Shallow- and Deep-Water Blackouts

[FPDocMatt]

Deep water blackout occurs as the surface is approached following a breathe-hold dive of over ten metres and typically involves deep, free-divers practicing dynamic apnoea depth diving usually at sea.[3] The immediate cause of deep water blackout is the rapid drop in the partial pressure of oxygen in the lungs on ascent.

Shallow water blackout only occurs where all phases of the dive have taken place in shallow water where depressurisation is not a factor and typically involves dynamic apnoea distance swimmers, usually in a swimming pool.[4] The primary mechanism for shallow water blackout is hypocapnia brought about by hyperventilation prior to the dive.

This confusion is exacerbated by the fact that in the case of deep water blackout hypocapnia may be involved even if ascent is the actual precipitator.

I'm starting a new thread because we kind of hijacked another thread with this discussion.

Can someone explain this to me? I'm a doctor and I don't understand it.

Hypoxia causing a blackout makes sense. So that explains deep-water blackout.

But why would hypocapnia (low carbon dioxide) cause a blackout? And why would it only occur when diving in shallow water?

 

[TSandM]

The hypocapnia doesn't cause a blackout. But because you are hypocapnic from hyperventilation before diving, you are unaware of the hypoxia that you are developing, because the hypoxic drive to breathe is so weak.

In the blackout on ascent from deep dives, you were hypoxic at depth, but because of the pressure, you had enough of a ppO2 to stay conscious until you ascended. In shallow water, the hypoxia isn't due to ambient pressure changes, but simply due to metabolism. You don't notice it because you have hyperventilated.

 

[FPDocMatt]

The hypocapnia doesn't cause a blackout. But because you are hypocapnic from hyperventilation before diving, you are unaware of the hypoxia that you are developing, because the hypoxic drive to breathe is so weak.

In the blackout on ascent from deep dives, you were hypoxic at depth, but because of the pressure, you had enough of a ppO2 to stay conscious until you ascended. In shallow water, the hypoxia isn't due to ambient pressure changes, but simply due to metabolism. You don't notice it because you have hyperventilated.

So are you saying that both shallow- and deep-water blackout are due to hypoxia? But with the shallow-water one, there's no expansion of the lung oxygen (and therefore no drop in ppO2)...?

You know, it sure seems like free diving is a foolhardy pursuit. But what do I know.

 

[E-diver]

I think what TSandM was saying is that hyperventilation dramatically reduces the amount of CO2 in the lungs, and since increasing CO2 levels in the lungs are the trigger telling us we need to breath, the shallow water breath-hold diver is lulled into a false sense of belief that they don't yet need a breath. When they finally feel the need to breath, the combination of lower than normal O2 levels (due too longer than normal breath hold) and reduction in O2 partial pressure causes the hypoxic blackout.

IMHO don't take this to mean that breath hold diving is a dangerous thing, simply be sure you avoid excessive hyperventilation to "maximize the dive" 3-4 deep breaths is all you need, and it really can be alot of fun. Also, treat it like any other recreational dive and go with a buddy...just in case.



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[feitr]

Terms aren't used very consistently also - what you are calling a deep water blackout is often called a shallow water blackout (given that you don't tend to black out until you get <5m to the surface).

With what you are calling a shallow water blackout - there used to be (and still is among some) a tendency to "purge" before doing free diving to remove CO2 and improve length of time in the water. There are basically two limiting factors in how long you can hold your breath - when CO2 levels rise to an uncontrollable level and you are forced to take a breath and when you get so hypoxic that you blackout (~50% of total hemoglobin having attached O2).

As for freediving being dangerous - you have to be a damn good freediver to actually black out because even if you are hypocapnic before starting your CO2 tolerance is still not going to be good enough to get to the point of blackout. Fwiw, it actually takes 2-4 minutes before your O2 attached to hemoglobin levels start to dip in a static (from 98-99%) - during that time you are using up the O2 in your breath hold and it is only really after that that it starts to get really uncomfortable and you have to withstand contractions/urge to breathe. Basically the only point at which some sort of hyperventilatoin makes sense is in competition where you basically want to push your body to the absolute limits (ie. as close to blackout without actually getting DQed) and more importantly there is will be a very very large group of safety divers/medical equipment close on shore, etc.

Fwiw, the most hypoxic you get is in the few seconds after you get out of the water in the lag period before you start to use the O2 again - if you watch freediving comps this is why there is always a period afterwards where they have to signal to the judges that they are control and then they get the all clear.

 

[Thalassamania]

Shallow water blackout is, as was observed, a problem that is almost exclusive to trained and experienced free divers. I deal with the issue by always diving with an alarm set on my watch ... three minutes and that's it, no matter what else is going on, I'm on my up. I have learned from experience that this works (for me) though I'd never attempt (or succeed at) a dive that long if I was working hard.

 

[FPDocMatt]

Terms aren't used very consistently also - what you are calling a deep water blackout is often called a shallow water blackout (given that you don't tend to black out until you get <5m to the surface).

Yes, I realize that. We need better terms. How about lung expansion blackout and hyperventilation blackout?

With what you are calling a shallow water blackout - there used to be (and still is among some) a tendency to "purge" before doing free diving to remove CO2 and improve length of time in the water.

It seems to me that hyperventilation is not the problem, but rather not knowing how long you hold your breath before you become hypoxic enough to lose consciousness. After all, overcoming the urge to breathe caused by hypercapnia sounds painful. It seems logical to hyperventilate to prolong breath-holding times in a comfortable manner. You just need to know when your hypoxia has reached dangerous levels.

As for freediving being dangerous - you have to be a damn good freediver to actually black out because even if you are hypocapnic before starting your CO2 tolerance is still not going to be good enough to get to the point of blackout.

I see.

Fwiw, it actually takes 2-4 minutes before your O2 attached to hemoglobin levels start to dip in a static (from 98-99%) - during that time you are using up the O2 in your breath hold and it is only really after that that it starts to get really uncomfortable and you have to withstand contractions/urge to breathe.

Perhaps what you're saying is only true for experienced breath holders. For the rest of us mere mortals, it becomes pretty uncomfortable long before 4 minutes.

...there is will be a very very large group of safety divers/medical equipment close on shore, etc.

Then why did that free-diver die? Perhaps they need scuba divers at every point along the route.

Fwiw, the most hypoxic you get is in the few seconds after you get out of the water in the lag period before you start to use the O2 again - if you watch freediving comps this is why there is always a period afterwards where they have to signal to the judges that they are control and then they get the all clear.

Yeah, I was wondering whether you are disqualified if you get to the surface, take a breath, then black out.

 

[dumpsterDiver]

Yes, I realize that. We need better terms. How about lung expansion blackout and hyperventilation blackout?



It seems to me that hyperventilation is not the problem, but rather not knowing how long you hold your breath before you become hypoxic enough to lose consciousness. After all, overcoming the urge to breathe caused by hypercapnia sounds painful. It seems logical to hyperventilate to prolong breath-holding times in a comfortable manner. You just need to know when your hypoxia has reached dangerous levels.



I see.




Perhaps what you're saying is only true for experienced breath holders. For the rest of us mere mortals, it becomes pretty uncomfortable long before 4 minutes.



Then why did that free-diver die? Perhaps they need scuba divers at every point along the route.



Yeah, I was wondering whether you are disqualified if you get to the surface, take a breath, then black out.

WOW, this guy makes comments like "freediving is foolhardy" and then wants to re-define the most basic vocabulary used in the sport of freediving?

He goes on to make comments like "hyperventilation is not a problem"....

The level of arrogance (or is it ignorance?) is remarkable.

 

[feitr]

Yes, I realize that. We need better terms. How about lung expansion blackout and hyperventilation blackout?



It seems to me that hyperventilation is not the problem, but rather not knowing how long you hold your breath before you become hypoxic enough to lose consciousness. After all, overcoming the urge to breathe caused by hypercapnia sounds painful. It seems logical to hyperventilate to prolong breath-holding times in a comfortable manner. You just need to know when your hypoxia has reached dangerous levels.



I see.




Perhaps what you're saying is only true for experienced breath holders. For the rest of us mere mortals, it becomes pretty uncomfortable long before 4 minutes.



Then why did that free-diver die? Perhaps they need scuba divers at every point along the route.



Yeah, I was wondering whether you are disqualified if you get to the surface, take a breath, then black out.

Hyperventilating before isn't going to result in a blackout for an inexperienced diver but it is a poor habit (it isn't likely somebody is going to learn to free dive by hyperventilating then all of a sudden stop because they've reached the point it is becoming very dangerous). Plus hyperventilating is not calming and raises your heart rate which doesn't help in a breath hold. Again I think it needs to be stressed that a blackout due to purging isn't something that "just happens" where you are fine one second then unconcious the next...we are talking about divers pushing the absolute limits. Most free diving preparations will involve deep breathing for a few minutes then several deep inhales, deep exhales (so purging without the hyperventilation).

Re: the difficulty of holding your breath - you'd be suprised it really isn't that hard. Most people should be able to do a 3 minute static in the pool after one day of practice. Successive breath holds get progressively easier (static, rest, static is easier, rest, static is even easier, etc.) and yes 3-4 minutes before the urge to breath really sets is about where the top guys would be. But I'd bet if you gave it a go for a day or two you'd break 3 minutes. Once you are actually doing work though obviously this becomes much less.

 

[FPDocMatt]

Most free diving preparations will involve deep breathing for a few minutes then several deep inhales, deep exhales (so purging without the hyperventilation).

So what is it that you're purging your body of if not the carbon dioxide? Medically, I think this is still hyperventilation. From Wikipedia:

Hyperventilation or overbreathing is the state of breathing faster or deeper than normal,^[1] causing excessive expulsion of circulatingcarbon dioxide.

Re: the difficulty of holding your breath - you'd be suprised it really isn't that hard. Most people should be able to do a 3 minute static in the pool after one day of practice. Successive breath holds get progressively easier (static, rest, static is easier, rest, static is even easier, etc.) and yes 3-4 minutes before the urge to breath really sets is about where the top guys would be. But I'd bet if you gave it a go for a day or two you'd break 3 minutes. Once you are actually doing work though obviously this becomes much less.

Interesting! Next time I go scuba diving and forget to bring my tanks I'll give it a try. :balloons:

Seriously, though, the information regarding free diving is much appreciated.