Flow dynamics of surgical and transcatheter aortic valves: Past to present

Abstract

Objective: To perform an in vitro characterization of surgical aortic valves (SAVs) and transcatheter aortic valves (TAVs) to highlight the development of the flow dynamics depending on the type of valve implanted and assess the basic differences in the light of flow turbulence and its effect on blood damage likelihood and hemodynamic parameters that shed light on valve performance.

Methods: A Starr-Edwards ball and cage valve of internal diameter 22 mm, a 23-mm Medtronic Hancock II SAV, a 23-mm St Jude Trifecta SAV, a 23-mm St Jude SJM (mechanical valve) SAV, a 26-mm Medtronic Evolut TAV, and a 26-mm Edwards SAPIEN 3 TAV were assessed in a pulse duplicator under physiological conditions. Particle image velocimetry was performed for each valve. Pressure gradient and effective orifice area (EOA) along with velocity flow field, Reynolds shear stress (RSS), and viscous shear stress (VSS) were calculated.

Results: The SJM mechanical valve exhibited the greatest EOA (1.96 ± 0.02 cm²), showing superiority of efficiency compared with the same-size Trifecta (1.87 ± 0.07 cm²) and Hancock II (1.05 ± 0.01 cm²) (P < .0001). The TAVs show close EOAs (2.10 ± 0.06 cm² with Evolut and 2.06 ± 0.03 cm² with SAPIEN 3; P < .0001). The flow characteristics and behavior downstream of the valves differed depending on the valve type, design, and size. The greater the RSS and VSS the more turbulent the downstream flow. Hancock II displays the greatest range of RSS and VSS magnitudes compared with the same-size Trifecta and SJM. The Evolut displays the greatest range of RSS and VSS compared with the SAPIEN 3.

Conclusions: The results of this study shed light on numerous advancements in the design of aortic valve replacement prosthesis and the subsequent hemodynamic variations. Future surgical and transcatheter valve designs should aim at not only concentrating on hemodynamic parameters but also at optimizing downstream flow properties.

Keywords: AV, aortic valve; EOA, effective orifice area; ID, internal diameter; PIV, particle image velocimetry; RSS, Reynolds shear stress; SAV, surgical aortic valve; SAVR, surgical aortic valve replacement; SJM, St Jude Medical; TAV, transcatheter aortic valve; VSS, viscous shear stress; ball and cage; blood damage; surgical aortic valves; transcatheter aortic valve replacement; turbulence.

Graphical abstract

The quest for the perfect aortic valve for replacement surgeries is still ongoing.

Figure 1

Image summarizing the objective of this study that is to perform an in vitro characterization of selected surgical and transcatheter valves that represent early to current valve designs to evaluate and understand the postimplant resulting flow and overall turbulence levels for these prostheses, and their potential relevance to blood damage. SJM, St Jude Medical.

Figure 2

Images of the valves. A, Starr Edwards Ball and Cage. B, Medtronic Hancock II. C, St Jude Trifecta. D, Masters Series St Jude Mechanical Valve. E, Medtronic Evolut. F, Edwards SAPIEN 3. These valves were selected in this study for downstream flow and flow properties assessment. This study focuses on understanding the major differences among these valves from the earliest designs to the ones currently available.

Figure 3

Phase-averaged velocity vectors and vorticity contours at different phases in the cardiac cycle of the various valves. The types of valves dictate the flow patterns downstream of each valve and the magnitude of the resulting jet velocity. The vectors denote the velocity, their length relative to each other represents their respective magnitude, and the contour represents the local spinning of the fluid. As the valves open, shear layers (shown in red and blue patches) form. The way these shear layers form and their intensity depend greatly on the way the valve is designed (ball-and-cage, trileaflet, bileaflet and with a long stent) and the size of the orifice. SJM, St Jude Medical.

Figure 4

Velocity component profiles of the different valves at peak systole at the Y location that corresponds to the exit from leaflets (A) V_x and (B) V_y. Different velocity profiles are obtained with the valves depending on the design and the size of the orifice. The Evolut TAV shows a greater velocity compared with the SAPIEN 3 of the same size, the Trifecta shows the lowest velocity compared with the SJM and the Hancock II. Hancock II and Evolut are characterized by more fluctuation compared to the Trifecta and SAPIEN 3, respectively. It is important to note that SAPIEN 3 is usually selected for larger annuli compared with the Evolut. SJM, St Jude Medical.

Figure 5

Velocity profiles of the different valves at peak systole at the Y location that corresponds to the exit from leaflets tips. In this figure, the overall magnitude of the velocity is shown. The Evolut yields higher velocity magnitude compared to the SAPIEN 3 of the same size. It is important to note that SAPIEN 3 is usually selected for larger annuli compared to the Evolut. The Trifecta shows the lowest velocity magnitude compared with SJM and Hancock II. SJM, St Jude Medical.

Figure 6

Principal Reynolds shear stresses (RSS) at different phases in the cardiac cycle for the different valves. The greater the turbulent stresses, the greater the turbulence and therefore the greater the losses. Elevated turbulent stresses indicate a suboptimal valve performance and correlate with a greater blood damage potential. The Evolut shows greater turbulent stresses compared with the SAPIEN 3 and the SJM shows lower RSS compared with the Trifecta and the Hancock II. SJM, St Jude Medical.

Figure 7

A, Principal Reynolds shear stresses (RSS) probability density function (PDF) at peak systole phase for the different valves and B, instantaneous viscous shear stresses probability density function throughout the cardiac cycle for the different valves. The probability density function or PDF displays all the values (all the range) of a certain parameter distributed over a certain region of interest and gives the relative or differential likelihood (frequency) of any parameter. The area under the PDF curve is always equal to 1 and therefore can also be considered as a normalized histogram. Complementing the results of Figure 5, the Evolut shows greater turbulent stresses compared with the SAPIEN 3 and the SJM shows lower RSS compared with the Trifecta and the Hancock II. SJM, St Jude Medical.

Figure 8

The evolution in aortic valve designs did not necessarily improve hemodynamic performance or downstream turbulence. SJM, St Jude Medical; EOA, effective orifice area.