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

Area of Science:

• Cardiovascular Physiology
• Biomedical Engineering
• Hemodynamics

Background:

• The augmentation index (AIx) is commonly used to assess the impact of arterial stiffness and wave reflection on left ventricular (LV) work.
• However, the validity of AIx as a sole indicator of wave reflection magnitude has been questioned.
• Previous assumptions overlooked the potential influence of cardiac factors on AIx.

Purpose of the Study:

• To investigate the independent and combined effects of LV myocardial shortening velocity (v_s,LV) and arterial stiffness on AIx using a computational model.
• To determine whether AIx accurately reflects wave reflection magnitude or is influenced by cardiac mechanics.
• To clarify the relationship between AIx, arterial stiffness, LV stroke work, and wave reflection pressure amplitude (p̂_bw).

Main Methods:

• A computational model of the circulatory system was employed to simulate blood pressure waveforms.
• Wave separation analysis was used to quantify reflected wave pressure amplitude (p̂_bw).
• Parametric analysis explored the influence of varying v_s,LV and arterial stiffness (indicated by pulse wave velocity) on AIx.

Main Results:

• A reduction in v_s,LV increased AIx from 21% to 42%, similar to the effect of increased arterial stiffness (21% to 41%).
• Combined reduction in v_s,LV and increased arterial stiffness resulted in a higher AIx of 54%.
• Both v_s,LV and arterial stiffness were found to be significant determinants of AIx, while p̂_bw was primarily determined by arterial stiffness.

Conclusions:

• The augmentation index (AIx) is influenced by both cardiac (LV contraction velocity) and vascular (arterial stiffness) properties.
• AIx should not be considered exclusively a measure of wave reflection or arterial stiffness.
• Reflected wave pressure amplitude (p̂_bw) is a more direct indicator of LV stroke work than AIx.