Abstract
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 (, obtained using wave separation analysis). Our reference simulation (normal vs,LV and arterial stiffness) was characterized by an AIx of 21%. A realistic reduction in vs,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 vs,LV and increased arterial stiffness resulted in an AIx of 54%. In a multistep parametric analysis, both vs,LV and arterial stiffness were about equal determinants of AIx, whereas was only determined by arterial stiffness. Furthermore, the relation between increased AIx and LV stroke work was only ≈50% explained by an increase in arterial stiffness, the other factor being vs,LV. The , 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.
Highlights
The augmentation index (AIx) is defined as the late systolic boost in the aortic pressure waveform divided by pulse pressure and is often expressed as a percentage [26] (Fig. 1)
The combined effect of k1 and vs,LV2 resulted in a waveform that can be considered an intermediate (Fig. 3, red dashed curve) curve of the k1 and vs,LV2 curves, with an AIx increase from 21 to 54%
Ejection duration increased to 309 ms, and the inflection point occurred 60 ms after the foot of the pressure waveform
Summary
The augmentation index (AIx) is defined as the late systolic boost in the aortic pressure waveform divided by pulse pressure and is often expressed as a percentage [26] (Fig. 1). The reflected wave will propagate with increased velocity, causing the augmentation to occur earlier in systole with a consequent increase in AIx [26]. An advantage of AIx is its nondimensionality, requiring neither calibration of blood pressure nor measurement of blood flow velocity [19]. Blood pressure waveforms can be obtained using noninvasive tonometry at the location of the carotid or radial arteries, or by oscillometric blood pressure recordings at the brachial level [18, 20]. Wilkinson et al [44] and Savage et al [33] reported good interobserver reproducibility of AIx, as measured by tonometry
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