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Hypocapnic Blackout...

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Well-Known Member
Sep 18, 2003
This last Monday night at our Freedive Toronto pool practice, I experienced my first ever blackout of any kind during static practice. This was not the standard hypoxic BO we think about when we're thinking about breathhold related blackouts however! I blacked out at about 5 seconds total time.

On my previous attempt I purged 5 times, and packed 6 times. I got a slight headrush, but it was in the "I've had worse" catagory! (I've had hypocapnic LMC several times, yes I know that this isn't ok!!:naughty ) I also noticed that I didn't feel any real pressure on my epiglotis during that static, so I decided to pack 8 times for my third attempt.

On my third and final attempt, I purged 5 times and packed 8 times after a 3-4 minute breathe-up. I went down, and almost immediatly felt the very early symptoms of hypocapnia, just a slight buzz. Much less than on my previous attempt. The next thing I knew I was being supported at the side of the pool being told to "breathe-breathe!" I was fully aware instantly that something had gone wrong, but I didn't know if I had finished my static and blacked out at the end, or anything else.

My spotter said that I went down, and at about 5-7 seconds I started releasing air. He said he remembered a time that he went down and a funny thought interupted his concentration and he laughed under water! He thought that possibly the same thing was happening to me, and he expected me to abort my attempt as I had obviously exhaled almost all my air. After 5 further seconds though when I failed to stand up or respond to a signal, he tried to lift me out of the water. I was obviously very limp at this point, and he had a bit of a struggle supporting my head above the water. At about 15-20 seconds total time I regained conciousness.

I have one point and a question as well.

To those of you who dive solo, (as I used to!) I had no warning that I was going to black out. It felt like a very slight buzz at most, not the "going down the tunnel" effect that I feel before a hypocapnic LMC. If I had been solo, I don't know if I would've regained conciousness before inhaling a lungfull of water or not.

Question to those of you who have experienced this type of blackout before: Did you know you were going out? What did you change to ensure that this didn't continue to happen? I know the obvious answer is to purge less, however I have purged 7 times and packed 10 times before with little to no headrush at all. Different day effect I guess.

Thanks for your imput,
I have yet to black out from hypocapnia, but I think I have been close. I used to really enjoy the buzz, because it seemed like time went bye fast at the beginning and when I came out of it, I was very relaxed. But then I got a pulse/oximeter and quickly found out the feeling, for me anyway, was just a feeling and not indicative of my O2 and heart rate. After getting a good buzz from purging and packing, my heart rate does drop well from 1 to 2 minutes, but after that, it goes up and my O2 falls faster than normal.

I have been experimenting with doing my purges before the end and then going back to slow exhales before doing just 3 purges right at the end. Apparently Tyler (tylerz) has been experimenting with this too. The idea is that once you lower your body’s CO2 it’s going to remain low for a while as long as you’re still breathing and not moving much muscle. You might give it try.

I too have found that on different days my tolerance to purging and packing is different. Some days it doesn’t matter how much I do, I can’t get a buzz and others I’m on the edge. By going back to slow breathes for 1 to 1:30 before a final 3 purges and packs, it gives me a chance to get past the buzz and regain composure before going into the water if the buzz was more than I anticipated.
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Are you sure this is hypocapnia.
I think you blacked out of Hypoxemia.

Low oxygene in the brain.

You might have packed so much that your heart and arteris got squeezed and could not pump fresh blood to the brain. BO

same buzz can occur (which in fact is the beginning of LMC) usually ones comes out of it specially if diving to depth and realaizing the pressure around the heart.
You will always regain consciousness on your own from a hypocapnic blackout.

There are many, many tricks to avoid these types of blackouts.

Eric Fattah
BC, Canada
I think this is hypocapnia because every time I have experienced it before (ie. buzz and LMC) I have come out of it around 30 sec during static. Also, after 8 packs I feel very mild pressure on my epiglotis.

I think that for hypoxemia I would have to have alot more pressure in my thorax (and against my epiglotis). Before CW dives I usually pack at least ten times, and as many as 18 max. On these dives I feel moderate to extreme pressure against my epiglotis and some in my chest until I pass 5-10m.

I'm not going to rule out hypoxemia though, as I imagine the symptoms would be almost identical to hypocapnia. The way I understand hypocapnia is that sensors in the neck identify the blood acidity from extremly low CO2 and in response restrict blood flow to the brain. So in both cases the blood flow to the brain would be restricted. Please correct me if my understanding is incorrect! I want my thinking to be as correct as possible.

Any tricks to not exhaling when you do black out Eric?? ;) I must say it would be cool to black out and wake up halfway through the static!

Definatly not something I want to try however! Right now I'm going to work on Co2 tolerance, not reducing my Co2 further as I've found my limit to low Co2.

I think your understanding is correct, but wouldn’t it be low blood acidity (alkalinity) from low CO2 that causes the blood restriction response? I think acidity from CO2 dilates blood vessels to increase flow, which is how the body tries to get more oxygen to the parts that need it.
I think you're right Don. I couldn't remember which way the acidity went, but that sounds correct. So low CO2 would cause low acidity and high CO2 would cause high acidity? My understanding of all this is growing, however still very limited!

I'm going to try your method for awhile during dry statics and see how it goes.

Interesting story, I blacked out for the first time last week about 10-15s into an apnea walk and it spooked be big time, luckily i wasn't under water. I'd like to know some tricks your talking about efattah, other than being more cautious in future coz this is obviously a bad look.

Last night i tried out some stuff and found a break between saturation breathes and purges helped. But i was thinking as high Co2 is the limiting factor for dynamics what would be the best way to reduce Co2 without blacking out? I did try a couple more purges and no packs and this seemed to work better. Anyone with more experience any advice they can pass on to me would be gratefully received.

I've never had the blackout at the start but, I only pack 50% for deep dives and just worked up to 20% for static. A few LMC's got my attention. 'Bad days' don't seem to give a warning and 'good days' often come after terrible warm ups. Train and experiment more.
What exactly do you do for purges? Do you follow them with 2-3 slow breaths?
So far I've been doing a 2-5 min breathe up consisting of 3-4 breaths/min. I've been doing purges at the very end, consisting of 3-7 fast shallow breaths concentrating on the exhale, followed by my inhale-packs and then hold.

I've had 7 or 8 LMC's at the beginning before, most dry, only two wet. The hold prior to my black out was a fairly mild headrush though, definatly not anywhere near the LMC stage. With that in mind I thought that adding two packs without changing the purges would be fine. Obviously I was wrong!

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Now you’re the sprinter/freediver/blackout guy!;) Has the freedive training done anything for your sprinting yet?

This is mostly speculation on my part, but I think the recovery phase from hypocapnic has as much to do with the body adjusting to the state of low CO2 and high O2 as it does with an increase in CO2. Its hard to know for sure, since CO2 is not as easily measured as O2. But I just can’t see CO2 increasing significantly in the first 10 seconds of static for the brain to sense and open the blood vessels.

The body adapts to changes in O2% of air and pressure pretty good, so I speculate that it has other sensors that are kicking in and telling it its okay and to unrestrict blood flow to the brain. If this theory is true then the idea of purging before the end and then going back to slow exhales, makes sense, because your giving the body time to adjust to the low CO2, which means you can further safely lower your CO2.

Bill statement about 2-3 slow breaths has me curious. It takes me 6 slow exhaling (saturation breathes as you called them) to do 1:30 of breathing, which is what I usually do after my main purges. But I have noticed that my heart rate start climbing back up after a good initial drop by then. Maybe 3 or 4 slow exhaling breaths would be better.

The only significance of heart rate is where it is after you begin the breathhold. A high heart rate is actually more capable of saturating the body during a breath up than a low one. But since we are probably capable of reaching saturation max pretty easy in the breathup, then too high of heart rate at this stage will likely lead to a higher heart rate and O2 consumption during the hold without any extra starting saturation benefit.
My breathe up is a little faster, probably not as deep and 'purges' are rapid (20 in 10 seconds), exhale only. Some one suggested three calm fairly deep breaths just before packing. You might give that a try, it really calms things down. A harder warm up seems to help too. On my last few PB's I started a 5+ hold less than 10 minutes before 'top'.
Don, good thinking. I'll give that a try too. If I could "condition" myself to even lower Co2 without blacking out, it would make the static that much easier.

"On my last few PB's I started a 5+ hold less than 10 minutes before 'top'. "
What do you mean by 'top'?

Some clarifications:

- High heart rate, low heart rate -- tell you nothing about the amount of blood pumped per second; that is cardiac output which is heart rate x stroke volume. So, don's trying to say that a high cardiac output is beneficial during the breathe-up.

- Blackout. All blackouts are caused by brain energy failure, which is normally preceded by lack of oxygen. The lack of oxygen can be caused by:
1. Holding your breath too long
2. Too low CO2 = too much vasoconstriction = too little blood flow to the brain
3. Too much packing = crushed arteries = too little blood flow to the brain
4. Combination of 2 & 3 = classically called a 'hypocapnic blackout', although it is more a combo of somewhat low CO2, and somewhat excessive packing, although neither enough of each to cause blackout on its own

- Methods of avoiding hypocapnic blackout
[too many to list, here are some]
1. Avoid vasodilating foods
2. Superhydrate with sodium & glycerol, + water
3. Pack with your body vertical in the water, head cocked back, causing water pressure to force blood into your head
4. Be cold, or at least not too warm (i.e. no wetsuit makes a huge difference here)
5. Avoid things which drop blood pressure such as excessive cardio, dehydration, or anything which makes you hot
6. Inhale, wait for 5 seconds, once any lightheadedness has passed, pack 5 times, wait til lightheadedness passes, pack a few more times
7. Alternatively, inhale rapidly, pack extremely fast, then immediately drop yourself to the bottom of the pool with heavy weights -- the pressure will crush your chest and improve blood flow to your head
8. More experienced breath-holders tend not to exhale during hypocapnic blackouts -- German record holder Stephanie Ortlepp would deliberately have a hypocapnic blackout in static, and wake up 5 minutes into the breath-hold

In all apnea competitions, I take my last breath at the T-30 warning. My lungs are full by T-26, and I spend the remaining 26 seconds doing the pack/wait/pack/wait in order to get the extra 3 litres from packing. It is worth the wasted 30 seconds.

Eric Fattah
BC, Canada
Fantastic explanation Erik, thanks!

Now I've got alot of experimentation to do. :)

Don, It's hard to quantify exactly the effects apnea training has had on running coz of so many other training factors however i have been getting some good improvements in anaerobic fitness with little specific training so that could certainly be due to the apnea stuff. I do deffinitly notice improvement in the weight room though. But the biggest benefit i have gained has been the additional awareness/control over my body. By the way as i was improving rapidly in my apnea training i've got hooked and i'll be diving soon. Can't wait.

Anyway, back to blackouts, thanks for the explanations you and eric have put forward, sounds like good stuff. Looks like slow breathes after the purges is the way to go. But what i did try successfully last night was purging and packing and then held onto a lamp post for 20s until the lighthediness had gone and then started the watch and started walking and managed an extra 20s walk on top of the 20s static at the beggining.

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This is terrific info and I have a couple of questions that I'm hoping someone can help with.

1. Why is high CO2 the limiting factor to Dynamics (Alanc post)

2. In a previous post Eric linked those that a susceptible to fainting are also often more susceptible to LMC. Why is this the case? I thought fainting was as much a Psychological response as a Physiological one. I've never fainted but am very susceptible to slight LMC(not severe enough for anyone to see)...and not something I'm all that happy about.
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Top is the posted time for you to start, in competition. It's actually the start of a small window and you will be penalized if you do not begin during that period.
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