Augmentation index (AIx) is widely used as a measure of wave reflection in clinical studies. The validity of AIx results rests on the ability to quantify the magnitude of the reflected pressure wave (P₋) in relation to the pressure pulse (PP), and this is traditionally carried out by determining the inflection point (Ip) on the pressure waveform. In this study we investigate the results of AIx and propose a modified augmentation index (mAIx) that is more sensitive to loading conditions.

In 11 anaesthetised dogs, total sequential occlusions were produced at 3 aortic sites (thoracic, diaphragm, abdominal), and at the left iliac artery. Pressure and flow were measured in the ascending aorta before and during the occlusion at each site, and Ip was ascertained using the 4th derivative of the pressure waveform allowing for the determination of AIx. Magnitude of P₋ was calculated using wave intensity analysis (WIA), and divided by PP giving a modified Augmentation index (mAIx = P₋/PP).

AIx during control were surprisingly not different from those determined during proximal occlusions. However, P₋ increased significantly during occlusions; resulting in a significant increase in mAx by 165% during thoracic, 48% during diaphragm and 20% during abdominal occlusions, all compared to control.

The Ip is not an accurate method for determining P₋. This can result in significant errors in the estimation of AIx. The mAIx, derived from pressure–flow relationship in the wave intensity analysis is more sensitive to the determination of P₋, than AIx which uses only the pressure waveform.