There is a very good book by Terry Maas titled BlueWater Hunting and Freediving that I would highly recommend. There are several keys to freediving, and one is not to try to hold your breath to the limit. I've found over the years that it is best to establish a rhythm to your freediving. Let me explain.
First, let me tell you how not to do it. When I was in high school, a long, long time ago (this was about 1962), I was on the high school swim team. A team member and I were very competitive with each other, and the coach had us do an underwater swim for distance. THIS IS SOMETHING YOU SHOULD NEVER DO, but we were too young to know this. Tom swam four lengths of a 20 yard pool underwater. I knew I could do better. I hyperventalated for about 30 seconds something else you should never do, until I was a bit dizzy, then dove in. I swam a modified breast stroke, and was feeling quite comfortable on my second turn-around underwater. At my third turn-around (going into the fourth length), I was feeling a bit of an urge. By the approach to the last turn (fourth turn, to beat Tom), the urge was very noticable, and I told myself that I would make the turn, push off, do one stroke underwater, surface, and I would have the team record for the year. And that's exactly what I did. BUT I DID NOT REMEMBER ANYTHING AFTER MY PUSHOFF. I had blacked out. I told the coach, and he promptly cancelled any further underwater distance swimming.
In college, I looked up what had happened in a physiology periodical, and what I found really scared me. There were recorded cases of swimmers swimming underwater for an extended time, then suddenly stopping and being essentially dead when brought to the surface. This is a form of "shallow water blackout." It occurs because you can blow off enough CO2 from your system by hyperventalating to postpone the "must breath" signal to your brain (which is based on CO2 buildup, and not O2 depletion), to the point where the oxygen levels are not enough to sustain consciousness. You can "black out" without the normally overwhelming "must breath" signal to your brain. In short, you can overstay your dive to the point of becoming unconscious.
This mechanism is somewhat different in freediving, but similar. However, because you are diving to depth, as you ascend the partial pressure of oxygen in the lungs can become less than that in the blood, and the oxygen can go from the blood to the lungs. this is the reverse of what normally happens, and the freediver can blackout as (s)he gets to about 10 feet below the surface.
Rhythm freediving is different, in that the diver does not do extended hyperventalation. Rather, the body slowly adapts to the breathholing over a period of time. My technique is to stay on the surface for a period and then dive, staying down until I become slightly uncomfortable. At first, that's as short as 30 to 45 seconds. As I repeat the dives, my body adapts and I can comfortably stay down longer. Tonight, after about 45 minutes of rhythm freediving, I stayed down pretty comfortably 2 minutes and 15 seconds on my last two dives. This was done in a pool, with a lifeguard watching after I had explained to him what I am doing.
I want to tell you this information so that you don't have to go through the trails that I did, and nearly loose it as I described above. Breath-hold diving is potentially dangerous, and shallow-water blackout is called an "expert's disease." It happens to very good swimmers and freedivers. You don't need to be trying for a record depth to have this happen. I have dived to 45 feet in the Puget Sound area, and nearly had this problem (you kinda know when you get that metallic taste in your mouth, and nearly loose bladder control). This was, again, when I was in high school and still "wet behind the ears." Experienced freedivers, who feel they may be in trouble, will take off their weight belt and hold it in their hand as they ascend. That way, if they do black out, they will automatically release the belt and get to the surface. That way, by the way, at least they have a chance of surviving the dive.
I hope this helps someone. It can happen to anyone, but "expert" divers are actually more vulnerable than novices. I've enclosed a 1960s photo of me with a catch while spearfishing off the Oregon coast in the 1960s.
SeaRat
