I don't know the actual hard scientific rational, but from a common sense standpoint, I think the reasoning for switching needles before each injection is to ensure you have a clean needle with just the regitine and no reminants from the dopamine you could be injected into tissue, this making the infiltration over a larger area. The way I think about it is you have the dopamine in the tissues you're wanting to treat with the regitine. If you use the same needle, on the 10th injection you've been penetrating skin that has dopamine then moving to a different site on the skin taking more dopamine there. If you change needles, you have regitine and only regitine you're injected each time with the "clean" needle. The pictures of dopamine infiltration are very ugly, and patient's can end up needing skin grafts, even amputation when an infiltration is not assessed/intervened quickly. I know in my own practice, I always cringe when we have a patient on dopamine with a PIV, I would prefer a central line for that type of infusion. If a PIV is all I have, I document that my PIV has good blood return and is infusing with no s/s infiltration and assess my IV site much more frequently than I would if I just had NS at KVO.