Location of a Reflection Site Is Elusive

Aortic pulse wave velocity (PWV), a measure of aortic stiffness, is an important indicator of cardiovascular risk. Derivation of PWV from uncalibrated proximal aortic or carotid pressure alone has practical advantages. However, when the time of return of the reflected wave, (Delta)t, is used to calculate PWV, inaccurate data are obtained. With aging PWV increases but (Delta)t hardly decreases, suggesting that the reflection site moves toward the periphery. We hypothesized that the forward and reflected waves in the distal aorta are not in phase, leading to an undefined reflection site. We derived forward and backward waves, at the entrance and distal end of a uniform tube, with length "L." With the tube closed at the end, forward and reflected waves are there in phase, and PWV=2L/(Delta)t. When the tube is ended with the input impedance of the lower body, forward and backward waves at its end are not in phase, and (Delta)t is increased, suggesting that the reflection site is further away (tube seems longer), and PWV calculated from 2L/(Delta)t is underestimated. Using an anatomically accurate model of the human arterial system, we show that the forward and backward waves in the distal aorta are not in phase. When aortic PWV increases, (Delta)t changes only little, and the reflection site appears to move to the periphery, similar to what is observed in humans. We conclude that to define the location of a reflection site is elusive and that PWV cannot be calculated from time of return of the reflected wave.