Question by Cody: Factors involved in a free dive blackout?
Ok so I’m thinking of getting into free diving and I’ve done a lit bit of researching into blackouts. I’m wondering what the main factors involved in a black out are. I thought people mostly blackout because they hyperventilate, but there seems to be some other causes.
What are the main causes of shallow water blackouts? Please explain factors such as oxygen and CO2 partial pressure during a ascent and how to avoid it.
Answer by moviebuff
Stimulation of the need to breathe is driven by the level of CO2 in the bloodstream, not by the O2 level. Hyperventilation (rapid, deep breaths) has the effect of lowering the CO2 levels in the bloodstream, and if performed in moderation before a free dive can help increase bottom time by reducing the breathing stimulus (CO2) which allows the diver to hold his/her breath comfortably for a longer period. This is a tightrope walk, however, because the diver continues to consume oxygen during the dive and so must return to the surface to breathe before the oxygen content drops below the level required to maintain consciousness (a PO2 of around .10 absolute atmospheres of pressure). As long as the CO2 levels in the bloodstream allow the diver to feel the need to breathe and return to the surface without the O2 partial pressure dropping below .10 ata, blackout probably will not occur.
The risk of blackout increases if the diver hyperventilates excessively before the dive, which reduces the CO2 in the bloodstream to very low levels. When the diver holds his/her breath and dives they will continue to consume oxygen from their lungs, but now the CO2 level may not rise to the point where the diver feels the need to breathe before the oxygen level drops below .10 ata and loss of consciousness occurs (blackout).
Blackout usually occurs in shallower depths during ascent, hence the term “shallow water blackout”. This is a result of the physics of gas under pressure and is most closely related to Dalton’s Law of Partial Pressures. Air is composed mostly of oxygen (20.9%) and nitrogen (78%), and so at normal atmospheric pressure (1 atmosphere absolute), the partial pressure of oxygen is about .21 ata (about twice the pressure needed to maintain consciousness). In salt water every 33 feet of depth increases the pressure surrounding the diver by 1 atmosphere, so at 33 feet the total pressure is 2 ata, at 66 feet it is 3 ata, etc. This increased pressure changes the volume & density of the air in the diver’s lungs (as described by Boyle’s Law), and this means the air pressure in the lungs will increase to match the ambient (external) pressure. Since the total pressure of the air in the diver’s lungs increases, Dalton’s Law states that the partial pressure of the component gases will also increase proportionally, therefore at 33 feet (2 ata) the PO2 is .42 ata, at 66 feet (3 ata) the PO2 is .63 ata, and so on.
So let’s say a diver fills their lungs with air and descends to 33 feet, where the PO2 of the air in their lungs is now .42 ata. As they stay on the bottom and consume oxygen, the PO2 level will drop and CO2 levels will rise. Let’s also say that the diver hyperventilated to such a degree that they are able to use a little over half of the oxygen in their lungs before feeling the need to return to the surface to breathe, at which point the PO2 in their lungs has dropped to .19 ata which is still sufficient to maintain consciousness. As they return to the surface, the ambient pressure drops from 2 ata towards 1 ata (surface pressure), which causes the air pressure in their lungs to drop proportionally also. Since the PO2 was .19 ata at 33 feet/2 ata, by the time the diver nears the surface the PO2 will have dropped to half of that, or .095 ata (insufficient to maintain consciousness), and so the diver blacks out.
Hopefully that helps illustrate the relationship between shallow water blackout and both CO2 and O2 partial pressures.
The following can help reduce the risk of shallow water blackout during free diving:
1) Avoid excessive hyperventilation before free dives. A maximum of 3 or four deep breaths before diving should help increase bottom time without reducing CO2 to unsafe levels.
2) Avoid heavy exertion at depth. Exertion increases the muscles’ need for oxygen and so will increase the rate of oxygen consumption from the lungs, which can result in an unsafe drop in PO2.
3) Don’t wait until you are desperate to breathe before surfacing. Surface while you are still comfortable to increase your chances of reaching the surface with a PO2 that is sufficient to maintain consciousness.
4) Increase your maximum depth and bottom times gradually so that you can learn what your physical limitations are in a controlled fashion.
Finally, always free dive with a buddy who can monitor your dive and provide aid if you should lose consciousness. Dive with someone more experienced or obtain training if possible. Use the “one up, one down” system (one buddy stays on the surface while the other dives), especially when trying for a new max depth or bottom time limit.
Good luck and safe diving.
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