Pressure-Dependent and Frequency Domain Characteristics of the Systemic Arterial System

Abstract

Pressure–volume relations in different arteries have been observed to be nonlinear. This gives rise to the pressure-dependent behavior of the overall arterial system compliance. Using a nonlinear pressure-dependent compliance arterial system model, we have shown that this compliance varies continuously throughout the cardiac cycle. However, the overall significance of this nonlinear compliance under different vasoactive conditions has been examined only to a limited extent. In addition, its frequency domain characteristics have not been investigated. We studied these aspects under varied physiological conditions. Systemic arterial compliance of the nonlinear compliance model was expressed as C(P) = a · exp(−bP). It is determined, through optimization, from the following equations: Pao(ti) = Qao(ti)Z0 + P(ti) and P(ti + 1) = P(ti) +Δt (Qao(ti) − P(ti)/Rs)/C(Pti), where Pao and Qao are aortic pressure and flow respectively, Z0 is aortic characteristic impedance, and Rs is the peripheral resistance. Results showed that C(P) increased during vasodilation and decreased during vasoconstriction, as expected. Frequency domain analysis of C(P) revealed a rapid and significant reduction in magnitude with increasing frequency, with a dominant presence at low frequencies, and a near absence at high frequencies, and its mean value approximates that of the windkessel. Thus, the systemic arterial system compliance varies continuously throughout the cardiac cycle with its predominant effect occurring at low frequencies. These features are not observed in the classical windkessel model.