Can you say anything more about the co2 effect? Always interesting to hear first(ish) hand accounts
I took a mild hit on a unit I was training on. The unit in particular has a crappy ADV that fires at the worst possible times. I was having some mask issues due to my beard and had to do a lot of clearing. That would then fire the adv way too easily. So it was a constant battle of fighting for ideal loop volume, which led to not cycling breaths as well as one would hope though the unit. I was in the process of doing a skill while fighting a leaking mask and the adv, when all of a sudden it felt like I got hit in the back of the head with a sledgehammer. It was a horrible instantaneous headache. Then suddenly my breathing rate started going up, but I was conscious of it and trying to slow it down. I never felt panicked or got panicked breathing. As hard as I would tell myself to slow my breathing it wouldn't happen and it kept going up. I never felt panicked, but I felt very uneasy and unsettled and felt I wanted the dive to be over. I thumbed it and exited. We were in the very front part of a cavern, so it was 20 seconds to the 20 foot mark where I could flush the hell out of the unit with oxygen. When we surfaced I breathed oxygen a while longer which helped make the headache go away.