Evaluation of aortic compliance in humans.

The central aorta acts as a compliant tube that buffers and conducts pulsatile ventricular output (7) and contributes mostly to total compliance of the arterial tree. The mathematical model introduced by O. Frank (7) describing pulse wave propagation and arterial mechanical properties assumes that the arterial tree is an elastic chamber (windkessel) in which the diastolic pressure decays exponentially with a time constant that is determined by total arterial resistance and compliance. This model led to approaches of deriving systemic vascular compliance in humans from the diastolic arterial pressure decay (10) and from pressure wave contour analysis (21). The classic definition of compliance is the change in blood volume relative to a given change in distending pressure. The direct measurement of regional aortic compliance is difficult, because there is no simple means of estimating regional changes in blood volume. However, assuming that there is mainly radial and negligible axial vessel movement during pulse pressure, compliance can be estimated as a change in radius, diameter, or cross-sectional area for a given change in pressure (11, 17, 19). This assumption of negligible axial vessel movement during pulse pressure appears to be a valid approximation when compliance is estimated from radial extension of vascular cross sections for several reasons. It should be noted that neglect of longitudinal vessel axis is accounted for in the theory of pulse wave propagation developed by O. Frank (7), assuming that volume changes occur mainly because of expansion of the vessel wall along its radius. Also, when aortic pressure and radius are analyzed in the living dog, the ratio of pressure changes to changes in vessel radius obtained at identical sites compared well with measures of impedance (19), justifying the use of simpler radius measurements for estimating dynamic elasticity of the aorta.