Though news of coastal shark attacks seem to preoccupy the media lately, there's something even more dangerous--and a lot more common--that gets much less notice. Its name: shallow-water blackout, a condition which commonly results in death by drowning. This is a very important topic this time of year as our Florida waters are teaming with divers looking for the tasty spiny lobster.
Shallow-water blackout often affects free divers and skin divers (those divers without any sort of breathing apparatus other than a snorkel). I used to think the only dangerous kind of diving was with tanks or compressed air because I had heard of and seen so many disastrous outcomes. However, studies have now shown that many fatalities among experienced divers and swimmers have occurred simply from free diving. Some medical experts believe many of the backyard pool drownings are caused by the physiologic changes that result in shallow-water blackout.
WHY DOES SHALLOW-WATER BLACKOUT HAPPEN?
It really is all about physics and gas pressure in the lungs, so here are the nuts and bolts. First, we all know we need oxygen (O2) to live but the body uses our levels of carbon dioxide (CO2) to tell us when to breath.
Most of us deplete our O2 stores more quickly than we build up CO2. This is even more pronounced in experienced divers who can hold their breath for longer periods of time (at least more so than us out-of-shape individuals).
Second, the deeper you are in the water, the more compressed the air in your lungs will become and the more concentrated the oxygen you are able to use.
As you rise in the water the lungs expand resulting in the decrease in concentration of usable oxygen. This expansion of the lungs is more pronounced as you reach 15 feet of the surface and continues to progress as you ascend in the water column. In effect, what can then happen is that your brain becomes starved of precious oxygen and you may blackout. Unless you are helped to safety immediately, you will drown.
WHO IS AT RISK FOR SHALLOW-WATER BLACKOUT?
As a rule, we all are at risk for shallow-water blackout. Statistics have shown, however, that shallow-water blackout tends to occur more often in experienced divers as well as younger divers. This may be due to experienced divers having become accustomed to ignoring that burning sensation in their lungs longer thus leading to lower levels of available oxygen as they attempt to surface.
Pre-dive hyperventilation is thought to be another dangerous practice that predisposes us to shallow-water blackout.
Just visiting any pool with a bunch of kids competing to see who can stay under the longest can show you this breathing practice when preparing to dive.
Physiologically it makes sense. Several deep breaths with prolonged expirations actually allows you to blow more carbon dioxide from you lungs thus staving off the sensation of needing to take a breath a little longer during a free dive.
Unfortunately, hyperventilating in this manner does not put more oxygen in the lungs and can lead to disastrous results. The U.S. Navy Diving Manual recommends hyperventilating no more than three or four breaths prior to a free dive for the same safety reasons.
Divers who use weights to descend--as well as spear fishermen and our lobster grabbers--are also at special risk because they will likely become focused on their prey and exhaust their oxygen supply beyond safe levels. Thinking "just a few more seconds" puts these sportsmen at risk of drowning due to shallow-water blackout as they try to surface.
WHAT CAN I DO TO PREVENT SHALLOW-WATER BLACKOUT?
If you are as hard headed as I am and intend to go diving any way, here is a summary and suggestions of what you can do to prevent disaster on what will hopefully be a fun day on (and under) the water:
- Always dive with a buddy and be sure to maintain contact while ascending to the surface.
- Do not hyperventilate more than three to four breaths (if at all) before diving.
- Do not over-exert yourself at depth.
- Pay attention to that burning in your lungs. Your body is trying to tell you something: "I need air!"
- Do not wear a weight belt while skin diving. If you need weight carry it in your hand.
- Avoid a competition with your buddy to see who can stay down the longest. You both may end up being down longer than either of you intended!
- Know basic CPR. (We should all be trained in this anyway.)
- Know your depth and realize the risks of shallow-water blackout increase as you ascend. This is especially true as you ascend to within 15 feet of the surface.
- As always, be safe. Live to fish another day!
