Arterial wave propagation phenomena, ventricular work, and power dissipation

Abstract

The effects of wave propagation phenomena, namely global reflection coefficient (Γ_G[ω]) and pulse wave velocity (c _ph), are studied in a model of the coupled left ventricle/arterial system. The left ventricle consists of a time-varying elastance, while the arterial system is modeled as a single, uniform, elastic tube terminating in a complex load. Manipulation of model parameters allowed for the precise control of (Γ_G[ω]) andc _phindependent of each other, peripheral resistance, and characteristic impedance. Reduction of Γ_G(ω) andc _ph were achieved through increases in load compliance and tube compliance, respectively. The equations describing the system were solved for left ventricular and aortic pressures and aortic flow. From these, stroke volume (SV), left ventricular stroke work (SW), and steady$(\dot W_s )$, oscillatory$(\dot W_o )$, and total power dissipation$(\dot W_t )$ in the arterial system were calculated. An index of arterial system efficiency was the ratio$\dot W_o /\dot W_t (\% \dot W_o )$, with lower values indicating higher efficiency. Reduction of Γ_G(ω) yielded initial increases in$\dot W_s$, while$\dot W_o$ increased for the entire range of Γ_G(ω), resulting in increased$\% \dot W_o$. This reduced efficiency is imposed on the ventricle, resulting in increasedSW without increasedSV. On the other hand, decreasedc _ph yielded in a steady increase in$\dot W_s$ and a biphasic response in$\dot W_o$, resulting in reduced$\% \dot W_o$ for most of the range of reducedc _ph. These results suggest that differential effects on arterial system efficiency can result from reductions of Γ_G(ω) andc _ph. In terms of compliance, changes in arterial compliance can have different effects on efficiency, depending on where the compliance change takes place. Reasons for these results are suggested, and the role of distributed compliances is raised as a new problem.