What is an "A5 Index marker" in Aortic root measurement?

I am a 58 yr old male with aortic root and ascending aorta dilatation (all the rest of the aortic structure being normal). My measurements have been unchanged at 46mm (root) and 36mm (ascending) for 4 years (measured by a mix of echo and CT).

Because I have no risk factors for heart desease my I am on annual monitoring. However this year I had a fright when I was told (by an apparently uninformed physician) that my recent echo produced a descending measurement of 54mm. He later wrote to me to say that this was a misunderstanding and the 54 is not a measurement of dilatation but an "A5 ....index marker with the aim of making future studies more comparable". What does this mean?

[Cleveland Clinic]

I have never heard of an A5 index marker.  Not sure if this is some institution specific.  I will ask around the imaging specialists and get back to you.  Echo's are not very accurate is assessing the descending aorta.