Fan Zhan

Effect of non-Newtonian and pulsatile blood flow on mass transport in the human aorta

To investigate the effects of both non-Newtonian behavior and the pulsation of blood flow on the distributions of luminal surface LDL concentration and oxygen flux along the wall of the human aorta, we numerically compared a non-Newtonian model with the Newtonian one under both steady flow and in vivo pulsatile flow conditions using a human aorta model constructed from MRI images. The results showed that under steady flow conditions, although the shear thinning non-Newtonian nature of blood could elevate wall shear stress (WSS) in most regions of the aorta, especially areas with low WSS, it had little effect on luminal surface LDL concentration (c(w)) in most regions of the aorta. Nevertheless, it could significantly enhance c(w) in areas with high luminal surface LDL concentration through the shear dependent diffusivity of LDLs. For oxygen transport, the shear thinning non-Newtonian nature of blood could slightly reduce oxygen flux in most regions of the aorta, but this effect became much more apparent in areas with already low oxygen flux. The pulsation of blood flow could significantly reduce c(w) and enhance oxygen flux in these disturbed places. In most other regions of the aorta, the oxygen flux was also significantly higher than that for the steady flow simulation. In conclusion, the shear shining non-Newtonian nature of blood has little effect on LDL and oxygen transport in most regions of the aorta, but in the atherogenic-prone areas where luminal surface LDL concentration is high and oxygen flux is low, its effect is apparent. Similar is for the effect of pulsatile flow on the transport of LDLs. But, the pulsation of blood flow can apparently affect oxygen flux in the aorta, especially in areas with low oxygen flux.

Swirling flow created in a glass tube suppressed platelet adhesion to the surface of the tube: Its implication in the design of small-caliber arterial grafts

To test the hypothesis that intentionally inducing swirling blood flow in a small-caliber arterial graft can suppress acute thrombus formation by affecting the adhesion of platelets to the internal surface of the graft, an experimental comparative study was designed to investigate the effect of swirling flow on the adhesion and activation of platelets in a straight glass tube coated with calf skin type I collagen. The experimental results showed that when compared with the normal flow, the swirling flow generated in the test tube significantly reduced the platelet adhesion to the surface of the test tube. Different from normal flow condition under which platelet adhesion increased simply with decreasing wall shear stress, the platelet adhesion density under swirling flow condition remained almost unchanged along the first 5 mm section of the tube in which the flow had relatively high rotation strength, even though the wall shear stress in this section of the tube dropped drastically. This suggests that when the swirling flow in the tube was strong enough, platelet adhesion was dominantly affected by the swirling flow itself, wall shear stress was secondary. The results also showed that there was no significant difference in the activation of platelets between the spiral flow group and the normal flow group. The present study therefore suggests that intentionally introducing swirling flow in small-caliber arterial grafts has no adverse effect on platelet activation and may indeed be a solution to improving the patency of the grafts by suppressing acute thrombus formation.

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.

Effect of swirling flow on the uptakes of native and oxidized LDLs in a straight segment of the rabbit thoracic aorta

To elucidate the physiological significance of the spiral flow in the arterial system from the viewpoint of atherogenic lipid transport, an ex vivo experimental comparative study was designed to investigate the effect of swirling flow on the distribution of native 3,3′-dioctadecylindocarbocyanine-low-density lipoprotiens (DiI-LDL) and DiI-ox-LDL uptakes by segments of the rabbit thoracic aorta. The experimental results showed that when compared with the normal flow, the swirling flow generated in the test arteries significantly reduced the DiI-LDL and DiI-ox-LDL uptakes by the arterial walls. The results also showed that the values of DiI-ox-LDL uptake were higher than those of DiI-LDL uptake at the same sample position in both the normal flow group and the swirling flow group. Most interestingly, the experimental results found that the percentage increase in DiI-ox-LDL uptake was much larger than that in DiI-LDL uptake when the perfusion duration increased from 3 to 24 h. In conclusion, the present study substantiated the hypothesis that the spiral flow in the arterial system plays a beneficial role in protecting the arterial wall from atherogenesis. Meanwhile, it supported the concept that the receptor-mediated bindings of LDL uptake, the barrier function of the arterial endothelial linings and the mass transport phenomenon of LDL concentration polarization are all involved in the infiltration/accumulation of atherogenic lipids within the arterial wall.

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).

The beneficial effect of swirling flow on platelet adhesion to the surface of a sudden tubular expansion tube: its potential application in end-to-end arterial anastomosis.

The size mismatch in an end-to-end vascular anastomosis between the host vessel and the graft may cause flow disturbance and possibly result in thrombosis. To test the hypothesis that intentionally induced swirling flow in an end-to-end anastomosis could suppress flow disturbance, impeding thrombus formation by affecting platelets adhesion, a comparative study was designed to investigate the effect of swirling flow on the adhesion and activation of platelets in a glass sudden tubular expansion tube coated with calf skin type I collagen. The results revealed that the swirling flow generated in the expansion could reduce the length of the flow recirculation zone distal to the expansion and significantly reduce the total number of adherent platelets in the test tube when compared with that for the normal flow. No significant difference was observed in the activation of platelets between the swirling flow group and the normal flow group. This study therefore suggests that intentionally introduced swirling flow in an end-to-end anastomosis has no adverse effect on platelet activation and may indeed be a solution to improve the patency of end-to-end microvascular anastomoses by suppressing thrombus formation.

A novel way to reduce thrombus build-up in vena cava filters.

Objectives and background: The build‐up of thromboses in vena cava filters after deployment presents serious problem to the patients. We proposed a novel way to overcome this problem in a belief that intentionally induced swirling flow can optimize blood flow patterns in Vena cava filters, enhance the stirring motion of flow, in turn accelerate the dissolution of blood clots captured in the filter and facilitate blood to flow pass through the filters. Methods: In this study, we experimentally compared the work efficiency of a vena cava filter under swirling flow condition with that of the same filter under normal flow condition. Results: The results show that when compared to the normal flow, the swirling flow indeed has a significantly beneficial effect on a VCF which can decrease its flow‐out time nearly 40% and reduce clot build‐up in the filter more than 50%. Conclusions: We therefore believe that the design of an ideal VCF should take how to create swirling flow in the filter into the consideration.

A new stent with streamlined cross-section can suppress monocyte cell adhesion in the flow disturbance zones of the endovascular stent

We proposed a new stent with streamlined cross-sectional wires, which is different from the clinical coronary stents with square or round cross-sections. We believe the new stent might have better hemodynamic performance than the clinical metal stents. To test the hypothesis, we designed an experimental study to compare the performance of the new stent with the clinical stents in terms of monocyte (U-937 cells) adhesion. The results showed that when compared with the clinical stents, the adhesion of U-937 cells were much less in the new stent. The results also showed that, when Reynolds number increased from 180 (the rest condition for the coronary arteries) to 360 (the strenuous exercise condition for the coronary arteries), the flow disturbance zones in the clinical stents became larger, while they became smaller with the new stent. The present experimental study therefore suggests that the optimization of the cross-sectional shape of stent wires ought to be taken into consideration in the design of endovascular stents.

Influence of proximal drug eluting stent (DES) on distal bare metal stent (BMS) in multi-stent implantation strategies in coronary arteries.

The aim of this study was to investigate the drug distribution in arteries treated with DES-BMS stenting strategy and to analyze the influence of proximal DES on distal segments of BMS. A straight artery model (Straight Model) and a branching artery model (Branching Model) were constructed in this study. In each model, the DES was implanted at the proximal position and the BMS was implanted distally. Hemodynamic environments, drug delivery and distribution features were simulated and analyzed in each model. The results showed that blood flow would contribute to non-uniform drug distribution in arteries. In the Straight Model the proximal DES would cause drug concentration in BMS segments. While in the Branching Model the DES in the main artery has slight influence on the BMS segments in the branch artery. In conclusion, due to the blood flow washing effect the uniformly released drug from DES would distribute focally and distally. The proximal DES would have greater influence on the distal BMS in straight artery than that in branching artery. This preliminary study would provide good reference for atherosclerosis treatment, especially for some complex cases, like coronary branching stenting.

SWIRLING FLOW CAN SUPPRESS MONOCYTES ADHESION IN END-TO-END ARTERIAL ANASTOMOSIS

To test the hypothesis that the monocytes adhesion would be suppressed by intentionally inducing swirling flow in end-to-end arterial anastomosis to inhibit the flow disturbance, the comparing experimental and numerical investigation under both normal flow condition and swirling flow condition were executed, in which the sudden expanded tube and U-937 cells were used. The numerical results reveal that, comparing to normal flow, the swirling flow could reduce the size of flow disturbed zones and enhance the wall shear stress (WSS) in the closed downstream of sudden expanded tube. The experimental results show that there are disturbed flow zones in the sudden expanded tube, where the adhesion number of U-937 cells is larger than other zones. More importantly, comparing to the normal flow, the swirling flow could reduce the adhesion of U-937 cells, in which the adhesion number become smaller with the increasing of the swirling intensity. Therefore, the present study suggests that intentionally introducing swirling flow in end-to-end arterial anastomosis may be a solution to solve the problem of intimal hyperplasia (IH) by suppressing the flow disturbance and restraining the adhesion of monocytes to keep the favorably unimpeded flow.