ELASTIC PROPERTIES AND WINDKESSEL FUNCTION OF THE HUMAN AORTA

An understanding of the role of the aortic elastic properties indicate s their relevance at several sites of cardiovascular function. Acting as an elastic buffering chamber behind the heart (the Windkessel funct ion), the aorta and some of the proximal large vessels store about 50% of the left ventricular stroke volume during systole. In diastole, th e elastic forces of the aortic wall forward this 50% of the volume to the peripheral circulation, thus creating a nearly continuous peripher al blood flow. This systolic-diastolic interplay represents the Windke ssel function, which has an influence not only on the peripheral circu lation but also on the heart, resulting in a reduction of left ventric ular afterload and improvement in coronary blood flow and left ventric ular relaxation. The elastic resistance (or stiffness), which the aort a sets against its systolic distention, increases with aging, with an increase in blood pressure, and with pathological changes such as athe rosclerosis. This increased stiffness leads to an increase in systolic blood pressure and a decrease in diastolic blood pressure at any give n mean pressure, an increase in systolic blood velocity, an increase i n left ventricular afterload, and a decrease in subendocardial blood s upply during diastole, and must be considered a major pathophysiologic al factor, for example, in systolic hypertension. The elastic properti es of the aortic Windkessel can be assessed in vivo in humans in sever al ways, most easily by measuring the pulse wave velocity along the ao rta. The higher this velocity, the higher the elastic resistance, that is, the stiffness. Other methods depend on assessment of the ratio be tween pulse pressure and aortic volume changes (Delta P/Delta V), whic h can be assessed noninvasively by ultrasonic or tomographic methods. All assessments of vessel stiffness have to take into account the dire ct effect of current blood pressure, and thus judgements about influen ces of interventions rely on an unchanged blood pressure. Alternativel y, to derive the ''intrinsic'' stiffness of the aortic wall one has to correct for the effect of the blood pressure present. Recently report s about pharmacologic influences on the elastic properties of the aort a have emerged in the literature. Angiotensin-converting enzyme (ACE) inhibitors and nitric oxide (NO) donors seem to directly reduce the el astic resistance of the aorta. This effect, in addition to other effec ts on blood pressure and the peripheral circulation, could have major clinical relevance as an additional mechanism for unloading the left v entricle, improving coronary circulation, and reducing the pulsatile s tress of the arterial system.