Commentary: The importance of understanding fluid dynamics.

Francisco Diniz Affonso da Costa, MD

The size of the ascending aorta is an important determinant of prosthetic aortic valve performance and should be considered when choosing the most appropriate prosthetic valve size to be implanted.

See Article page 28.

Pressure recovery (PR) is a physical phenomenon of fluid mechanics in flow-through nozzles and orifices. PR has been extensively characterized in cases of native aortic valve stenosis and surgical bioprosthetic valves, and its magnitude shown to be influenced by several factors such as flow rates, the geometry of the funnel-shaped stenotic orifice, aortic compliance, morphology and diameter of the sinus of Valsalva, and that of the ascending aorta. However, data on PR past different transcatheter aortic valve (TAV) models is relatively scarce. In this issue of the JTCVS Open, Samaee and colleagues have meticulously characterized, in a left heart simulator model, the fluid dynamics after implantation of an Evolut self-expandable Medtronic 26-mm transcatheter aortic valve. In addition, keeping other parameters constant, the influence of different ascending aorta diameters on the measured PR was also analyzed.

At first glance, one may argue that, despite the obvious alterations in flow patterns imposed by the various ascending aorta diameters, the net differences in measured gradients at the vena contracta and downstream in the ascending aorta are small and, thus, not relevant in clinical practice. However, the data bring to attention important new information that may help interventional cardiologists in choosing the correct size of the transcatheter valve (TAV) to be implanted to optimize not only valve hemodynamic performance but to improve the potential durability of the implanted device.

In everyday practice, sizing of TAV is mostly based in the diameter and perimeter of the aortic annulus as measured by computed tomography scans. In borderline cases, a slight oversizing is generally the preferred approach. The study by Samaee and colleagues suggests that other parameters, including the design of the TAV device and the diameter of the ascending aorta, should also be considered. Their data show that, depending on the size of the ascending aorta, a slightly oversized Evolut self-expandable valve may not be the best choice, and indeed may be associated with increased gradients due to incomplete stent expansion owing to the intrinsic design of this device. Although this may be partially compensated by a greater PR in small ascending aortas, the pinwheeling effect on the cusps will affect the opening and closing mechanisms of the prosthetic valve cusps and increase leaflet fluttering during systole, which in turn may have important implications in long-term durability.

The importance of “in vitro” testing prosthetic heart valves in pulse duplicators and fatigue-testing systems should not be underestimated and is essential to completely characterize their performance under different hemodynamic conditions. In addition, they provide surgeons a broader understanding of the complex interplay between prosthetic valve design, aortic annulus diameter, and the anatomy of the aortic root and tubular portion of the ascending aorta that may be helpful when choosing the most appropriate surgical technique depending on anatomical and physiological variabilities.

The work of Samaee and colleagues certainly has some limitations but offers an important piece of information that is useful not only for daily clinical practice but also for further improvement in the design of the currently available TAV devices.