Influence of mean pressure on aortic impedance and reflections in the systemic arterial system

The present investigation sought to determine the extent to which primary changes in mean arterial pressure (MAP) might influence the calculated aortic impedance. In seven open-chest, anesthetized, autonomically blocked dogs, we measured aortic impedance using white-noise analysis at various levels of MAP achieved by adjusting the height of a left atrial reservoir rather than by pharmacological intervention. Impedance spectra so obtained were parameterized according to best-fit wind-kessel models (Ra, Rc, Ca) and three parameters to characterize wave reflections: the frequency at which the phase of the impedance approached or crossed through 0 degree (phi 0), the frequency of the first minimum of the impedance modulus (fmin), and the amplitude of the first oscillation of the impedance modulus (a 1). The dependency of each of these parameters on MAP was investigated. In addition, we calculated the reflection coefficient spectrum (RCS). Results show that windkessel parameters were not significantly influenced by MAP and that the reflection parameters and the RCS depended on increased levels of MAP in a manner consistent with an increase in pulsewave velocity.