Computational analyses and design improvements of graft-to-vein anastomoses

An extended convection diffusion model for red blood cell-enhanced transport of thrombocytes and leukocytes

Transport phenomena of platelets and white blood cells (WBCs) are fundamental to the processes of vascular disease and thrombosis. Unfortunately, the dilute volume occupied by these cells is not amenable to fluid-continuum modeling, and yet the cell count is large enough that modeling each individual cell is impractical for most applications. The most feasible option is to treat them as dilute species governed by convection and diffusion; however, this is further complicated by the role of the red blood cell (RBC) phase on the transport of these cells. We therefore propose an extended convection-diffusion (ECD) model based on the diffusive balance of a fictitious field potential, Psi, that accounts for the gradients of both the dilute phase and the local hematocrit. The ECD model was applied to the flow of blood in a tube and between parallel plates in which a profile for the RBC concentration field was imposed and the resulting platelet concentration field predicted. Compared to prevailing enhanced-diffusion models that dispersed the platelet concentration field, the ECD model was able to simulate a near-wall platelet excess, as observed experimentally. The extension of the ECD model depends only on the ability to prescribe the hematocrit distribution, and therefore may be applied to a wide variety of geometries to investigate platelet-mediated vascular disease and device-related thrombosis.

Wall shear stress measurement using phase contrast magnetic resonance imaging with phase contrast magnetic resonance angiography in arteriovenous polytetrafluoroethylene grafts

PURPOSE

The purpose of the present article was to determine the changes in luminal vessel area, blood flow, and wall shear stress in both the inflow artery and the venous stenosis of arteriovenous polytetrafluoroethylene (PTFE) grafts.

METHODS AND MATERIALS

Polytetrafluoroethylene grafts were placed from the carotid artery to the ipsilateral jugular vein in 8 castrated juvenile male pigs. Contrast-enhanced magnetic resonance angiography (MRA) with cine phase-contrast magnetic resonance imaging (MRI) was performed 2 weeks after graft placement.

RESULTS

The mean wall shear stress at the venous stenosis was 4 times higher than the control vein, while the inflow artery was only 2-fold higher. By day 14, venous stenosis had formed, which was characterized by narrowed area and elevated blood flow.

CONCLUSION

By day 14, there is venous stenosis formation in porcine arteriovenous PTFE grafts with increased shear stress with decreased area when compared to control vein.

Impact of Cuffed, Expanded Polytetrafluoroethylene Dialysis Grafts on Graft Outlet Stenosis

BACKGROUND

The objective of this study was to determine prospectively the difference between the graft outlet strictures of a polytetrafluoroethylene (ePTFE) graft with a cuff at the graft-vein anastomosis (Venaflo; Bard industries, Tempe, Ariz.) and that of the regular ePTFE graft (Stretch Gore-Tex; Gore, Flagstaff, Ariz.) placed for hemodialysis access.

METHODS

Between January and April 2005, 36 consecutive patients (average age: 63.3 years) underwent ePTFE graft implantation (36 implantations) for hemodialysis at the Vascular Surgery Section of Chang Gung Memorial Hospital. The patients of the study cohort were randomly assigned to two groups based on the graft used: cuffed graft group (Venaflo graft) and non-cuffed standard graft group (Gore-Tex graft). Each patient underwent antegrade venography at the 3-month follow-up to demonstrate the graft outlet stricture. Results of the graft outlet angiography analysis were examined, and all medical records were reviewed at end of the study. The degree of the graft outlet stenosis was compared between the two groups.

RESULTS

Average stenosis of the cuffed graft group and non-cuffed (standard) graft group were 22.76 +/- 26.37%% and 44.95 +/- 27.48%%, respectively; the difference between the two groups was statistically significant (P < 0.05).

CONCLUSIONS

The graft outlet stricture of cuffed ePTFE grafts for hemodialysis 3 months after implantation was less severe than that for the standard ePTFE graft. The correlation between the stricture level and dialysis graft patency requires further clarification.

Numerical Simulation of Wall Shear Stress and Particle-Based Hemodynamic Parameters in Pre-Cuffed and Streamlined End-to-Side Anastomoses

A number of research studies have related multiple hemodynamic parameters to the formation of distal anastomotic intimal hyperplasia (IH) at the sub-cellular, cellular, and tissue levels. Focusing on mitigating WSS-based parameters alone, several studies have suggested geometrically modified end-to-side anastomoses with the intent of improving synthetic graft patency rates. However, recent clinical trials of commercially available versions of these grafts indicate persistently high rates of failure. Furthermore, recent evidence suggests that platelet-wall interactions may play a significant role in the formation of IH, which is not captured by WSS-based parameters alone. In this study, numerical simulations have been conducted to assess the potential for IH formation in conventional and geometrically modified anastomoses based on both wall shear stress (WSS) conditions and platelet-wall interactions. Sites of significant particle-wall interactions, including elevated concentrations and stasis, were identified by a near-wall residence time model, which includes factors for platelet activation and surface reactivity. Conventional, pre-cuffed, and streamlined distal end-to-side anastomoses were considered with proximal and distal arterial outflow. It was found that a pre-cuffed anastomosis, similar to the Distaflo configuration, does not offer a hemodynamic advantage over the conventional design considered with respect to the magnitude of the WSS field and the potential for platelet interactions with the vessel surface. Streamlined configurations largely consistent with venous confluences resulted in an advantageous reduction of wall shear stress gradient values; however, particle-wall interactions remained significant throughout the anastomosis. Results of this study are not intended to be directly extrapolated to surgical recommendations. However, these results highlight the difficulty associated with designing an end-to-side distal anastomosis with two-way outflow that is capable of simultaneously reducing multiple hemodynamic parameters. Further testing will be necessary to determine if the observed elevated particle-wall interactions in a pre-cuffed anastomosis provide the stimulus responsible for the reported high failure rates of these grafts.

Local haemodynamics and shear stress in cuffed and straight PTFE-venous anastomoses: an in-vitro comparison using particle image velocimetry

OBJECTIVES

To use particle image velocimetry (PIV) to study the haemodynamics and shear stress associated with cuffed and straight PTFE-venous anastomoses.

METHODS

Silastic models of a straight and cuffed (Venaflo) PTFE-venous anastomoses were attached to a pulsatile flow 'Berlin Heart' circuit filled with glycerine/water and hollow glass tracer spheres. Instantaneous velocity fields were obtained PIV and shear rates and patterns calculated from frame-by-frame analysis.

RESULTS

A high velocity jet struck the anastomotic 'floor' and was deflected toward the venous outflow. Shear stresses near the floor were significantly higher, in the straight anastomosis. Sites of high shear stress correlated well with the known sites of intimal hyperplasia.

CONCLUSIONS

A cuffed anastomosis type may be favourable in terms of local haemodynamics so enhancing the long-term patency of PTFE-venous grafts.

Prospective, randomized evaluation of a cuffed expanded polytetrafluoroethylene graft for hemodialysis vascular access

BACKGROUND

A cuffed expanded polytetrafluoroethylene (ePTFE) hemodialysis graft was developed to address the problem of recurrent stenosis at the graft-vein anastomosis. The purpose of this study was to compare graft patency and blood flow rates of cuffed and noncuffed (standard) ePTFE grafts placed for hemodialysis access.

METHODS

Forty-eight patients were prospectively randomized and followed for up to 24 months after placement of a cuffed or standard ePTFE graft for hemodialysis access. Study end points included time to graft failure and blood flow rates on hemodialysis.

RESULTS

Risk factors for graft failure were similar in both groups. However, the overall incidence of graft failure was significantly lower in the cuffed ePTFE graft group (P =.039). Graft patency rates in the cuffed versus standard groups were 64% versus 32% at 12 months (P =.037) and 58% versus 21% at 24 months (P =.0213). No cuffed ePTFE graft failed as a result of venous outflow stenosis. Average graft flow rates were similar when first measured 3 months postoperatively (845 mL/min, cuffed vs 715 mL/min, standard; P =.51) but declined more rapidly in the standard group (12 months, 623 vs 253 mL/min [P =.037]; 24 months, 531 vs 121 mL/min [P =.012]).

CONCLUSIONS

The cuffed ePTFE graft was associated with increased blood flow rates during hemodialysis and improved graft patency compared with a standard ePTFE graft. Our results suggest a beneficial effect of the cuffed venous geometry for hemodialysis vascular access.