The trunk diameter ratio (TDR) for retinal arterioles compares an arterioles actual diameter with that predicted from its branches using the Parr equation (see previous section).
Arteriole selection: trunk arterioles >= 80 microns in diameter
trunk diameter ratio =
= (observed trunk diameter in microns) / (predicted trunk diameter from its branches)
Ratio
Interpretation
< 1
the trunk is small relative to the branches, either due to constriction of the trunk and/or passive dilatation of the branches
1
the trunk and branches are proportionate (normal)
> 1
the trunk is large relative to the branches, either due to constriction of the branches and/or passive dilatation of the trunk
A mean TDR can be calculated for one or both eyes. Sharrett et al calculated the mean using arterioles with a diameter of 80 – 99 microns.
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