I don't know if you were doing any other bondage with it, so this may not help your situation, but here is what my lover/Master does when I am tied up: one hand gets a safety knot--he puts the loose end of the rope in my hand, so all I have to do is pull and I have a hand free. I would think that if you were really in need of air, that hand just might jerk, and then come free, which would certainly signal something was wrong. My biggest problem with it is that part of me wants to really let go into his domination, and I am tempted to let go of that loose end. Since my l/M is all about safety, I have to remember this is a big no-no. BTW, unless I pull the end, on purpose, I do not come untied, so I can still have the feeling of being restrained and be safe. Hope this helps, if not you, then someone!