I'm no master at bdsm (no pun intended) but the usual trick of a safe word or, in this case, an action to perform when you're running out of breath seems to be the obvious thing.
start off with a lower threshold, especially since it's your first time at asphixiation.
By the way, with the action, I'd make a hand gesture which couldn't be mistaken for a normal action, and wouldn't be ignored or be too difficult to do.
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