I've seen, first-hand, several instances of individuals aborting technical dives due to "dark narc" on reaching bottom depth... and in every instance the diver concerned was breathing heavily prior to, and during, descent. Descent speeds were moderate-fast (at, or exceeding 15m per minute).
Andy, while I agree with a big chunk of what you wrote, I'd be curious to see how many of them were also deep breathing versus shallow breathing. I've seen a number of people that were breathing very rapidly ("heavy breathing") but also breathing very shallow get spaced out on deep dives. The worst case was when I witnessed a friend whose eyes rolled up in his skull at 210' in Cozumel during a fairly rapid descent.
The problem with this is they're not breathing deep enough to flush enough CO2 to prevent getting narc'd and tunnel vision. I see it all the time in student cave divers when they're in charge of running a reel in a high flow system.
One trick I teach my cave students is that as soon as they tie into the permanent line, I want them to stop and take three deep breaths before they proceed further into the cave. I tell them I want to see them counting "1-2-3" as they're breathing. The purpose behind this is so they stop and completely ventilate their lungs, clearing out the CO2.
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@Remy B. -- the problem you're experiencing isn't due to the descent rate as much as it is to your own nervousness and apprehension, which is probably leading you to hyperventilate. You probably feel more in control during the slower descent, which is helping you to stay relaxed and breathe deeper, and during the quicker descent you probably feel out of control, which is causing you to breathe shallow and rapidly. While my "stop and breathe 1-2-3" trick isn't really as applicable during a rapid descent in open-ocean, you can definitely use it once you've hit your target depth to clear your head. You can also try using it from the surface before you descend -- stop and the water column, say at 5', and take 3 deep breaths before your rapid descent.
Another suggestion I would make is to visualize in your mind the dive, including the descent, before you even get on the boat, and then visualize it in your mind again before you splash into the water. I used to think that visualization was new-age mumbo jumbo, but after spending five years racing a bike on the SE bike circuit, I learned a lot about how successful athletes use visualization techniques to "improve their game." I've since then incorporated visualizing techniques on "big" dives and found them to be a very powerful tool.
