If you're playing with breath, or working in a situation where verbal communication might be compromised, you can use a "mechanical safe word." "Borrow," or otherwise acquire a kid's bouncy-ball-- one of those super balls, preferably a glow-in-the-dark or brightly colored one, and hold it in your hand. If at some point you start to feel like you need to pause, get out of scene, need help, whatever, let go of the ball. It will bounce, be visible and make noise. Your top, who will know about this, can be attentive to that cue and act accordingly.

Hope this helps.

Don