Blood Flow Conditions and Sounds in AVFs

The native arteriovenous fistula (AVF) is the lifeline for patients on hemodialysis treatment, but it is still affected by high non-maturation and early failure rates, requiring in most of the cases, the creation of a new vascular access.

Despite the exact mechanism underlying stenosis development and consequent AVF failure remains tentative, there is a general consensus that hemodynamic conditions play a key role. The hemodynamic conditions can be studied using computational fluid dynamic simulations (CFD), advanced computational techniques that allow to simulate blood flowing in virtual 3D models generated from medical images. The current gold standard in the clinical studies with CFD is to obtain reliable 3D AVF models from non-contrast enhanced MRI and our group developed a novel MRI protocol for this purpose. However, recent studies performed by other groups suggest that US technique can also provide accurate and reliable models and the hit on the market, and the tUS Piur Device, which was recently made available to the investigators' research group, offers new avenues for non-invasive and inexpensive 3D patient-specific AVF model generation. Previous computational fluid dynamics investigations inside patient-specific AVF models conducted by the investigators revealed transitional turbulent-like flow in the vein. In particular, the investigators evaluated the venous surface areas occupied by high values of the Oscillatory Shear Index (OSI), a well-accepted hemodynamic metric for the identification of disturbed flow conditions, and they found that wide areas of the venous segment of AVFs are characterized by OSI > 0.1. More recently, by using fluid structure interaction simulations, the investigators have shown that such turbulent-like blood flow conditions cause the venous wall to vibrate at high frequencies and that wall vibrations phenotypically collocate with typical regions of stenosis formation. The investigators' hypothesis is that flow-induced vibrations are transmitted to the skin surface of the patient and then result in those palpable thrills and audible bruits that, over the years, nurses and nephrologists got used to qualitatively evaluate using their stethoscopes. However, up to now sound evaluation has only been qualitative and therefore very subjective, but it may provide a strong indication of aberrant hemodynamic conditions and could have a potential as a non-invasive and unexpensive surveillance method.

Therefore, studies aimed at clarifying the relationship between the blood flow conditions and the AVF sound will help advancing the knowledge in the field, providing indications on the role of hemodynamics in AVF failure and bringing out novel methods such as sound analysis for AVF surveillance.