hmm. when ds was in the NICU, and his O2 sat was way down, they would super swaddle him and put him in different positions. The nurses would swaddle him so tight, it made me a little nervous, so I can't imagine a sling being much different.
What I do know is that being in an upright position may not provide the best O2 saturation, infants sat O2 much better whne laying down. It took ds a few trys to pass his carseat test before they let me bring him home, but when he was laying down, or being held, he did fine with his levels when he wasn't on a vent.
I wonder if anyone has done any studies?