What's going on? Exhale, FRC, BO, CO2, safety.

baiyoke · Sep 24, 2012

Can anyone explain in laymans terms what is going on during "less-than-full" apnea? I have searched and searched allready.

Taken from another thread:

baiyoke said:
There's something I don't understand well, even though I have read a very scientific explanation before (perhaps too difficult for me).

Is it easier to BO when doing less-than-full/exhale/FRC breathholds???

Some say it is safer, fx Sebastion Murat in this video because of stronger DR as I understands it.

Others say it is easier to BO along the lines that less CO2 is building up, and therefore it is easier to reach blackout.

Now I know Sebastion is very much into benefits of exhaling in depth-diving. So just for a second, to try separate different contributing factors, how about just a regular static? It seems it comes down to STRONG DR vs. CO2 BUILDUP.

Are there different believes and experiences about wich is "safer"?

Or is there perhaps like someone mentioned a "sweetspot" in exhaling, an optimal degree/amount of both DR and CO2?

Now, this is what I think I get: For clarity, I consider empty-lung apnea first.
- We have less oxygen overall initially.
- DR is stronger because of fast CO2 buildup in beginning
- CO2 i high because it cannot move from bloodstream to lungs (not much anyway)
- later in the (static emptylung) dive it is easier to BO because......???

Wouldn't the higher CO2 make it "safer" because of urge to breeth? Or is CO2 only high until a point when vasoconstriction is strong, and CO2 "stops" building up fast, and only builds up from the core, very slowly.

Could someone please explain it, starting with simple, straight forward information to begin with?

Thanks

cdavis · Sep 24, 2012

The subject is pretty counter-intuitive and makes no sense unless you understand the physics and physiology. Sorry, the physiology is not simple. Its a lot of little pieces that fit together.

Try reply #5 in the Murat Video thread. As an addition, the two of the biggest 02 users in the core are the brain and heart. Blood shift, which is both stronger and sooner with exhale, slows the heart down: beats less, uses less 02. Exhale diving is a very calm style, slows down 02 consumption in the brain, at least some.

The physics is straightforward. Less buoyancy, less buoyancy change, less fight to get off the surface, shallower neutral, less powering down, less 02 consumption. This is particularly relevant because the DR hasn't kicked in strongly early in the dive and the muscle effort induced low 02(and high c02) blood goes straight to the core. Correct exhale technique is minimal effort on the way down. That is inherently more energy efficient. Coming up can also be easier, depending on how you weight and depth. Effort on the way up matters much less because, by that point, DR has mostly cut off circulation to the leg muscles.

The idea that full exhale statics lead to early and unexpected B0s comes from experience reported in these forums. There a few that were statics at depth. The first makes sense from a physiological perspective with minimal DR. The second I don't understand. I know from experience the around half lung works great for me. I've explored fairly long static/swim pool dives at full exhale plus reverse packing with no adverse results, but I don't trust them. All I can tell'ya.

Don't feel bad if all this is hard to bend your head around. I probably read all Sebs old posts twenty times before it began to penetrate my skull.

Connor

baiyoke · Sep 24, 2012

Thanks.

I know it can be very complicated. So let's just focus on a "simple" emptylung static.

What you're saying is - oversimplified - that we get high CO2 (CO2/O2 ratio) after short time, but finish lower-than-normal CO2 (CO2/O2 ratio), compared a full-lung?

If this is it, what is hard to understand is, that something that starts high, can turn low, because in absolute measures, CO2 must still be much higher at end. I guess things are relative in apnea, so even if CO2 is still rising, it FEELS relatively low compared to the end in full-lung apnea.

A bit like this (red=empty, blue=full, y=CO2 level, x=time)

Is it like that?

cdavis · Sep 24, 2012

I don't do long statics, so don't have much knowledge to share on that. Exhale diving is not designed for static, its designed for diving.

That said, your graph looks like what I conceive is happening to c02 level(with 02 level being the flip side) when exhale diving. It matches my experience and how it feels.

Connor

Siku · Sep 24, 2012

baiyoke said:
..in absolute measures, CO2 must still be much higher at end.

Your missing something though, Baiyoke. Absolute CO2 is lower at the end of an empty/FRC dive in the CORE because
(if I understand correctly):

a) Vasoconstriction means blood isn't circulating to the limbs and bringing CO2 back to the core and

b) DR means muscles are working anaerobically so generating less CO2

efattah · Sep 24, 2012

Blood CO2 is always lower on an exhale STATIC (not dive) because the ratio of gas:blood is lower. Since hemoglobin can both attach O2 and CO2 then the blood has both O2 storage and CO2 buffering, whereas the lungs have strongly unequal O2 storage and CO2 storage.

Forget analysis, just do a simple experiment; hyperventilate then do a full exhale static vs. a full inhale static. Drastically easier to BO on the exhale static. Even if you don't hyperventilate, use an oximeter; MUCH easier to reach critically low O2 levels on an exhale static vs. inhale static.

Now whether BO occurs earlier or later DURING A DIVE is a totally different story. Also blood CO2 during active motion is not so simple due to bloodshift considerations.

baiyoke · Sep 24, 2012

Allrighty Cdavies, then perhaps I'm getting closer to understand it...

Siku said:
Your missing something though, Baiyoke. Absolute CO2 is lower at the end of an empty/FRC dive in the CORE because
(if I understand correctly):

a) Vasoconstriction means blood isn't circulating to the limbs and bringing CO2 back to the core and

b) DR means muscles are working anaerobically so generating less CO2

Yes, my sentence can be misleading. I mean absolute CO2 level within each type of dive must be higher at the end (for both dive-types CO2 increases). But I understand that CO2 is absolute lower in exhale compared to full-lung. Wich is what the graf is showing also...

Eric, now things are getting more complicated What do you mean by this "hemoglobin can both attach O2 and CO2 then the blood has both O2 storage and CO2 buffering, whereas the lungs have strongly unequal O2 storage and CO2 storage." How are the lungs storage capabilities described? I would suspect "more oxygen, less CO2 buffer"-capabilities...? Right? That is why CO2 level increase more on full lungs right? Anf if someone had say 40 litres lungs, that person would overwhelmed by too high CO2 levels long before oxygen would run out, right?

If the above is rigth, then I think finally I get it (simplified), because that diffence i storage capability is then a very important factor...

EDIT: Oh wait.. If the CO2 inreases much faster on exhale, it's because CO2 cannot go to the lungs as a storage place as usual. On a full-lung apnea the lungs would absorbe a lot CO2, and hence the small/slower DR. Now this is where things don't add up to me...

Or is it like this on full lungs: The lungs absorb some CO2, and therefore DR is slow/small, but this is outweighted by the huge O2 amount in the long run... Wich then makes a breathhold with small DR, high CO2 in the end, and overall big O2 expense thruout the dive..?

growingupninja · Sep 24, 2012

efattah said:
Forget analysis, just do a simple experiment; hyperventilate then do a full exhale static vs. a full inhale static. Drastically easier to BO on the exhale static. Even if you don't hyperventilate, use an oximeter; MUCH easier to reach critically low O2 levels on an exhale static vs. inhale static.

Good advice; this was my experience with exhale (complete exhale and FRC vs full lung). I think it is worth mentioning that DR is highly individual, as are DR triggers. I do some FRC and exhale training... DR comes on about 50% quicker in tests I've done on myself, but besides speed of onset, I seem to get about the same DR whether I am full lung (provided I begin the dive very slowly and/or spend some time in freefall) or empty. Best case for me is a solid DR while I still have fully saturated blood O2 and plenty of air in my lungs, but I am not diving to depths where DCS is an issue, at least after a single dive. Many people however report a staggering DR on exhale holds but not much happening on full lung holds.

cdavis · Sep 24, 2012

Right with'ya ninja. DR response is VERY individual.

Quick onset of DR is what I strive for first, along with as much air in my lungs as is consistent with the first. For me, the two together result in the longest, most comfortable dives. I get there best at about 60 percent of full lung and a minimal breathup style.

Connor

efattah · Sep 26, 2012

You need to differentiate between:
- QUANTITY of CO2 in the body (# of molecules)
vs.
- CONCENTRATION of CO2 in the body (# of molecules per unit storage area)
vs.
- RATIO of Quantity CO2 to Quantity O2 in the body
- RATIO of Concentration CO2 to concentration O2
- RATIO of Quantity CO2 to concentration O2
etc...

When you say 'CO2 goes up faster' it is not clear if you mean the quantity of CO2 or the concentration of CO2.

It is easy to BO on an exhale static because the concentration of CO2 does not reach high levels by the time the O2 concentration falls to blackout levels.

Siku · Sep 26, 2012

efattah said:
Blood CO2 is always lower on an exhale STATIC.

Surely if the lung volume is smaller then any given amount of CO2 offloaded from the blood into the lungs will represent a higher proportion of the total gas there, i.e. a higher concentration, leading to slower offloading of the blood CO2, leaving greater quantities of CO2 in the blood as the hold progresses. No?

baiyoke · Sep 26, 2012

cdavis said:
Right with'ya ninja. DR response is VERY individual.

Quick onset of DR is what I strive for first, along with as much air in my lungs as is consistent with the first. For me, the two together result in the longest, most comfortable dives. I get there best at about 60 percent of full lung and a minimal breathup style.

Connor

That is interesting, because it indicates a "sweetspot" below full lungs... (depending on what you do, i.e. depth, pool, sta.).

efattah said:
You need to differentiate between:
- QUANTITY of CO2 in the body (# of molecules)
vs.
- CONCENTRATION of CO2 in the body (# of molecules per unit storage area)
vs.
- RATIO of Quantity CO2 to Quantity O2 in the body
- RATIO of Concentration CO2 to concentration O2
- RATIO of Quantity CO2 to concentration O2
etc...

When you say 'CO2 goes up faster' it is not clear if you mean the quantity of CO2 or the concentration of CO2.

It is easy to BO on an exhale static because the concentration of CO2 does not reach high levels by the time the O2 concentration falls to blackout levels.

Now it's getting way beyond what I'm trying to grasp right now. Let me ask these two questions then, to try keep it simpel, one factor at a time:

1) "hemoglobin can both attach O2 and CO2 then the blood has both O2 storage and CO2 buffering, whereas the lungs have strongly unequal O2 storage and CO2 storage." How are the lungs storage capabilities described?

2) When I exhaled completely, I quickly get an urge to breat. Why is that? (If = CO2 rises quickly, then why does it rise quickly on emptylungs?)

If anyone has a link to a thread they remember, where things are explained, maybe it's easier to post it, I will then read it (I don't know why search function haven't helped me).

I get a feeling, that this area of apnea is not very well undertstood... Is that the reason of the lack of a straightforward explanation. I know things can get very complicated quickly, but for example when talking about hyperventilation and blackout, it's easy to explain the basic, main factors behind it (Something like hyp. lowers CO2 dramatically, to a point, that it does not get high enough again to warn you before O2 is hitting BO-level). Is it not possible to give a similar straight forward explanation of why DR kicks in quickly on empty lungs????

Appreciate the efford though

baiyoke · Sep 29, 2012

Ok, with about 250 views, and nobody able to explain for 2 days now why DR is triggered quickly during exhale-apnea, it seems that perhaps nobody knows I guess... I have read many, many, many threads the past two days, and it seems plausible from those threads, that co2 is not the reason... So even if the graph I made further up gives a picture of something experienced during exhale apnea, it is probably wrongful... CO2 is not the trigger... Low O2 is more likely... allthough the picture is not clear. (Plus a lot of other triggers when submerged in water - temperature - depth/pressure - nasal receptors - etc.)

http://forums.deeperblue.com/general-freediving/95013-no-dive-reflex-warm-water.html#post881932

trux said:
Diving reflex is triggered by a number of causes, some of them mentioned by Growingupninja and X_yeti. First of all, the absolute temperature is not that important, it is rather the difference of the temperature (especially on the face) before the immersion, and during the dive. Then the fact of stopping to breathe alone is already a very strong trigger for the DR which can act alone without the other contributing factors. Then it is also the depth that amplifies the effect. And then, it is primarily hypoxia (low O2) that drives the DR stronger. According to some studies hypercapnia contributes too, but there was another study denying it.

The denial of CO2 as a trigger is a bit surprising, since it contradicts the experience of many freedivers. There was an interesting information from Johan Andersson about it in this thread, but frankly told I have still doubts that hypercapnia has no influence on the vasoconstriction and hence on the DR. Since the time of the discussion I found many studies describing closer the influence of CO2 on periferal vasoconstriction, so there is apparently a conflict in all those claims. I'd like to see a new study looking at this topic closer and more critically.

.

and http://forums.deeperblue.com/freedi...s/62899-hypercapnia-hipoxia-4.html#post625745

trux said:
I think that everyone agrees that carbon dioxide plays important or rather primary role in diving reflex, but reducing it just to that is definitely over-simplification. There are more factors playing a role - as shown in the laboratory experiment I already linked before, and which was written by a scientist who reproduced the experiments with thousands of students during 15 years, bradycardia is very well reproducible with very basic simple conditions: face exposed to cold/wet and apnea. And it works even if only one of the factors is present. Also artificially induced hypercapnia (without apnea) leads to bradycardia.

In the document, there is the following diagram displaying the mechanism of diving reflex in simplified form:

image credit © Physiology.org
(click for a bigger version)
Surprisingly it tells that PAO2 chemoreceptors are actually significantly more important for starting the diving reflex than pH and PACO2 receptors. That's in direct contrast to the breathing urge feeling and contractions that are more relying on pH and PACO2 chemoreceptors. Frankly told, it surprised me - from other documents I read I was under the opposite impression.

However, as shown not only in these experiments, but also in other researches (i.e. thesis of Dr. Lindberg, or researches on Ama divers), there are huge differences of the strength of the diving response not only depending on the basic conditions, but there are many other factors that play important role, and there are huge differences between individuals.

Apparently the diving reflex is strongly influenced also by stress (moderate stress helps, but extreme stress is counterproductive), and many other physiological, physical, and psychical conditions, but it was proved that the strength and extent of the diving reflex may be learned by experience and training (it is stronger at professional freedivers like Amas or at top competitors).

So you are certainly right that CO2 is a very important clue here, but telling that all the diving reflex, bradicardia, blood shift, vasoconstrictions, and also contractions are nothing else than hypercapnic reaction is on my mind little bit exaggerated. I do not think though that you meant it literally when you wrote it, and I am sure that you are well aware of the complexity.

And the perceived urge to breath is instead DR kicking in:

http://forums.deeperblue.com/general-freediving/95059-lung-stretching.html#post883688

growingupninja said:
..... limit due to low O2 blackout, it would be much easier to cross this threshhold when stretching on an exhale because you won't get a strong CO2 warning signal. Low O2 without high CO2 is not particularly uncomfortaable, although massive vasoconstriction and brachycardia may feel strange when it first comes on. I know my body adjusts quickly to the sensation though (within maybe less than a minute), and with continued exhale work I will feel pretty good even though my blood O2 sat is in the low 80's, etc. ...'

This explains also why BO is easier on exhale... Nice, Thanks

On top of that, some people write explicitly, that not much is known about exhale-apnea, because the EXHALE part has not been targeted by medical researchers etc.

So, for now, I have to conclude, that it is unknown (?) why DR is triggered quickly during exhale, but that it might have something to do with lowered O2, and perhaps many other factors... Stress perhaps... I would personally speculate, that the very feeling of less-than-full lungs, might trigger DR also... Maybe through receptors measuring lungvolume... It would make sense, because the body then starts to prepare for emergency, because of little volume...

Any comments?

cdavis · Sep 29, 2012

The phsiological details I would not want to speculate on, but for sure, contracting lung volume strongly contributes to blood shift, a major factor in DR. Start with small lungs and go down, pressure reduces lung volume farther and the simple physics of the situation creates a degree of blood shift. I think the body adds to the physics with vasoconstriction of the perifery.

growingupninja · Sep 29, 2012

Baiyoke, part of the reason you get a strong urge to breath immediately upon exhale is probably the 'empty' feeling in your lungs. Training, diving deeper, FRC diving, exhale stretches, etc, would make this a more comfortable state.

Perceived urge to breathe in my case is frequently DR kicking in very strongly--this I have seen in action while doing holds with a pulse O2 meter, but that is my experience and we do know that DR is experienced very differently among individuals, and 'urge to breathe' is often purely mental and very subjective--sometimes I experience strong DR very quickly on a full lung, without an accompanying urge to breathe, but only when immersed/swimming. I had never tested this in myself (measuring pulse and O2 blood sat) until I had already been diving and training regularly so in my case it may just be some benefit from training. The last time I tested, I was getting strong full lung DR on slow 50M dynamic swims in a swimming pool, which surprised me greatly since when I do facial immersion in swimming pool temp water, my numbers are pretty close to dry holds (very inconsistent or absent DR until truly hypoxic). This would speak to a certain level of CO2 triggering DR, but I also think the body just learns, and as Trux stated, temperature differential could have a great deal to do with it--when I have done facial immersion tests I am indoors, but tests in a pool were performed in sunlight when the air temp was probably mid eighties.

As far as DR being triggered by exhale, this too is variable; in some individuals it seems to be a stronger trigger than in others.

Siku · Oct 1, 2012

Can anyone explain why CO2 is lower on empty lung. This seems counter intuitive, as I asked a few days ago:

Siku said:
Surely if the lung volume is smaller then any given amount of CO2 offloaded from the blood into the lungs will represent a higher proportion of the total gas there, i.e. a higher concentration, leading to slower offloading of the blood CO2, leaving greater quantities of CO2 in the blood as the hold progresses. No?

Thanks

cdavis · Oct 1, 2012

Depends on what part of the body you are looking at.

As a simplistic description: On an exhale dive, done right, you start off with slightly higher level of c02 in the whole body and a smaller lung size. After the dive begins and before DR kicks in hard, there is less room to offload c02 into the lungs, so c02 level goes up fast and kicks in vaso constriction, DR, blood shift much quicker than during a full lung dive. At this point, co2 levels are still low enough to be reasonably comfortable(takes a little practice). Once DR is well established, exercise related c02 production is kept away from the core (and the c02 sensors), heart rate goes way down, producing less co2 in the core and so c02 levels in the core go up much more slowly than a full lung dive. This is why you can get such long comfortable dives out of exhale diving. Baiyoke posted a graph that pretty well illustrates the principal. I think it's in this thread.

Connor

baiyoke · Oct 1, 2012

Cdavis, I concluded in my last post that the graph I posted is probably wrongful as a model of "CO2-build-up", but most likely more a model of "perceived urge to breath".

In all respect I agree with what you very well describes above, except one thing: I don't think CO2 is the trigger of fast DR in exhale as explained in my last post. And I must say, that even if CO2 unloads slower into lungs because they are smaller, I can't see how it could rise SO FAST... I belive (for now) that CO2 only rises in just about the same pace, or slightly faster as on full lungs.

But this is in line with what you describe above, except that it can't be CO2 that is the trigger for DR, but instead the other things mentioned in my post before. That's my humble opinion until explained otherwise...

cdavis · Oct 1, 2012

No respect needed. I just stumbling around in this stuff, trying to figure out what works best.

"Perceived urge to breath" is a reasonable way to look at that graph. Perception is affected both by actual c02 level at the sensors in the chest and by getting used to the feeling. Taking down a bit more c02 enables the diver's body to "perceive" higher co2 and help the body into DR

Early DR kick in is a combination of small lung size, c02 level, little exercise in the early part of the dive, and, I think, the residual blood shift left over from previous dives. There are probably other factors as well; it all works together. Getting the most out of exhale diving is an exercise in balancing these factors, among others. Balance requires practice and will probably be different depending on what you are seeking out of exhale.

From experience,I'm pretty sure about c02 contributing to DR. Other things being equal, there is a substantial improvement in dive time when you take down a bit more co2 than a full lung dive(and practice enough to get used to the different feeling)

EricF made a good point earlier, forget the theory and go test. Most of this stuff is very testable, especially if you have a pool available with 4 meters or more of depth.

Connor

Siku · Oct 1, 2012

Connor

Thanks for the answer, it makes sense and is in fact exactly what I posted above, see post #5 in this thread.

But what is confusing me is Eric's comment:

efattah said:
Blood CO2 is always lower on an exhale STATIC (not dive) because the ratio of gas:blood is lower.

Again, surely if the lung volume is smaller then any given amount of CO2 offloaded from the blood into the lungs will represent a higher proportion of the total gas there, i.e. a higher concentration, leading to slower offloading of the blood CO2, leaving greater quantities of CO2 in the blood as the hold progresses.

Remember Eric is talking about a static at the surface, so I'm not sure DR is a factor. Or maybe it is. Eric?

What's going on? Exhale, FRC, BO, CO2, safety.

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