Augmentation index is not a proxy for wave reflection magnitude: mechanistic analysis using a computational model

The augmentation index (AIx) is deemed to capture the deleterious effect on left ventricular (LV) work of increased wave reflection associated with stiffer arteries. However, its validity as a proxy for wave reflection magnitude has been questioned. We hypothesized that, in addition to increased wave reflection due to increased pulse wave velocity, LV myocardial shortening velocity influences AIx. Using a computational model of the circulation, we investigated the isolated and combined influences of myocardial shortening velocity vs, LV and arterial stiffness on AIx. Aortic blood pressure waveforms were characterized using AIx and the reflected wave pressure amplitude ((P) over cap (bw), obtained using wave separation analysis). Our reference simulation (normal v(s),(LV) and arterial stiffness) was characterized by an AIx of 21%. A realistic reduction in v(s),(LV) caused AIx to increase from 21 to 42%. An arterial stiffness increase, characterized by a relevant 1.0 m/s increase in carotid-femoral pulse wave velocity, caused AIx to increase from 21 to 41%. Combining the reduced v(s),(LV) and increased arterial stiffness resulted in an AIx of 54%. In a multistep parametric analysis, both v(s),(LV) and arterial stiffness were about equal determinants of AIx, whereas (P) over cap (bw) was only determined by arterial stiffness. Furthermore, the relation between increased AIx and LV stroke work was only approximate to 50% explained by an increase in arterial stiffness, the other factor being v(s),(LV). The (P) over cap (bw), on the other hand, related less ambiguously to LV stroke work. We conclude that the AIx reflects both cardiac and vascular properties and should not be considered an exclusively vascular parameter. NEW & NOTEWORTHY We used a state-of-the-art computational model to mechanistically investigate the validity of the augmentation index (AIx) as a proxy for (changes in) wave reflection. In contrary to current belief, we found that LV contraction velocity influences AIx as much as increased arterial stiffness, and increased AIx does not necessarily relate to an increase in LV stroke work. Wave reflection magnitude derived from considering pressure, as well as flow, does qualify as a determinant of LV stroke work.