And it's one of many reasons why I dive solo most of the time and rely on my redundancy (which, admittedly, would be of no use should I have a heart attack or other serious problem).
Dr. Bill,
I wanted to ask, how often do you check your buddy bottle during the dive? In doubles, my common configuration means I am going to hand off my primary to an OOA diver, and I'll be using the regulator hung around my neck. I breath that backup reg on the surface, and I breathe it at least twice during the underwater portion of the dive. Generally after initial descent (the wreck on a wreck dive, or the cave entrance on a cave dive) and again, just after agreeing to turn the dive (after reaching thirds in the cave). In this way, I assure myself that should the worst happen, I am going to have a fully functional regulator. I see some divers prepare to dive with a bailout bottle, or with stages, and they never breathe them. This always struck me as odd.