A Comparative Study Of Numerical Simulations Of Mechanical Stress On Blood Flow In The Left Atrium Of A Porcine Atrial Fibrillation Model

Objective(s):Based on the pre-and post-modelling numerical simulations of the porcine Atrial Fibrillation(AF)model,the flow field,flow velocity and Wall Shear Stress(WSS)and Wall Pressure Stress(WPS)in the Left Atrial(LA)were obtained.The quantitative values of the two mechanical stresses(MS)were obtained to provide a mechanical reference for subsequent in vitro cell-based experiments loaded with the above MS to investigate the mechanism of MS influencing the development of AF.Methods:The Yunnan small-eared pigs modelled by surgery were randomly divided into transepicardial electrical stimulation(TES)and transendocardial electrical stimulation(TES)model groups,with six in the TES group and six in the TES group.Before modelling,the group was defined as Sinus Rhythm(SR)group and after modelling,the group was defined as AF group.Velocity spectral images of the inlet(pulmonary vein orifice)and outlet(mitral valve orifice)were acquired by echocardiography in the SR and AF groups respectively,and the peak flow velocities of the inlet S and D peaks and the outlet E and A peaks were recorded in the SR and AF groups.CT angiography(Computer Tomograghy Angiography(CTA))was performed by320-row multilayer spiral CT and scanned to acquire cardiac imaging data in the SR and AF groups.A Computational Fluid Dynamics(CFD)framework model in both steady-state and dynamic modes was constructed,consisting of the following main components:(1)Importing the CT data from the SR and AF groups into the reverse engineering software MIMICS(Materialise’s interactive medical image control system)software to identify the 2D images of the SR group and AF group LA from the CT images;(2)a finite element mesh generation program to convert the 2D geometry into a 3D volumetric mesh and correctly identify the boundary planes;(3)defining the pulmonary vein port as the inlet and the mitral valve port as the outlet,and using the CFD software FLUENT was used to carry out CFD simulations under flow boundary conditions;(4)the flow lines and flow velocities,WSS and WPS variation characteristics of the LA and the left atrial appendage(LAA)as a whole and at several different locations were measured and analysed by post-processing software.The data obtained were also statistically analysed by two independent samples t-test,two correlation samples Wilcoxon test and box line plots in SPSS 23.0.Results:1.Of the 12 Yunnan small-eared pigs in this experiment,6 received epicardial placement of pacing leads for electrical stimulation and 6 received endocardial placement of pacing leads for electrical stimulation,with a 100% modeling rate.The same transient AF ECG was acquired for the AF models obtained from the two different surgical procedures.The two AF models compared the blood flow velocities during the periods of inlet S and D peaks and outlet E peak captured by echocardiography and the differences were not statistically significant(P > 0.05).2.Electrocardiography performed before and after the modelling of the 12-head AF model revealed that the heart rate was faster during AF compared to SR.Echocardiography revealed that when the rhythm changed from SR to AF,the inlet and outlet flow velocities decreased,the AR wave flow in the pulmonary vein orifice disappeared,the D-peak > S-peak,and the A-peak in the mitral orifice disappeared.The steady-state model cloud and numerical results at the end-diastolic moment showed that when the rhythm changed from SR to AF,the flow velocity and WSS decreased and the WPS increased throughout the wall in the LA.Analysis of the dynamic model using numerical simulation clouds,the Wilcoxon test for two correlation samples and the box plot statistical method showed that the flow velocity was reduced throughout one whole cardiac cycle within the LA in these locations(near the left pulmonary vein orifice,near the right pulmonary vein orifice,anterior wall,posterior wall,lateral wall,atrial septum,near the mitral valve orifice,LAA orifice,middle LAA and deep LAA)when AF was compared to SR,with a difference of WSS decreased with decreasing blood flow velocity,with a statistically significant difference(P < 0.05),with a higher degree of change in the proximal pulmonary vein orifice,proximal mitral valve orifice and LAA orifice than in the other locations;WPS increased to varying degrees,with a statistically significant difference(P < 0.05),with a higher degree of change in the proximal pulmonary vein orifice,proximal mitral valve orifice and LAA orifice than in the other locations;WPS increased to varying degrees,with a statistically significant(P < 0.05).Conclusion(s):1.The AF models for the epicardial and endocardial methods obtained the same boundary conditions required for numerical simulations.2.The numerical model allows quantification of the changes in flow velocity and flow lines,WSS and WPS parameters within the LA at SR and AF.The flow lines become disordered at each location point AF compared to SR,the flow velocity decreases,the WSS decreases in proportion to the flow velocity and the WPS increases in an inhomogeneous distribution.