As a general rule of thumb, with spinals drug mass is more important than volume and the reverse is true with epidurals.
Look at it this way: CSF volume is roughly 200 ml in an adult (middle of the range). 2 ml vs 3 ml in that volume is a drop in the bucket. It's more important the amount of drug you put in and the drug distribution (baricity).
Epidurals are segmental blocks, usually requiring at least 2 ml/segment. Obviously drug mass will have an effect (you'll get a more intense block with a more concentrated solution), but volume is important. In OB I'll sometimes use a larger volume of a more dilute solution over a smaller volume of a concentrated solution to provide analgesia but minimize the motor block.A
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