Arterial stiffness
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Arteries stiffen as a consequence of age and atherosclerosis. The two leading causes of death in the developed world, myocardial infarction and stroke, are both a direct consequence of atherosclerosis. Increased arterial stiffness is associated with an increased risk of cardiovascular events. The World Health Organisation predicts that by 2010 cardiovascular disease will also be the leading killer in the developing world and represents a major global health problem.
Once considered by the ancient Greeks as inert conduits within which air flowed, William Harvey is generally credited with being the first to describe the circulation of the blood through arteries. When the heart contracts it generates a pulse or energy wave that travels through the circulation. The speed of travel of this pulse wave (pulse wave velocity or PWV) is related to the stiffness of the arteries. Other terms that are used to described the mechanical properties of arteries include elastance, or the reciprocal (inverse) of elastance, compliance. The relationship between arterial stiffness and pulse wave velocity was first predicted by Thomas Young in his Croonian Lecture of 1808 (Young T: On the function of the heart and arteries: The Croonian lecture. Phil Trans Roy Soc 1809;99:l-31), but is generally described by the Moens - Korteweg equation (Nichols WW, O'Rourke MF. Vascular impedance. In: McDonald's Blood Flow in Arteries: Theoretical, Experimental and Clinical Principles. 4th ed. London, UK: Edward Arnold; 1998:54–97, 243–283, 347–395.) or the Bramwell-Hill equation (Bramwell JC, Hill AV. The velocity of the pulse wave in man. Proc R Soc Lond (Biol) 1922;93:298-306).Typical values of PWV in the aorta range from approximately 5m/s to >15m/s.
Measurement of aortic PWV provides some of the strongest evidence concerning the prognostic significance of large artery stiffening. Increased aortic PWV has been shown to predict cardiovascular, and in some cases all cause, mortality in individuals with end stage renal failure (Blacher et al.,Circulation. 1999; 99: 2434–2439), hypertension (Laurent et al., Hypertension. 2001; 37: 1236–1241), diabetes mellitus (Cruickshank et al., Circulation. 2002; 106: 2085–2090) and in the general population (Mattace-Raso et al. Circulation. 2006;113:657-663, Hansen et al., Circulation. 2006;113:664-670.). However, at present, the role of measurement of PWV as a general clinical tool remains to be established.
