Rusty on the greek alphabet, but I think delta means change or difference, so that would be between lying down and upright. Single measurement (one vein, one position) would be the CSA by itself, yes?
Assuming the vein is a cylindrical tube CSA is π*r*r
n being the length and r being a single radius (half a diameter). Same units of length all round, please.
So if by some miracle, you see delta r, it's probably the difference between the radius upright, and the radius lying down. Still the same units, but there's probably a convention for meaning of negative that I don't know. I don't know how to type that nifty delta symbol but it's sometimes written as a little triangle. Oh, and once you have multiplied two lengths, you get square units (area) and the third (n) makes it cubic units (volume).
We're talking circular tubes, and delta r expressed as a ratio between two radii, is analogous to a delta CSA, which could be between sitting up and lying down (which can change), or between not narrowed and narrowed (which usually doesn't).
So n*r*r of that cylindrical section is really volume of a cylindrical vein of length n and radius r. Clear as mud?
Add valsalva and my eyes get a nice reddish glaze.
Good thing Dr. Z. specified narrowing as a ratio as well, between the narrow part and the un-narrow part. In % of the un-narrow vein width.
Good thing I don't have to read it all. I'd get it confused (at least pre-Liberation I would