**4. Arteriovenous grafts**

AVGs constitute about 12–13% of vascular accesses in Europe, Japan, Australia, and New Zealand, compared to 25% in the United States [22]. Although AVFs are much more common than AVGs, both are preferred over CVCs. AVGs may be preferable over AVFs when a patient has unsuitable veins for AVF or AVF maturation issues. A study examining mortality in maintenance hemodialysis patients showed that transitioning from a CVC to an AVG of AVF was associated with reduced mortality (hazard ratio [HR] = 0.69), and transitioning from AVG or AVF to a CVC was associated with higher mortality (HR = 2.12) [23].

In the past, AVG advances have focused on reducing vein stenosis and graft clotting, as well as altering flow dynamics. Recent advances in AVGs include early cannulation AVGs (eAVGs), anti-neointimal hyperplasia AVG therapy, hybrid AVGs, and bioengineered vessels as AVGs.

The eAVG is a graft that can be cannulated within 72 hours of placement for dialysis. It is designed to with materials to prevent back wall puncture, and with self-sealing properties to allow for immediate access [24]. One 2015 systematic review showed that eAVGs had similar complication and patency rates to standard AVGs made of expanded polytetrafluoroethylene (ePTFE) [25].

Another advance in AVG technology relies on surface modification. Heparin coatings have been created to lower the risk of thrombosis, although the evidence on whether it reduces thrombosis is controversial [26]. Other efforts have been made to make grafts more biocompatible through strategies such as outer wall modification using electrospinning, nanotopography, or lithography. To combat neointimal hyperplasia, which causes graft failure, sirolimus-eluting devices and paclitaxel coatings have been designed. Although a few studies have demonstrated successful use of these anti-proliferative agents, large comparative trials are lacking [27, 28].

Hybrid AVG systems have allowed navigation through central vein stenosis. The Hemodialysis Reliable Outflow device (HeRO, Merit Medical Systems, South Jordan, UT), uses a combination of a tunneled CVC and AVG to provide hemodialysis access while bypassing a venous stenosis or occlusion (**Figure 3**) [29]. Although expensive,

#### **Figure 3.**

*Hemodialysis reliable outflow (HeRO) Graft. This illustrates the HeRO Graft, which utilizes an expanded polytetrafluoroethylene (ePTFE) component to bypass venous stenoses and occlusions. The graft is anastomosed to an artery on one end and is inserted into a central vein on the other end, bypassing the stenosis/occlusion in between. Reprinted from: HeRO Graft [Internet]. Merit Medical; 2022. Available from: https://www.merit. com/peripheral-intervention/access/renal-therapies-accessories/merit-hero-graft/. Copyright 2022, from Merit Medical. © Merit Medical, Reprinted by Permission.*

this device may save cost due to lower complications compared to a tunneled CVC alone [30].

Over the last few decades, tissue bioengineered vessels have been created to replace prosthetic grafts. Blood vessels may be chemically treated to decrease immunogenicity. Human vascular cells may be grown on biodegradable scaffolds. One study in the US and Poland investigating human acellular vessels found that after a year, primary patency was 28% and secondary patency was 89% [31]. Overall, these bioengineered vessels have demonstrated better patency than standard AVGs in a few studies, but evidence of clinical benefit over standard AVGs is lacking.
