Clinical applications of arterial stiffness; definitions and reference values
Arterial stiffness is emerging as the most important determinant of increased systolic and pulse pressure in our aging community, and therefore, the root cause of a host of cardiovascular complications and events, including left ventricular hypertrophy and failure, aneurysm formation and rupture, and a major contributor to atherosclerotic and small vessel disease and thus to stroke, myocardial infarction, and renal failure.
Although appreciated for years, it is only in recent times, after acceptance of ill effects of systolic pressure in the elderly, that serious attention has been directed at precise measurement of arterial stiffness. The issue, although superficially simple, is complex, as older treatises on the subject will attest. The purpose of this review is to introduce the different terms that are used to describe arterial stiffness, and note their pitfalls and limitations and to provide normal values, where possible, as a function of age. Because some terms, which refer to global properties, imply models of the circulation, it will be necessary initially to refer to these models.
The oldest model of the arterial system is the Windkessel—the inverted air-filled dome of old fashioned fire engines that transformed pulsatile flow from a steam or hand-activated pump into a steady stream through the fire hose nozzle. In this model, the dome represents the cushioning function of the arteries, and the nozzle, the peripheral resistance. Although conceptually useful, this model is unrealistic because elastic properties are not present at just one site but are distributed along the aorta and major arteries. The pressure wave has a finite wave velocity in arteries, and in addition, pressure waveforms are different in amplitude and contour in central and peripheral arteries. Physical properties of arteries are different as well, and different arteries at different sites respond differently to aging, to hypertension, and to drugs. Value of the Windkessel model is seriously limited as a comprehensive explanation of arterial behavior under different circumstances, although under some specific circumstances—the very elderly, the very hypertensive—it may appear realistic.
The cushioning and conduit functions of the arterial system may be represented separately by a proximal Windkessel with peripheral distributing tube or by a single distensible tube in which both functions are combined. (Reprinted with permission from the publisher Churchill Livingstone for O'Rourke MF: Arterial Function in Health and Disease. Edinburgh, 1982). The left end of the tube represents the ascending aorta, and the right end, the summation of all arterial/arteriolar junctions.
The most realistic model of the arterial system is a simple tube with one end representing the peripheral resistance, and with the other end, receiving blood in spurts from the heart. A wave generated by cardiac activity travels along the tube toward the periphery and is reflected back from the periphery. The pressure wave at any point along the tube is a resultant of incident and reflected wave. When the tube is distensible, as in youth, the wave velocity is slow, therefore reflection returns late to the heart, in diastole. When the tube wall is stiffened, as in the elderly, wave travel is fast, and the reflected wave merges with the systolic part of the incident wave, causing a high pressure in systole and corresponding low pressure in diastole throughout the tube.
Source: https://academic.oup.com/ajh/article/doi/10.1016/S0895-7061(01)02319-6/165843/Clinical-applications-of-arterial-stiffness