The elucidation of the mechanical characteristics of blood flow is important toward the understanding of cardiovascular physiology and disease. Unfortunately, the pulsatility of blood flow, its strong interactions with the surrounding vasculature and its turbulent nature make it particularly difficult to study. The MSCBL has pioneered in new tools to quantify the flow in realistic models of normal, diseased or anatomically abnormal valves. A particular focus has been put on the hemodynamics of the normal (i.e., tricuspid) aortic valve (TAV) and the congenital bicuspid aortic valve (BAV). The BAV is the most common congenital heart defect and exists in many anatomic variations. While BAV patients can live a normal life, this valvular defect is a major risk factor for aortic dilation. The recent demonstration of the dependence of the dilation patterns on the BAV morphotype has suggested the possible role played by hemodynamics in the etiology of this complication. Our laboratory has implemented novel techniques to quantify the degree of flow abnormality on TAV and BAV leaflets and in TAV and BAV aortas. The knowledge gained from this research effort can be directly exploited toward the elucidation of the mechanical regulation of organ physiology and pathology.
Our lab has designed a steady flow loop to assess valvular function and quantify energy loss in tricuspid and bicuspid aortic valves, under normal and disease (calcified) states. The flow setup is driven by a centrifugal pump capable of delivering flow rates between 5 and 20 L/min and features a valve chamber accomodating a tissue valve within a realistic aortic root geometry. Observation of the valve orifice at the maximum flow rate of 20 L/min indicates that the TAV generates a circular orifice while the BAV generates an elliptical orifice. Quantification of the energy loss index (ELI), which characterizes valvular function and valvular disease severity, also demonstrates the intrinsic degree of stenosis of the BAV relative to the TAV as well as a dramatic increase in resistance to blood flow under calcific conditions.
The MSCBL has also invested in the development of computational models to characterize the temporal and spatial mechanical stress characteristics on the surface of TAV and BAV leaflets. The models were designed to account for the complex transfer of momentum occurring between the moving valve leaflets and the surrounding pulsatile blood flow. The fluid-structure interaction models were developed in the commercial software package ANSYS. The valve geometries investigated thus far include a TAV, a type-0 BAV (i.e., BAV with two identical leaflets) and a type-I BAV (i.e., BAV with one large fused leaflet and one normal non-fused leaflet). The three models operated under a physiologic transvalvular pressure gradient and were used to quantify the wall shear stress environments on both surfaces of each leaflet.
|Flow simulations in a TAV and in type-0 and type-I BAVs|
|Wall shear stress predictions on the fibrosa of TAV, type-0 and type-I BAV leaflets|
To measure the pulsatile flow characteristics in TAV and BAV aortas, we designed a left-heart simulator capable fof replicating the flow conditions of the left heart in terms of aortic and ventricular pressures and cardiac output. The flow loop is driven by a programmable pulse generator and a series of diaphragm accumulators mimicking ventricular function. The compliance and resistance of the vasculature are replicated using adjustable compliance and resistance modules. The test section consists of an optically accessible valve-aorta chamber capable of accomodating native tissue aortic valves and an anatomically realistic and compliant aorta phantom. This setup was used along with a PIV system (LaVision Inc) to measure the flow characteristics in the aorta downstream of a TAV and three BAV morphotypes (LR: left-right coronary leaflet fusion, NL: non-left-coronary leaflet fusion, RN: right-non-coronary leaflet fusion). The measurements reveal the alignment of the TAV jet parallel to the axis of the aorta and the skewness of the BAV jets toward the aortic wall convexity (LR- and NL-BAV cases). The BAV jet skewness also increases the degree of flow rotationality between the aortic root and the middle section of the ascending aorta.
Our group has also designed unified valve-aorta FSI models to model the interactions between valve leaflets and aortic flow. These sophisticated models have been used to quantify for the first time global and local hemodynamic differences between TAV and BAV aorta flow patterns. The simulations reveal the increased flow skewness generated by type-I BAVs relative to the TAV, as well as the dependence of the degree of jet skewness on the BAV morphotype. As already observed clinically using PC-MRI, the BAVs also generate peripheral streamlines that wrap around the central orifice jet to generate right-handed helical flow patterns.
|FSI simulations in TAV and BAV aortas|
The aortic valve achieves unidirectional blood flow between the left ventricle and the aorta. It normally consists of three leaflets that open during systole and close during diastole, under the pressure difference established between the ventricle and the aorta.
Calcific aortic valve disease is the most common aortic valve disorder. It affects 4% of adults over 65 years of age and consists of the formation of calcific lesions on the valve leaflets.
The bicuspid aortic valve is the most common congenital valvular defect and affects 2% of the population. While a normal aortic valve consists of three leaflets, the bicuspid aortic valve forms with only two, as a result of fusion between two adjacent leaflets.