Where does the Q come from??

I love being a nurse, and specifically a critical care nurse, for many reasons, most of which I have bored my readers with over the years. I don't think, though, that I've recently addressed the part where you get to use your brain to good effect. I LOVE THAT. It started out with someone asking why they use Q for "perfusion" in a V/Q scan. Now I have to stop and say I KNOW the V should have a dot over it, but the old SGML entity I remember is not rendering, and I have spent my limit of 10 minutes looking for the damn code. So IMAGINE the dot over the V.

Hypotheses abounded, but the rest fell off when the doc and I got into a smackdown over what the Q stood for in Poiseuille's law (my theory was that that Q came from there). A secondary discussion occurred over whether change in pressure was related inversely to the radius or radius squared. The doc was pulling for radius squared, and I felt it was the inverse of the radius to the fourth power but dropped it. I should have stood my ground because that part was correct. But had absolutely nothing to do with the original question.

Anyway, our theory was that the Q came from this equation on account of P is already used for pressure so using it for perfusion too was just silly. Anyone know any real answers for this? Google is not helping me out.

For the curious, here is Pouseuille's law (from a reputable Wikipedia source):

Screen Shot 2012 06 24 at 2 44 34 PM

Yes, kids, you too can be involved in an incredibly heated intellectual discussion about the physics of fluid dynamics at 6AM!

So who knows the answer? I still want to know.