Aortic valve replacement (AVR) remains a major treatment option for patients with severe aortic valve disease. Clinical outcome of AVR is strongly dependent on implanted prosthetic valve size. Fluid-structure interaction (FSI) aortic root models were constructed to investigate the effect of valve size on hemodynamics of the implanted bioprosthetic valve and optimize the outcome of AVR surgery. FSI models with 4 sizes of bioprosthetic valves (19 (No. 19), 21 (No. 21), 23 (No. 23) and 25 mm (No. 25)) were constructed. Left ventricle outflow track flow data from one patient was collected and used as model flow conditions. Anisotropic Mooney–Rivlin models were used to describe mechanical properties of aortic valve leaflets. Blood flow pressure, velocity, systolic valve orifice pressure gradient (SVOPG), systolic cross-valve pressure difference (SCVPD), geometric orifice area, and flow shear stresses from the four valve models were compared. Our results indicated that larger valves led to lower transvalvular pressure gradient, which is linked to better post AVR outcome. Peak SVOPG, mean SCVPD and maximum velocity for Valve No. 25 were 48.17%, 49.3%, and 44.60% lower than that from Valve No. 19, respectively. Geometric orifice area from Valve No. 25 was 52.03% higher than that from Valve No. 19 (1.87 cm2
Aortic valve disease affects more than 60 million people worldwide, with prevalence growing resulting from an ageing population [
We hypothesize that properly selected bioprosthetic valve with size larger than that recommended by current AVR guidelines may lead to better hemodynamics and cardiac function post AVR surgeries. However, due to high cost and risk involved in AVR surgery, it is not practical to test the hypothesis directly on patients. It is desirable to use computer-aided simulations to test the feasibility of the hypothesis before actual patient studies could be performed.
Recent advances in computational modeling techniques have made it possible for computational models to be constructed and used to simulate innovative high-risk surgical procedures and evaluate the impact of valve size on AVR surgical outcome. Previous numerical simulation of valve dynamics mainly focused on three categories: Structure mechanical analysis, computational fluid dynamics and fluid-structure interaction (FSI) modeling. Structure mechanical analysis mainly focuses on the dynamics of the leaflets, where a constant pressure value or a time-varying pressure was applied to the leaflets, ignoring the interaction between leaflets and blood flow [
In this study, ALE based Two-Way FSI models of aortic root were introduced to investigate the effect of valve size on hemodynamics of the implanted bioprosthetic valve and seek optimal outcome post AVR surgeries. Leaflets were described by nonlinear anisotropic constitutive laws, and we explored AVR optimize strategies by simulating entire cardiac cycle for different bioprosthetic valve sizes.
The aortic root geometry included valvular leaflets, sinuses, interleaflet triangles and annulus. The aortic root model with 4 different sizes of bioprosthetic valves (19 (No. 19), 21 (No. 21), 23 (No. 23) and 25 mm (No. 25)) were constructed.
Parameter | No. 19 (mm) | No. 21 (mm) | No. 23 (mm) | No. 25 (mm) |
---|---|---|---|---|
Inner diameter of sinutubular junction | 16.8 | 18.4 | 20.2 | 21.9 |
Inner diameter of annulus | 19.0 | 21.0 | 23.0 | 25.0 |
Maximum inner diameter of sinus | 21.8 | 23.9 | 26.2 | 28.4 |
Height of sinutubular junction | 16.0 | 16.0 | 16.0 | 16.0 |
Effective height of the valve | 9.0 | 9.0 | 9.0 | 9.0 |
Thickness of the leaflet | 0.4 | 0.4 | 0.4 | 0.4 |
Thickness of the aortic wall | 1.5 | 1.5 | 1.5 | 1.5 |
Left ventricle outflow track (LVOT) flow data from one patient was collected and used as model flow conditions for our valve models (
For aortic root structural model, we assumed that the aortic root material is hyperelastic, nearly-incompressible, homogeneous and anisotropic (valvular leaflets)/isotropic (aortic wall). The governing equations for aortic root models were as follows:
where
The Mooney–Rivlin model was used to describe the nonlinear anisotropic (valvular leaflets) and isotropic (aortic wall) material properties. The strain energy function for the five-parameter Mooney–Rivlin model was given below [
where
where
Blood flow was assumed to be laminar, Newtonian, viscous and incompressible. The Navier–Stokes equations with ALE formulation were used as the governing equations. Our flow model is given below [
where
No-slip boundary conditions and natural force boundary conditions were specified at all interfaces to couple fluid and structure models together. Heart rate of the patient was 108 bpm. The systolic velocity waveform at the midpoint of the LVOT cross-section was measured by pulse Doppler (
A geometry-fitting mesh generation technique was employed to generate meshes for aortic root with complex geometry [
Hemodynamics plays an important role in valve cardiac functions [
FSI models with 4 different valve sizes were used in this study to quantify transvalvular pressure gradient, flow shear stress and geometric orifice area from those models with the goal to optimize valve size selections in AVR. Blood flow velocity and aorta root dimension were obtained from one patient for modeling use. FSI aortic root models with 4 different bioprosthetic valve sizes (19, 21, 23 and 25 mm) were constructed to evaluate the impact of prosthetic valve size on AVR outcome. The systolic transvalvular pressure gradient and geometric orifice area were calculated to seek the optimal surgical design for potential AVR outcome improvements. The flow shear stress conditions on the leaflets were calculated which could provide important information for AVR design and improve the durability of bioprosthetic valve.
Blood pressure, velocity, systolic valve orifice pressure gradient, systolic cross-valve pressure difference, geometric orifice area and flow shear stress of 4 valves models with different sizes were compared. Details are given below.
Computational blood pressure values at an annulus cross-section and a sinotubular junction cross-section of valves were recorded and averaged over those cross-sections for analysis (
Model | MSVOPG (mmHg) | PSVOPG (mmHg) |
---|---|---|
No. 19 | 54.52 | 86.47 |
No. 21 | 38.23 | 61.35 |
No. 23 | 31.38 | 51.24 |
No. 25 | 27.76 | 44.82 |
The true forces that the valve leaflets are subjected to are determined by the flow pressures on the two sides of the leaflets: The side facing LV (inlet) and the side facing aorta (outlet). Flow pressure data on the two sides from the FSI models (they are actually the FSI interfaces between the flow model and the structure model) were recorded and averaged (over the leaflets) for comparison analysis. The systolic cross-valve pressure difference (SCVPD) was the pressure difference from the two sides of the leaflets.
Geometric orifice area (GOA) was used as the main parameter to assess valve kinematics. It was calculated directly from the deformed valve geometry [
No. 19 Valve | No. 25 Valve | |||
---|---|---|---|---|
Parameters | FSI Model data | FSI Model data | ||
Percent flow acceleration (%) | 44.44 | 46.15 | ||
Forward flow period (ms) | 270 | 260 | ||
Closing flow period (ms) | 35 | 62 |
Flow shear stress (FSS) on valve leaflet surface plays an important role in valve functions, disease initiation, development and healing. FSS on all nodes on the supraaortic surface (the side of the leaflets facing aorta) and the subaortic surface (the side of the leaflets facing LV) were recorded, and mean FSS (averaged on the two leaflet surfaces, respectively) were obtained foranalysis. Time-varying mean FSS values in systolic phase are plotted in
Model | Subaortic MMFSS (dyn/cm2) | Supraaortic MMFSS (dyn/cm2) |
---|---|---|
No. 19 | 32.87 | 13.28 |
No. 21 | 26.75 | 10.54 |
No. 23 | 23.35 | 8.26 |
No. 25 | 19.39 | 7.17 |
MMFSS: Maximum mean flow shear stress.
This study applied a FSI modeling approach to patients undergoing AVR to investigate the effect of valve size on the outcome of AVR surgery. The computational modeling approach could be using to test the feasibility of novel high-risk surgical procedures to avoid performing those surgical procedures on humans. Aortic root valve FSI models were built sharing the same material properties of aortic root and inlet flow rate to simulate the AVR with different valve sizes for the same patient. Non-linear anisotropic material model was used to characterize the mechanical properties of aortic valve leaflets, which is consistent with the anisotropic properties of aortic valve leaflets reported in literature [
In both routine clinical and research applications, evaluations of the hemodynamic performance of bioprosthetic valves are mainly focused on transprosthetic pressure gradient and orifice area which directly affect ventricle cardiac function [
Kinematics of the valves with different sizes were compared by GOA (
FSS is closely related to ventricle remodeling and valve disease initiation and development such as inflammation and calcification.
Our first major limitation is that image-based patient-specific 3D valve geometry reconstruction was not included and data from root bioprosthesis and previous work were used [