Augmentation Index

What is augmentation index?

The augmentation index is a measure of wave reflection and arterial stiffness and calculated as a ratio from the BP waveform. Augmentation index is commonly measured as the ratio of the central Pulse Pressure (cPP) and the reflected pulse pressure which augments the central BP, called central Augmentation Pressure (cAP) shown in blue in the first graph below. When arteries are stiff, a reflected wave is formed where arteries split. (Like the ripple effect when you throw a pebble in a pond). This reflected wave moves back at the heart and increases the pressure at which the heart has to pump.

Why is augmentation index important?

Augmentation index is a proposed marker of arterial status:

• Augmentation index is a predictor of adverse cardiovascular events in a variety of patient populations, and higher augmentation index is associated with target organ damage [8].

• Augmentation index can distinguish between different vasoactive medications when upper arm BP and pulse wave velocity do not [9].

Why are there different ways of calculating augmentation index?

Central Augmentation Index (cAIx) is defined for a central pressure waveform as the ratio of Augmentation Pressure (cAP) to Pulse Pressure (cPP): cAIx = cAP / cPP

The wave shape is different for a pressure waveform obtained at the upper arm or wrist. In some cases, late systolic pressure may be higher than early systolic pressure. In other cases, as shown below, a small pressure augmentation is visible in late systole and is lower than early systolic pressure. Peripheral augmentation index (pAI) is defined as the ratio of late systolic pressure (P2) to early systolic pressure (P1): pAI = P2 / P1

How does BP+ measure augmentation index?

BP+ augmentation index is calculated using the peripheral augmentation index formula above.

What is the accuracy of BP+ measurement?

Accuracy of BP+ algorithms have been checked against expert analysis of 50 measurements taken from a cross-sectional clinical study. The maximum likely error is just 4% of the typical measurement range.

Is augmentation index related to age?

Both augmentation index and central BP are known to increase with the age of the subject and be related to cardiovascular outcome.

Is augmentation index related to gender?

On average, women have a higher augmentation index than men due in part to their smaller arteries.

Is augmentation index related to central BP?

Augmentation index and central BP are thought to measure different aspects of arterial stiffness.

Is augmentation index related to height?

Some evidence suggests that the timing of the reflected wave is related to the dimensions of the body. In shorter individuals, a shortened return time for reflected waves leads to an increase in central pressure augmentation [10].

Does BP+ correct for heart rate when calculating augmentation index?

The BP+ algorithm does not correct for heart rate as the relationship between heart rate and peripheral augmentation index has not been established. Low heart rates tend to be related to increased central augmentation index. It has been proposed that higher heart rates shorten systole (the ejection phase) and therefore the reflected wave will arrive at a relatively later moment in the cardiac cycle, lowering central augmentation index [11]. Advanced BP+ analysis of PWA components can provide direct insight into these effects.

Is cholesterol related to augmentation index?

Cholesterol is a bio-indicator associated with cardiovascular diseases, but its relationship with central BP is yet to be established. Nevertheless, central augmentation index has been shown to positively correlate with cholesterol levels [12,13].

Does brachial artery, arm or cuff variation affect the measurement?

As augmentation index is a ratio, variations in the arm, cuff or artery do not significantly affect the measurement.

How do variations in BMI affect augmentation index?

Body mass index (BMI) does not affect the measurement of augmentation index. However, studies have found that that increased body fat content, but not BMI, is strongly associated with increased large artery stiffness and wave reflection [14].

Does brachial artery variation affect the readings?

As augmentation index is a ratio, variations in the brachial artery produce minimal changes for the measurement. Additionally, the brachial artery isn't nearly as affected by disease as other major blood vessels and thus still provides the gold standard site for measurement. 

Reference

[8] M. Shimizu and K. Kario - Review: Role of the augmentation index in hypertension - Therapeutic Advances in Cardiovascular Disease, vol. 2, Feb. 2008, pp. 25-35.

[9] P. Boutouyrie, A. Achouba, P. Trunet, S. Laurent, and for the EXPLOR Trialist Group - Amlodipine-Valsartan Combination Decreases Central Systolic Blood Pressure More Effectively Than the Amlodipine-Atenolol Combination: The EXPLOR Study - Hypertension, vol. 55, Jun. 2010, pp. 1314-1322.

[10] B.P. McGrath, Y.L. Liang, D. Kotsopoulos, and J.D. Cameron - Impact of physical and physiological factors on arterial function - Clinical and Experimental Pharmacology & Physiology, vol. 28, Dec. 2001, pp. 1104-1107.

[11] I.B. Wilkinson, H. MacCallum, L. Flint, J.R. Cockcroft, D.E. Newby, and D.J. Webb - The influence of heart rate on augmentation index and central arterial pressure in humans - The Journal of Physiology, vol. 525, May. 2000, pp. 263-270.

[12] I.B. Wilkinson, K. Prasad, I.R. Hall, A. Thomas, H. MacCallum, D.J. Webb, M.P. Frenneaux, and J.R. Cockcroft - Increased central pulse pressure and augmentation index in subjects with hypercholesterolemia - J Am Coll Cardiol, vol. 39, Mar. 2002, pp. 1005-1011.

[13] J. Nürnberger, A. Keflioglu-Scheiber, A.M. Opazo Saez, R.R. Wenzel, T. Philipp, and R.F. Schäfers, - Augmentation index is associated with cardiovascular risk - Journal of Hypertension, vol. 20, Dec. 2002, pp. 2407-2414.

[14] A. Wykretowicz, K. Adamska, P. Guzik, T. Krauze, and H. Wysocki - Indices of vascular stiffness and wave reflection in relation to body mass index or body fat in healthy subjects - Clinical and Experimental Pharmacology & Physiology, vol. 34, Oct. 2007, pp. 1005-1009.