Anqiang Sun

An S-type bypass can improve the hemodynamics in the bypassed arteries and suppress intimal hyperplasia along the host artery floor

The development of distal end-to-side anastomotic intimal hyperplasia (IH) has been attributed to the flow disturbance and abnormal wall shear stress (WSS) distribution there. The geometry of the bypass has a strong influence on the flow pattern and WSS distribution. Using a canine model of end-to-side anastomosis, a 45 degrees S-type bypass was compared with 60 degrees , 45 degrees and 30 degrees conventional bypasses in the term of IH along the host artery floor. Numerical blood flow simulations were also carried out to characterize the flow patterns at the distal parts of the bypassed arteries for the 4 models. The results showed that the averaged intima thicknesses of the host artery floors for the 4 bypass models were 119.50+/-10.30 microm (60 degrees ), 65.56+/-6.53 microm (45 degrees ), 45.26+/-5.99 microm (30 degrees ) and 47.64+/-4.85 microm (S-type), respectively, vs. 9.81+/-1.88 microm in the control group (without bypass surgery). Compared with the control group, neointima thickness in all 4 bypass models was significantly increased, but the neointima thickness of the 45 degrees S-type bypass was apparently much better than its 45 degrees conventional counterpart, and was as good as the 30 degrees conventional bypass. The numerical simulation revealed an apparent swirling flow pattern in the S-type bypass, which was very different than the flow patterns in the 3 conventional bypass models. This swirling flow altered the overall flow pattern in the distal part of the bypassed artery and eliminated the low WSS zone along the host artery floor. The improvement in the term of IH for the S-type bypass is most likely due to the alteration of the overall flow pattern and WSS distribution by the geometrical configuration of the S-type bypass.

Physiological Significance of Helical Flow in the Arterial System and its Potential Clinical Applications

Helical flow in the human aorta is possibly a typical example of ‘form follows function’ in the vascular system. The helical blood flow may provide guaranties for the inner surface of the ascending aortic wall to get smooth and even washing by the blood so that atherosclerotic plaques can hardly form in the area of the ascending aorta. It has been documented that the phenomenon of helical flow of blood is not just localized in the ascending aorta, it also exists in several large arteries and veins as well. Preliminary studies demonstrated the widely existing helical flow might play positive physiological roles in facilitating blood flow transport, suppressing disturbed blood flow, preventing the accumulation of atherogenic low density lipoproteins on the luminal surfaces of arteries, enhancing oxygen transport from the blood to the arterial wall and reducing the adhesion of blood cells on the arterial surface. These roles of helical blood flow may lessen the burden of arteries and protect the arteries from the pathology of atherosclerosis, thrombosis, and intimal hyperplasia. The great development of time-resolved three-dimensional phase contrast MRI (flow-sensitive 4D-MRI) and the advent of dimensionless indices such as helical flow index proposed to characterize helical flow make clinic quantification of the helical flow in the human large arteries possible. Moreover, researchers probed into the possibility to apply the mechanism of helical flow to the design of vascular interventions to reduce thrombus formation and intimal hyperplasia caused by abnormal flow conditions.

Numerical investigation of blood flow in the distal end of an axis-deviated arterial bypass model

Blood flow characteristics in an axis-deviated artery bypass model were studied numerically and compared with those in a conventional bypass model. The numerical results revealed that a pronounced swirl flow was created in the host artery by the geometrical configuration of the axis-deviated bypass. The swirl flow created in the axis-deviated bypass model altered the overall flow pattern and eliminated the slow recirculation zone with low wall shear stress existing in the conventional model on the host artery floor where there is disturbed flow known to be associated with the development of intimal thickening and hyperplasia. The numerical study therefore suggests that by intentionally introducing swirl flows into the bypassed arteries, the adverse effects along the host artery floor can be suppressed.

Nitric oxide transport in normal human thoracic aorta: effects of hemodynamics and nitric oxide scavengers.

Despite the crucial role of nitric oxide (NO) in the homeostasis of the vasculature, little quantitative information exists concerning NO transport and distribution in medium and large-sized arteries where atherosclerosis and aneurysm occur and hemodynamics is complex. We hypothesized that local hemodynamics in arteries may govern NO transport and affect the distribution of NO in the arteries, hence playing an important role in the localization of vascular diseases. To substantiate this hypothesis, we presented a lumen/wall model of the human aorta based on its MRI images to simulate the production, transport and consumption of NO in the arterial lumen and within the aortic wall. The results demonstrated that the distribution of NO in the aorta was quite uneven with remarkably reduced NO bioavailability in regions of disturbed flow, and local hemodynamics could affect NO distribution mainly via flow dependent NO production rate of endothelium. In addition, erythrocytes in the blood could moderately modulate NO concentration in the aorta, especially at the endothelial surface. However, the reaction of NO within the wall could only slightly affect NO concentration on the luminal surface, but strongly reduce NO concentration within the aortic wall. A strong positive correlation was revealed between wall shear stress and NO concentration, which was affected by local hemodynamics and NO reaction rate. In conclusion, the distribution of NO in the aorta may be determined by local hemodynamics and modulated differently by NO scavengers in the lumen and within the wall.

3,3'-Dioctadecylindocarbocyanine-low-density lipoprotein uptake and flow patterns in the rabbit aorta-iliac bifurcation under three perfusion flow conditions.

The aim of this study was to elucidate which of the following two factors plays a more important role in the localization of atherogenesis: the barrier function of the arterial endothelium modulated by wall shear stress or flow-dependent low-density lipoprotein (LDL) concentration at the blood/wall interface. To determine this, the rabbit aorto-iliac bifurcation was perfused with 3,3′-dioctadecylindocarbocyanine (DiI)-LDL solution under three different flow conditions: (i) forward flow (perfused in the in vivo flow direction); (ii) backward flow (perfused in a reversed flow direction); and (iii) static group (no flow). The results showed that there was a peak in the curve of DiI-LDL uptake distribution along the lateral wall of the bifurcation for all three groups, which was located in the branching areas where the endothelial cells were round and polygonal with no preferred orientation. Nevertheless, the peak of the forward flow group was much sharper than those of the other two groups. The overall DiI-LDL uptake was the highest for the static group. The present experimental study supports the concept that both the barrier function of the endothelium modulated by wall shear stress and the mass transport phenomenon of LDL concentration polarization are involved in the infiltration/accumulation of atherogenic lipids within the arterial wall. Nevertheless, the latter might play a larger role in the localization of atherogenesis.

Hemodynamic study of overlapping bare-metal stents intervention to aortic aneurysm

To investigate the hemodynamic performance of overlapping bare-metal stents intervention treatment to thoracic aortic aneurysms (TAA), three simplified TAA models, representing, no stent, with a single stent and 2 overlapped stents deployed in the aneurismal sac, were studied and compared in terms of flow velocity, wall shear stress (WSS) and pressure distributions by means of computational fluid dynamics. The results showed that overlapping stents intervention induced a flow field of slow velocity near the aneurismal wall. Single stent deployment in the sac reduced the jet-like flow formed prior to the proximal neck of the aneurysm, which impinged on the internal wall of the aneurysm. This jet-like flow vanished completely in the overlapping double stents case. Overlapping stents intervention led to an evident decrease in WSS; meanwhile, the pressure acting on the wall of the aneurysm was reduced slightly and presented more uniform distribution. The results therefore indicated that overlapping stents intervention may effectively isolate the thoracic aortic aneurysm, protecting it from rupture. In conclusion, overlapping bare-metal stents may serve a purpose similar to that of the multilayer aneurysm repair system (MARS) manufactured by Cardiatis SA (Isnes, Belgium).

Compositional or charge density modification of the endothelial glycocalyx accelerates flow-dependent concentration polarization of low-density lipoproteins

We hypothesized that diminished endothelial glycocalyx (GCX) at atherosclerotic lesion-prone sites accelerates flow-dependent concentration polarization of low-density lipoproteins (LDLs) at the luminal surface, and in turn contributes to vulnerability of these sites to atherosclerosis. A parallel plate flow chamber was applied to expose cultured endothelial monolayers to three different levels of shear stress (3, 12, 20 dyn/cm2). Heparinase III (Hep.III) was employed to degrade heparan sulfate proteoglycans selectively and 3-(N-morpholino) propanesulfonic acid-buffered physiological salt solutions (MOPS-PSS) were used at either normal ionic strength (Normal-MOPS), low ionic strength (LO-MOPS) or high ionic strength (HI-MOPS) to modify the effective charge density of the endothelial GCX. Water filtration velocity (V w ) across the endothelial monolayer, the luminal concentration of LDLs (C w ) and the uptake of LDLs by endothelial cells were measured and compared among the following five groups of cells: (1) Control; (2) Hep.III treatment; (3) LO-MOPS; (4) Normal-MOPS; and (5) HI-MOPS. The results obtained substantiated the aforementioned hypothesis and demonstrated that compositional or charge density modification of the endothelial GCX facilitated water filtration across the endothelium, enhanced the accumulation of LDLs on the luminal surface and increased the uptake of LDLs by endothelial cells, therefore contributing to atherogenesis.

Intentionally induced swirling flow may improve the hemodynamic performance of coronary bifurcation stenting.

Objectives and Background: Bifurcation stenting represents a challenge for the high rate of restenosis that has close correlations with local hemodynamics. This study sought to test the hypothesis that by intentionally inducing swirling flow the hemodynamic performance of bifurcation stenting can be improved. Methods: The hemodynamics of bifurcation stenting with and without swirling flows were numerically simulated and compared. Results: The results revealed that swirling flow significantly suppressed flow disturbance of blood at the bifurcation and WSS at two critical regions, the outer wall of the bifurcation ostium and the floor wall opposite the branching, was enhanced. Conclusions: This study therefore suggests that intentionally inducing swirling flow might be a good strategy in bifurcation stenting.

Numerical simulation of nucleotide transport in the human thoracic aorta

Extracellular adenine nucleotides ATP and ADP on vascular endothelial cells may play a role in the localization of atherogenesis by regulating the release of nitric oxide from endothelial cells and modulating intracellular calcium levels. To quantitatively investigate the concentration distribution of nucleotides on the luminal surface of the human thoracic aorta, we numerically simulated the transport of nucleotides using an aorta model constructed based on MRI images and analyzed the effects of different factors on nucleotide transport, such as ATP release rate (S(ATP)), pulsatile flow and the absence of ATP in the main blood stream. The numerical results revealed that the combined concentration of ATP and ADP (c(w-ATP+ADP)) on the aortic surface varied from place to place, being relatively low in disturbed flow regions. In addition, c(w-ATP+ADP) was significantly affected by S(ATP). For relatively slow S(ATP), such as the moderate sigmoidal release model, c(w-ATP+ADP) was very low in certain flow regions with low wall shear stress. However, for very rapid S(ATP), such as the rapid linear release model, c(w-ATP+ADP) was relatively high in these same regions. The results also demonstrated that for relatively slow S(ATP), pulsatile blood flow enhanced c(w-ATP+ADP). However, for very rapid S(ATP), pulsatile blood flow would reduce c(w-ATP+ADP). Moreover, the absence of ATP within the main blood stream would not influence the distribution of c(w-ATP+ADP). In conclusion, the concentration distribution of nucleotides along the aortic wall was quite uneven and determined by both the ATP release rate and the blood flow pattern in the aorta.

Hemodynamic Performance of a New Punched Stent Strut: A Numerical Study.

Local flow disturbance by arterial stent struts has been shown to play an important role in stent thrombosis. To reduce the local flow disturbance near a stent strut, we proposed a new concept of stent design with small holes in the stent struts. The present study evaluated the new design numerically by comparing it with the traditional stent in terms of local hemodynamic parameters such as flow velocity, flow recirculation area, time-averaged wall shear stress (TAWSS), oscillating shear index (OSI), and relative residence time (RRT). The results demonstrated that when compared with the traditional strut, the new design could significantly enhance flow velocity and reduce the flow recirculation zone in the vicinity of the strut. Moreover, the new design would significantly elevate TAWSS and remarkably reduce OSI and RRT along the host arterial wall. In conclusion, the new design of stent struts with punched holes is advantageous over the traditional one in the aspect of improving local hemodynamics, which may reduce thrombosis formation and promote re-endothelialization after stenting.