Keywords

Introduction

Calcific aortic stenosis is the most common acquired valvular heart disease, and its prevalence is increasing significantly with the aging population. Once symptoms of angina, syncope, or heart failure develop, average survival is 2–3 years [1]. Traditionally, surgical aortic valve replacement (SAVR) has been the definitive treatment for severe symptomatic aortic stenosis; however, many patients do not undergo surgery as they are considered high risk or inoperable due to age and comorbidities [2].

Percutaneous transcatheter aortic valve replacement (TAVR) has been developed as an alternative to surgical AVR in those who are high risk or not candidates for SAVR. TAVR was first described in humans by Cribier in 2002 [3]. The transcatheter valve was delivered via an anterograde technique from the venous circulation, by subsequent transseptal puncture into the left heart and passage of the valve through the mitral and into the aortic position. Later, a transarterial retrograde approach was developed—a technique in wide use today [4]—and followed by additional approaches including transapical, transaxillary, and transaortic techniques.

Since its advent, TAVR has been performed in over 50,000 patients worldwide. TAVR has evolved into a reproducible procedure with steadily improving clinical outcomes and is now emerging as the standard of care in patients with severe aortic stenosis who are considered inoperable [5] and an accepted alternative to surgery for patients at high risk for conventional aortic valve replacement [6].

Techniques of Transcatheter Aortic Valve Replacement

TAVR is most often performed utilizing the transfemoral retrograde approach, with alternative access approaches usually reserved for patients with concomitant severe peripheral arterial disease.

Transfemoral Approach

The common femoral artery, at the level of the femoral head, is the primary access site for the transfemoral approach owing to its relatively large size and compressibility. The side with the largest and least diseased, tortuous, or calcified iliofemoral artery—as assessed by a screening angiogram and/or multidetector computed tomographic (CT) angiography—is selected for placement of the sheath. The potential site of access is assessed first with fluoroscopy and/or ultrasound, and arterial access is gained by percutaneous puncture. Alternatively, a surgical cutdown is utilized to access the femoral artery. A smaller percutaneous sheath is inserted into the femoral artery on the contralateral side for placement of a pigtail catheter in the ascending aorta for root angiography.

The TAVR procedure may be performed under local or general anesthesia. Following balloon valvuloplasty, the valve prosthesis is passed across the aortic valve and positioned under fluoroscopic and transesophageal echocardiographic (TEE) guidance. Balloon-expandable valves are deployed under rapid ventricular pacing at a rate of 160–220 bpm to minimize cardiac output and therefore minimize unintentional motion of the valve during balloon dilatation. Self-expanding valves are advanced across the aortic valve within a delivery catheter and deployed by retracting the outer sheath of the delivery catheter generally without rapid pacing. Once the delivery system is retrieved, the femoral access site is closed utilizing previously inserted percutaneous sutures or repaired surgically if a surgical cutdown was used.

Transapical Approach

The transapical approach was first described in 2006 with balloon-expandable valves [7]. A sheath is placed surgically in the left ventricular apex, accessed through a small left anterolateral minithoracotomy. Following balloon valvuloplasty, the valve prosthesis and balloon catheter are passed over a wire into the left ventricle and positioned within the aortic annulus under fluoroscopic and transesophageal echocardiographic guidance.

This approach may be considered if the iliofemoral arterial system is of sufficiently small diameter, calcified, or tortuous and not technically suitable for delivery of the device. Also taken into consideration is the angulation of the aorta and arch. In particular, a transverse or extremely unfolded ascending aorta may increase the difficulty of delivery and positioning of the balloon-expandable devices. It has been suggested that with advances in device technology and a reduction in delivery system profiles for the transarterial approach, alternative access approaches will be limited to less than 30 % of TAVR procedures [8]. Contraindications to the transapical approach include previous left ventricular surgery using a patch, calcified pericardium, severe respiratory disease, and a non-reachable left ventricular apex [9].

Axillary/Subclavian Approach

The axillary/subclavian approach is an alternative access route in those excluded from the transfemoral approach; however, it has the similar limitations with respect to vessel caliber, calcification, and tortuosity. The left axillary artery is surgically isolated through a sort subclavicular incision although percutaneous access and closure are occasionally utilized. The sheath is passed through the axillary artery and into the ascending aorta, and the transcatheter heart valve (THV) is delivered in a retrograde fashion across the native aortic valve [10, 11].

Transaortic Approach

Recently, direct access through the ascending aorta has been described [12]. This involves a minimally invasive right second or third intercostal minithoracotomy or ministernotomy with introduction of the THV directly into the ascending aorta and retrograde passage across the diseased aortic valve. This approach has been utilized for patients in whom transarterial approaches are suboptimal and in those with chest wall abnormalities, severe respiratory disease, or poor left ventricular function, excluded from the transapical approach [13]. Potential advantages over the transapical approach include reduced myocardial injury and bleeding, in addition to a more familiar surgical technique to cardiac surgeons.

Transcatheter Valves

Two THV designs have been widely used in clinical practice over the last few years: the balloon-expandable Edwards SAPIEN and SAPIEN XT valves (Edwards Lifesciences Inc., Irvine, USA) and the CoreValve ReValving System.

Edwards SAPIEN and SAPIEN XT Valves

The Edwards transcatheter heart valves (Edwards Lifesciences Inc., Irvine, USA), available in SAPIEN™ and the newer-generation SAPIEN XT™, consist of bovine pericardial leaflets, mounted on a balloon-expandable frame (Fig. 4.1). The pericardium is treated with an anti-calcification treatment used in surgical heart valves. The ventricular portion of the frame is covered with a fabric sealing cuff to provide an annular seal for minimization of post-TAVR paravalvular leak.

Fig. 4.1
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Current balloon-expandable transcatheter heart valves: Edwards SAPIEN THV (a) schemata and (b) following deployment with aortography and Edwards SAPIEN XT THV (c) schemata and (d) following deployment with aortography

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The SAPIEN™ THV utilizes a stainless steel frame and is available in two sizes, with external diameters of 23 and 26 mm and heights of 14.3 and 16.1 mm, respectively, once fully deployed. The newer SAPIEN XT™ THV consists of a cobalt chromium frame; the strength of the alloy is greater allowing a reduction in strut thickness with fewer stent struts and therefore a lower-profile device for delivery. As a result, radial strength may be lower. It is available in four sizes, with external diameters of 20, 23, 26, and 29 mm and heights of 13.5, 14.3, 17.2, and 19.1 mm, respectively (Fig. 4.2).

Fig. 4.2
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Edwards SAPIEN XT dimensions and recommended annulus sizes

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The current manufacturer’s recommendations are to select device size according to two-dimensional long-axis TEE measurements of the aortic annulus, although emerging evidence by CT has challenged the accuracy of such measurements. The 23 mm valve is recommended for use in patients with annulus diameters of 18–22 mm and the 26 mm in patients with annulus diameters of 21–25 mm, while the 29 mm SAPIEN XT valve is recommended for use in patients with annulus diameters of 24–27 mm.

The Edwards THVs are generally implanted utilizing a transarterial or transapical approach. For transarterial implantation (Fig. 4.3), the SAPIEN THV is compressed onto the Retroflex 3 delivery system that is introduced into the femoral artery via a 22 or 24 F sheath for the 23 and 26 mm valves, respectively. The newer-generation SAPIEN XT is crimped onto the lower-profile Novaflex delivery system, resulting in lower sheath sizes of 16, 18, and 20 F for the 23, 26, and 29 mm valves, respectively (Fig. 4.4). This lower-profile delivery system has expanded the number of patients eligible for the transfemoral approach who would otherwise have been excluded due to small vessel diameter and has resulted in a significant reduction in vascular complications [14]. Recommended minimal luminal diameters of the iliofemoral system are demonstrated in Table 4.1 for each valve and sheath size. These recommendations, however, do not take into account vessel calcification, atheroma, and tortuosity. In the absence of severe disease, a relatively compliant artery may accommodate a slightly larger sheath size.

Fig. 4.3
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Transfemoral delivery of Edwards balloon-expandable transcatheter heart valve. (a) Passage of the valve across the aortic arch. (b) Crossing the native aortic valve. (c) Positioning of the valve utilizing aortography and transesophageal echocardiography. (d) Partial balloon expansion of the transcatheter heart valve. (e) Full balloon expansion of the transcatheter heart valve. (f) Post implantation aortography

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Fig. 4.4
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Edwards balloon-expandable delivery systems. Retroflex 3 delivery system with (a) crimped valve and (b) balloon-expanded valve and Novaflex + delivery system with (c) valve crimped and (d) balloon-expanded valve

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Table 4.1 Edward SAPIEN and SAPIEN XT sizes, sheath dimensions, and recommended minimum artery diameter
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For the transapical approach, the THV is crimped onto the balloon and delivered using the shorter Ascendra delivery system for the SAPIEN THV and the Ascendra 2 delivery system for the SAPIEN XT.

CoreValve

The CoreValve ReValving System™ (Medtronic Inc., Irvine, USA) consists of porcine pericardial leaflets mounted within a self-expanding nitinol frame (Fig. 4.5). The current-generation device is compressed within its Accutrak™ delivery catheter and introduced through an 18 French sheath. It is manufactured in 26, 29, and 31 mm outer diameters, as measured at the ventricular part of the frame, recommended for use in patients with annulus diameters of 20–23, 23–27, and 26–29 mm, respectively. The frame has an overall length of up to 55 mm, and in addition to anchoring at the level of the annulus and valve leaflets, the CoreValve extends superiorly to anchor in the ascending aorta.

Fig. 4.5
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CoreValve ReValving System (a) schemata and (b) following deployment with aortography

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Careful evaluation of the aortic root and ascending aorta, in addition to that of the aortic annulus, is required to avoid compromise of the coronary artery ostia and avoid valve embolization (Fig. 4.6). It is recommended that a coronary ostial height of 15 mm is required to ovoid coronary obstruction if the 12 mm skirt is deployed too aortic. Additionally, the sinus of Valsalva width needs to be greater than 27 mm for the 26 mm device and greater than 29 mm for the larger devices, to ensure a space surrounding the central waist of the device, to house the displaced native valve leaflets. The recommended maximum diameter of the ascending aorta is 40, 43, and 43 mm for the 26, 29, and 31 mm devices, respectively, to allow for satisfactory anchoring of the device at its upper portion (Table 4.2). Care must be taken in those patients with bypass grafts to ensure the device does not interfere with graft ostia.

Fig. 4.6
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Aortic root and annulus measurements required for CoreValve demonstrated on (a) schemata and (b) aortography. The required measurements include (1) aortic annulus diameter, (2) sinus of Valsalva width, (3) ascending aorta width, and (4) sinus of Valsalva height

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Table 4.2 CoreValve dimensions and manufacturer’s recommendations
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Since the CoreValve is self-expanding, it has the potential benefit of partial retrieval if positioned incorrectly (Fig. 4.7). Disadvantages when compared with the SAPIEN valves include a higher rate of atrioventricular block, pacemaker insertion, paravalvular regurgitation, and the potential late consequences of covering the coronary ostia.

Fig. 4.7
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Delivery of the CoreValve ReValving System: (a) Crossing the native aortic valve. (b) Retraction of the outer sheath. (c) Partial release of the transcatheter heart valve at which point it is still possible to retrieve the valve. (d) Full expansion of the prosthesis extending from just below the insertion of the native leaflets to the supracoronary aorta

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Newer Valves

Several newer transcatheter heart valves are under evaluation, including the Lotus™ valve (Boston Scientific Inc., USA), Portico™ (St Jude Medical Inc., USA), Engager™ (Medtronic Inc., USA) [15], JenaClip™ (JenaValve Inc., DE) [16, 17], Acurate (Symetis Inc., CH), and Direct Flow™ valves (Direct Flow Medical Inc., USA) (Fig. 4.8) [18, 19]. Generally, these next-generation valves are self-expanding and include features that may facilitate valve positioning and improve annular sealing, with lower-profile delivery systems to accommodate smaller vessel diameters. These valves are currently under clinical evaluation and it remains unclear whether outcomes will be inferior, comparable, or superior to current-generation THVs.

Fig. 4.8
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Newer transcatheter heart valves under evaluation: (a) Lotus valve (Boston Scientific Inc. USA), (b) Portico valve(St. Jude Medical Inc. USA), (c) Engager valve(Medtronic Inc. USA), (d) JenaClip valve (JenaValve Inc. DE), (e) Acurate valve (Symetis Inc. Ch), and (f) Direct Flow valve (Direct Flow Medical Inc. USA)

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Patient Selection and Evaluation

Although feasible in most patients with severe aortic stenosis, TAVR is generally utilized in patients not suitable for surgical AVR, who are likely to derive functional and survival benefit. The evaluation of potential TAVR patients is a complex process involving a multidisciplinary team approach; key members of the team include the primary cardiologist, interventional cardiologist, cardiothoracic surgeon, vascular surgeon, cardiac anesthesiologist and perfusionist, echocardiographers, and cardiac imaging specialists, as well as specialized nursing. This multidisciplinary approach is also applied during performance of the procedure and postoperative care of the patient [20].

“High-risk” surgical patients are often defined as having a Society of Thoracic Surgeons Predicted Risk of Mortality (STS-PROM) at 30 days of greater than 10 % or logistic EuroSCORE of greater than 20 % (Box 4.1). These risk calculators are often limited, however, and do not account for several pertinent clinical risk factors, such as previous coronary artery bypass grafting with patent grafts, porcelain aorta, previous chest radiotherapy, severe lung disease, and liver cirrhosis.

A comprehensive evaluation involving transthoracic echocardiography, coronary angiography, aortic angiography, and CT is required prior to TAVR to assess the aortic annulus dimensions and geometry, access site, and approach. Transthoracic echocardiography is used to assess valve morphology, transaortic gradient, valvular regurgitation, calcification, orifice area, and other concomitant cardiac pathologies. It is also used as a screening tool to evaluate aortic annulus dimensions prior to device selection. Final valve sizing, however, is usually dependant on intraoperative two-dimensional long-axis TEE measurements. Three-dimensional annular sizing with 3-D echocardiography and CT is emerging as a potentially more accurate way of sizing the aortic annulus, the latter of which is utilized by many operators including our own. Generally, the implanted valve should be slightly larger than the annulus size to minimize the risk of malapposition, paravalvular leak, and valve embolization associated with undersizing. Excessive oversizing, however, may result in incomplete stent expansion potentially affecting valve hemodynamics or aortic root injury or rupture.

Arterial access is assessed with the combination of invasive angiography and contrast-enhanced CT (Fig. 4.9). As discussed above, the evaluation of arterial dimensions and the presence or absence of atheroma, calcification, and tortuosity is fundamental in the assessment of the TAVR patient in minimizing potential major vascular complications.

Fig. 4.9
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Iliofemoral assessment with (a) angiography and (b) MDCT

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The aortic root is assessed using invasive angiography and contrast MDCT to evaluate root and valvular calcification, left main height from the left coronary cusp insertion (due to risk of coronary obstruction), and technical issues related to each valve type and delivery system. Left and right heart catheterizations are also performed to assess the presence of pulmonary hypertension and coronary ischemia and the need for revascularization prior to TAVR. Percutaneous revascularization may be considered as a staged procedure in the presence of a large ischemic burden; however, in the majority of cases, this is not required. Measurement of the aortic transvalvular pressure gradient and calculation of aortic valve orifice area may be useful in cases of aortic stenosis of uncertain severity on transthoracic echocardiography.

Ideally, TAVR should be performed in regional centers of excellence with a dedicated heart valve program and high procedural volumes [20]. The procedure may be undertaken in a cardiac catheterization laboratory with modifications or in a hybrid operating room equipped with high-quality fluoroscopic imaging; the facilities need to be large enough to accommodate sophisticated X-ray imaging integrated with echocardiography, cardiopulmonary bypass and intra-aortic balloon pump machines, and anesthesia equipment, with surgical sterility standards mandatory.

Complications of TAVR

Procedural Complications

Vascular Complications

Vascular events, primarily unplanned iliofemoral repair, are not an infrequent complication of TAVR and have been reported in up to 34 % of patients [21]. These vascular events are not limited to access sites but may involve any site at which instrumentation occurs. Potential complications include aortic dissection, ventricular perforation, and access site complications including dissection, stenosis, perforation, arteriovenous fistula, pseudoaneurysm, hematoma, nerve injury, and compartment syndrome. With the transapical approach, potential access site complications include bleeding, left ventricular pseudoaneurysm formation with or without rupture, and hemodynamic instability requiring urgent cardiopulmonary bypass support. Additionally, excessive balloon dilatation and oversizing of the valve may result in aortic root or annulus rupture.

More recently, a significant reduction in the incidence of vascular events has been reported, with the frequency of major vascular complications now reported as low as 1 % in experienced centers [14]. This explanation that underlies this significant improvement has been largely attributed to increased operator experience, better vascular screening, and smaller sheath sizes with improved delivery systems. As a general rule, vascular access complications are more likely to occur when the minimum artery diameter is less than the sheath external diameter, particularly in the presence of significant atheroma and calcification.

Stroke and Neurological Events

The risk of stroke due to dislodgement and subsequent embolization of atheromatous debris from the aortic arch or from the calcified valve itself has been reported to range between 2 and 11% [22–25]. Additionally, new foci of microinfarction have been reported in up to 85 % as detected by diffusion-weighted magnetic resonance imaging performed soon after TAVR [26]. Although this is reported more frequently following TAVR as compared with conventional SAVR, TAVR patients are generally older with more comorbidities. The clinical significance of these cerebral perfusion defects remains unclear with no apparent association with neurological events or clinical impairment of neurocognitive function.

Cerebral protection devices that divert debris downstream in the aorta are currently in development [27, 28]. These filter devices are delivered percutaneously, typically via the right radial or brachial artery, and positioned within the aortic arch to shield the great vessels. They allow ongoing cerebral perfusion but aim to divert emboli away from the cerebral circulation, where they are less harmful or can be treated effectively without sequelae. Results of clinical trials to address this real concern are keenly anticipated.

Coronary Occlusion

Mechanical coronary obstruction following TAVR is a rare but potentially catastrophic complication, yet occurring in less than 1 % of patients. Possible mechanisms of obstruction are numerous and include inappropriate valve positioning leading to impingement of the coronary ostia by the THV sealing cuff; displacement of bulky native valve leaflets by the THV device; embolization of calcium, thrombus, or air; and dissection of the aortic root. Patients at increased risk of coronary obstruction include those with low-lying coronary ostia (often defined as <12 mm from the basal leaflet insertion), bulky native leaflets, and a narrow aortic root with shallow sinuses of Valsalva, limiting lateral displacement of the native valve leaflets. If this is a concern, aortic root angiography performed at the time of balloon valvuloplasty will help assess the risk [29].

Early identification of ischemia following valve implantation is vital and may be identified by the presence of unexplained hypotension, electrocardiographic changes, and hypokinesis of the left ventricle as seen on echocardiography. Urgent coronary angiography and revascularization are sometimes required, either by percutaneous stent implantation or coronary artery bypass grafting.

Valve Malposition and Embolization

Valve malposition is described as being subvalvular when it is placed “too low” or supravalvular when placed “too high.” If positioned subvalvularly, annular sealing may be compromised resulting in significant paravalvular regurgitation with hemodynamic compromise. If deployed lower in the left ventricular outflow tract, ventricular arrhythmias or interference with the mitral valve apparatus may result. In contrast, supravalvular malpositioning may result in aortic regurgitation, coronary occlusion, or valve embolization. Malposition has been attributed to poor visualization and imaging, poor THV positioning, ineffective rapid ventricular pacing resulting in a suboptimal reduction in transaortic flow and cardiac motion, valve undersizing, and potential displacement from other cardiac structures such as subaortic stenosis or a mitral valve prosthesis [30].

With improved operator experience, there has been a significant reduction in balloon-expandable valve malpositioning, with current rates of valve embolization at less than 0.5 % [31]. Embolization can be successfully managed by partially inflating the deployment balloon within the prosthesis and withdrawing both the balloon and prosthesis into the smaller diameter portion of the transverse or descending aorta. The prosthesis is then deployed in an area of the aorta where it will not impede normal flow in any vascular territory. If THV embolization occurs into the left ventricle, conversion to open heart surgery with removal of the device is required. Malpositioning of self-expandable valves is less likely to result in embolization; however, the relatively longer and larger-diameter prostheses may require snaring and withdrawal into the ascending or descending aorta.

Renal Impairment

Acute kidney injury has been reported in 12–28 % of patients undergoing TAVR, with up to 2 % requiring dialysis [22]. Potential causes of acute kidney injury include the administration of contrast media, transfusion, periods of severe hypotension during rapid ventricular pacing, balloon valvuloplasty, and valve deployment, and the risk of cholesterol embolization as a result of catheter manipulation within diffusely atheromatous aortas. Additionally, many TAVR patients are elderly with an element of underlying renal impairment prior to the procedure. Although many patients show an improvement in their eGFR following TAVR, the presence of acute kidney injury has been associated with a 4-fold risk in postoperative mortality [32].

Conduction Disturbance

Atrioventricular (AV) block is a well-described complication of TAVR. Since the AV conduction system passes superficially through the interventricular septum in close proximity to the aortic valve, injury may occur during valve implantation resulting in partial or complete heart block [33]. Up to 6 % of patients undergoing TAVR with the Edwards SAPIEN THV require pacemaker insertion [34, 35], and this is comparable to rates of pacemaker insertion following surgical AVR [6]. Rates of pacemaker insertion are significantly higher with CoreValve, occurring in up to one third of patients [36, 37]. Risk factors include advanced age, preexisting conduction abnormalities (particularly right bundle branch block) [38], and prosthesis oversizing. In the majority of patients requiring pacemaker insertion, this is apparent shortly following TAVR, although late heart block may occasionally occur [39].

Prosthesis-Related Complications

Prosthesis–Patient Mismatch

Prosthesis–patient mismatch (PPM) occurs when the effective orifice area (EOA) of a normally functioning prosthetic valve is too small in relation to body size, resulting in suboptimal hemodynamic performance of the valve with higher transvalvular gradients. It is often described as being present when the indexed EOA (iEOA) is less than or equal to 0.85 cm2/m2 (moderate PPM); severe PPM is defined as an iEOA less than or equal to 0.65 cm2/m2. PPM in surgical prostheses has been shown to have a negative effect on left ventricular regression and systolic function, NYHA class, and freedom from congestive heart failure. Moreover, it has been associated with increased early and late mortality [40]. Although, conceptually, PPM following TAVR should be largely avoided by appropriate sizing prior to surgery, it is frequently encountered following SVR with moderate PPM reported in 20–70 % and severe PPM in up to 28 %.

Following TAVR, severe PPM has been reported in 11–16 %, while moderate PPM has been reported in 18–32 %, with higher rates occurring following CoreValve implantation [41–44]. Although this incidence compares favorably with surgical prostheses, the effect of PPM after TAVR on clinical outcome is largely unknown.

Paravalvular and Valvular Regurgitation

Paravalvular aortic regurgitation (PAR) is common with the current THVs and is most commonly encountered as a result of incorrect positioning or due to undersizing of the valve in relation to the native annulus. Approximately 85 % of patients have some degree of PAR [45], but the majority of these have trivial or mild AR with no impact on clinical outcomes. Significant PAR, however, has been shown to be a strong predictor of in-hospital mortality [46]. If the valve is incompletely expanded, balloon valvuloplasty may reduce the leak; however, an undersized SAPIEN-type valve cannot be expanded larger than its fabric sealing cuff, and an undersized CoreValve cannot be expanded above its predetermined dimension of the nitinol frame. If severe PAR is present, implantation of a second THV overlapping the first may be useful.

Prosthetic Valve Thrombosis and Endocarditis

As with surgical prostheses, valve thrombosis and endocarditis can occur following TAVR, with isolated cases reported in the literature [47, 48]. Additionally, mitral valve injury with native valve endocarditis has been described [49]. Although a rare complication, it is likely that more cases will be reported as the procedure becomes more widely used. Criteria for diagnosis and management guidelines are similar as those used for surgical prostheses.

Transcatheter Valve-in-Valve Implantation

When surgical bioprosthetic valves fail, requiring reoperation, perioperative morbidity and mortality are high—especially in the setting of increased age, poor left ventricular ejection fraction, renal impairment, pulmonary disease, and the need for coronary artery bypass grafting in addition to valve replacement. Encouraging results have been reported with transcatheter valve-in-valve (ViV) implantation, in which a THV is inserted within the degenerated bioprosthesis.

ViV implantation has been successfully performed within the aortic, mitral, tricuspid, and pulmonary positions. Aortic bioprostheses may be accessed by the transfemoral, transaortic, transaxillary, or transapical routes. The mitral bioprosthesis is most often accessed via the transapical approach; however, the transvenous–transeptal approach has also been successfully described. Pulmonary and tricuspid bioprostheses may be accessed transvenously through the internal jugular, superior vena cava, or femoral vein; an open transatrial route with direct puncture into the right atrium may also be utilized to access the tricuspid valve.

The THV is positioned so that the stent frame anchors on the prosthetic sewing ring. Many surgical bioprostheses have radiopaque markers or rings providing fluoroscopic landmarks for transcatheter valve placement. Since these landmarks differ for each type of bioprosthesis, a good understanding of the radiographic appearance of the valvular prosthesis and the relationship between the markers and the sewing ring is required to aid with positioning. Additionally, knowledge of the internal diameter of the surgical bioprosthesis is imperative, since the THV should be sized accordingly; in most cases, the internal diameter of a surgical prosthesis is significantly smaller than the labeled valve size. As a result, those with smaller surgical bioprostheses (<21 mm) may not be eligible for THV implantation [50–53]. Although the long-term durability of ViV implants remains unknown, early experience suggests that TAVR may be a viable option and particularly desirable in those with prohibitive surgical risk.

Future Directions

SVR remains the standard of care for most patients with symptomatic severe aortic stenosis; however, excellent outcomes have been demonstrated with TAVR. TAVR has evolved as the standard of care for inoperable patients with severe aortic stenosis and is a viable alternative for selected high-risk operable patients. As device technology continues to improve, and longer-term follow-up is obtained—particularly regarding valve durability—it is possible that TAVR will become the preferred treatment option in moderate and, potentially, some low-risk patients with severe symptomatic aortic stenosis.