AVF-MONITOR: a New Wearable Device for Vascular Access Monitoring in Hemodialysis Patients - Proof-of-concept Study

End-stage renal disease (ESRD) is an increasingly prevalent global health issue, closely linked to the aging population. The majority of individuals with ESRD requiring renal replacement therapy undergo hemodialysis (HD) treatment . A successful HD procedure requires a well-functioning vascular access (VA) to ensure a safe and durable connection between the patient's circulation and the artificial kidney. To date, VA dysfunction remains the leading cause of morbidity and hospitalization among HD patients, representing a major limitation of this treatment modality. The current recommended VA for hemodialysis is the native arteriovenous fistula (AVF), which is surgically created in the forearm by connecting a vein and an artery through an anastomosis. Although the AVF is considered the first-choice access, it is still associated with high rates of non-maturation and early failure, with up to 40% failing within the first year post-intervention, mainly due to vascular stenosis. Identifying patients at risk of AVF failure is crucial, yet current surveillance strategies remain inadequate. In practice, AVF function is rarely monitored using the gold standard Doppler ultrasound (US) after surgical creation and initiation of HD, unless nurses report significant cannulation difficulties. By that time, however, the AVF is often already occluded, requiring the urgent placement of a central venous catheter to continue HD, followed by surgical intervention to create a new AVF. Moreover, Doppler US is time-consuming and requires a dedicated nephrologist with specific imaging expertise. Furthermore, in smaller dialysis centres, the lack of access to a Doppler US machine further restricts effective surveillance.

Continuous monitoring of AVF function could enable early identification of reduced blood flow or stenosis development, allowing timely salvage intervention by interventional radiologists before the complete closure of the AVF.

Over the years, nurses and nephrologists got used to assess AVF by palpation and auscultation, qualitatively evaluating its vibration (thrill) and the sounds it emits using a stethoscope. A non-stenotic AVF typically produces a soft, continuous murmur: a strong sound during systole and a weaker one during diastole, both associated with laminar blood flow. In contrast, a malfunctioning AVF often lacks a palpable thrill and emits a systolic-only high-pitched, hissing sound caused by turbulent and disturbed blood flow. In more severe cases of stenosis, a distinct 'whistling' sound may be heard, indicating critical dysfunction or complete failure of the AVF.

Based on these clinical evidences, recent studies have revealed significant differences in the acoustic characteristics of AVF sounds, recorded with electronic stethoscopes, between functioning and non-functioning AVFs. The frequency spectra of these sounds show that patent AVFs are characterized by low-frequency sounds, whereas stenotic AVFs exhibit higher frequencies, reaching up to 700-800 Hz. Furthermore, it has been demonstrated that AVF sounds acquired with an electronic stethoscope can be used to extract key information about hemodynamic conditions, providing indications of complex flow dynamics and adequate blood volume for effective HD. These results suggest that sound analysis has significant potential to improve clinical AVF surveillance and enhance patient outcomes.

However, the monitoring technique involving the use of an electronic stethoscope requires expert personnel, as it demands numerous precautions during both sound recording (e.g., avoiding friction and excessive pressure) and analysis, which requires technical expertise. These factors limit the use of this technological solution to only a small subset of HD patients.

To overcome these limitations, a prototype wearable device called AVF-MONITOR has been designed which allows to record AVF sound, possibly enabling simple and continuous AVF surveillance. In the future, after adequate testing, AVF-MONITOR could be potentially used in a home setting, allowing patients to independently monitor their AVF function on a daily basis.