2.5 Does Wave Reflection Protect the Microvasculature from High Pulse Pressure?

Abstract

Background

Wave reflection (caused by a stiffness increase from large to small arteries) has been considered to protect against high microvasculature Pulse Pressures (mPP) (1). However, according to transmission line theory, Transmission (T) and Reflection (R) coefficients are proportional (T = 1+R), implying that reflection would not be protective. Proximal arterial stiffening with aging is associated with reduced Total Arterial Compliance (TAC) and increased forward Pressure (Pfw). We hypothesized that a high TAC and low Pfw, rather than high R, are responsible for protection from mPP.

Methods

We constructed a fractal arterial tree containing 5008 vessels across 14 generations (fractal exponent 2.76, asymmetry ratio 0.8). Wave speed in each vessel was prescribed to achieve a uniform reflection coefficient (R = −0.025, 0, 0.025 or 0.05) at every junction, achieved by progressively stiffening distal vessels while keeping aortic wave speed constant (“distal-stiffening”) or by progressively stiffening proximal vessels while keeping average wave speed in all terminal vessels constant (“proximal-stiffening”, see Figure). An elastance heart model was applied at the inlet and simulations were performed with a one-dimensional flow solver (2).

Results

Proximal-stiffening and distal-stiffening had opposing effects on R but the same effects on mPP, whereas mPP increased monotonically with decreasing TAC and increasing Pfw in both settings (Figure).

Conclusion

Wave reflection per se does not provide protection from high mPP since greater reflection also entails greater transmitted pressure. Although a decreased R may accompany proximal arterial stiffening, the likely mechanism of increased mPP with aging is decreased TAC and greater Pfw.