Development of systemic arterial mechanical properties from infancy to adulthood interpreted by four-element windkessel models

Aortic impedance data of infants, children and adults (age range 0.8-54 yr), previously reported by others, were interpreted by means of three alternative four-element windkessel models: W4P, W4S, and IVW. The W4P and W4S are derived from the three-element windkessel (W3) by connecting an inertance (L) in parallel or in series respectively, with the aortic characteristic resistance (R-c). In the IVW, L is connected in series with a viscoelastic windkessel (VW). The W4S and IVW (same input impedance) fit the data best. The W4S, however, suffers from the assumption that R-c is part of total peripheral resistance (R,). The IVW model offers a new paradigm for interpretation of resistive properties in terms of viscous (R-d) properties of vessel wall motion, distinguished from R, Results indicated that rapid reduction of R-d/R-p during early development is functional to modulation of decay time constant (iota(d)) of pressure in diastole, such that normalization over heart period(iota(d)/T) is independent of body size. Estimates of total arterial compliance (C) vs. age were fitted by a bell-shaped curve with a maximum at 33 yr. With body weight (BW) factored out by normalization, the C/BW data scattered about a bell-shaped curve centered at 66 mmHg. Inertance was significantly higher in pediatric patients than in adults, in accordance with a lower cross-sectional area of the vasculature, commensurate to a lower aortic flow. Changes of arterial properties appear functional to control the ratio of pulsatile power to active power and keep arterial efficiency as high as 97% in infants and children.