OR 0.73, 95% CI 0.37–1.44). The risk of thromboembolic events was reported in only one randomized trial (1.6% stentless vs. 7.3% stented), and it suggested possible reduction in risk (OR 0.20, 95% CI 0.04–1.00).
No difference was found for overall composite estimates of complications for stentless versus stented groups (13.0% vs. 13.8%; OR 0.97, 95% CI 0.52–1.83). Early endocarditis did not differ significantly (0.8% vs. 2.2%; OR 0.52, 95% CI 0.07–4.06) and late endocarditis did not differ significantly (0.8% vs. 1.2%; OR 0.85, 95% CI 0.21–3.41) between stentless and stented groups, but was generally poorly reported in randomized trials. Similarly, no difference was found for risk of atrial fibrillation, myocardial infarction, atrioventricular block, permanent pacemaker insertion, intra-aortic balloon pump, heart failure, renal dysfunction, ventricular arrhythmia, wound infection, bleeding complications, reoperation for bleeding, and reoperation for any reason (Table 4).
Prosthesis Patient Mismatch and EOAI
Prosthesis-patient mismatch was numerically lower in the stentless group (11.0% vs. 31.3%, OR 0.30, 95% CI 0.05–1.66), but this parameter was reported in only four randomized trials and did not reach statistical significance. EOAI was significantly greater for patients receiving stentless aortic valve compared with stented valves at 30 days (WMD 0.12 cm2/m2, 95% CI 0.03–0.21 cm2/m2), at 2 to 6 months (WMD 0.15 cm2/m2, 95% CI 0.02–0.28 cm2/m2), and at 1 year (WMD 0.26 cm2/m2, 95% CI 0.10–0.41 cm2/m2). Randomized studies did not provide EOAI for follow-up beyond 1 year.
Mean and Peak Gradient
Mean gradient at 1 month was significantly lower by over 6 mm Hg in the stentless valve group compared with the stented valve group (WMD −6 mm Hg, 95% CI −10 to −2 mm Hg). The mean gradient remained significantly lower at 2 to 6 month follow-up (WMD −4 mm Hg, 95% CI −7 to −1 mm Hg), at 1 year follow-up (WMD −3 mm Hg, 95% CI −6 to −1 mm Hg), and up to 3 year follow-up (WMD −3 mm Hg, 95% CI −3 to −2 mm Hg; Fig. 2F). Similarly, peak gradient was lower at 1 month by 8 mm Hg (WMD −8 mm Hg, 95% CI −14 to −3 mm Hg), and the significant difference was maintained at 2 to 6 month follow-up (WMD −8 mm Hg, 95% CI −13 to −3 mm Hg), 1 year follow-up (WMD −8 mm Hg, 95% CI −14 to −3 mm Hg), and at 3 to 8 years’ follow-up (WMD −10 mm Hg, 95% CI −16 to −5 mm Hg).
LVMI and EF
Although the LVMI was generally lower in the stentless group versus the stented valve group, the aggregate estimates of mean difference did not reach significance during any time period of follow-up (1 month, 2 to 6 months, 1 year, and 8 years) as shown in Table 4. However, there was significant heterogeneity among the trials for this outcome with some studies suggesting large and significant reductions while others suggested no reduction in LVMI. Percent EF was not significantly different between stentless and stented groups at any time point (1 month, 6 months, 1 year, and 3 years). The proportion of patients achieving NYHA class I or II at follow-up did not show a significant difference between stentless and stented valve groups (Table 4).
Resource- and Procedure-Related Outcomes
Cross-clamp time was significantly greater by 23 minutes (WMD 23 minutes, 95% CI 18–27 minutes) and bypass time was significantly greater by 24 minutes (WMD 24, 95% CI 19–30 minutes) in the stentless group compared with the stented valve group. Time in the operating room was not significantly different, but was reported in only one randomized trial. Other resource-related outcomes, including time on the ventilator (WMD −11 hours, 95% CI −42 to +22 hours), intensive care unit length of stay (WMD −0.1 day, 95% CI −1 to +1 day), and total hospital length of stay (WMD −0.2 day, 95% CI −2 to +2 days) did not reach statistical differences between stentless and stented groups.
Clinical Outcomes for Non-RCT
Outcome estimates did not differ significantly between randomized and nonrandomized trials for many outcomes including death at most time points and most clinical complications including overall complications, stroke, atrial fibrillation, atrioventricular block, permanent pacemaker insertion, intra-aortic balloon pump, heart failure, renal dysfunction, ventricular arrhythmia, wound infection, bleeding complications, reoperation for any reason, early endocarditis, and late endocarditis.
There were a number of exceptions, where nonrandomized trials suggested notably different effect sizes than randomized trials (as indicated by a significant test of interaction between randomized and nonrandomized data). However, most of the differences noted between nonrandomized and randomized data were highly susceptible to bias because few trials had reported on the outcomes and definitive conclusions were not possible. For example, the estimate for myocardial infarction from nonrandomized trials suggested lower rates of myocardial infarction overall and in the opposite direction as with the randomized trials (P value for interaction = 0.06); however, this was based on a single nonrandomized trial and only three randomized trials and may not reflect a balanced view of effect size. A number of continuous outcome data differed significantly between randomized and nonrandomized trials (ie, mean gradient, peak gradient, EOAI) and in a number of these cases the heterogeneity occurred because of the differences in magnitude of reduction over time (as opposed to differences in the direction of effect over time). This heterogeneity between randomized and nonrandomized trials is also highly susceptible to bias, given that these outcomes were rarely reported in nonrandomized trials (ie, 1 or 2 only), and the more complete reporting from randomized trials should be relied on preferentially given this information. In addition, differences in loss to follow-up in the nonrandomized trials may have resulted in heterogeneous results.
Importantly, the outcome of death at 30 days was found to be heterogeneous between randomized and nonrandomized trials. The non-RCT studies tended to report lower incidences of 30-day mortality in the stentless versus stented groups (4.0% vs. 6.8%, respectively; OR 0.60, 95% CI 0.39–0.91, P = 0.02). This result was discrepant with randomized trials which showed no significant difference 30-day mortality between groups, with a direction of effect in the opposite direction than for nonrandomized trials (3.7% vs. 2.9% for stentless and stented valve groups, respectively; OR 1.36, 95% CI 0.68–2.72). The P value for interaction between the randomized and nonrandomized trials was 0.05, suggesting that they are not compatible with each other. Given that some of the nonrandomized trials were retrospective, with potential for loss to follow-up in both groups, the prospective randomized trials with adequate follow-up should be considered to provide the most representative estimates of relative effects.
Most differences between randomized and nonrandomized data were due to differences in magnitude of effect size rather than direction of effect size (cross clamp time, bypass time, EOAI, mean gradient, and hospital length of stay). In no case did the results of nonrandomized data provide compelling data that improved on the estimates provided by randomized trials.
To date, RCT do not show significant reductions in all-cause mortality during early or late follow-up for aortic valve replacement with stentless valves when compared with stented valves. Similarly, significant differences in stroke, myocardial infarction, arrhythmias, organ failure, and endocarditis, and risk of any complication were not found. Additionally, structural valve dysfunction, regurgitation, and reoperation for valvular complications were not measurably reduced with stentless versus stented aortic valves. It is noteworthy that for each of these outcomes there was insufficient power to rule out the possibility that a real and important difference might exist. Further RCT are required to definitively rule in or rule out whether differences exist. At this time, however, the totality of the evidence shows that no differences in clinical outcomes have been proven.
Although none of these important clinical outcomes were significantly impacted by the insertion of stentless instead of stented valves, there were a number of intermediate hemodynamic and cardiac parameters that were improved with stentless valves. The EOAI was increased at all time periods of follow-up beyond 30 days (Table 4). Mean and peak gradients were significantly reduced by 6 and 8 mm Hg, respectively, at 30 days, and this difference was maintained at maximal follow-up (3–10 years). Nevertheless, despite these reductions in effective orifice area and peak gradients, LVMI reductions did not reach significance for stentless valves versus stented valves in the randomized trials. Longer term data from nonrandomized trials suggests there is a potential for significant reduction in LVMI in survivors. However, there was heterogeneity between the single randomized trial reporting data up to 8 years (WMD −2, 95% CI −17 to +13 mm Hg) when compared with the aggregate estimate from three nonrandomized trials reporting this information at 2 to 10 years follow-up (WMD −19 mm Hg, 95% CI −35 to −2 mm Hg). This prevents definitive conclusions about whether the difference in the nonrandomized trials is due to real effects attributable to stentless valves or whether other biases played a role in the differences measured. Nonrandomized trials are particularly susceptible to selection bias (ie, differences in prognostically significant baseline characteristics due to differences in clinicians’ decisions to implant stentless valves in patients with different risk characteristics than for stented valves) and differential loss to follow-up between groups (ie, only survivors who are healthy enough to attend clinics will be followed up for hemodynamic measures over time, and retrospective trials are particularly susceptible to bias due to lost follow-up over time).
Unfortunately, the clinical relevance of reductions in EOAI, LVMI, and EF over time is unknown. Whether small or large reductions in any one or a combination of these parameters is sufficient to improve clinically important outcomes such as reduced need for reoperation, reduced adverse cardiac events, improved survival, and quality of life remains unknown. Insufficient data were provided in the trials regarding NYHA functional status, which would provide a more clinically relevant estimate of cardiac functionality than the intermediate outcomes of EOAI, LVMI, and EF. Quality of life was not measured in any of the trials.
For each clinical outcome reported, the CI were wide, suggesting that the available evidence to date provides insufficient power to rule out the possibility of clinically important differences, even when the data is aggregated through meta-analysis to maximize the power across existing studies. Much larger randomized trials will be necessary to determine whether true differences exist in the clinically relevant outcomes of death, stroke, myocardial infarction, endocarditis, patient function, and need for surgical reintervention.
Randomized Versus Nonrandomized Trials
It was encouraging to note that, for this meta-analysis which ascertained all comparative trials of stentless versus stented aortic valve replacement, there were more randomized trials that contributed data than nonrandomized trials. Examining the graphs for each of the outcomes reveals that the nonrandomized analysis provides little information beyond the randomized trials alone. This is encouraging, because it is more common for nonrandomized trials to outnumber randomized trials for many surgical interventions, particularly early in the development of the technology and learning curve associated with newer interventions, and surgeon investigators are to be congratulated for having conducted randomized trials before wide-spread dissemination of this new technology. Randomized trials are generally preferred to nonrandomized trials to circumvent the shortcomings that are inherent in nonrandomized data, including biases which result in prognostic differences due to differential selection and follow-up between intervention and control groups in nonrandomized reports. Furthermore, many nonrandomized trials compare noncontemporaneous groups of patients (either retrospectively or prospectively), with the risk of outcome differences being driven by variable historical practices and loss to follow-up.
Although it was planned a priori in the protocol for this meta-analysis to report both randomized and nonrandomized data, it became apparent after the analysis was complete that the nonrandomized studies provided little strength to the analysis.
Although randomized trials also had important shortcomings related to generalizability (patients were excluded due to difficult morphology, and aortic root configuration not amenable to stentless valves) and lack of power to adequately rule out the possibility of important difference between stentless and stented valves. Each included study was relatively small, and even after maximizing power through meta-analysis, the total amount of information remained insufficient to definitively rule out the possibility of clinically significant differences. The finding of no significant differences does not prove equivalence because there was insufficient power and subsequently wide CI for a number of the clinically important outcomes.
Strengths and Limitations
This comprehensive meta-analysis attempts to assess all relevant clinical outcomes in addition to intermediate hemodynamic and cardiac function outcomes between subcoronary stentless aortic valves and stented aortic valves. The analysis followed methodologically rigorous guidelines as recommended by QUOROM6 for creating a protocol for the analysis, ascertaining all relevant comparative trials, and reporting all clinically relevant outcomes. This analysis provides the best available estimate of the impact of stentless versus stented aortic valve replacement to date, and also highlights the limitations of the existing evidence which should be addressed by further randomized trials with sufficient power to rule in or rule out whether important differences exist between survival and patient function.
A number of limitations exist in this analysis, as a direct result of the shortcomings in the existing trials which provided the data for this analysis. The most important limitation is that very few trials reported late outcomes at greater than 1 year. As a result, no definitive conclusion regarding the impact (or lack thereof) of stentless valves on late morbidity and mortality can be given. If stentless valves improve hemodynamics, these differences may not translate into measurable differences in clinically relevant morbidities or survival until longer-term follow-up. The overall impact of stentless versus stented valves on survival differences, quality of life, valve durability, NYHA class, and patient functional differences over time remains unknown because studies have not addressed these outcomes over sufficient duration to allow potential differences to be assessed.
There are inherent limitations in patient generalizability, because patients who were thought to be unsuitable for stentless surgery were excluded from randomized trials (usually before randomization, but some after randomization if difficult morphology became known during surgery). In particular, patients were excluded if stentless valve implantation was not possible due to severe root wall calcifications that could not be removed surgically, or if the aortic root was dilated. Many studies did not detail exclusions explicitly, and it was not possible to determine whether exclusion criteria (ie, for difficult or borderline morphology or other high-risk patient characteristics) were similar across trials.
In a number of trials with follow-up beyond hospital discharge, the completeness of follow-up was not transparently reported. This may particularly affect the estimates of hemodynamics and survival beyond 1 year, if differential follow-up occurred between groups and if loss to follow-up was significant. Sensitivity analysis to explore the potential impact of loss to follow-up was not possible because insufficient data were provided in the studies.
There were insufficient trials comparing the same stentless valves with the same stented valves to adequately perform subanalyses among all of the different comparisons. As a result, the meta-analysis does not provide information that is specific for each valve type. Rather, it compares a variety of stentless valves with a variety of stented valves, in aggregate. Some studies have suggested that mortality seems to vary across different stented biologic valve substitutes, which are not explained by differences in structural deterioration and hemodynamic performance alone, and this raises the possibility that some of the heterogeneity measured within this meta-analysis might be due to differences in valve performance. This meta-analysis could not assess noncomparative observational longitudinal studies.
Evidence from randomized trials shows that subcoronary stentless aortic valves improve hemodynamic parameters of EOAI, mean gradient, and peak gradient over the short and long term. These improvements have not led to significant improvements in patient morbidity, mortality, and resource-related outcomes; however, few trials reported on clinical outcomes beyond 1 year and definitive conclusions are not possible until sufficient evidence addresses longer-term effects.
The authors acknowledge Ms. Aurelie Alger and Ms. Elizabeth Chouinard for their professional assistance in organizing the consensus conference; Dr. Guyan Wang, Dr. Kathy Fang, Dr. Myoung Kim, and Dr. Ling Pei for their expert data extraction; and Ms. Jennifer Podeszwa-deOliviera and Ms. Karla VanKessel for their services in facilitating the literature searches and retrieval.
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This is a beautifully written meta-analysis comparing stentless to stented bioprosthetic aortic valves. It is an important contribution to the literature and accompanies the consensus statement that appears in this issue. The evidence from the randomized trials showed that subcoronary stentless aortic valves improved the hemodynamic parameters of effective orifice area, mean gradient and peak gradient over the short and long term. However, these improvements did not have any measurable impact on patient morbidity, late mortality and resource-related outcomes. The principal weakness of the conclusions of this report is that few trials reported late outcomes of greater than 1 year. It may be that the improved hemodynamics of the stentless valves would not begin to show any differences in morbidity and mortality until late follow-up was available. Moreover, most of these studies only reported late mortality, and few looked at late functional outcomes in these patients, which may have been improved in the stentless population. Whether there were late improvements in patient morbidity or resource-related outcomes is unclear because of the infrequency of these variables being reported. This represents an inadequacy of our surgical literature, which tends to focus only on early mortality and may mask the benefits of new technology and techniques.
Keywords:© 2009 Lippincott Williams & Wilkins, Inc.
Meta-analysis; Aortic valve surgery; Stentless valve surgery