#### PWV Definition

The pulse wave velocity is defined as the ratio between the length of an arterial segment and the related transit time of the pressure wave: PWV = Distance / DeltaT. PWV must not be confused with the flow speed of the blood. The greater the stiffness of the arterial wall, the greater the PWV will be.
The nature of the walls of the blood vessels plays a key role in determining the pulse wave velocity inside them:
Aorta and large elastic Arteries have a high content of elastin fibers having the task of making the blood flow continuous in the rest of the body: this is achieved by dilation during the systolic phase to store excess energy which is then returned during the diastolic phase, with the reduction of their lumen.
The below image was derived from a healthy adult population and refers to the Carotid-Femoral segment with PWV percentile curves in meters / second referred to the age: note how PWV increases with age. This is due to the change of composition of the arterial walls with a reduction of the ratio between elastin fibers and collagen fibers.
Peripheral arteries, on the other side, are mainly of muscular type, adapting their lumen according to the sympathetic system.
The below image was derived from a healthy adult population and refers to the Carotid-Radial segment with PWV percentile curves in meters / second referred to the age: note how PWV remains almost constant with age.
In summary, we can say that:
The Carotid-Femoral PWV increases with age and may be considered an index of biological age of the vascular system. Its increase is also caused by situations of arterial stiffness due to inflammation, calcification, ...
For this reason, it's considered the gold standard for assessing Arterial stiffness.
On the other side, the Carotid-Radial or the Femoral-Tibial PWV does not change with age and does not provide information on the biological age of the vascular system. The PWV in this case is mainly related to the activity of the sympathetic system.