DCI and freediving

[Fitz-Clarke]

I’m not sure what you are saying Seb. I think it has been fairly well established that blood flow to the brain increases by a small amount during both BH dives and prolonged apnea. The assumption is rising carbon dioxide levels cause cerebral artery vasodilation. That’s why I said the diving response doesn’t protect the brain from DCS. This considers only the perfusion aspect.

If you lower brain temperature more than 4 or 5 degrees you will have significant cognitive decline. Nitrogen solubility increases with cooling by about 0.7 to 0.9 percent per degree (1), which translates to maybe about 5 percent change. Not a very big effect over that temperature range. How this difference might influence bubble nucleation or growth and translate to DCS risk is not clear, but I wouldn’t anticipate a big difference.

Regardless, I don’t see how you could significantly change brain temperature over the course of a dive of 3 to 4 minutes. I would be interested to hear more. Do you have some data?

Reference:
(1) Weathersby PK, Homer LD. Solubility of inert gases in biological fluids and tissues: a review. Undersea Biomed Res. 7(4):277-96;1980

 

[sebastien murat]

What I was getting at was the fact that the brain is NOT exempt from the DR....it doesn't merely benefit from it, it contributes to it. Blix's group showed that brain cooling is a component of the DR in animals. The (to be published) results we obtained on myself showed that that this seems to be an evolutionarily conserved response. Perhaps not surprising really, since considering the brain's disproportionately elevated O2-consumption and allotment of cardiac output in apnea, any mechanism (e.g., cooling) that would suppress its O2-consumption rate and increase hypoxia tolerance would significantly prolong survival. One of the misconceptions about cerebral vasodilation, at least in apnea, is that it merely maintains cerebral oxygenation, i.e., the brain merely benefits from the DR. But if you view the brain as being an active player in the DR then it opens up a whole bunch of new possibilities, e.g., suppression of shivering thermorgenesis and the associated increase in O2-consumption; this was reported in animals, we reported (unpublished except in this forum) similar effects which were corroborated by others (in this forum).

Further, as you're no doubt aware, Fahlman showed, the DR would limit N2 uptake. Is there a better way to juggle and manage strictly limited and diminishing O2 stores and DCI in the face of such a dire situation as apnea diving, as by cooling? Seems not (?)

I did not have a noticeable cognitive decline with brain cooling of 1, 2, 3, 4 or 5 degrees; I was slow and speech was impaired, but I could promptly follow instructions and run the experiment on myself for as long as half an hour. It's possible that maximal cooling will only occur in the unconscious state (?). I haven't rendered myself unconscious, so can't say. Importantly, this is not a hypothermic response but a fully orchestrated bradythermic state. Further, we monitored regional temperature differences, i.e., these levels of brain cooling were not global. Hypothermia, i.e., loss of thermoregulation, has an undifferentiated thermodynamic effect on all processes, i.e., there would be homogenous global cooling. I suspect, but do not know, whether such a level of global cooling in hypothermia would result in cognitive decline. Results in hypothermic rats show that the decline occurs at about 30C.....but rodents are not humans and have some rather unique qualities in regards to tolerance of certain conditions.

In regards to your question: how could brain temp (unbelievably) drop so quickly? I have only theories. Its probably a combination of things, but either way, it seems that nature still has much to teach us. I'm aware of the bio-heat equation and have looked at this and I think there's more to it than that (maybe).

 

[sebastien murat]

Here's the reference on those hypothermic rats (for completeness) Fitzy:

Moser, E. I. and Andersen, P. (1994). Conserved spatial learning in cooled rats in spite of slowing of dentate field potentials. J. Neurosci. 14, 4458-4466.

 

[rafacuatic]

After reading this thread, I have some questions for the experts based on my experience with the DCS
Do people who have suffered DCS are more likely to suffer more?
What type of injury can occur after a dcs in the brain, some type of necrosis, so that, in a case of having another dcs, will repeat the same symptoms more easily, as neural symptoms?
For me, and in some cases, the same depth on several dives, finished with DCS symptoms by increasing the ascend speed, regardless of the amount of air in the lungs.
In my experience, surface interval over successive descents and ascend speed are crucial to avoid dcs, and may influence cold, tiredness,etc
It would be good for people to share their experiences with dcs to avoid it in the future.
Thanks

 

[sebastien murat]

I think you need input from clinicians, e.g., spaniards, french and italians.They get plenty of cases. But, if you burn the toast, how can one expect ever it being fresh again!

 

[growingupninja]

Re: HR while diving as a way for instructors to quickly identify students who might be extremely susceptible to blackout. There is guy I have trained with in the pool who is a beginner and to my knowledge he has ended any max dynamic in samba, and usually at a distance that seems short given his reasonable fitness level. He says he does not experience contractions except had one once right before samba and he doesn't seem to be hyperventilating. I have wanted to see what his pulse looks like during a breathhold. I suspect he doesn't experience hypoxia/hypercapnic DR.

This fella, he sounds like he has a high metabolic rate all the way through (=> poor DR), in which case stimulation from hypercapnia (=> contractions) is greatly depressed, because CO2 is very soluble in peripheral tissue, or like you suggest he's CO2-insensitive...either way not good...but HR monitoring should id the problem.

I just wanted to follow-up, re: diver who is samba prone. I hadn't talked to this guy in a while, but he's been wanting to push his limits a little further in the depth department (I think his current PB is around 70') and we go to talking... so I ended up in ocean with him last weekend. I was not super-enthused with the prospect of acting as his safety on a max attempt given his history, so I asked to monitor him on an pulse O2 meter before we even got in the water....

Results were interesting; once we got to actually talking about it, he explained that he has asthma, primarily 'exercise induced', and a lung capacity very much on the smaller end of the scale. Pulse oxy test seemed to reflect this (I'm not a doctor but makes sense).

Basically, we walked down a steepish trail to the water, and he stuck his finger in the meter and started a breath-up. What was interesting to me was that his pulse was 115bpm, more or less reasonable since we'd just walked down a steep hill and he was a bit nervous. But his blood oxygen was only about 94-95%. This on the low end of 'at rest' normal, but I know in myself if my breathing/pulse is even slightly elevated my blood oxygen would be instantly saturated. It took him close to a minute of breath-up before his blood O2 got up to 97-98%. I assume this is the asthma.

Once he actually started his breath-hold, his blood O2 as shown in the meter began to drop within about 30-45 seconds, and once it began to drop, it fell pretty fast. He aborted the hold when he got to around 90%. I assume the rapid fall-of was the result of lung capacity. His HR during the hold did drop significantly and was in the 50's when aborted. Also I was seeing some vasoconstriction in the read-out.

I suspect that his body is accustomed to running on low(ish) O2 due to the asthma, and perhaps higher CO2 for the same reason. So he feels hypoxia later than the average diver, at a point when his blood O2 is already falling quite rapidly.

 

[sebastien murat]

People are like that: when they get desperate some will have no hesitation jeopardizing other people's safety just so they can get to do their thing. Can't be trusted, doesn't have your back ...ditch him, presuming he's not some teenager still working it out!

Anyway, the issue at hand: his asthma + approach is resulting in the worse possible alignment of conditions for tripping a BOs and risk of drowning

- Low lung volume = low lung O2 stores => earlier tapping into blood O2 stores => earlier desat.
- If airways readily collapse, which they would in asthmatics, he'll hardly be able to access lung O2 stores (probably gets worse in-water and at depth; so-called, physiologic shunt) => more rapid desat
-Swims down from outset (presumably) => attenuated DR (HR in the 50s no good) => high burn rate + increased risk of DR reversal during ascent (because of high HR, i.e., not really in DR mode, just exercise mode).
-On the CO2 side, it's also double-edged: the effect of low lung volume, swimming, lung collapse => `relative´ down-shift in hypoxia reserve safety margin (CO2 build-up delayed; = lower hypoxia tolerance)

Remedy: nothing except tools to maximize DR, but if DR not at least in low 40s still not very optimistic.

 

[growingupninja]

Thanks for the response, Seb. Not so much worried about my own back as I am not really pushing anywhere near limit on those kind of dives and outside of competition ALWAYS dive as if I am entirely alone, but I have never really been able to accept BO and samba as normal like many freedivers. But while I've hauled fish up, I've never drilled deep water BO rescue of a person....

The day we went out I actually forgot my long fins so we kept it super conservative, but worked a bit on technique, duck dives and such. Seems like better overall fitness/technique would help maximize DR since it would mean less effort during work phases.

I have always scoffed at a 2:1 surface interval as being ridiculously conservative unless DCS is a risk due to depth, but in this case I see it as pretty freaking necessary. Learn something new everyday....

 

[fcallagy]

Hi Lance how does he relax pre dive?

 

[growingupninja]

Hi Lance how does he relax pre dive?

Breathing through a snorkel, motionless horizontal float, I suppose as he was taught in FII class. Seemed very relaxed. I thought it to be an extremely long breathe-up, but in this case seems warranted and I tend to be on the other end of that spectrum. He wanted to keep his eyes closed during breathe-up but I don't consider that safe or practical outside of strict competition/line-diving.

 

[Mullins]

Some people just aren't suited to freediving. They can have perfect technique, be fit, do a lot of training.... and BO after a 20m dive, every time.