Author + information
- Received February 18, 2016
- Revision received May 13, 2016
- Accepted May 16, 2016
- Published online August 8, 2016.
- Prem A. Midha, MSa,
- Vrishank Raghav, PhDa,
- Jose F. Condado, MD, MSb,
- Ikechukwu U. Okafor, BSa,
- Stamatios Lerakis, MDb,
- Vinod H. Thourani, MDb,
- Vasilis Babaliaros, MDb and
- Ajit P. Yoganathan, PhDa,∗ ()
- ↵∗Reprint requests and correspondence:
Dr. Ajit P. Yoganathan, Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology & Emory University, Technology Enterprise Park, Suite 200, 387 Technology Circle, Atlanta, Georgia 30313-2412.
Objectives The purpose of this study was to optimize hemodynamic performance of valve-in-valve (VIV) according to transcatheter heart valve (THV) type (balloon vs. self-expandable), size, and deployment positions in an in vitro model.
Background VIV transcatheter aortic valve replacement is increasingly used for the treatment of patients with a failing surgical bioprosthesis. However, there is a paucity in understanding the THV hemodynamic performance in this setting.
Methods VIV transcatheter aortic valve replacement was simulated in a physiologic left heart simulator by deploying a 23-mm SAPIEN, 23-mm CoreValve, and 26-mm CoreValve within a 23-mm Edwards PERIMOUNT surgical bioprosthesis. Each THV was deployed into 5 different positions: normal (inflow of THV was juxtaposed with inflow of surgical bioprosthesis), −3 and −6 mm subannular, and +3 and +6 mm supra-annular. At a heart rate of 70 bpm and cardiac output of 5.0 l/min, mean transvalvular pressure gradients (TVPG), regurgitant fraction (RF), effective orifice area, pinwheeling index, and pullout forces were evaluated and compared between THVs.
Results Although all THV deployments resulted in hemodynamics that would have been consistent with Valve Academic Research Consortium-2 procedure success, we found significant differences between THV type, size, and deployment position. For a SAPIEN valve, hemodynamic performance improved with a supra-annular deployment, with the best performance observed at +6 mm. Compared with a normal position, +6 mm resulted in lower TVPG (9.31 ± 0.22 mm Hg vs. 11.66 ± 0.22 mm Hg; p < 0.01), RF (0.95 ± 0.60% vs. 1.27 ± 0.66%; p < 0.01), and PI (1.23 ± 0.22% vs. 3.46 ± 0.18%; p < 0.01), and higher effective orifice area (1.51 ± 0.08 cm2 vs. 1.35 ± 0.02 cm2; p < 0.01) at the cost of lower pullout forces (5.54 ± 0.20 N vs. 7.09 ± 0.49 N; p < 0.01). For both CoreValve sizes, optimal deployment was observed at the normal position. The 26-mm CoreValve, when compared with the 23-mm CoreValve and 23-mm SAPIEN, had a lower TVPG (7.76 ± 0.14 mm Hg vs. 10.27 ± 0.18 mm Hg vs. 9.31 ± 0.22 mm Hg; p < 0.01) and higher effective orifice area (1.66 ± 0.05 cm2 vs. 1.44 ± 0.05 cm2 vs. 1.51 ± 0.08 cm2; p < 0.01), RF (4.79 ± 0.67% vs. 1.98 ± 0.36% vs. 0.95 ± 1.68%; p < 0.01), PI (29.13 ± 0.22% vs. 6.57 ± 0.14% vs. 1.23 ± 0.22%; p < 0.01), and pullout forces (10.65 ± 0.66 N vs. 5.35 ± 0.18 N vs. 5.54 ± 0.20 N; p < 0.01).
Conclusions The optimal deployment location for VIV in a 23 PERIMOUNT surgical bioprosthesis was at a +6 mm supra-annular position for a 23-mm SAPIEN valve and at the normal position for both the 23-mm and 26-mm CoreValves. The 26-mm CoreValve had lower gradients, but higher RF and PI than the 23-mm CoreValve and the 23-mm SAPIEN. In their optimal positions, all valves resulted in hemodynamics consistent with the definitions of Valve Academic Research Consortium-2 procedural success. Long-term studies are needed to understand the clinical impact of these hemodynamic performance differences in patients who undergo VIV transcatheter aortic valve replacement.
Valve-in-valve (VIV) transcatheter aortic valve replacement (TAVR) has emerged as a treatment for high surgical risk patients with failing aortic surgical bioprostheses (1,2). Recently, the Food and Drug Administration has approved both balloon-expandable (SAPIEN XT, Edwards Lifesciences, Irvine, California) and self-expanding (CoreValve, Medtronic, Dublin, Ireland) transcatheter heart valves (THV) for this indication in the United States. Although VIV-TAVR may restore valve function and improve symptoms, adverse events such as increased post-procedural gradients (28.4%), coronary obstruction (3.5%), device malpositioning (15.0%), and valve leaflet thrombosis (4%) have been reported (3–6). A lack of understanding of how VIV deployment location affects THV hemodynamics may explain some of these untoward events.
Current sizing and deployment recommendations are on the basis of reference guides that use valve true internal diameters for THV size selection. As a consequence, commonly used guides, such as the VIV Aortic app (7) and the THV manufacturer’s instructions for use (IFU) for deployment in native aortic valves can recommend a different THV size for the same surgical bioprosthesis size (refer to the Online Appendix). At this time, no evidence-based industry sizing or positioning guidelines for VIV-TAVR exist, although it is approved by the U.S. Food and Drug Administration. Furthermore, recent studies suggest that in cases of extreme oversizing of the THV, a supra-annular deployment can result in superior hemodynamics for a balloon-expandable valve in a small bioprosthesis than in the deployment location recommended by the existing guidelines (8–10). In the current study, we investigate whether the drastic effects of supra-annular deployment seen in a small bioprosthesis were still present when there was less prosthesis–patient mismatch. We performed an in vitro study to better understand THV hemodynamics according to valve type, degree of oversizing, and deployment location for balloon- and self-expanding VIV-TAVR.
This study was conducted in a validated pulse duplicator (Figure 1) that simulates physiologic and pathophysiologic conditions of the heart (11). A noncalcified surgical bioprosthesis was mounted into an idealized rigid acrylic chamber designed to simulate the aortic sinus and ascending aorta (Figure 2). The chamber dimensions were based off of published average anatomic measurements (12,13). The aortoventricular angle in the left heart simulator is 0°, which is the standard configuration for in vitro TAVR testing for Food and Drug Administration submissions. The flow rate and the aortic and ventricular pressures were tuned to physiologic levels through a lumped systemic resistance and compliance and measured through a custom data acquisition system. The working fluid was a 3.5-cSt saline–glycerine solution (approximately 36% glycerine by volume in 0.9% NaCl) to match the kinematic viscosity of blood. Further details of the flow loop are provided in our previous publication (8).
Valve models and deployment
A 23-mm Edwards PERIMOUNT surgical bioprosthesis was implanted in the in vitro model. This surgical bioprosthesis type and size was chosen because it is the among the most commonly encountered in general practice (14,15). In addition, this surgical valve type and size has multiple recommended THV sizes depending on the guidelines used. For the VIV-TAVR model, THV size selections were on the basis of the recommendations by the VIV app and IFU for deployment in native aortic valves. For the 23-mm Edwards PERIMOUNT, both guidelines recommend a 23-mm SAPIEN valve, but the VIV app recommends a 23-mm CoreValve Evolut and the IFU recommends a 26-mm CoreValve. In the current study, a 23-mm SAPIEN, a 26-mm CoreValve, and a 23-mm CoreValve Evolut were deployed within a 23-mm Edwards PERIMOUNT surgical bioprosthesis in the following 5 positions: normal (0 mm; bottom of the THV stent aligned with the bottom of the surgical bioprosthesis sewing ring, as indicated by the ViV Aortic app); −3 and −6 mm below the normal position; and +3 and +6 mm above the normal position (Figure 3).
All valves used in this study were previously unused and noncalcified, and the same THVs were used for all 5 deployment positions. To minimize unnecessary deployments of the balloon-expandable SAPIEN, the valve was never crimped beyond what was required to insert it into the surgical valve (∼21 mm).
Migration was assessed visually by observing the relationship between THV stent struts and the surgical valve stent posts through the optically clear flow chamber before and after flow testing. For the 23-mm CoreValve Evolut, the THV migrated to an extreme subannular position under physiologic conditions (Figure 2). This was believed to be due to the lack of calcification and decreased distal aorta anchoring force in our model and, thus, the testing of this THV was performed with the valve artificially tethered in the desired deployment location.
VIV performance was characterized using mean transvalvular pressure gradients (TVPG) and regurgitant fraction (RF). The flow loop was tuned to mean arterial pressure of 100 mm Hg (Figure 1), a heart rate of 70 beats/min, and a cardiac output of 5 l/min with a peak instantaneous systolic flow rate of 25 l/min and a systolic duration of 35%. Two hundred consecutive cardiac cycles of hemodynamic data (aortic pressure, ventricular pressure, and flow rate) were collected for each test condition at a 1 kHz data sampling rate using a custom LabVIEW Virtual Instrument.
The level of aortic regurgitation was assessed through comparison of the RF, and was computed from the measured flow waveforms using the following equation:(Equation 1)where LV is the leakage volume and SV is the forward stroke volume. The leakage volume is obtained by subtracting the closing volume from the total regurgitant volume.
Aortic valve area
Effective orifice area (EOA) was computed through the Gorlin equation (16):(Equation 2)where Q is the flow through the aortic valve. This metric is used to evaluate residual stenosis after VIV-TAVR.
On the basis of ISO heart valve guidelines, localized bending of leaflet material, or pinwheeling, should be avoided due to potential for premature tissue degradation (17). This was quantified using en face data by tracing the length of the leaflet free edges and comparing them to their unconstrained, ideal configurations as described in Figure 4. A pinwheeling index (PI) was computed by following equation:(Equation 3)where is the length of the leaflet free edge from the perimeter of the valve to the coaptation center, and is the straight line distance between the endpoints of the leaflet free edge. The value is presented as a percentage.
Relative embolization risk for each VIV deployment was evaluated by measuring the pullout force required to dislodge the valve in a different apparatus than the flow studies, and without the downstream tethering. The apparatus and method used to measure pullout force are similar to those described in our previous study (8); however, the downstream region of the CoreValve’s stent provides resistance to embolization and necessitated the inclusion of the ascending aorta in our test fixture. The ascending aorta was coated in polytetrafluoroethylene matching the coefficient of friction between nitinol and calcified aortic tissue (approximately 0.15) as reported by previous studies (18–20). Force was applied gradually until the valve migrated and the peak force measured by a digital force gauge was recorded. Each test condition was repeated 6 times.
The data are presented as a mean ± SD. Normality of all the data were tested using the Anderson-Darling method. One-way analysis of variance was used for analyzing independent sample sets with Tukey’s post hoc test for comparisons between multiple groups. The analysis was done using SPSS Statistics for Mac, Version 22.0 (IBM Corp., New York); p < 0.05 was considered significant. A full presentation of the p values from the post hoc tests can be found in the Online Materials.
The TVPG is reported as a mean ± SD measured over 200 consecutive cardiac cycles (Figure 5). Although we observed that all the measured mean gradients were below the Valve Academic Research Consortium-2 criterion for success of <20 mm Hg (21), there were differences between deployment locations according to the valve type. Under the VIV app defined normal deployment positions, the 26-mm CoreValve displayed a lower mean gradient than the 23-mm CoreValve and 23-mm SAPIEN (7.76 ± 0.14 mm Hg vs. 10.27 ± 0.18 mm Hg vs. 11.66 ± 0.22 mm Hg; p < 0.01). Both CoreValves performed optimally in the normal to -3-mm deployment range, with the mean TVPGs increasing with supra-annular and subannular implantation of the THV. On the other hand, the SAPIEN performed better with increasingly supra-annular deployment. The mean TVPG at the 6-mm supra-annular deployment is approximately 20% lower than at the normal position (9.31 ± 0.22 mm Hg vs. 11.66 ± 0.22 mm Hg; p < 0.01), whereas a 6-mm subannular deployment increased the gradient by a similar amount.
The RF is reported as a mean ± SD measured over 200 consecutive cardiac cycles, and was observed to change with valve type and deployment position (Figure 6). Because all deployments demonstrated good coaptation and the closing volume was removed for calculation of RF, the RF value is paravalvular leak (PVL). For the SAPIEN valve, RF progressively decreased as the deployment height increased from −6 to +6 mm (7.40 ± 0.21% to 0.95 ± 0.60%; p < 0.01). However, for both CoreValves, subannular and supra-annular deployment resulted in higher RF, with the minimum RF observed at the normal deployment position (26-mm: 4.79 ± 0.67%; 23-mm: 1.98 ± 0.36%; p < 0.01). Furthermore, the RF of the 26-mm CoreValve was greater than that of the 23-mm CoreValve at all deployment positions.
Aortic valve area
The EOA is presented as a mean ± SD measured over 200 consecutive cardiac cycles. The variation of EOA with deployment height for all the THVs (Figure 7) mirrored the corresponding changes in mean TVPG. For each THV, the lowest TVPG corresponded to the highest EOA (26 mm CoreValve: 1.66 ± 0.05 cm2; 23 mm CoreValve: 1.44 ± 0.05 cm2; 23 mm SAPIEN: 1.51 ± 0.08 cm2; p < 0.01).
The PI values were computed as a means of quantifying the level of excessive leaflet deformation occurring at each deployment, and are presented as a mean ± SD (n = 10) (Figure 8). The SAPIEN had the least amount of pinwheeling when compared with both CoreValve sizes at all the deployment positions (p < 0.01). At the normal deployment position, the 26-mm CoreValve had significantly greater (approximately 4.5 times) pinwheeling when compared with the 23-mm CoreValve (29.13 ± 0.22% vs. 6.57 ± 0.14%; p < 0.01). Representative extreme pinwheeling cases for each valve can be seen in Online Video 1.
The pullout forces are reported as a mean ± SD (n = 6). The pullout forces were measured only for supra-annular THV deployments because for both the SAPIEN and CoreValves the hemodynamic performance was suboptimal at subannular deployment (Figure 9). In all cases it was observed that the pullout forces decreased with supra-annular deployment. Furthermore, at all deployment locations, the pullout force was the highest for the 26-mm CoreValve, followed by the 23-mm SAPIEN, with the 23-mm CoreValve having the lowest pullout forces. None of the THVs experienced antegrade migration under any of the study conditions; however, the 23-mm CoreValve Evolut did migrate to an extreme subannular position unless it was restrained physically (Figure 2).
In the case of the SAPIEN, retrograde migration was not a concern because the supra-annular “flower pot” deployment provides additional “geometric” resistance. Systolic hemodynamic forces on the THV were estimated on the basis of the work from Dwyer et al. (22) and are detailed in our previous work (8). A baseline safety threshold was computed to be 0.25 N by Equations 4 and 5(Equation 4)(Equation 5)where is the maximum surgical bioprosthesis geometric orifice area (GOA) on the basis of the internal diameter of the valve, and is the geometric orifice area determined via en face imaging of the deployed THV. This is interpreted as the amount of force the fluid imposes on the THV under a peak systolic gradient and represents the theoretical lower limit of pullout force necessary to avoid antegrade embolization. Under a conservative diastolic gradient of 100 mm Hg; however, the self-expandable THVs could migrate toward the left ventricle, as was seen with the 23-mm CoreValve (Figure 2). This diastolic fluid force in Newtons may be computed by(Equation 6)where the mean arterial pressure (MAP) is the diastolic gradient in mm Hg, is the true internal diameter of the surgical bioprosthesis in millimeters, and SAVR is the surface area to volume ratio). This theoretical maximum diastolic fluid force amounts to 4.62 N, which is in the similar range to that of the 23-mm CoreValve deployment pullout force measurements.
In an in vitro model simulating VIV-TAVR in a 23-mm surgical bioprosthesis, we observed that THV hemodynamics met Valve Academic Research Consortium-2 definitions of success (21) for both the SAPIEN and CoreValve THVs. However, hemodynamic performance varied with THV type, size, and deployment position. The results of this study (Table 1) suggest that optimal VIV deployment positions exist for both the balloon- and self-expandable THV designs. These optimal positions were determined through an analysis of benefits (mean gradient, valve area) and risks (PVL, leaflet deformation, and embolization risk). In the case of the SAPIEN, the optimal position is at a supra-annular deployment around 6 mm above what is recommended by the ViV Aortic guidance application (7). In contrast, the optimal deployment for the CoreValve exists at the ViV Aortic application’s recommended implantation height. It should be noted that, whereas THVs were developed and tested under current ISO standards, existing recommendations for VIV-TAVR were not developed under such rigorous testing standards. In addition, a deployment that resulted in “device success” in this study is likely to be affected by the characteristics of a stenotic, calcified bioprosthesis in vivo. Thus, these results provide important information on where not to implant THVs, and provide a guide for the in vivo evaluation of possible deployment locations.
The improvement in gradients at supra-annular positions for the balloon-expandable valve can be explained by a more complete expansion of the distal portion of the THV, resulting in a “flower pot” deployment. This finding is similar to our previous study; however, the effects of supra-annular deployment are not as drastic as the 20 mm Hg reduction in mean TVPG seen in a 19-mm PERIMOUNT (8). However, in the case of the CoreValve, the leaflets are already in a supra-annular position (Figure 3), and thus, are less constrained by the bioprosthesis and surrounding anatomy, resulting in optimal gradients at the normal position. Furthermore, the IFU-recommended 26-mm CoreValve had significantly lower gradients when compared with the ViV Aortic application recommended 23-mm CoreValve. This discrepancy highlights the need for hemodynamic performance-based guidelines for VIV-TAVR, as opposed to the ViV Aortic app guidelines, which are strictly on the basis of geometric observations. It also shows that an oversized THV could yield significant improvements in mean post-procedural gradients. As expected, a higher EOA correlated with a lower TVPG for all THVs at all positions.
The occurrence of regurgitation, especially in the form of PVL, is an important concern for both native valve and VIV-TAVR, because moderate or greater PVL has been associated with worse outcomes after TAVR (23,24). The lowest leakage was observed at the same positions that resulted in the lowest TVPG, further supporting our recommendation for a supra-annular deployment of the SAPIEN valve, and for a normal deployment of the CoreValve. The 23-mm CoreValve valve had substantially lower leakage than the 26-mm CoreValve; however, both THVs had <5% leakage at the normal deployment position. This difference could be due to incomplete expansion of the oversized 26-mm CoreValve in the semirigid bioprosthesis. Although THV oversizing results in higher rates of conduction disturbances and annular rupture (25,26), it has been hypothesized that the “protective effect” of the surgical bioprosthesis ring explains the lower rates of these complications observed with VIV-TAVR (3,27). Although the leakage for all the valves reported in this study would be classified as mild to mild/moderate regurgitation (21,28,29), it should be noted that the in vitro setting represents a conservative scenario. Patient specific anatomic variations and leaflet calcification profiles could result in suboptimal expansion of the valve leading to higher PVL. Notably, for all valves the highest leakage was observed at the lowest subannular position (−6 mm), where the sealing of the THV skirt is lower/minimal. Due to aortic stenosis patients’ inability to tolerate PVL, it is critical to avoid suboptimal deployment during VIV-TAVR.
In the case of the CoreValve at subannular and supra-annular positions, the leaflets were observed to deform to a greater extent when compared with the PERIMOUNT. Hence, the PI was developed as a novel metric to quantify the severity of this phenomenon. The very mild levels of leaflet deformation exhibited by the control surgical valve were used as a reference for an acceptable PI. The SAPIEN had a lower PI when compared with both CoreValves at all deployment positions. The incomplete expansion of the oversized CoreValve could result in higher deformation of the THV stent and leaflets leading to higher PI. A counterintuitive finding of this study was that supra-annular deployment of a 26-mm CoreValve resulted in higher gradients, yet little change in the PI than the normal deployment. We speculate that, due to the supra-annular leaflet position of the CoreValve, the theoretical benefit of decreased leaflet distortion with a high deployment is lost due to concomitant inlet constriction observed with this deployment location (Figure 10). Greater pinwheeling is associated with increased fatigue loading and accelerated failure of the bioprosthetic leaflets (17). Recent in vitro work on understanding the effect of valve oversizing during TAVR has demonstrated 2 important results: first, the level of pinwheeling increased with THV oversizing; second, increased pinwheeling was associated with increased leaflet stress levels (30). Although THVs are oversized by necessity to generate adequate anchoring force, excessively increased stress could lead to decreased leaflet durability, and is a very important concern when considering TAVR for a lower risk patient population. More detailed in vitro and clinical studies are critically needed to determine if such quantitative findings will have a deleterious effect on short-term (<5 years) and/or long-term (∼10 years) THV durability. As medical imaging advances, such as high-speed 4-dimensional computed tomography (6), it may be possible to quantify pinwheeling in vivo.
To assess the safety of our hemodynamic performance-based deployment recommendations, we evaluated the risk of THV embolization. The greatest pullout force was observed at the normal position for all the THVs which is due to the higher contact area between the THV and surgical bioprosthesis, which is consistent with previous work (8). Due to the minimal systolic fluid forces on the THV, antegrade migration does not seem to be a high-risk concern. However, under a diastolic gradient, the valve that holds the greatest risk of retrograde embolization is the 23-mm CoreValve, where the pullout force at all positions were very close to the 4.62 N diastolic threshold. Although a calcified bioprosthesis may provide additional anchoring for the valve in vivo, this particular VIV combination cannot be recommended by this in vitro study due to observed retrograde migration (Figure 2) in a noncalcified bioprosthesis. This finding underscores the need to weigh the risks and benefits when using this valve combination in vivo.
We performed a controlled parametric in vitro study of VIV-TAVR in a 23-mm PERIMOUNT. On the basis of an analysis of benefits (mean gradient, valve area) versus risks (PVL, leaflet deformation, and embolization), the optimal deployment for the 23-mm SAPIEN valve is at +6 mm supra-annular position, and at the normal position for both the 23-mm and 26-mm CoreValves. Furthermore, the 26-mm CoreValve had better hemodynamic performance than the 23-mm CoreValve at the normal position, but at the risk of higher PI and potentially reduced leaflet durability. It is reassuring that all VIV-TAVR deployments were consistent with the definitions of Valve Academic Research Consortium-2 procedural success (21). We acknowledge that extreme supra-annular deployment may not always be possible on the basis of anatomic and procedural constraints, and that it may be risky for operators with limited TAVR experience. In addition, newer THV and surgical valve design features may pose additional challenges to be overcome in the pursuit of an optimal VIV deployment. For instance, it is unknown what affect the SAPIEN 3 skirt may have on forces resisting migration, as well as inflow resistance in a high implantation VIV scenario. Although this study has demonstrated the robustness of these THVs, it underscores the need for rigorous performance- and safety-based recommendations. The clinical, industrial, and regulatory communities need to be aware that such risk/benefit analyses are critical going forward for the use of THVs in VIV applications, especially in the lower risk patient population.
WHAT IS KNOWN? Currently, contradicting and/or incomplete recommendations for valve-in-valve transcatheter aortic valve replacement leave many outcomes up to chance. This study performs an in vitro risk–benefit analysis to highlight that optimal performance may lie outside existing recommendations.
WHAT IS NEW? On the basis of the evidence from this work, the authors would recommend a supra-annular 23-mm SAPIEN or normally (defined by ViV Aortic App) positioned 26-mm CoreValve in a failing 23-mm PERIMOUNT. However, clinicians are strongly urged to consider patient-specific anatomic characteristics and carefully weigh the benefit of high THV implantation in reducing post-procedural gradients against the potential risks of coronary obstruction and device migration in potential candidates for valve-in-valve transcatheter aortic valve replacement.
WHAT IS NEXT? Similar in vitro studies utilizing other surgical bioprostheses are critically important to optimize VIV performance and patient outcomes.
The authors acknowledge the members of the Cardiovascular Fluid Mechanics Laboratory for their assistance and feedback. The work at the Cardiovascular Fluid Mechanics Laboratory at the Georgia Institute of Technology was funded through discretionary funds available to the Principal Investigator, such as the Wallace H. Coulter Endowed Chair.
For supplemental tables and a video, please see the online version of this article.
Dr. Vinod H. Thourani, is a consultant or researcher for Edwards Lifesciences, Medtronic Corporation, Claret Medical, St. Jude Medical, LivaNova, Boston Scientific, and Abbott Medical. Drs. Babaliaros and Lerakis are consultants or researchers for Edwards Lifesciences and Abbott Medical. Dr. Yoganathan is a consultant or researcher for St. Jude Medical and Boston Scientific. All other authors have reported that they have no relationships relevant to the contents of this article to disclose.
- Abbreviations and Acronyms
- effective orifice area
- geometric orifice area
- instructions for use
- pinwheeling index
- paravalvular leak
- regurgitant fraction
- transcatheter aortic valve replacement
- transcatheter heart valve
- transvalvular pressure gradient
- Received February 18, 2016.
- Revision received May 13, 2016.
- Accepted May 16, 2016.
- American College of Cardiology Foundation
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