Aortic Stiffness and Disease: Location is Key
Increased risk of cardiovascular disease and events associated with aortic stiffening has been attributed both to the haemodynamic consequences of stiffening and to this being a marker for an age-related degeneration of the arterial wall predisposing to arterial disease.1 There are several measures of arterial stiffness and indeed the term "stiffness" which does not have a physical definition was coined to encompass properties of arteries that tend to lead to an increase in many biomechanical measures of stiffness, particularly the speed of propagation of the pulse wave, pulse wave velocity (PWV). For any individual segment of an idealized artery, PWV is determined, not only by the intrinsic stiffness of the arterial wall as represented by its Young's elastic modulus (E, the ratio of stress to strain) but the effective thickness of the arterial wall and is inversely related to (the square root of) arterial diameter.2 PWV measured between accessible points in the arterial tree such as the carotid and femoral arteries provides an average measure of stiffening over the whole of this carotid-femoral pathway but is a simple robust measure of large artery stiffening that is closely related to major adverse cardiovascular events.3 E is also inversely related to the distensibility of the artery: the fractional change in arterial diameter produced by a given pressure change. This provides a measure of stiffness at a single location but requires knowledge of the local pulse pressure which is often difficult to obtain. Whilst E is the property most closely related to the behavior of the arterial wall, it represents the behavior of a composite material comprising individual elements that each have differing elastic properties and as such behaves in a very different fashion from a homogenous elastic material. One consequence is the pressure dependence of stiffness: as the arterial wall is stretched by an increasing distending pressure, load is transferred to stiffer elements so that E and hence PWV increases with increasing blood pressure.4
- Received April 16, 2015.
- Accepted April 17, 2015.