We previously demonstrated that the linear portion of the pressure-velocity loop (PU-loop) corresponding to early systole could be used to calculate the local wave speed. In this work we extend the results of this method to show that determination of the time at which the PU-loop first deviates from the linearity provides a convenient way to determine the arrival time of the reflected wave.

Pressure and flow were measured in elastic tubes of different diameters, where a strong reflection site existed at known distances away form the measurement site. Pressure and flow were also measured in the ascending aorta of 11 anaesthetised dogs where a strong reflection site was produced through total arterial occlusion at 4 different sites. The arrival time of the reflected wave was determined using a new algorithm that detects the sampling point at which the initial linear part of the PU-loop deviates from linearity by comparing the relative difference of slopes to an empirically determined threshold.

In elastic tubes the arrival time of reflected waves detected using the PUloop was almost identical to that detected using wave intensity analysis and foot-to-foot methods with a maximum difference of 5%. Arrival time of reflected waves detected using the PU-loop in vivo highly correlated with that detected using wave intensity analysis (r² = 0.94, P < 0.001).

We conclude that the new technique described in this paper is easy to use, its results are comparable to those of traditional techniques, and allows for the dynamic determination of wave speed and the arrival time of reflected waves, simultaneously.