Given the introduction of the 4Fr WavelinQ system in 2018 that followed the original 6 Fr system, there has been a limited window of time to assess longer-term outcomes for patency and functionality. Furthermore, from the original prospective clinical studies examining outcomes of the 4Fr device, the protocol did not require longer-term follow-up [6]. This retrospective multi-center study with a larger patient population addresses the lack of longer-term outcomes. Furthermore, the patient population presented in this study was not part of any prior clinical trial and reflect predominantly nonacademic practice.

From the two studies published by Berland, the EASE study (Endovascular Access System Enhancements Study) which combined patients from three prospective studies that utilized the 4Fr system had a technical success rate of 100% with an 87% cumulative patency at 6 months [7]. A later post-market approval study (which included the EASE study population) had a technical success rate of 96% with 88% cumulative patency at six months [6]. Patency beyond 6 months was not examined. In another comparative retrospective single-operator study of the Ellipsys and WavelinQ devices by Shahverdyan et al., cumulative patency was not assessed [12]. A single-center retrospective study of 30 patients who had fistulas created with the WavelinQ 4 Fr device had a 100% technical success rate, with an 87% maturation rate with longer-term 96% one-year and 82% two-year cumulative patency rates [8]. Cumulative patency at 2 years was 93% in this study.

Functional patency was 93% at two years indicating that once two-needle cannulation was achieved it was maintained with a high level of longer-term durability, which was not ascertained in the prior published studies. Interventions per patient year were 0.73 that included maturation interventions. While the intervention rate is similar to Kitrou’s 0.53 and the post-market approval study of 0.55 [6, 8], the relatively low rate of interventions highlights the need for minimal intervention to maintain functional patency up till two years. By comparison, surgically created accesses require approximately 1.9 interventions per patient year [2]. Time to maturation ranges from 41 to 130 days in prior studies [6, 8, 12, 13], whereas we observed median maturation time of 95 days. Although within the observed range, the later time to maturation may be the result of an inconsistent follow-up protocol between the different sites and the COVID pandemic impacting the ability to evaluate patients in person.

Univariate analysis of collected variables did not identify any factors that influenced cumulative or functional patency. This included an assessment of ulnar artery/ulnar vein and radial artery/radial vein fistulas indicating that fistula location did not impact patency. However, improved maturation was associated with male gender and coil embolization during endoAVF creation. Improved maturation in male patients has also been observed with surgical AVF creation [14, 15]. Twenty-five percent of patients (25/98) in this study were black, a larger percentage than prior study populations with the WavelinQ device yet ethnicity did not influence patency or maturity.

The selection of the vein for embolization was guided by flow rate observations during the verification of fistula creation. Coil embolization was performed in 80.4% of patients to enhance blood flow through the new AVF, a number that is higher than presented in Kitrou’s study, where coil embolization was performed in 33.3% of the cases [8]. This observation is likely related to operator preference as the decision to embolize is somewhat subjective based on observed flow patterns post-endoAVF creation and may be related to site of fistula creation. Major complications were rare, with only one instance observed—pseudoaneurysm formation at the wrist ulnar artery access site. This was the likely result of patient noncompliance during post-procedure recovery. No infectious complications were observed, whereas a 4.1% rate was observed with surgical fistulas in a meta-analysis [16]. The low complication rate correlates with previous studies with 1–4 complications reported overall [6, 8, 17].

Regarding technical challenges observed during this study period, patients with calcified arteries at the intended site of endoAVF creation should be avoided. It is believed that the technical failures in this study were attributed to arterial calcification, and it is unclear what degree of calcification presents a problem. Also, the authors gravitated to brachial nerve blocks as endoAVF experience matured. Brachial blocks reduce vascular spasm, lead to vessel dilation, and likely reduce the IV analgesia requirements. Patience is also required for retrograde venous access as traversing venous valves can be difficult. For patient safety, arterial access obtained at wrist level resulted in no arterial access complications with the exception of one event described above.

A direct comparison to the Ellipsys endoAVF device (Medtronic, Minneapolis, MN) may not be appropriate given differences in the location of fistula creation and energy used to create the fistula (between the radial artery and the venous perforator; thermal energy) and that longer-term outcomes are from patients selected prospectively for the pivotal US prospective study initiated in 2015. However, in 85 patients cumulative and functional patency at 2 years was 88% and 92%, respectively, which are similar to outcomes observed in this study suggesting that endoAVF creation is a durable access solution [18]. Intervention rates are not comparable as the Ellipsys study intervention rate did not include interventions within the first two years of creation for those patients who had a mature fistula. Also, a comparison to surgically created autogenous fistulas is limited by differing definitions, patient populations, and fundamental differences in the anatomic locations of endoAVFs. Despite these considerations, a meta-analysis of surgically created AVFs found a primary failure rate of 23%, with subsequent patency rates of 71% and 64% at the one- and two-year marks, respectively [19]. Another surgical fistula meta-analysis found one-year cumulative patency to be 79% [16]. In this study, if the cases of failure to mature (intent to treat) are included, one- and two-year cumulative patency would be 84% and 82%.

This study has several limitations. Given the retrospective nature of the study and nonconformity in data points collected pre- and post-procedure, certain variables could not be collected that may be relevant to outcomes such as the patients prior access history, time to dialysis catheter removal, vein diameters pre fistula creation, and brachial artery flow volumes over time. Also, all three centers had different follow-up algorithms which likely influenced time to maturation and functional patency.