Intimal Hyperplasia and Wall Shear in Arterial Bypass Graft Distal Anastomoses: An In Vivo Model Study

Abstract

The observation of intimal hyperplasia at bypass graft anastomoses has suggested a potential interaction between local hemodynamics and vascular wall response. Wall shear has been particularly implicated because of its known effects upon the endothelium of normal vessels and, thus, was examined as to its possible role in the development of intimal hyperplasia in arterial bypass graft distal anastomoses. Tapered (4-7 mm I.D.) e-PTFE synthetic grafts 6 cm long were placed as bilateral carotid artery bypasses in six adult, mongrel dogs weighing between 25 and 30 kg with distal anastomotic graft-to-artery diameter ratios (DR) of either 1.0 or 1.5. Immediately following implantation, simultaneous axial velocity measurements were made in the toe and artery floor regions in the plane of the anastomosis at radial increments of 0.35 mm, 0.70 mm, and 1.05 mm using a specially designed 20 MHz triple crystal ultrasonic wall shear rate transducer Mean, peak, and pulse amplitude wall shear rates (WSRs), their absolute values, the spatial and temporal wall shear stress gradients (WSSG), and the oscillatory shear index (OSI) were computed from these velocity measurements. All grafts were harvested after 12 weeks implantation and measurements of the degree of intimal hyperplasia (IH) were made along the toe region and the artery floor of the host artery in 1 mm increments. While some IH occurred along the toe region (8.35+/-23.1 microm) and was significantly different between DR groups (p<0.003), the greatest amount occurred along the artery floor (81.6+/-106.5 microm, mean +/- S.D.) (p < 0.001) although no significant differences were found between DR groups. Linear regressions were performed on the paired IH and mean, peak, and pulse amplitude WSR data as well as the absolute mean, peak, and pulse amplitude WSR data from all grafts. The mean and absolute mean WSRs showed a modest correlation with IH (r = -0.406 and -0.370, respectively) with further improvements seen (r = -0.482 and -0.445, respectively) when using an exponential relationship. The overall best correlation was seen against an exponential function of the OSI (r = 0.600). Although these correlation coefficients were not high, they were found to be statistically significant as evidenced by the large F-statistic obtained. Finally, it was observed that over 75 percent of the IH occurred at or below a mean WSR value of 100 s(-1) while approximately 92 percent of the IH occurred at or below a mean WSR equal to one-half that of the native artery. Therefore, while not being the only factor involved, wall shear (and in particular, oscillators wall shear) appears to provide a stimulus for the development of anastomotic intimal hyperplasia.

Boundary control and shape optimization for the robust design of bypass anastomoses under uncertainty

“…In Figure 13 we give the values of the J 5 functional for the four different cases: ω = 0, 1, 2, 3. The optimal angles recovered are in the range [30][31][32][33][34][35] • depending on the residual flow, and again the largest values of J 5 (v) were observed for the case ω = 0. Moreover, the optimal angles obtained by using a 2D model are very similar to those computed by exploiting a full 3D flow simulation (see also Tab.…”

“…We consider throughout this section a simplified 2D bypass configuration Ω = (0, 5) × (0, 1), where Γ in = {(x, y) : x = 0, y ∈ (0, 1)} and Γ bc = {(x, y) : x ∈ (1, 3/2), y = 1}, respectively, thus considering the graft-tohost diameter ratio to be fixed at its (near-)optimal value 1.5 as discussed in [32].…”

“…The major factors known to strongly influence the recurrence of intimal hyperplasia are related to low or oscillating Wall Shear Stress (WSS) [32,33] occurring near the anastomosis. Hence, a typical attempt to design a bypass graft is apt at minimizing some cost functionals related to haemodynamic quantities.…”

“…Many works [1,2,16,17,21,35,38,43,49,51,56,58] have focused in the last decade on the optimal shape design of end-to-side anastomoses, typically by acting on the wall shape near the anastomosis by local shape variations. The three most significant design variables in end-to-side anastomoses are [39]: the anastomosis angle, the graft-to-host diameter ratio [32], and the toe shape (see Fig. 1).…”

A reduced computational and geometrical framework for inverse problems in hemodynamics

“…The mechanobiological response of the tissue to flow disturbances is a complex phenomena to model mathematically. Clinical evidence indicates that increased plaque growth is observed in regions of abnormally low or oscillating WSS [31,33]. In conjuction the tissue undergoes a maladaptive process, resulting in a marked change in the mechanical properties of the arterial wall.…”