An old water pipe hums differently from a new one. A pipe that has lost its flexibility answers every surge of water with a sharp echo that races back to the pump faster than in a young system. Something similar happens with our arteries: over the years their walls harden, and the pulse wave, having reached the small vessels, reflects back to the heart almost without delay. This early return of the wave can be picked up by a sensor on the neck, and from it you can estimate the age of the vascular system.
From the resulting velocity curve, eight features were extracted, among which the reflected-wave index VRI is especially interesting. It shows how large the reflected wave is compared with the main one. In young arteries the reflection is weak, because flexible walls damp the wave the way a soft pillow absorbs a blow. In older people the walls are stiff, and the reflected wave comes back almost as strong as the original. The correlation of VRI with age turned out to be the highest among all features: r = 0.738, and after adjustments for sex and exercise habits it rose to 0.798.
Before this work, it was unclear which of the eight carotid Doppler features is most strongly linked to age and whether it would hold up after adjustments. Now it is clear: the reflected wave is the most reliable marker. It is measured with a simple portable sensor that can be used in a general practitioner's office or in the field. However, until VRI has been compared with reference measures of arterial stiffness, such as pulse wave velocity from the neck to the thigh, it is too early to call it a ready clinical tool.
The echo of an old water pipe returns quickly because the pipe has lost its flexibility. Likewise, the reflected wave in an artery hurries back when the walls harden. A sensor on the neck hears this echo and turns it into a number that grows with age. Perhaps soon this is exactly how the biological age of vessels will be checked, without complex equipment and lengthy examinations.
The gap between the strongest and the weakest feature in the random forest was almost 20-fold: VRI received an importance of 0.536, while d-min got only 0.025. This shows how much more informative the reflected wave is compared with the diastolic minimum.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.