Trisha Uselton:
Here's a hypothetical situation -- my husband and I are diving with a group and suddenly someone starts to have convulsions. Choking, shaking, spitting out the reg, that sort of thing. I'm close by, so should I offer my octopus, even though I don't know what is in that person's tank?
The reason I ask is a friend of ours and his wife were diving in the Caribbean and got some bad air on their first dive (headaches, nausea) so they switched to Nitrox for the rest of their trip. If we had been with them would it have been a bad idea to give them Nitrox during a safe accent? Or visa versa? We are OWD certified by SSI with just 15-25 dives so far, but this is something that could come up in the future.
To recap: can you share air with a Nitrox diver? Any exceptions? Can you share Nitrox with an air diver? Any exceptions?
I understand that this is an "until we can both make a safe accent" situation, depending on what our maximum depths/time/air-remaining-in-my-tank are saying.
Trish
You KNOW your gas is ok to breathe - you're breathing it.
You have no idea what is in the other diver's tank. If HIS gas is causing him problems, you are helping him.
The potential issue comes if he is not coherent enough to rationally exchange regulators. That can be a problem.
If he is convulsing and has spat out the reg, then he's in imminent danger of drowning. Some folks say "get him to the surface." I say "get him something to breathe, THEN get him to the surface, but in a controlled way once the convulsion abates and he begins respiration."
Realize that if he is convulsing he may be experiencing an O2 hit.
If he is then it is critical that you not ascend with him - not even a foot - until he begins to breathe. It is also critical that when he DOES begin to breathe that there is a REG IN HIS MOUTH.
During the convulsion his glottis will be locked closed. If you ascend with him during this time he will suffer pulmonary barotrauma - quite possibly fatal. In other words, you may KILL him. In addition if he does not have a reg in his mouth and begins to breathe again after the convulsion abates he will inhale water, and that is extremely likely to kill him.
If you SEE someone convulse you should assume that it is an O2 hit and keep the reg in his mouth if he has one. The convulsion ITSELF will not harm him - drowning, however, will. If he has ejected the reg, I'd give him MINE, since I KNOW my gas is safe to breathe at the depth we are at (I'm breathing it now!) Keep it in his mouth and prepare to ascend with him FROM BEHIND when he comes around; it should not take more than a minute or so - be prepared for the victim to quite possibly have no idea where he is or what's going on, and to be highly combative (e.g. a full-on panic!) Once he is breathing or goes limp, it is then safe to ascend (protect his airway if he is not breathing on his own!) So long as he is "rigid" his airway is almost 100% certain to be locked closed and you must NOT ascend.
The logic behind not replacing his reg if he still has his is that there is a risk of flooding his airway, and the convulsion itself won't harm him - but flooding his airway will. He may convulse again once he comes around, but once he's coherent you can switch regs with him in control (offer him yours), and further, once you ascend significantly his PO2 will drop and in a couple of minutes the risk of convulsion will go down. If he has already ejected his reg then you can't make the situation worse by giving him yours, and there's a very good chance that he ejected it with his glottis locked shut, and thus there is little if any water that has gotten in there. As such you can't make the situation worse - and insuring that he has AIR to breathe when he comes around might save his life.
This assumes you SEE the convulsion. If you come across an unconscious diver then it becomes much more difficult, as you can assume nothing about why it happened. You can't "fix" him on the bottom, so you are forced to ascend with him - realize that he may have a deco obligation, he may have inhaled a huge amount of water, all kinds of things could have happened. But standard protocol is that you take him up, because you can't help him underwater if you have no idea what is going on, and there is no way to assess that on the bottom. The best you can do is to check and make sure he's not locked-rigid (obviously in the middle of a convulsion), protect his airway so that any air in his lungs can expand out his mouth as you ascend, keep his reg in his mouth if he has it still in there, and get him to the surface at a controlled rate.
That's something that I left out of my post. 
