Shallow Water Blackout
Ever since I first learned to swim as a child, I have loved swimming underwater. I am not talking here about snorkeling or scuba diving; I suppose the technical term for what I am describing is free diving. You simply immerse yourself in the water of a swimming pool or better a pond or lake and enjoy the sense of visiting a new and slightly alien world.
You find your senses challenged. Your sight is reduced to a few yards. Even in a backyard pool finding a small coin underwater requires you to approach much closer than you would in open air and, looking up, the water surface blocks out almost all of the world above. There is also a sense of pressure on your body that seems to contradict a wonderful feeling of weightlessness. If you do not move, you find yourself hung in the water, slowly rising to the surface.
Your hearing is more profoundly affected. The sound of stones struck together is deeper than it would be in open air and there is a kind of rattle associated with it. Perhaps this is an echo effect. Much has been written about underwater sound and the noises or songs of sea creatures like whales and dolphins. In freshwater lakes, however, the general sense is of absolute quiet. I enjoyed using that as an excuse for failing to hear parental calls to get out of the water.
Unfortunately there is a more serious side to this. I served with a chief boatswain who had been at Pearl Harbor when the Japanese attacked on the morning of December 7, 1941. He was a diver at that time - they went down then in those heavy suits with steel-framed helmets - and was frightened that the bombers would return. "A bomb exploded anywhere within a half mile would have killed me," he told me.
Where I most enjoyed underwater swimming was at our camp at Fish Creek Ponds in the Adirondacks. I would swim out past the sandy bottom where a few clams made trails to deeper water with more debris. There I could watch catfish and sunfish swim by and occasionally find a crayfish scuttling backward among the weeds.
One reason for my love of underwater swimming was my failure on the surface. My college roommate, Cromwell Anderson, was captain of the swim team, but I was simply a flop at the crawl or backstroke. I could, however, swim farther underwater than he could. I was able to make it twice the length of our Olympic pool, 100 meters in all, before surfacing. I thought this was great until another friend did three laps.
It turns out, I now learn, that we were doing something very foolish. And I exacerbated that foolhardiness by often swimming those laps alone in the pool. There is an effect associated with this kind of underwater swimming so important that it has been assigned a name, shallow water blackout. People die from it. Last summer, for example, Annapolis midshipman Kyle Hurdle passed out while doing so and lifeguards were unable to revive him.
Christian Swezey described this phenomenon in The Washington Post: "Normally, a person can hold his breath for about a minute before the respiratory center reacts to growing levels of carbon dioxide in the blood and demands a breath. In order to prolong the time between breaths, some athletes hyperventilate on purpose, meaning they breathe more than necessary before going underwater, trying to rid the blood of carbon dioxide. This fools the brain into thinking it doesn't need to breathe even when its oxygen stores are dangerously low."
With no carbon dioxide to trigger breathing, the swimmer experiences muscle cramps, tingling, dizziness and blurred vision. More important, the brain finally shuts down, the swimmer faints and his or her lungs involuntarily take in water.
It is important to understand that this is not just a concern for beginners. As Hurdle's accident indicates, it is even more a problem for intermediate or advanced swimmers who consider this kind of breathing part of their training.
The 1999 Navy Diving Manual includes instructions related to shallow water blackout. Navy divers take no more than two or three breaths before diving.
All swimmers should be aware of this phenomenon.-- Gerry Rising