Hypocapnea

[Matbase]

Can being overly hypocapnic shorten a breath hold time due to the vascular constriction causing the brain to become hypoxic prematurely resulting in a BO?

 

[trux]

Well, if you black out at the beginning of the breath-hold because of hyperventilation, then you either can stay breathless forever (if you had no buddy), or the buddy will pull you out and hence the breath-hold will be very short.

I assume though that your question was not properly asked, and you rather wanted to know what influence on the length of the breath hold such hyperventilation (or hypocapnia) has. Well, there are many different effects. Besides the constriction of carotids, there are many other negative effects (i.e. muscle tension, tetanism, myoglobin depletion / reduced oxygenation of tissue, delayed or suppressed diving response, convulsions at low PaO2, etc). The most important is lower hypoxic tolerance (mainly due to shifted Bohr effect curve), and hence earlier blackout, which is also more likely because the urge to breath is delayed.

 

[efattah]

Well, if you black out at the beginning of the breath-hold because of hyperventilation, then you either can stay breathless forever

Actually if you blackout from hypocapnia you will always wake up, even with no buddy. I suppose there is a 1 in 1000 chance you might not, but as a general rule you wake up. CO2 will accumulate during unconsciousness, and the O2 will become available again.

 

[trux]

Actually if you blackout from hypocapnia you will always wake up, even with no buddy. I suppose there is a 1 in 1000 chance you might not, but as a general rule you wake up. CO2 will accumulate during unconsciousness, and the O2 will become available again.

Yes, you are certainly right here, but there may be a problem anyway if you are submerged and inhale water. Hopefully natural reflex would prevent it, but you never really know. Even worse it would be if you are not buyant and sink.

 

[Matbase]

I didnt word my question correctly but youve answered what I was getting at, thanks. My question wasnt relating to BO from hyperventalating or in water practise. Just wanting a better understanding of the effects of lowering ones CO2 levels.

Is there a definable line between a standard pre-hold breathe up and over doing it?

Also is it correct that packing delays ones dive response?

Thanks
Matt

 

[Mullins]

Definite line between 'standard' and 'overdoing it'? No. Many would argue that 'standard' breathups usually result in hypocapnia in any case, and that freedivers sacrifice performance for comfort.

I don't think that packing would 'delay' the dive response. However some of its effects counteract those of the dive response. In particular tachycardia (although stroke volume is reduced so overall cardiac output might not rise). Ultimately though, packing increases performance in the pool because the extra oxygen outweighs the cost in efficiency. For constant weight and FIM packing increases performance up to a point, but will eventually become counter-productive simply because of the buoyancy change of the higher air volume. E.g. if you could pack 20 litres you wouldn't be able to get off the surface without seriously overweighting yourself.

 

[trux]

E.g. if you could pack 20 litres you wouldn't be able to get off the surface without seriously overweighting yourself.

Dave, you must work more on your technique! Just pressure up the air in your lungs, without increasing the volume. Just like when you fill a scuba tank. In such way the packing will work to your advantage - less volume and buyancy change in depth.

 

[Bill]

'Just pressure up the air in your lungs, without increasing the volume.'

Does anyone have any numbers on this? Just curious.

 

[trux]

No, I was just joking. There is certain overpressure in the packed lungs, but it is relatively small. Too big overpressure would lead to a barotrauma (and it indeed ends so sometimes). I know I saw some numbers in some of the documents listed on my website, but do not remember them exactly. If you want to read more on the topic try to look them up here:

documents packing @ APNEA.cz

 

[Matbase]

Thanks thats been a great help, can you guys let me know what your typical breathe up is or is that a trade secret?

 

[trux]

... back to the diving response: yes, packing works against the DR. As Dave wrote, it causes tachycardia, but also the higher pressure increases PaO2, and acts against blood shift and vasoconstriction. It is often also connected with some previous hyperventilation (hence hypocapnia) which also acts against the DR. So yes, you will likely get a stronger and quicker reflex on empty lungs, FRC, or even with full lungs without packing, but as Dave wrote, it does not yet mean you will manage to hold your breath longer.

As for breath up - there are so many different ways, and it also depends very much on the purpose - whehter it is for training, for a maximal attempt, serial diving, for spearfishing or recreational diving, ... Unless you are a competitive diver going for his maximal performance, I would strongly advice against both packing and any kind of hyperventilation. Otherwise, you can find plenty of recipes for just "the right" breath-up, if you dig the forum archive.

 

[TRITON]

I've found out that breathing really shallow and slow before a dive works for me.I almost feel the urge to breathe before I start my dive.
Contractions come a bit sooner ,but aren't so strong.......all PBs i've done except STA are with this type of breathing.