Acoustical Monitoring of Model System for Vascular Access in Haemodialysis

Stenosis is the main reason of the failure of a vascular access in haemodialysis. Its presence engenders an increase of the morbidity and an increase of the hospitalization for hemodialysed patients and its early detection consists thus in an important issue. Digital phonoangiography or digital acoustical analysis is an alternative to the current techniques combining the advantage of its low-cost, its easy implementation as well as its user- and skill-independency. This technique, automated version of the auscultatory technique used by the practitioners with their stethoscope, is based on the idea that the stenosis engenders modifications of the hemodynamic in the vascular access at the origin of sounds or “bruits”. This study proposed to assess the influence of different parameters on the acoustics by mean of an in vitro model system that allows controlling independently each of these parameters. Assessed parameters included the severity or degree of the stenosis, the flow, the pressures, the viscosity and the velocity. Results indicated that the essential parameter was the velocity at the stenosis which determines the appearance of spectral features, starting from a value of about 1.4 m/s while a value around 2.6m/s could be almost always associated with certitude to their presence. Spectral features were found in the frequency bandwidth from 70 to 800Hz with three main regions located at 180-300Hz, 310-390Hz and 440-700Hz. Moreover results indicated that a minimum severity of the stenosis was required, corresponding to an obstruction of the lumen of the vessel of at least 44%. These results suggested that the underlying reason was the modification of the type of flow from the transitional or laminar to the turbulent state confirmed by the Reynolds number. This study also further investigated the possibility of detection of sounds directly on the extracorporeal blood circuit by transmission through the needle.