ATI indicates where within one heartbeat the blood flow peak occurs at the observation point, and typically shows a value of approximately 25–30%.
In peripheral regions where the pathway is longer and it takes more time for the blood flow peak to arrive, ATI becomes higher. Therefore, ATI becomes higher in cases such as venous regions where the waveform peak position is delayed compared to arteries.
Additionally, when the heart rate is high and the period of one heartbeat is short, ATI also becomes higher.
When expressed as a percentage, the value ranges from 0 to 100, but in some cases (such as Cobitos), it may be expressed as a decimal from 0 to 1.0, such as 0.3.
ATI stands for "Acceleration Time Index."
ATI was originally called "Peak," but the name was changed to "ATI" to make it easier to use in English-language publications.