Transcatheter aortic valve-in-surgical aortic valve implantation: current status and future perspectives

During the last decade, the relative use of surgical bioprosthetic valves has increased by nearly 80%, an observation likely explained by improved surgical techniques, valve durability, avoidance of anticoagulation and patient's preference [1]. Nevertheless, surgical bioprosthetic valves are known to fail; actuarial freedom from reoperation for a failing bioprosthetic valve is ∼95, 90 and 70% at 5, 10 and 15 years, respectively [2, 3]. The lifetime risk of reoperation actually decreases with increasing patient age at the time of the index procedure. For example, a 50- and a 60-year old patient undergoing surgical bioprosthetic aortic valve replacement will have a lifetime risk of reoperation of 45 and 25%, respectively [4]. The gold-standard treatment for a failing surgical bioprosthetic valve is a redo operation. The operative mortality following an elective redo operation in low-to-intermediate risk patients is 2–7% [5, 6]. In high surgical risk or non-elective cases, however, the mortality can be as high as 30% [7]. Even in low-risk and elective redo scenarios, the risks of wound infection, blood transfusions, postoperative pain and delayed functional recovery are not negligible. Transcatheter aortic valve-in-surgical aortic valve (TAV-in-SAV) implantation was first reported in 2007, and since then, numerous case series and registries have demonstrated its safety and efficacy [8, 9]. Since TAV-in-SAV avoids sternotomy and cardiopulmonary bypass, it can improve resource utilization by accelerating patient recovery and by reducing the length of hospital stay. Furthermore, it may obviate or reduce the number of repeat surgical procedures in a patient's lifetime. Indications for TAV-in-SAV implantation include high or excessive surgical risk patients with stented or stentless bioprosthetic aortic valve failure due to calcification, wear and tear and/or pannus; endocarditis and thrombosis are contraindications. Patients may present with predominant aortic stenosis or regurgitation, or a combination. In our practice, octogenarian patients at intermediate surgical risk are also considered for TAV-in-SAV. Keeping certain caveats in mind, procedural steps for TAV-in-SAV implantation are similar to those for native aortic valve stenosis. Physicians need to acquire basic knowledge about the construction and dimensions, radiographic identification and potential failure modes of SAV bioprostheses. For stented bioprostheses, the internal stent diameter is crucial for transcatheter valve size selection. Currently, manufacturers recommend applying existing sizing chart criteria to TAV-in-SAV procedures. Intuitively, the compliance of stented or stentless bioprostheses is dissimilar to calcified native aortic valves. This would suggest, therefore, that the interference mechanics between a transcatheter valve and surgical valve bioprosthesis and that between a transcatheter valve bioprosthesis and a native aortic valve are dissimilar. Whether historical sizing charts for the Edwards and Medtronic transcatheter heart valves are optimal for TAV-in-SAV procedures is a topic of debate, because the compliance of the bioprosthesis and native annulus are different so that intuitively, the sizing algorithm should also be different. Pre-implant balloon valvuloplasty is rarely needed, especially in those patients presenting with predominant aortic regurgitation. According to the ACC/AHA Valvular Heart Disease guidelines, balloon valvuloplasty of a surgical bioprosthetic valve is considered a Class III indication [10]. Unless there is severe calcification of the surgical prosthesis that may impede expansion of the transcatheter valve, we do not routinely perform pre-implant balloon valvuloplasty, especially since tear and embolization of parts of the leaflets and leaflet rupture from the valve stent may be more imminent than with native valves. Radio-opaque components of stented valves serve as perfect markers for positioning and deploying the transcatheter valve. Other possible aids include the use of repeat aortic angiograms, transoesophageal echocardiography, a pigtail catheter lying in the base of the prosthetic leaflets and/or identification of calcific spots. Surprisingly, malpositioning can be observed in up to 15% and the use of a second transcatheter valve has been documented in up to 8% of patients [11]. Initial learning curve, misunderstanding of radio-opaque markers on the stented prostheses and the inability to re-position a self-expanding valve into a higher position once contact with the base ring has been made may explain these alarming observations. The largest reported TAV-in-SAV registry to date included 202 patients with a mean age of 77 years from 38 centres [11, 12]. Procedural success was achieved in 93% of cases while the 30-day mortality rate was 8.4%. Coronary ostial obstruction occurred in 3.5% of cases, while the authors observed that certain surgical prostheses such as the stented Sorin Mitroflow (Sorin Group, Milan, Italy) and stentless Sorin Freedom bioprostheses had a greater propensity towards coronary obstruction. The leaflets of the Sorin Mitroflow continue to become an outer single layer of the pericardium covering the stent posts, which increases the risk of coronary ostial obstruction during valve expansion. Average maximum and mean transprosthetic gradients were 28 ± 14 and16 ± 9 mmHg, respectively. Of note, the mean transprosthetic gradient was 5 mmHg lower with the Medtronic CoreValve than with the Edwards-SAPIEN valve, probably due to the lower profile of the Medtronic CoreValve within the surgical bioprosthesis. The Medtronic CoreValve anchors within the surgical bioprosthesis with its inflow portion, composed of nitinol and a single layer of porcine pericardial skirt only; valve function is supra-annular. Nearly the entirety of the Edwards-SAPIEN valve anchors within the surgical bioprosthesis. This means that the cobalt–chromium stent, skirt and bovine pericardial leaflets are within the surgical bioprosthesis and contribute to the higher gradients seen with this valve. Excessive oversizing may lead to an under-expanded transcatheter valve with consequent leaflet redundancy that increases leaflet stresses and may negatively influence durability. On the other hand, the rigid base ring of a stented valve may provide the necessary platform to produce a nearly circular transcatheter valve that allows for optimal leaflet geometry and durability. To the best of our knowledge, there are no reports of a failed TAV-in-SAV prosthesis. TAV-in-SAV implantation may ‘disrupt’ conventional surgical practice patterns. It is foreseeable that a larger number of younger patients (age <60 years) will be referred for bioprosthetic valve replacement as opposed to mechanical valve replacement, given the option of a TAV-in-SAV implantation in case of future structural valve failure. There is also interest in transcatheter valves for failing surgical mitral valve bioprosthesis, surgical mitral valve repair (i.e. valve-in-a-ring) and surgical pulmonary valve bioprosthesis. Transcatheter valve replacement with the Medtronic Melody or Edwards-SAPIEN prosthesis for a failing right ventricle–pulmonary artery homograft is well established. We recently reported on a transatrial antegrade approach for double mitral and tricuspid TAV-in-ring implantation using the Edwards-SAPIEN valve (Fig. 1) [12]. (A) Mini-right lateral thoracotomy for access to the mitral and tricuspid valve. Notice the extra-stab for the transcatheter delivery of the Edwards-SAPIEN valve into the mitral ring. (B) The Ascendra 2 delivery catheter entering the left atrium via the right superior pulmonary vein. (C) The Ascendra 2 delivery catheter entering the right atrium directly through the right atrial wall. Note that the temporary pacing lead is introduced into the left ventricle via the right superior pulmonary vein. (D) Edwards-SAPIEN valve in a mitral and tricuspid annuloplasty ring. The following case example demonstrates some of the challenges that can be encountered during the preprocedural screening phase of a TAV-in-SAV procedure. A 78-year old male presented with symptomatic severe aortic regurgitation due to a failing Sorin Mitroflow bioprosthesis that was implanted 11 years earlier. The operative report noted a 23-mm valve size (internal stent diameter 19 mm) (Fig. 2A). Preoperative MSCT measurements estimated an effective internal stent diameter of ∼21.5 mm (perimeter 6.8 cm), suggesting a 25-mm Sorin Mitroflow; this contradicted the operative report (23 mm). The Medtronic CoreValve sizing chart recommends a 23- and 26-mm Medtronic CoreValve for the 23- and 25-mm Sorin Mitroflow, respectively. The patient did not have a valve identification card. After contacting Sorin directly, records confirmed a 23-mm prosthesis, thereby discounting the MSCT measurements. The base ring of the Sorin Mitroflow is internally covered with material not appreciated by the MSCT. Thus, we overestimated the internal stent diameter. Another point of contention lies in the transcatheter valve size selection. Anecdotal experience strongly suggests that a 23-mm Medtronic CoreValve may be fitting for either a 23- or a 25-mm Sorin Mitroflow, making the discussion above ‘academic’. The patient underwent a successful implantation with a 23-mm Medtronic CoreValve. (A) Sizing chart for various stented surgical bioprosthetic valves. The red box highlights the internal stent diameter for the 23- and 25-mm Sorin Mitroflow bioprosthesis. (B) The multiplanar, multislice reconstruction demonstrates an internal stent perimeter of 6.8 cm, which is equivalent to an effective diameter of 21.5 mm (circular). In the absence of direct comparisons, it is probably fair to say that mortality and stroke rates are similar between patients undergoing TAV-in-SAV and TAVI for native aortic valve stenosis. Having said that, the issues of relatively higher gradients, malpositioning and coronary ostial occlusion are probably more frequent with TAV-in-SAV. The safety and efficacy of this approach compared with conventional redo surgery in high-risk patients is currently unknown. Although a randomized controlled trial would be ideal, it is unlikely that sample size requirements would be achieved; a multicentre propensity score-matched analysis could, however, be contemplated. We believe that TAV-in-SAV is an excellent alternative to conventional redo surgery for high surgical risk patients. In the very near future, the indications will expand towards intermediate surgical risk patients with failing surgical bioprosthesis.

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