Takeru Naiki

A 3D-LDA Study of the Relation Between Wall Shear Stress and Intimal Thickness in a Human Aortic Bifurcation

A realistic model experiment on hemodynamics was performed to study correlations between wall shear stresses measured in a cast model of the aortic bifurcation and intimal thickness at each corresponding site of the native blood vessel from which the cast had been made. An elastic model of a 54 year old human aortic bifurcation was made of a polyurethane elastomer using a dipping method, and was perfused with Newtonian or non-Newtonian fluid under physiologic pulsatile flow condition. Local flow velocities were measured with an optical-fibered, 3-dimensional laser Doppler anemometer (3D-LDA) to determine wall shear stresses. Distribution of intimal thickness was determined using histological specimens of the native blood vessel. The results obtained are: 1) Non-Newtonian fluid rheology increased wall shear stresses; 2) Positive correlations were observed between intimal thickness and the maximum instantaneous wall shear stress, and 3) However, if we take only the data from the circumference at the level of the flow divider tip, there were negative correlations between them.

Flow Visualization Analysis in a Model of Artery-Graft Anastomosis

Flow characteristics near the end-to-end anastomosis of vascular graft were studied in model tubes by flow visualization techniques. Artery and vascular graft were modelled by an elastic tube fabricated from an elastomeric polymer and a rigid plastic tube, respectively. Anastomotic models were made by connecting these two tubes, which had compliance mismatch at their anastomoses. These model tubes were installed into a mock circulatory loop and flow was visualized using hydrogen bubbles and aluminum powder as the tracer. Flow disturbances including flow separation and eddies were observed near the modelled distal anastomosis (graft-to-artery anastomosis). Peak values of the wall shear rate were high in the proximal anastomotic area (artery-to-graft anastomosis) and low in the distal region. These phenomena were enhanced in the models with increased compliance mismatch. The local abnormal flow observed in the anastomotic zone might cause thrombus formation and subintimal hyperplasia. To improve the patency in small-calibered arterial grafts, it is important to match their compliance to that of natural arteries.

Flow visualization analysis of pulsatile flow in elastic straight tubes

Effects of wall compliance on the flow characteristics were studied by visualizing pulsatile flow in two straight elastic tubes having different compliance and in a rigid tube. The elastic tubes were made of segmented polyether polyurethane and their compliance was adjusted by varying the wall thickness. Their diameter changes were +/- 3.3 and +/- 4.9% for the pressure pulsation between 20 and 250 mm Hg. An acrylic pipe was used for the rigid model. An air-driven artificial heart was used to generate the pulsatile flow having the mean Reynolds number and frequency parameter of 740 and 11.4, respectively. The flow was visualized by the hydrogen bubble method at every 5% of the pulsatile flow cycle. Velocity distributions along the tube diameter were determined from still images of time lines taken with a CCD camera. The ratio of the wall shear rate in the elastic tubes to that in the rigid tube at each phase correlated well with the radial velocity of tube wall, while it had no significant correlation with the instantaneous tube diameter. These results suggest that the wall compliance either increases or decreases the wall shear rate depending on the phasic relation between the flow and pressure waves. When studying the hemodynamic effects on vascular diseases by model experiments, it may be important to take wall elasticity into account.

Effects of long-term feeding of α-glucosylhesperidin on the mechanical properties of rabbit femoral arteries

Many people are sensitive to cold, resulting in poor blood circulation. There is evidence that hesperidin results in increased peripheral circulation and skin temperature. A transglycosylated hesperidin, α-glucosylhesperidin, is more bioabsorbable than hesperidin. In the present study, biomechanical studies were performed on the effects of long-term feeding of α-glucosylhesperidin on the contractile response (diameter response) and stiffness of femoral arteries excised from rabbits. Animals in the normal (non-treated), low, and high groups were fed 0, 150 and 4500 mg/day, respectively, of α-glucosylhesperidin for about 24 weeks. The feeding of α-glucosylhesperidin did not change arterial stiffness nor mean blood flow rate in the femoral artery; however, it increased mean aortic blood pressure and decreased arterial diameter at 100 mmHg in the high group. The diameter responses developed by 10-5 M of norepinephrine were significantly lower in the high and low groups than in non-treated group. This result indicates that, due to the long-term feeding of α-glucosylhesperidin, arterial contraction induced by the neurotransmitter of sympathetic nerves decreases. It was estimated that blood flow in such muscular arteries as the femoral artery is maintained at normal by α-glucosylhesperidin even under the conditions of autonomic imbalance and cold intolerance.