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What really happens in SWB?

J Campbell

Well-Known Member
Sep 17, 2001
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OK. So there is this guy at the pool where I swim who is a retired Navy doctor. He used to take care of divers back in the 1950s. He says that death is caused in shallow water blackout because the low oxygen causes the heart to spasm and the person dies of heart failure. I told him , no, they usually just suffocate. But him being the doctor "expert" (50 years ago) won't listen to me. So what is the real mechanism of death during shallow water blackout - suffocation or heart failure, or something else?
 

hank49

Member
Jun 25, 2010
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If the heart fails, causing death, at what point does it fail? As long as one gets to the surface before inhaling water, they will very likely recover.
I had a black out victim in my arms during the PFI course. He didn't die He woke up and asked me why I was holding him. I said, "because I love you, son".
 
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Kars

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Oct 24, 2003
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I'm no doctor, don't have any degree's, but I suspect the only person to turn this doctor around is either a famous or higher qualified doctor or he himself. You're just a layman, too stupid to understand his level of magic.
The only small chance you have is go through the details of the process step by step by asking him how parts work. There will be contradictions (failing logic), don't pass judgement, but give trust to him to explore alternatives.

Considering your SWBO question. I believe it's like you said, simple suffocation. After the BO starts the diver sinks down (or floats head down) and the body eats up the remaining O2 until all processes have stopped, heart and brain flat-line. I'm not sure which one stops first.

Old minds are often like old bodies, shrunk and stiff.
 

cdavis

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Jan 21, 2003
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probably neither, per se.

Usual process: the B0 victim loses consciousness, laryngospasm sets in, that lasts for a time during which the diver is still alive and can be resuscitated. At some point, the urge to breathe overcomes the larynospasm( or, at least the laryngospasm relaxes) and the diver inhales water, curtains. Did he die of heart failure, suffocation or the effects of breathing water? Does it matter?
 
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Simos

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Feb 15, 2009
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Sorry don't really get it, when you die of 'heart failure' or 'heart attack' you still 'suffocate' so what's the difference? Ultimately there is a point beyond which the brain suffers unrecoverable damage because of no oxygen...

I guess the question is more one of cause and effect because i would take a guess that he is right in that the heart would also stop at some point and 'fail' if you are really low on oxygen - I would guess that it's probably unlikely that the heart will keep beating after your brain was destroyed by not having oxygen.

The only difference I guess is what happens if you are brought up in the surface - if you've blacked out for long enough, your heart will have stopped, hence why sometimes CPR is needed to revive someone. If you haven't blacked out for long enough, the heart is still beating and one has the ability of recovering by themselves by resuming to breathe without aid and circulating O2.

So I guess you are both right lol I would guess that heart failure is probably the ultimate desperate attempt of the body to conserve oxygen....
 

Deano 647

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Jul 29, 2019
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I'm only a new freediver (40m PB in CWT) but I come from a tech diving background and have had a long held amateur interest in decompression and hyperbaric effects on humans. In my short experience talking to freedivers, they of course know the term of shallow water blackout, but I'm not sure there is a wide understanding of the actual difference between a straight forward hypoxic blackout, versus one resulting from the mechanism of falling PO2 driven by reduced hydrostatic pressure. I actually writing a piece on this and humbly offer the following for 'peer review' by the experts on this forum. I would appreciate all feedback, especially clarifications/corrections. Bear in mind this is to be published in a magazine for tech and cave divers, hence some of the explanatory notes.

********************************************************

Since the ‘bad old days’ of scuba diving many long-held practices have rightly fallen into disrepute. Being good at Deep Air was once a badge of courage and toughness, whereas today you’d be ridiculed for suggesting say a dive to 100m on a single 80cuft bottle. I learnt to plan an ascent utilising US Navy tables which were based on decades old data and long before the first wearable dive computers (Yes I am THAT old) Today we can pick and choose our algorithm of choice and tune it to our liking.

Likewise we now have a better understanding of how breath hold diving works in practice though there is still much to learn. Indeed, getting hard data on freediving is much more difficult than compressed air diving as the commercial imperative to fund solid scientific research on freedivers simply is not there. With compressed gas, there was great interest from the various fields of military, scientific, commercial and recreational communities to know with a high degree of certainly whether a certain dive profile could be conducted safely. So funding research had an easily justifiable ROI (Return On Investment).

Freediving however is more of a niche recreational activity and it’s a tough sell for research scientists to prove an ROI on expensive studies for a recreational pursuit like freediving, other than for reasons of pure research. Nonetheless progress is being made [quote example papers and work by DAN Europe]

Hyperventilating for instance, once a mainstay of breath-hold diving is now understood to be not only counterproductive but fatally dangerous. Counterproductive, because the boffins now understand that not only do elevated levels of CO2 drive a stronger MDR, but the presence of elevated levels of CO2 in the blood trigger the body to release extra oxygen molecules that would otherwise be still bound to the haemoglobin, thus realising an increase in diver performance. Sure, increased CO2 and the resultant contractions don’t feelvery nice. But like a lot of things in life, sometimes those things which is good for us in the long term, can cause us discomfort in the short term!

But the most deleterious effect of hyperventilation is far more sinister. Humans have evolved so that under normobaric conditions in the average person, the rising levels of CO2 reach a point where the urge to breathe becomes undeniable beforeoxygen levels fall to a point unable to sustain consciousness.

By hyperventilating before the dive, the diver blows off CO2, artificially lowering the starting point from which the CO2 starts to build. But the hyperventilation does not result in any big increase in O2. So, as your O2 level drops as per usual towards the danger zone, the CO2 is building from a much lower base. Since the vast bulk of urge-to-breathe comes from the level of CO2, there is every chance you will ‘feel’ fine even though your O2 levels are perilously low, you haven’t yet got that big urge to breathe and you may black out.

All the above in relation to hyperventilation is still true in a pool, without any depth change, but diving vertically to depth introduces ambient pressure changes, ramping up the danger logarithmically by the insidious effect of partial pressure. Air of course is largely comprised of O2 and N, but the nitrogen can be dismissed in this discussion so really, it’s all about the O2. Nitrogen as an inert relates to DCS for sure, but not applicable to this discussion of Shallow Water Blackout (SWB).
PP02 is a subject well understood by tech divers but at this early stage of free dive instruction, I don’t think it as universally understood by students or perhaps even some instructors, at least not to the extent that we do as tech divers. In my experience, there is tendency to lump it all under ‘hypoxia’ without a proper appreciation of the crucial difference between normobaric hypoxia resulting from a simple denial of urge to breathe, versus SWB, driven by falling hydrostatic pressure and resultant PO2.

So, consider a breath hold dive to depth after hyperventilation. The CO2 floor has been lowered reducing the urge to breath but oxygen levels start from more or less the same point and are consumed as usual. BUT, at say 30m the PO2 of 21% is under 4ATA of pressure, resulting in a PO2 of 0.84. It actuality, it is never that high by the time the freediver has arrived at 30m because unlike SCUBA, the freediver is metabolising the O2, from the second they leave the surface, but for the purposes of this discussion let’s leave the numbers at that.

As the freediver who hyperventilated is at depth consuming that O2, the higher concentration or PP02 driven by the partial pressure allows them to function up to a point. But as they return to the surface, the falling hydrostatic pressure reduces the PO2, most dangerously in the final seconds of the dive when the diver Is the most stretched on their breath hold. Between 10m and the surface the PO2 drops by a whopping 50%, when they are only 10 seconds from safety. All of this, possibly without yet a single strong recognisable urge to breathe because of hyperventilation.

Note that the following numbers are not scientific but anecdotal, to illustrate the problem. Even if our diver arrives at 30m with an effective arterial PO2 of .84 he hangs out for a while, or even if he turns the dive and commences his ascent, he has consumed a large % of his accessible O2. How much by the time he leaves the bottom … maybe 25%? Which means they will need at last 25% more to get bac to the surface. In practice this might be more as the effort to get down is less than the effort to fin up, against the negative buoyancy at depth. But if we say a consumption of 50% during the dive, then has he approaches the surface his PO2 of 0.84 if half consumed at depth would leave the diver with an arterial PO2 of 0.42 on the bottom. But let’s break down the ascent and PO2 in stages of ascent.

30m/4ATA 0.42
20m/3ATA 0.32
10m 2ATA 0.21
0m/1ATA 0.11

Since the average person cannot endure a concentration of O2 lower than about 13%, that diver is in danger of blackout.

It will not surprise you then to learn that the clear majority of blackouts occur in the last 10m, and indeed on the surface. After ‘completing’ the dive. This phenomenon explains almost all breath hold deaths in the ocean, of which in the USA there are 50+ per year. Almost all of those fatalities are untrained divers such as long time spearos who scorn structured training, do not follow ‘one up, one down’ freediving practice, and fail to perform basic safety procedure such as meeting buddy at 6 – 10m then observing them for minimum of 30 seconds post-dive. Those of you familiar with the early deaths in cave diving n the 60s and 70s will immediately note the similarity of untrained divers entering an environment, about which they don’t know, what they don’t know. Truly a recipe for disaster and frustratingly easy to avoid if simple protocols were followed which have been categorically proven to almost eliminate than chance of a fatality.

Talking to survivors of shallow water blackout, the overwhelming consistency in their recounting is they had ZERO warning of the event. They felt fine right up until they blacked out with no recollection of the incident. As yours truly has experienced what was later explained to me as a 20 second blackout in pool training, I can likewise assure you, there was no panic, or impending sense of doom. I felt stretched for sure with a very strong urge to breathe, but as I got within 5m of the wall, I felt “Yeah, I got this”. The next thing I recall was being on my back over the lane rope with people saying loudly; “BREATHE DEAN!, BREATHE DEAN!” Wow, what just happened ? Absent a buddy/ trained in how to recover a blacked out diver, I was dead.

As WKPP pioneer George Irvine was fond of saying; “Diving solo removes any second chance you ever had” Wiser words have never been spoken whether recreational, tech, cave, OC, RB or freediving. We must respect the fact that whilst we retain a biological connection to our ancient aquatic past, that is not our native domain. Any journey into water, whether by compressed gas or breath hold, must be treated with the full respect for our underwater domains.

As it is with technical and cave diving, we freedivers owe it to those payed with their lives, to learn from their mistakes and dive safely in the future.
 
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cdavis

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Jan 21, 2003
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Nice job and well written. The last several paragraphs describing what happens on ascent get a little long winded. Maybe thats because I know where you are going and the science behind it, but I suspect your audience will have a similar reaction. The last 3 paragraphs are great.

While I completely agree with everything you write, especially for the vast majority of divers, there is some strange stuff out there that the above doesn't explain. I know personally two divers who survived BOs unaided. check out Amphibious's account on DB. The other is one of my longest and best dive buddies. Also, extreme, I mean really extreme hyperventilation appears, in some cases to be beneficial to long deep dives. Check out https://forums.deeperblue.com/threads/the-actual-impact-of-low-co2.111310/ That is even wierder to me, but seems to be a solid result, supported by history of crazy levels of hyperventilation by Mediteranian freedivers of the past.
 
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hteas

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Mar 9, 2005
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Deano, not sure that you want to add this, but there are 3 avenues to drown yourself freediving, Your explanation of a standard blackout is great, but there are 2 others, drowning, and blood pressure loss. Drowning occurs when you simply don't come up when you hear the surface calling. That is what happened to you in the pool. There was no sudden change of partial pressure, you simply kept pushing through the contractions or whatever. The other is blood pressure related. When you have a strong dive reflex, your blood pressure may hit 300/200 early in a dive, according to an old article from a DAN Symposium. This happens because your heart is pumping (blasting) blood to only a limited volume, your torso and head. Your arms and legs have been abandoned. Exhaling as you reach the surface, or shortly thereafter, releases blood to those starving muscles. The result can be a sudden blood pressure loss blackout. Mine was as I reached the surface after a long hard dive. A friend, during a world record attempt, lasted a couple of seconds after reaching the surface before she sank back into the water and the arms of one of her safety divers.
 

cdavis

Well-Known Member
Jan 21, 2003
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Good points hteas, reminds me I need to do hook breathes every dive. I got out of that habit because half lung diving doesn't produce as strong an urge to breath as full lung.
 

Bill McIntyre

San Clemente, CA
Staff member
Forum Mentor
Jan 27, 2005
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You guys know a lot more about this than I do, but I just want to make a comment. Everyone says that they had no warning of a blackout. Is it at least possible that they had a warning but don't remember it?

Many years ago I had a head on collision with another cyclist. The top tube of my bike was bent and the foam in my helmet was crushed. That was in the morning, and the first thing I can faintly recall was waking in the hospital that night. I had no recollection of the crash. But the guy I hit didn't get a concussion, so he says that I was coming fast downhill around a left turn with high brush on the inside of the turn obscuring my view. He was coming the other way. We saw each other, and I swerved right while he swerved left and we collided. It seems pretty obvious that I had a warning but I didn't remember anything about it. A nurologist explained that the memory was in the short term memory bank of my brain and wouldn't stick until it was passed to the long term memory bank, and the collision happened before that could happen.

If that's true, could it be possible that blackout victims also had a warning, but don't recall it because it wasn't in their long term memory bank?

If anyone can put better names on my "memory bank" terms, feel free to do so. I know I've read more precise descriptions of brain function, but I don't remember them.

Edit- an afterthought. When I first started diving as a teenager in the early 1950s, no one knew anything. I hyperventilated like mad. Very often before and after I surfaced I saw stars and was dizzy and I thought that was just normal. I'm probably lucky that I never quite blacked out, but it seems to me I was getting warnings. I remembered them because I never quite blacked out.

Then I was almost exclusively a scuba diver from age 15 to 57, when I resumed freediving. By then there was more knowledge available and I was old enough to be cautious, so I can recall just one instance when I was probably close to black out. I friend had a fish wrapped up in kelp on the bottom and asked me to cut it out. By the time I did it I knew I was in danger. On the way up my legs got numb and I got light headed. When I surfaced I was dizzy and saw stars but I didn't black out. So it seems to me that I did get a warning and I remembered that warning because I didn't loose consciousness. Does that make sense?
 
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cdavis

Well-Known Member
Jan 21, 2003
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You guys know a lot more about this than I do, but I just want to make a comment. Everyone says that they had no warning of a blackout. Is it at least possible that they had a warning but don't remember it?

Many years ago I had a head on collision with another cyclist. The top tube of my bike was bent and the foam in my helmet was crushed. That was in the morning, and the first thing I can faintly recall was waking in the hospital that night. I had no recollection of the crash. But the guy I hit didn't get a concussion, so he says that I was coming fast downhill around a left turn with high brush on the inside of the turn obscuring my view. He was coming the other way. We saw each other, and I swerved right while he swerved left and we collided. It seems pretty obvious that I had a warning but I didn't remember anything about it. A nurologist explained that the memory was in the short term memory bank of my brain and wouldn't stick until it was passed to the long term memory bank, and the collision happened before that could happen.

If that's true, could it be possible that blackout victims also had a warning, but don't recall it because it wasn't in their long term memory bank?

If anyone can put better names on my "memory bank" terms, feel free to do so. I know I've read more precise descriptions of brain function, but I don't remember them.

Edit- an afterthought. When I first started diving as a teenager in the early 1950s, no one knew anything. I hyperventilated like mad. Very often before and after I surfaced I saw stars and was dizzy and I thought that was just normal. I'm probably lucky that I never quite blacked out, but it seems to me I was getting warnings. I remembered them because I never quite blacked out.

Then I was almost exclusively a scuba diver from age 15 to 57, when I resumed freediving. By then there was more knowledge available and I was old enough to be cautious, so I can recall just one instance when I was probably close to black out. I friend had a fish wrapped up in kelp on the bottom and asked me to cut it out. By the time I did it I knew I was in danger. On the way up my legs got numb and I got light headed. When I surfaced I was dizzy and saw stars but I didn't black out. So it seems to me that I did get a warning and I remembered that warning because I didn't loose consciousness. Does that make sense?
Makes a lot of sense. The general opinion seems to be that there is a loss of memory just before a B0, so the diver would not remember a warning. That is not always true. At least oneB0 I was involved in, the diver did not lose any memory. We know because he had a camera running and remembered it all except the B0 itself. I had him up, conscious and breathing in only a few seconds, so maybe thats a special case. That one also had zero warning. Absolutely nothing, a normal dive, well within the divers capability. Still scares me spitless.

Bill, I also had the stars thing a couple of times as a young diver, combining very short surface intervals with hyperventilation and staying down as long as possible( lots of fish around). We are both lucky to still be here.
 

Deano 647

New Member
Jul 29, 2019
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Hi Hteas, thanks for suggesting a clearer explanation of the different mechanisms to BO. I think that is very worthwhile.

1. Simple (sic) hypoxia. Unrelated to pressure effects. Could be (and is done regularly) on dry land.

2. Hyperbaric driven SWB. Requires depth-driven rapidly falling PO2/Ambient water pressure.

3. Hypovolemic hypoxia, exhale induces a final critical reduction in material PO2 and BO occurs. In some way 2 and 3 are related. Arguably 3 is the final state of 2, but nonetheless well worth defining. Thank you ! And as Davis says, 3 suggests a continued use of hook breathing to maintain intra-thoracic pressure in those critical post surfacing 10 seconds to drive re-oxygenaion of the brain.

When you have a strong dive reflex, your blood pressure may hit 300/200 early in a dive, according to an old article from a DAN Symposium.
My understanding is that heart rate increases in breathe up, but decreases almost immediately upon immersion and descent. (as a result of MDR) Accompanying effect of the MDR is reduced heart stroke volume, created by the immersion effect. This is an adaptation to the increased venous return. i.e., as more blood returns to the heart, compared to when the body is not immersed in water, the body's homeostatic reaction is to reduce the amount of blood pumped with each stroke aka reduced VOLUME (as well as bradycardia driven by MDR) aka reduced Heart rate (SPEED). My understanding is these are two complimentary and simultaneous reactions, but not related. Reduced stroke volume from immersion happens to scuba divers and freedivers. too) But the bradycardia (driven by MDR) is NOT experienced by Scubadivers.

Can anyone confirm that as I have positioned it above is true, or have I misinterpreted ?
 

MarcinB

Well-Known Member
Oct 26, 2012
255
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When you have a strong dive reflex, your blood pressure may hit 300/200 early in a dive, according to an old article from a DAN Symposium.
These values come from a study done in a hyperbaric chamber and are greatly overestimated due to factors related to pressure sensor construction which was proven in a subsequent study.

Exhaling as you reach the surface, or shortly thereafter, releases blood to those starving muscles. The result can be a sudden blood pressure loss blackout.
In all studies I've seen blood pressure either decreased gradually when the breath hold was over (in the case of static apnea) or even continued to rise for a few seconds (dynamic apnea) before starting to decrease. In my opinion this mechanism is not a major factor contributing to blackout considering the fact that brain receives well oxygenated blood ~10s after the first inhale when blood pressure is still much higher compared to the baseline value.
 

cdavis

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Jan 21, 2003
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Hi Marcin,

Curious as to how the mechanism actually works. I dive half lung and have a very strong DR as a result. When I start breathing again, I can feel blood shift relaxing and my urge to breathe goes way up after I start breathing. It feels like blood shift relaxes pretty fast. Seems like that could lead to a sharp drop in BP, even if heart rate picks up. What is your take on this?
 

MarcinB

Well-Known Member
Oct 26, 2012
255
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Bialystok/Poland
Arterial blood pressure is a product of total peripheral resistance (TPR) and cardiac output (CO). Looks like increasing CO is able to compensate the drop in TPR (HR rises very quickly when you start to breath again, the same is true for stroke volume due to a drop in chest pressure). Take a look at the graphs below (taken from Sivieri et al. Eur J Appl Physiol. 2015 Jan;115(1):119-28., Perini et al. Eur J Appl Physiol. 2008 Sep;104(1):1-7, and Palada et al. Respir Physiol Neurobiol. 2008 Apr 30;161(2):174-81.)
1573217935263.png
1573217994193.png
1573218494247.png
 
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Nathan Vinski

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Apr 19, 2015
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Hi Marcin,

Curious as to how the mechanism actually works. I dive half lung and have a very strong DR as a result. When I start breathing again, I can feel blood shift relaxing and my urge to breathe goes way up after I start breathing. It feels like blood shift relaxes pretty fast. Seems like that could lead to a sharp drop in BP, even if heart rate picks up. What is your take on this?
Was just reading through and thought this was interesting.. When im in 'top-shape' for deep diving and do no-warm-up, my first urge to breath comes 10-15 seconds after surfacing. The first 5 breaths are 'forced' (don't feel like I need to breath).

On +90% dives I also notice a very significant increase in mental clarity in the last 25m when this happens (again.. only in top-shape and on no warm-up, so maximum DR..). In not-so-top-shape I feel a bit foggy in the last 15m, but also get the urge to breathe at about -30 -25..

I've not seen many people report increased urge to breath after starting to breath, but it's a very consistent thing for me at the end of a serios training camp...
 

Laurawebster

New Member
Sep 19, 2019
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I remember when my uncle died due to heart attack, suddenly all of his sons, started caring about aunty, and this might be the case of wanting a part of an estate and assets. But uncle has already planned about the estate by talking to an estate lawyer by knowing all the trusts and estate planning lawyer.
 

cdavis

Well-Known Member
Jan 21, 2003
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Was just reading through and thought this was interesting.. When im in 'top-shape' for deep diving and do no-warm-up, my first urge to breath comes 10-15 seconds after surfacing. The first 5 breaths are 'forced' (don't feel like I need to breath).

On +90% dives I also notice a very significant increase in mental clarity in the last 25m when this happens (again.. only in top-shape and on no warm-up, so maximum DR..). In not-so-top-shape I feel a bit foggy in the last 15m, but also get the urge to breathe at about -30 -25..

I've not seen many people report increased urge to breath after starting to breath, but it's a very consistent thing for me at the end of a serios training camp...
Nathan, my pattern is a bit different. When I come up, I definitely have a pretty good urge to breathe. The first breath feels like a normal recovery breath(as if I was full lung diving), sometimes the second also. After that I'm panting, can't get enough air. Its a function of half lung diving. My understanding is that relaxing blood shift brings hi c02 blood back to the core and causes the sudden increase in urge to breath. I thought it was pretty weird at first, but other exhale divers told me it was what to expect and I'm used to it now.
 

cdavis

Well-Known Member
Jan 21, 2003
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684
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Sarasota, Fla
Arterial blood pressure is a product of total peripheral resistance (TPR) and cardiac output (CO). Looks like increasing CO is able to compensate the drop in TPR (HR rises very quickly when you start to breath again, the same is true for stroke volume due to a drop in chest pressure). Take a look at the graphs below (taken from Sivieri et al. Eur J Appl Physiol. 2015 Jan;115(1):119-28., Perini et al. Eur J Appl Physiol. 2008 Sep;104(1):1-7, and Palada et al. Respir Physiol Neurobiol. 2008 Apr 30;161(2):174-81.)
View attachment 55103View attachment 55104View attachment 55105
thanks Marcin, I need to study this.
 
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