Transcatheter aortic valve replacement (TAVR) is a percutaneous procedure that emerged relatively recently as a substitute for the invasive open heart aortic valve replacement surgery. Instead of performing a highly invasive surgical procedure to replace the damaged aortic valve, the TAV is inserted through an artery via a flexible catheter and deployed in such a way that the metallic stent-like structure of the device forces open the damaged aortic valve while the flexible tissue flaps within the stent takes over the role of healthy leaflets, thus restoring valve function.
For patients who already have bioprosthetic surgical aortic valves (SAV), calcification and ingrowth of scar tissue can jeopardize the normal function of the surgical valve which necessitates another intervention. For this reason, and to avoid a redo open heart surgery, TAV implantation inside the failing bioprosthetic surgical aortic valve — also known as valve-in-valve (ViV) — offers an alternative for high-risk patients.
Despite the novel and less invasive aspect of ViV, the procedure has not been perfected yet and consequently, some adverse effects still occur during and post-implantation. Poor valve opening and valve leakage are some of the drawbacks. High-pressure gradients from poor valve opening and high valvular leakages impose an expensive work load on the left ventricle that may lead to heart failure. In addition, several clinical studies have pointed out the likelihood of reduced leaflet mobility as an additional drawback. Reduced leaflet mobility occurs when a thrombus or blood clot forms on one or more leaflets thus compromising valve operation. Leaflet thrombosis has been related in literature to sinus flow stasis and poor washout.
The study by Hatoum et al tackles these selected adverse effects with the goal of understanding the interplay between various adverse outcomes. The interplay comes into picture because ViV can be performed in various arrangements of the TAV device relative to the failing bioprosthetic valve; it remains to be established how different implant depths and rotational orientations affect some parameters of valve function namely pressure gradient across the valve, leakage fractions (indicators of valvular leakage), and other parameters that predict blood blot formation, namely shear stress, sinus flow stasis, and sinus washout.
Several clinical and in-vitro studies have demonstrated that supra-annular deployment of the TAV with respect to the SAV is associated with lower pressure gradients, yet with higher leakage fractions. Commissural alignment (valve “rotation”) has been less well characterized but to date has not been implicated in ViV hemodynamics. Despite a clinical preference for supra-annular deployment, elevated pressure gradients (>20 mmHg) do occur in approximately 15% of these cases compared with 34% of sub-annular deployments.
These observations suggest that other determinants of pressure gradient after ViV exist and open the door to considering other surrogates of valve function when determining target implant depth. Although higher implants have been associated with more favorable pressure gradients, conflicting reports about their association with less sinus washout and, potentially, a higher risk of flow stasis and thrombosis have been published. Accordingly, a ViV implant that incorporates favorable hemodynamics in tandem with sinus washout may be optimal.
Using an aortic root model, a 23mm Evolut TAV (Medtronic, Minneapolis, MN) was implanted in a degenerated 23-mm Perimount Magna SAV (Carpentier-Edwards, Irvine, CA) extracted from a patient who underwent a redo operation. The TAV was deployed at 16 different combinations consisting of four axial positions of +6.0mm, 0mm, –6.2mm, and –9.8mm and four angular rotations of 0, 30, 60, and 90 degrees. Hemodynamic parameters were evaluated under pulsatile flow conditions ensured by a left heart pulse duplicator yielding physiologic flow and pressure curves. Base hemodynamics for all conditions were maintained with a systolic/diastolic pressure of 120/80 mmHg, a 1 beat/s heart rate, a systolic duration of 33%, and a cardiac output of 5 L/min.
The working fluid in this study was a mixture of water and 99% pure glycerin producing a density of 1,080 kg/m3 and a kinematic viscosity of 3.5 centistokes, similar to blood properties. For all ViV cases, the velocity field within the sinus region, including the region adjacent to the TAV leaflets, was measured using high spatial and temporal resolution particle image velocimetry (PIV).
This study shows that sub-annular deployments do not necessarily lead to higher pressure gradients or worse ViV performance. In addition, supra-annular deployment is associated with lower PGs irrespective of commissural alignment. Higher leakage fractions were obtained when the TAV is deployed supra-annularly. This may be due to the minimal sealing provided to the TAV skirt and the loose grip of the SAV on the TAV. Although decreased washout in itself does not directly lead to thrombogenesis, once the clotting process is triggered (perhaps by blood contact with the foreign surfaces such as a prosthetic valve), thrombosis is most likely to occur in low-flow and low shear stress regions characterized by longer particle/cell residence times. Sub-annular axial positions offer an advantage over supra-annular positions when it comes to evaluating shear stress and velocity in the sinus and sinus washout.
The presence of the 60-degree angle in ViV improves shear stress and yields higher magnitudes and wider ranges of shear stress values. This gives a certain commissural rotational orientation advantage over having the commissures aligned. Although rotational alignment is not currently controllable, enabling such control would certainly be advantageous for future generation TAVs. Coronary flow enhances sinus flow velocity, shear stress, and sinus washout which may account for the higher likelihood of leaflet thrombosis in non-coronary sinus leaflets.
This study along with a previous one entitled “Effect of severe bioprosthetic valve tissue ingrowth and inflow calcification on valve-in-valve performance” lay the ground for more thorough in-vitro assessment via accounting for patient-specific calcification and inflow valve ingrowth effects on ViV performance. Further insight about the appropriate balance to achieve an optimal sinus hemodynamics-valve performance balance is still needed from in vivo studies.
These findings are described in the article entitled Implantation Depth and Rotational Orientation Effect on Valve-in-Valve Hemodynamics and Sinus Flow, recently published in the journal Annals of Thoracic Surgery. This work was conducted by Hoda Hatoum, Jennifer Dollery, Scott M. Lilly, Juan A. Crestanello, and Lakshmi Prasad Dasi from The Ohio State University.
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