Aortic Stiffness and Disease: Location is Key.

I ncreased risk of cardiovascular disease and events associated with aortic stiffening has been attributed both to the hemodynamic 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. 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 bio-mechanical 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 elastic modulus (E, the ratio of stress to strain) but also by the effective thickness of the arterial wall, and PWV 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. Although 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 thus behaves in a very different fashion from a homogeneous 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 increase with increasing blood pressure. In this issue of Circulation, Raaz and colleagues 5 examine the role of aortic stiffness in abdominal aortic aneurysm (AAA) formation. As with other cardiovascular conditions, this is a pathology in which aortic stiffness could act as a marker for aortic wall pathology predisposing to aneurysm or could be implicated in the pathogenesis of AAA through mechanical forces imposed by the stiffening. In an elegant series of experiments, they show that stiffening is causally related …