Computational estimation of fluid mechanical benefits from a fluid deflector at the distal end of artificial vascular grafts

Abstract

Intimal hyperplasia at the distal anastomosis is considered to be an important determinant for arterial and arteriovenous graft failure. The connection between unhealthy hemodynamics and intimal hyperplasia motivates the use of computational fluid dynamics modeling to search for improved graft design. However, studies on the fluid mechanical impact on intimal hyperplasia at the suture line intrusion have previously been scanty. In the present work, we focus on intimal hyperplasia at the suture line and illustrate potential benefits from the introduction of a fluid deflector to shield the suture line from unhealthily high wall shear stress.

Introduction

In vascular bypass surgery, as well as surgery for venous access, autologous vessels are used as far as possible. However, because of occasional lack of autologous material of sufficient quality, artificial grafts are often used as a substitute. Unfortunately, artificial grafts are more prone to occlude postoperatively [1]. Anastomotic designs can be categorized as being either of the end-to-end or the end-to-side type (Fig. 1). The latter is typically employed in vascular bypass surgery as well as in constructions of vascular access grafts, used for hemodialysis. Anastomotic intimal hyperplasia (IH) at the lower anastomosis (Fig. 1) is considered to be one of the principal causes of graft failure. By attaching a vein patch or a vein cuff at the lower anastomosis, the rate of graft patency seems to be improved [2], [3], [4]. The mechanism of the improvement has been suggested to be related to factors such as fluid mechanical adjustments, compliance mismatch, and to the venous material itself [5], [6], [7], [8], [9], [10].

Artificial graft ends that mimic the geometries obtained by the surgical techniques mentioned above have been suggested to significantly improve the patency [11]. However, still there is room for further geometric design improvement [12], [13]. In particular, fluid mechanical effects around the suture line need to be drawn into focus [12], [13], [14]. It is therefore desirable to include protrusions representing the suture line in the geometric model (Fig. 2, Fig. 3, Fig. 4).

IH seems to be closely coupled primarily to low or high wall shear stress (WSS), but also to related measures [15], [16]. A WSS outside the range 0.5–3 Pa is considered closely related to IH, as disclosed by van Tricht et al. [17]. Arteries seem to establish a radius that, under normal conditions, results in a mean WSS in the range 1–2  Pa [18]. Therefore, IH induced by low WSS may be a way for the arteries to restore the local WSS back to the normal range [19], [20]. However, the process of IH stimulated by high WSS most likely receives positive feedback from the increased WSS that follows, which means the process does not end until the site is occluded.

The aim of the present study is to computationally evaluate the idea of redirecting a potentially high WSS away from the suture line by introducing a slight stricture—a fluid deflector (FD)—to shield the suture line from excessive fluid mechanical forces. The design change due to the FD will affect the manufacturing process of the artificial graft, that is, the FD is introduced before the surgical procedure. The result will be increased WSS just proximal to the suture line, within the artificial graft, where intimal hyperplasia is not expected to be stimulated by a high WSS [21].