RESUMO
Objective To propose and numerically simulate a novel stent with triangular wire cross-section so as to compare the hemodynamic effect of endovascular stents in aneurysm with circular, rectangular and triangular cross section, respectively. Methods Three aneurysm models were constructed by implanting 3 kinds of stents separately, including one with bare circular wire cross-section stent (named CM), and one with bare rectangular wire cross-section stent (named RM), and one with bare triangular cross-section stent (named TM). An unstented aneurysm model was also constructed to serve as the control (named UM). Four models were preformed with numerical simulation of fluid-structure interaction under the same boundary conditions using finite element method. Results Compared with TM and CM, RM showed lower velocity, higher flow resistance and longer turn over time, which demonstrated that the effect of blood flow pattern on three stents was decreased in the order of RM, TM, CM. RM could reduce the magnitude and fluctuation of wall shear stress (WSS) more significantly than that of TM and CM. However, the pressure rise of TM and CM was lower than that of RM. WSS distribution and deformation results indicated the possibility of aneurysm development along the distal wall was higher than that along the proximal wall, and the top of aneurysm was in the highest risk of rupture. Conclusions Three kinds of stents show differences of hemodynamics in aneurysm treatment, which provides significant references for the structural design and optimization of endovascular stent.
RESUMO
OBJECTIVES: To investigate the "brain sparing effect", a paradoxical increase in fetal cerebral perfusion in pregnant women with uteroplacental insufficiency through fetal middle cerebral artery(MCA) waveform analysis, and to construct new reference ranges for fetal MCA Doppler flow velocity resistance indices in nomal preganacies to be used in the assessment of fetal well-being. MATERIALS AND METHODS: The subjects were selected from pregnant women receiving antenatal care at Severance Hospital between March 1996 and December 1999. Fetal MCA reference resistance index range according to gestational age were obtained through routine Doppler velocimetry for 4621 normal pregnant women between 24-42 gestational weeks. Those manifesting multiple pregnancy, Diabetes Mellitus, drug abuse, chronic hypertension, fetal congenital anomaly, fetal growth restriction(FGR) with discrepencies of more than 7days and pregnancies complicated by pregnancy induced hypertension (PIH) were excluded. Additionally, the fetal MCA hemodynamic changes were compared for 140 pregnant women with FGR or PIH. The normal ranges for fetal MCA Doppler resistance indices were expressed as mean+/-SD, and a comparative analysis between the PIH or FGR group and the normal control group was performed. Statistical analysis was done using the SPSS 8.0 program, t-test and the Pearson correlation. RESULT: The fetal MCA doppler flow resistance indices for normal pregnancies were 0.77+/-0.006 at 24 weeks and 0.79+/-0.006 at 28 weeks, demonstrating a statistically significant increase(p<0.05). At 40 weeks the index was 0.71+/-0.009 showing a significant decrease after 28weeks(p<0.05). There was a tendency for the resistance index to be lower in the PIH/FGR group compared to the normal controls. CONCLUSION: Constructing a normal reference resistance index range for fetal MCA flow is of high clinical value with regards to predicting fetal well being and understanding the hemodynamics of fetal cerebral perfusion.