Correlation Between Blood Flow Patterns And Intimal Hyperplasia At The Distal End-to-Side Anastomosis

Arterial bypass surgery is a common vascular surgical procedure in modern medical practice. However, arterial restenosis due to intimal hyperplasia (IH) at the distal anastomosis is the major cause in the late grafting failure. Statistics shows that the procedure only has a moderate long-term patency rate of 33% at 60 months. IH is characterized by the invasion and proliferation of vascular smooth muscle cells and fibroblasts, causing hyperplasia in the subendothelial plane. Anastomotic restenoses occur predominantly at the heel and toe of the end-to-side anastomosis and on the floor of the host artery opposite the anastomosis. IH formation at the suture line can be attributed to two major factors, namely surgical injury and compliance mismatch. However, IH on the floor of the host artery has been thought to be entirely due to the abnormal flow patterns created as the blood flows from the graft into the artery, impinging on the floor of the junction. Such flow behaviour is unphysiological, as end-to-side junctions do not occur naturally in adults. Thus, hemodynamic flow patterns in distal end-to-side anastomoses are widely implicated in the initiation of the IH formation processes. In order to optimize the bypass surgery increasing the patency rates and better understand the correlation between IH at the distal end-to-side anastomosis and its hemodynamic environment, we constructed 30o, 45o, 60o and S-type anastomoses at the canine common carotid artery site with autogenous graft from the canine external jugular vein and compared the effects of the four different procedures on intimal hyperplasia formation. In order to study the flow fields in the four different bypassed arterial models, a flow visualization method by combining transparent blood vessel technique with particle image velocimetry (PIV) has been established. The flow visualization study was carried out using an in-house designed measurement system. Our finding from the present study is summerized as fllows: (1) Different bypass procedures have a great influence on intimal hyperplasia formation at the distal anastomosis. IH in 60 o anastomosis is the thickest, 45 o the second, 30 o and S-type the least; (2) The flow visualization method utilized in the study may best maintain the 3-D geometry and configuration of the bypassed vessels, and most closely mimic the characteristics of the in-vivo blood flow fields, hence enabling us to get better understanding of the role of hemodynamic factors in the development of intimal hyperplasia; (3) PIV can collect most information about the instantaneous whole-flow-field in a bypass model; (4) Vascular intimal hyperplasia has a close correlation with the blood flow pattern in the bypassed artery. Neointima is much easier to form in the disturbed flow zone with flow recirculation and low fluid velocity distal to the end-to-side anastomosis. The thickness of the neointima is most likely to correlate with the wall shear stress. The lower the wall shear stress, the thicker the neointima and vice versa. This coincides with the low shear stress theory. (5) In processing series images, the feature points in two continual particle images were caught with feature tracing algorithm, which laid a solid groundwork for obtaining the actual velocity and other hemodynamic parameters.