The advent of endovascular arteriovenous fistula (endoAVF) creation with the development of the Ellipsys (Medtronic) and WavelinQ (BD) systems marks a “major innovation in dialysis access”— and a “mounting evidence base” demonstrates their high functional patency rates. So said interventional radiology lead at East and North Herts NHS Trust (Stevenage, UK) Kate Steiner, who led a discussion of the next generation of endoAVF devices at the Charing Cross Symposium 2023 (25–27 April, London, UK). Joining her was Robert Shahverdyan (Asklepios Clinic Barmbek, Hamburg, Germany) and Simon Hogan (Pathfinder Medical, London, UK), who offered a preview of the future of the technology.
Hogan opened proceedings with a dive into the ePath system currently in development at Pathfinder Medical, which he explained has its roots in the work of the company’s electrical engineer CEO Sorin Popa at Imperial College London. He explained that it is a twin-catheter system with a 3Fr target that goes into the target artery, and a 5Fr crossing catheter for the target vein.
Under fluoroscopic guidance, the ePath system sees the two catheters inserted “either from a parallel point of view or anti-parallel way depending on the access sites that are being used”. The target device has a basket on the end that acts as a sensor, while there are four electrodes printed into the surface of the crossing device,” Hogan outlined. These electrodes generate electrical fields in which the alignment of the target basket can be detected.
“You know exactly where the needle is pointing when you rotate the handle. The display is very intuitive—it is just two arrows that point to each other, and when it turns green you know the devices are aligned. Then, the needle pops out straight into the middle of the basket. You deploy a guide wire, after which you can deploy a stent or balloon over the top of that: whatever the operator desires to create the fistula.”
Hogan added that preclinical trials in cadaveric and animal models had tested the system in the wrist and other sites in the forearm. “We hope to be able to advance our clinical research in the coming months,” he said.
Shahverdyan spoke next, offering an overview of the Velocity device (Venova Medical) from his point of view as a member of the scientific board advising on the product’s development. Its designers, he detailed, looked to build on the first generation of endoAVF devices by resolving their “reintervention and cannulation difficulties”. He explained that, unlike the ePath system, the Velocity is a single-catheter device that is guided by ultrasound only and without a sheath. It is used in the proximal radial artery, and the procedure utilises a 2.1cm long 4–5mm diameter tapered and fenestrated stent graft.
“When you deploy the implant and remove the catheter, you do not need to use external force for the anastomosis as in previous devices,” Shahverdyan stated. “You also cover the major branches during AVF creation, which otherwise steal the flow into the deep veins—this may help to achieve faster and better maturation rates of the superficial veins without losing flow into the deep veins. Hopefully, this will also reduce the risk of secondary interventions due to juxta-anastomotic issues.”
Animal studies produced “great results”, before first-in-human studies with a total of nine patients saw 100% procedural technical success, Shahverdyan said. The procedure should also be less painful than some alternatives as it can be performed under local anaesthesia.
Queried by Steiner on the unique advantages of these second-generation devices relative to earlier endoAVF technologies, Shahverdyan argued that, in the USA, the current rate of reinterventions with endoAVF is high. These fistulas frequently require balloon angioplasty and/or deep vein coil-embolization to assist maturation, and this is something which Shahverdyan envisions to be less of an issue with newer technologies.
Hogan weighed in here to say it was flexibility around the site of fistula creation that was unique to the ePath system, enabled by the small size of the devices involved: “We can get down into some very small peripheral arterial and venous trees […] I think that should open up a broader range of patients that includes patients with vessels that are too far apart for either an Ellipsys or a WavelinQ procedure.” Preclinical trials in an animal model found even distances of 7–8mm were surmountable, he said, while adding as an aside that adjunctive procedures such as brachial vein embolisation can be done through the same access with which the device is inserted. These features both mark advances on the offering of first-generation devices.
Steiner stated in summary that, with second-generation systems, vessels could be further apart, resolving a limitation with both existing devices. With both available systems, Shahverdyan averred, one of the exclusion criteria is the limited distance between the vessels at the anastomosis site. A stent graft would hence increase the number of suitable patients. With that in mind, he said, Venova Medical decided to broaden the length between the artery and the vein to 3mm in their VENOS 1 first-in-human trial. Asked where he saw the Velocity device fitting into his practice, Shahverdyan said that, if it lived up to its promise (high maturation and patency rates with low reintervention rates), it could be a complete replacement for both existing endoAVF devices.
Stenting and surgical options
Noting that Velocity uses stent graft and ePath offers them as an option, Steiner posed the question: “Will we see some of the issues we have seen with stents elsewhere—acute thrombosis, in-stent stenosis, edge stenosis?”
Admitting he remained unsure, Shahverdyan said the initial trial in nine patients yielded only one early occlusion. Stent grafts, he suggested, benefitted from low resistance in the venous system to pull blood from the proximal or distal radial artery, leading to the implant remaining open for the longest six-week follow-up period recorded so far by the Velocity developers. “We will see in the future whether it comes to edge stenosis. If we achieve unassisted maturation of the target vein and can cannulate early, hence removing the dialysis catheters as soon as possible, that possible secondary hyperplasia would not be so relevant anymore.”
“Stent technology is moving on at great pace,” Hogan added. “In our case, the ePath allows us to deploy the stent that is most appropriate for a patient and their anatomy. We are not limited to one type or design of stent.”
Steiner suggested both devices create something that more closely resembles a surgical AVF (sAVF) than Ellipsys or WavelinQ. “But if it does,” she asked, “why not create an sAVF?”
“The idea of endoAVF is not only that it is minimally invasive now,” Shahverdyan answered, “but it is also that low flow into the superficial veins might keep them from aneurysm or stenosis—especially downstream cephalic arch stenosis. We think that with a tapered formation of the implant it almost mimics the VasQ (Laminate Medical) anastomotic external support device in that we have tapered laminar flow—VasQ AVFs tend to feel less pressurised than typical sAVFs, probably due to the reduction of that turbulent flow. We anticipate similar haemodynamic changes. It is worth remembering that sAVF will still be there, and if endoAVF fails it will still be an option. It is an addition and not a direct replacement unless the results are significantly better for endoAVF.”
Hogan echoed this desire to preserve laminar flow by creating a gentler turn for the blood. This, he hopes, will limit flow issues. Steiner put it that, though it was a “very similar anatomical location [with ePath] to a sAVF, it was still a very different way of making the fistula” compared with a sAVF. Agreeing, Hogan said it was about “standing on the shoulders of giants, learning from previous endoAVF technologies as well as sAVF. It is about trying to combine the best of all of those approaches into one offering for patients and for clinicians.”
Asked to “put his business head on”, Hogan answered the issue of adoption and cost by saying that the upfront cost of endoAVF technology sometimes prevents clinicians from seeing the long-term cost benefit offered by their improved outcomes. Shahverdyan concurred, saying that fast procedures and good outcomes— and the avoidance of failed sAVF procedures— could save on costs in the long run.
Quizzed finally by Steiner on the prospects for their devices, Hogan said that, from a pivotal study and regulatory point of view, things would be “much clearer” in around a year. On Velocity, Shahverdyan said a multicentre pilot study was “hopefully planned” for autumn or winter 2023. Steiner stated simply: “Watch this space.”
