Preventing Blackout

[fishon]

I was wondering what techniques could be used to help minimize the chance of blackout. Obviously diving within your limits is the best, but what other things can be done? I read about exhaling as you approach the surface- the theory being to reduce the expanding volume of oxygen poor air (I guess). Does anyone think this method is good or bad? I also have talked to someone who likes to suck in air from his mask as it expands. Does anyone do this. I was trying it and I felt like I got little air for my effort, but maybe I am just not good at it.
Also, I read that some people go fast near the surface (where blackout is more likely) in order to speed through that zone. On this forum I have also read that some people like to go slow, I think to allow the body to remain calm and adjust. What is everyone's opinion about these methods?
Just to give context, I am talking about diving for fun or fishing to about 120 feet (about 35 meters) or less.
Thanks

[efattah]

Packing your mask air through your nose on the ascent definitely helps, if you can learn to do it.

The consensus is not clear on whether going fast in the danger zone is better than going slowly.

The biggest dangers for blackout are generally caused by:
- Being too hot
- Being dehydrated, even if you don't feel thirsty (the longer you are in the water, the more dehydrated you get, this is UNAVOIDABLE)
- Over breathing
- Taking too short rests between dives
- Physical exertion underwater
- Eating a low carb diet

In terms of resting between dives, if you really want to minimize your blackout risk you should rest for 4-8 minutes between dives, depending on how strenuous they were. But, it is extremely important that during those 4-8 minutes you barely breathe at all. In fact a 60-second static near the end of the recovery phase might be a good idea (this allows you to rest, and recover, without blowing off too much carbon dioxide).

[Adrian]

A couple of days ago I was in the water with Johnny and had a samba after:

- Being dehydrated, even if you don't feel thirsty (the longer you are in the water, the more dehydrated you get, this is UNAVOIDABLE)
- Taking too short rests between dives
- Physical exertion underwater

3 out of 6 conditions were present.

We had been around three hours in the water with many dives, and had found a large anchor with a long (20 meters at least) thick 18 mm. rope and 2 meter chain stuck in a cave at around 10 meters. Johnny manages to untangle the anchor from the cave and when I start to pull on the rope to recover the loose end, it gets stuck in a long thin lateral crevice on a rock (must have been made to measure!). I quickly go down again, have trouble equalizing cause I'm getting congested after 3 hours, spend a good half minute hanging in mid water equalizing, finally clear my ears and decide to go on down to get that blasted rope out which involves quite a lot of pulling and messing around. I finally free it, go back up and have a samba during my recovery breathing. For a few seconds my left side was basically out of control. Johnny was right there and watchful and I was keeping afloat with my right side so no problem there. That was the last dive for the day and I felt really tired afterwards.

I also sniff in the expanding air on ascent, it makes quite a difference.

The moral here for me is to be patient and make sure you have a good long recovery period, only going back down when you feel rested.

Adrian

[subaquaticus]

Originally Posted by efattah

The biggest dangers for blackout are generally caused by:
.........
- Being dehydrated, even if you don't feel thirsty (the longer you are in the water, the more dehydrated you get, this is UNAVOIDABLE)

I enjoy drinking fruit juice or some beverages like fanta in "normal life" ; no beer and no wine...

These beverages are no good for the stomach before a comp ; and I do not like plain water...

I found a fantastic beverage named "Vittel Energy plus" ; unfortunately you can find it only in specific sports stores... ; This beverage (I guess) is kind of isotonic, assimilates immediately in your body, is sugared..., tastes good...

I usually consume 3 bottles of it (3 X 75 cl) the day before the comp...

I am going to look at the formula on the bottle to try to understand why it is so good...

Originally Posted by efattah

- Eating a low carb diet

I am afraid I do not understand very well what a "low carb diet" is...

Could you be more specific and give some concrete examples of such a diet ?

Up to now, it has never been very clear to me what you should eat and what you should NOT eat before apnea...

Some say that you should have an empty stomach in order to minimize your metabolism... but then what about the risk of hypoglycemia ?

Last year I did a marahon in roller skating... I had carefully prepared my alimentation and brought with myself some energetic things to eat during the race... I remember another competitor very close to faint begging for some sugar in the middle of the comp...

Is an empty stomach advisable ?

[fishon]

Well, I drink plenty of beer and Fanta, but I'm pretty sure that's not helping with hydration. Thanks for all the tips. I am going to work on "sniffing" that air back in from my mask. When I'm fishing I hate staying up top for 4-8 minutes. Usually just 2 or 3 minutes. What are the best exercises/training tips for getting the best recovery time?
Thanks

[DeepThought]

Originally Posted by subaquaticus

Some say that you should have an empty stomach in order to minimize your metabolism... but then what about the risk of hypoglycemia ?

I never got hypglycemic while diving , even for hours, even with my last meal being yesterday. It's a matter of metabolism and how your body is being used to that. The liver holds enough glycogen to keep the blood sugar levels for a few days I guess, it's just a matter of what your body is used to do.

Though maybe hard contractions are more prone to induce hypoglycemia, I don't know, I've never been to a competition and I'm not much of a trainer.

[subaquaticus]

Originally Posted by DeepThought

I never got hypglycemic while divin , even for hours, even with my last meal being yesterday.

Supposing you are diving for hours... don't you drink or eat something ?

I am surprised....

[DeepThought]

Usually I won't take food and even water with me. But I don't get to dive longer than 2.5 hours at a session usually (mostly like 1.5 before it's too cold). I'm a recreational diver so I'm not sure that's the way to approach a competition event though.

It all depends on what your body is used to. There has been many periods in my life when I used to eat one meal per day, so I'm more likely to get hypoclycemic after eating a small simple-carb treat (even fruit) after not eating for a long time than if I don't eat at all.

EDIT: I have been for a few 4-5 hours dive sessions as well with this approach.

[flyingshark]

Originally Posted by fishon

I was wondering what techniques could be used to help minimize the chance of blackout. Obviously diving within your limits is the best, but what other things can be done? I read about exhaling as you approach the surface- the theory being to reduce the expanding volume of oxygen poor air (I guess). Does anyone think this method is good or bad? Thanks

I am wondering about this too. I recently finished reading Pelizzari's 'Manual of Freediving' and he says under no circumstances to ever exhale while still submerged...and he says once you break the surface to never exhale forcefully, but to exhale in a controlled manner and then take a slow breath. In my experience I have done the slow breathing without reading this book. Just seems like common sense I guess. As far as the exhalation underwater goes...well, I remember that in the past I somtimes let out a bubble trail in the last 10 feet of water to "slow" my ascent...I guess this was wrong. I guess I just wanted to stay down just a little longer, haha. Oh well, it makes sense not to exhale while submerged, why waste any air?

Any thoughts on this people?

[subaquaticus]

Originally Posted by flyingshark

I recently finished reading Pelizzari's 'Manual of Freediving' and he says under no circumstances to ever exhale while still submerged...

I also read Pelizzari's book... highly advisable !

I feel it is bad to exhale underwater because :

1. it may be interpreted by safety divers as a beginning black out or loss of motor control ;

2. it disminishes your floatability and hence delays your coming back to fresh air...

and I see no advantage in doing so...

I have seen people being eliminated in a pool comp for breathing forcefully at the end of their apnea, static or dynamic... again it can be interpreted as a LMC... If I do so, I tend to feel a bit dizzy...

[cdavis]

This is an area of disagreement, Some experts, Performance Freedivng for instance, teach exhaling in the last few meters before surfacing. The idea is to reduce the volume into which reverse o2 exchange can occur. It also reduces the very rapid reinflation of the lungs near the surface. There is some evidence that reinflation, especially after packing, can slightly reduce blood preasure to the brain, favoring BO. It gets the diver closer to inhale once he (she) hits the surface, a significant advantage for me which fits in with old spearfishing habits.

It is probably not a fair example, but last summer my buddy, who normally exhales near the surface, did not on a long, deep dive. He BOed on the surface, a most unexpected experiance for all of us.

Connor

[Panos Lianos]

The only reason to exhale before you surface would be to do it in the last 30cm-50cm of your ascent, in order to inhale right after you surface and not having to exhale on the surface before you inhale.

[efattah]

Exhaling near the end has never been shown to prevent 'reverse O2 transfer.' In fact, reverse O2 transfer during the last few meters is extremely unlikely. The reason is that the venous blood is so profoundly desaturated (i.e. blue) that as it passes the lungs, even if the lungs have only 4% oxygen in them, the venous blood will still pick up some oxygen.

What WILL happen is that during the end of the ascent, the AMOUNT of oxygen that the venous blood picks up as it passes the lungs will decrease.

Here is an example:

Deep part of dive:
Venous blood oxygen saturation = 60%
Venous blood passes lungs, becomes arterial blood
Arterial blood oxygen saturation = 90%
Gain in oxygen after passing lungs = 30%

Shallow part of ascent (risk of SWBO):
Venous blood oxygen saturation = 40%
Venous blood passes the lungs, becomes arterial blood
Arterial blood oxygen saturation = 50% (near B/O)
Gain in oxygen after passing lungs = 10%
(reverse O2 transfer would imply a NEGATIVE gain after passing the lungs; this situation would be most likely after you had already blacked out)

So, you can see in this example that the amount of O2 picked up by the blood as it passes the lungs will decrease as the diver ascends, but the blood still picks up O2. If you exhale at 10m on the ascent, then the venous blood will still pick up a bit of O2 as it passes the lungs, but this will drain the lungs much faster of their oxygen, meaning subsequent venous blood will pick up EVEN LESS oxygen than it would have, if you hadn't exhaled.

I agree with Panos; allowing the air to spontaneouly come out of the lungs in the last 1-3m is good, only because it is natural and it prevents huge pressure in the lungs. However I would never say that it prevents reverse oxygen transfer.

[fishon]

I usually breathe out a little in the last meter or two in order to clear my snorkel. My head never really comes out of the water. If I am fighting a fish I usually go ahead and come up to breathe without the snorkel, because I might not have a free hand to put the snorkel back in my mouth. Do you always pop up and take a breath without the snorkel?
Thanks

[Panos Lianos]

I agree with Eric, there is no such thing as "reverse O2 transfer" or "vacuum effect".

Yes, ALWAYS come up and breathe without the snorkel.

I know a story where a guy came up, gave the fish to his budy and then blacked out on the surface. His budy took the fish right away and only saw him black out after he took the fish back to the boat. The guy was out for a few minutes before his budy realized that he had a B/O. It took him a while to bring him back after taking him back to the boat and giving him Artificial Ventilation.

So, never mind the fish. Always surface and breathe and give ok sings to your budy before you do anything else.