Understanding Augmentation Pressure and Augmentation Index (AIx)

Each heartbeat generates a forward-travelling pressure waveform that moves through the arterial wall. When this waveform encounters points of impedance — such as arterial branching points, narrowing, or plaque buildup — a portion of the wave is reflected back toward the heart. This reflected wave combines with the original forward-travelling wave to produce what we observe as the arterial pulse waveform.

Augmentation pressure and AIx both represent the degree to which reflected waves influence the overall waveform shape. The timing of reflection depends on arterial stiffness:

• In individuals with elastic (compliant) arteries, the wave travels slowly and combines during diastole (when the heart is relaxed), resulting in lower values of both AIx and augmentation pressure.
• In individuals with stiffer arteries or higher cardiac load, the wave travels faster and returns during systole (when the heart is working), resulting in higher values.

Together, AIx and augmentation pressure offer complementary information about vascular health: AIx reflects arterial stiffness more directly, while augmentation pressure reflects the vascular load on the heart and organs. Normal values vary across the population by age and sex.

Reference: Normal Vascular Aging: Differential Effects on Wave Reflection and Aortic Pulse Wave Velocity — The Anglo-Cardiff Collaborative Trial (ACCT) | JACC. DOI: 10.1016/j.jacc.2005.07.037