Valve expansion: the next frontier for lifetime management in transcatheter aortic valve implantation

Transcatheter aortic valve implantation (TAVI) has evolved from a procedure reserved for inoperable or high-surgical-risk patients to now being recommended across the entire spectrum of surgical risk [1]. Consequently, an increasing proportion of younger patients with longer life expectancy are now being treated with TAVI [2]. In this cohort of patients, achieving a successful valve implant without a major procedural complication is no longer sufficient, as longer-term outcomes such as valve function and durability represent key cornerstones to ensuring an optimal patient lifetime management [3]. In this context, prior studies have demonstrated how the final geometry of the implanted transcatheter aortic valve (TAV) may impact the prosthetic valve function and the risk of subclinical leaflet thickening or thrombosis, both of which can impact long-term valve durability.

In this issue of the European Journal of Cardio-Thoracic Surgery, Moscarelli et al. [4] analysed post-implantation computed tomography (CT) scans of 100 patients who underwent TAVI using the self-expanding Evolut TAV (Medtronic, USA) in patients with native tricuspid (n = 84) or bicuspid (n = 16) aortic valve stenosis. The eccentricity index was calculated at six pre-defined cross-sectional planes and used as a marker of prosthesis expansion and deformation. Additionally, CT was used to assess leaflet as well as sub- and peri-valvular thrombosis, with 6-month follow-up echocardiography to check the valve function. The principal findings of the study were as follows: (i) the greatest eccentricity was observed at the TAV inflow level, which progressively improved (more circular) towards the TAV outflow level, (ii) the extent of TAV eccentricity was significantly greater in patients with bicuspid morphology, (iii) 44% of patients had detectable thrombus at any TAV location, with valvular and peri-valvular thrombosis (in the sinuses) being more frequent in the bicuspid group (81%) than in the tricuspid group (37%) and (iv) the observation of increased eccentricity and thrombosis in bicuspid patients did not correlate to differences in valve function at the 6-month follow-up.

A few limitations of this study deserve attention. First, the sample size was limited to only 100 patients, of whom only 16 patients had a bicuspid phenotype. This limits the possibility of additional analyses linking valve eccentricity to subsequent thrombosis and valve function. Second, a key element missing was the reporting of the extent and distribution of calcification, which could have provided important mechanistic insights into reasons for TAV deformation and thrombosis, particularly in bicuspid patients in whom the extent of calcification is often more severe. Third, only the Evolut valve was evaluated; therefore, the findings cannot be applied across the spectrum of different TAV platforms. Finally, as the authors commented, the duration of follow-up was limited to 6 months. A longer-term follow-up may be more useful to detect any longer-term differences in valve function.

Nevertheless, the authors should be congratulated for conducting a detailed study of valve geometry, particularly in different native aortic valve phenotypes. By evaluating valve geometry at multiple planes, the authors were able to demonstrate how TAV eccentricity, and therefore under-expansion, can vary regionally from the TAV inflow to the outflow level. This phenomenon, which the authors described as ‘stent-frame decoupling’, may reflect a potential advantage of using TAVs with a supra-annular leaflet position, particularly in bicuspid anatomies, whereby pronounced TAV under-expansion at the inflow level may have less impact on the higher positioned leaflets. Additionally, the finding of increased leaflet and peri-valvular thrombosis in bicuspid patients may provide a pathophysiological explanation for the increased stroke rates observed in bicuspid TAVI cohorts.

These observations are consistent with prior post-TAVI cardiac CT studies, which have consistently demonstrated that TAV under-expansion or deformation is associated with an increased risk of leaflet thickening and/or thrombosis [5, 6]. Computational modelling studies suggest that this may arise due to increased blood stasis on the surface of the TAV leaflets, thereby promoting platelet activation and thrombotic events [7]. TAV under-expansion may also impact valve function, as recently reported in a fluoroscopic analysis of 1216 balloon-expandable SAPIEN implants (Edwards Lifesciences, USA), whereby significant frame asymmetry—a surrogate for TAV under-expansion—was detected in nearly one in five patients and was associated with an increased incidence of bioprosthetic valve dysfunction [8]. The potential clinical consequences of TAV under-expansion were further demonstrated in the recent ACURATE IDE randomized controlled trial, whereby TAV under-expansion was associated with a 2-fold increased risk of all-cause mortality and 3-fold increased risk of stroke [9]. Similar findings were also recently reported amongst bicuspid AS patients treated with the SAPIEN valve, whereby the combination of TAV under-expansion and eccentricity was associated with increased rates of leaflet thrombosis and all-cause mortality [10].

Taken together, these data provide compelling evidence for the importance of achieving a uniform and well-expanded TAV. In anatomies where the risk for TAV under-expansion may be deemed high, consideration should be given to the type of TAV implanted. Moreover, techniques such as effective predilatation, fluoroscopic detection of TAV under-expansion and corrective post-dilatation should be considered routinely—especially in case of excessive leaflet calcification, bicuspid anatomy and/or use of self-expanding TAVs. Future development of calcium debulking technologies or the possible ‘tricuspidization’ of bicuspid aortic valves by means of leaflet modification techniques may prove to be useful adjunctive techniques, particularly when treating more challenging heavily calcific anatomies.

In conclusion, as we continue to expand TAVI towards treating younger patients and more bicuspid anatomies, the definition of a successful procedure needs to evolve to consider the impact of the index TAVI on longer-term valvular and clinical outcomes. Achieving a uniform and well-expanded TAV will benefit both the acute procedural outcome and have a favourable impact on long-term valve function and durability, providing an optimal lifetime management strategy for patients with aortic stenosis.

FUNDING

None declared.

Conflict of interest: Ole De Backer has received Institutional research grants and consultancy fees from Medtronic, Abbott and Boston Scientific and Arif Khokhar has received Speaker fees from Boston Scientific and Abbott.

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