Theory (as presented by my friend's CNM prof): Reduced blood-flow to the placenta during a contraction means that less undesirable drug X will be delivered to the fetus than if you give the IVP not during a contraction. Therefor, only give IVP during contractions.
My friend didn't think it made sense. He asked if half-life had a role. The professor said no. He asked me. My reasoning was this:
The professor's statements make no sense because contractions are usually, what, 30-120 seconds, and circulation time from a PIV to the placenta is going to be something like, say, 30ish seconds (or more). For a IVP given during a 30sec contractions, most of the drug would arrive at the placenta after the contraction ended.
But it would also continue to arrive.... fluid dynamics and half lives and all that... bioavailability for IVP peaks AFTER the IVP is completed.
The largest variable would be the pharmacokinetics of the drug. And I cannot think of very many drugs that have a short enough plasma half-life (seconds to a few minutes) where it would matter even if you had 120second contractions and pushed for the first 40 seconds only. In fact, the only drugs I can think of off the top of my mind with such short half-lives are epi, adenosine, or maybe fentanyl.
Counter-hypothesis: Pushing only during contractions has a negligible effect on the dose delivered to a fetus unless delivering drugs with extraordinarily short plasma half-lives during the first part of long contractions.
I have a a vast experience of NOTHING in L&D, nor was it my best subject. So, after a fruitless search for any evidence that supports either position, I thought I'd ask here.