3. Mechanisms for Age-Change in Aortic Augmentation Pressure

Abstract

Augmentation index (AIx) is a widely used measure of wave reflection and aortic stiffness. It rises with age in a curvilinear manner whereas central augmentation pressure (AP) and pulse pressure (PP) from which it is determined (as AIx = AP ÷ PP) rise linearly with age. Apparent flattening of AIx over age 60 has been attributed to decreased peripheral wave reflection in older subjects. We sought a purely mathematical explanation of this phenomenon — that two positively sloped linear equations with different intercepts on the y-axis yield a curvilinear change when one is divided by the other. Data were from 1601 patients previously described attending an outpatients clinic and aged from 25 to 93 years. The rise in AP and PP could be described by straight lines (top figure), whereas their ratio as AIx was curvilinear (bottom figure) and approximated the change with age as described in other studies. Change in AIx with age cannot simply be attributed to decrease in peripheral wave reflection. The phenomenon described here must be considered together with change in shape of the left ventricular ejection pattern with age, since wave reflection may cause a decrease in late systolic flow.