BP+ Education

Augmentation Index

Augmentation Index

What is augmentation index?

The augmentation index is a ratio calculated from the blood pressure waveform, it is a measure of wave reflection and arterial stiffness.

Augmentation index is commonly accepted as a measure of the enhancement (augmentation) of central aortic pressure by a reflected pulse wave (shown in green in the graph).

Why is augmentation index important?

Augmentation index is a sensitive marker of arterial status:

Why are there different ways of calculating augmentation index?

Referring to the figure above, central augmentation index (cAI) is defined for a central pressure waveform as the ratio of augmentation pressure (AP) to pulse pressure (PP):

cAI = AP / PP

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, no pressure augmentation is visible (late systolic pressure is lower than early systolic pressure).

Peripheral Augmentation Index (pAI)

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, given above.

What is the accuracy of BP+ measurement?

Accuracy of BP+ algorithms has 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 blood pressure 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.

Is augmentation index related to central blood pressure?

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

A statistical relationship between augmentation index and central blood pressure has also been established [3].

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 [4].

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.

Decreases in heart rate relate to increases in 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 [5].

Advanced BP+ analysis of pressure wave components can provide direct insight into these effects. A white paper on BP+ pressure wave component analysis is available.

Is cholesterol related to augmentation index?

Cholesterol is a blood chemical indicator, which may or may not relate to functional measures of cardiovascular health such as augmentation index.

Nevertheless, central augmentation index has been shown to positively correlate with cholesterol levels [6], [7].

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 [8].

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.

References

  1. [1] 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.
  2. [2] 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.
  3. [3] S. Munir, A. Guilcher, T. Kamalesh, B. Clapp, S. Redwood, M. Marber, and P. Chowienczyk - Peripheral Augmentation Index Defines the Relationship Between Central and Peripheral Pulse Pressure - Hypertension, Nov. 2007, p. HYPERTENSIONAHA.107.096016.
  4. [4] 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.
  5. [5] 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.
  6. [6] 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.
  7. [7] 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.
  8. [8] 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.