Experimental evaluation of local wave speed in the presence of reflected waves.

Wave speed (also called pulse wave velocity) is the speed by which disturbance travels along the medium and it depends on the mechanical and geometrical properties of the vessel and on the density of the blood. Wave speed is a parameter of clinical relevance because it is an indicator of arterial stiffness and cardiovascular diseases. The aim of this work is to compare different methods for the determination of local wave speed in bench experiments and investigate their relative accuracy when reflections are present. Pressure (P), flow (Q) and diameter (D) were measured along a flexible tube far and close to three positive and three negative reflection sites. Wave speed was calculated using PU-loop, (lnD)U-loop, QA-loop, D(2)P-loop, sum of squares and characteristic impedance methods. Results were compared to the foot-to-foot method. We found that far from the reflections almost all methods give uniform results. Close to positive reflections the methods that rely on P and Q (or U) overestimate the wave speed value, while techniques based on D (or A) and Q (or U) underestimate it. On the contrary, close to negative reflections the methods that rely on P and Q (or U) underestimate the wave speed value, while techniques based on D (or A) and Q (or U) overestimate it. The D(2)P-loop does not seem to be affected by positive or negative reflections. Most of the methods currently used to determine local wave speed are affected by reflections, but the (lnD)U-loop remains the easiest technique to use in the clinic.