What a stupid name.

Nonetheless it’s what we’re stuck with. It is the ratio of LVOT VTI to AV VTI.

Given that ALL ratios are dimensionless, you can see my chagrin.

VTI LVOT / VTI AV

LVOT is measured with pulse wave, to localise it to the LVOT

AV VTI is measured with continuous wave, because the AV VTI will be the highest in that column.

DI >0.5 = Mild AS

DI 0.25-0.5 = Moderate AS

DI <0.25 = severe AS

Here’s a helpful diagram by Professor Catherine M Otto from this article.