I'm probably the only one here who's had her baby hooked up to heart and breathing monitors while in the sling.
His alarms went off a lot because of the way his chest skin rolled inward from side to side while he was scrunched up, but it was because they couldn't pick up well. I didn't notice any problem from the pulse oximeter that measured his O2 levels. I was pretty careful to keep his head extended back instead of scrunched up though. It's not hard to do if you wear them high up in the sling so their head is outside of the fabric and can tilt back a little rather than being pushed into his chest. I found it ideal if the top rail of the sling was just below ear level and curving down around the back of the neck to give support while still allowing for neck extension.