Quiz - 12 - Diving Knowledge Workbook - Diving Physiology

[Angelo Farina]

Only thing I can think of is someone may be breathing excessively, perhaps in a current or during panic and then ascend?

If you breath a lot, don't your risk HYPERoxygenation? Too much oxygen, which can cause you to have convulsions or to pass out, but that's not hypoxia, which means too small amount of oxygen...

 

[TMHeimer]

If you breath a lot, don't your risk HYPERoxygenation? Too much oxygen, which can cause you to have convulsions or to pass out, but that's not hypoxia, which means too small amount of oxygen...

You're probably correct. I was thinking you take in a lot of oxygen at depth where there is enough in the plasma stream to keep you conscious. Then ascend to where the is not enough 02 because you are inhaling at shallow ambient pressure, plus not enough C02 to trigger breathing (or something like that). But that scenario probably has to do with free diving or what happens if you're breathing Carbon Monoxide at depth then ascend and back out. At any rate, I think you are correct in that it would be hard to run into hypoxia breathing Air or Nitrox rec. diving to "normal" depths.
Tech. diving, CCRs-- different story I guess. The experts there can explain how it can happen.

 

[Pedro Burrito]

Spoiler: Answer

a. True

This is precisely why an overextended, breath-holding diver blacks out. Although the tissues are hypoxic - in need of oxygen - the urge to breathe is suppressed because of the low level of carbon dioxide influenced by hyperventilation. If the tissues continue to be "air starved," they begin to cease functioning. Eventually, the breath-hold diver will become unconscious.

 

[Couv]

Now that the spoiler is posted I have a question. If the stimulus to breathe is solely dependent on CO2 levels, why is it one has a greater urge to breathe with a low lung volume than with a full lung volume?

 

[Mike1967]

So, one can get hypoxia while breathing Nitrox in open circuit? This is something I really did not know...

Rebreather, who said OC?

 

[Angelo Farina]

Rebreather, who said OC?

So the TDI Advanced Nitrox course regards using Nitrox in rebreathers? I did not know that....
well, tye truth is that I do not know much about TDI...
However, the only rebreather I did use operating with Nitrox cylinders was the Caimano IV SC, which is semi-closed, hence it cannot go in Hypoxia.
Never heard of using Nitrox as a diluent with CC rebreathers, but of course all can be done...

 

[Mike1967]

Never heard of using Nitrox as a diluent with CC rebreathers, but of course all can be done...

I know bugger all about rebreathers but don't they have a 100% O2 bottle? Something regulates the O2 level and if things go wrong you don't get enough, that's hypoxia right?

 

[Angelo Farina]

I know bugger all about rebreathers but don't they have a 100% O2 bottle? Something regulates the O2 level and if things go wrong you don't get enough, that's hypoxia right?

Yes, in CC rebreathers there is an high risk of hypoxia, particularly if the diluent is hypoxic (as common in deep diving). Most common causes are the oxygen valve left closed, or a failure in the system which should release oxygen in the mixture.
However, your post was referring an Advanced TDI Nitrox course. In my knowledge, Nitrox is usually not employed in rebreathers, for normal depths you simply use air as the diluent, and for very deep you can have hypoxic heliox or trimix. No Nitrox...
What the point of using Nitrox as a diluent in a CC rebreather? You have an oxygen bottle, so you can use it for increasing the ppO2...
As said, I have seen just one rebreather using Nitrox cylinders, the OMG Caimano-IV rebreather. A military apparatus, employed by our Italian Comsubin. It operates in pure oxygen in CC, or in pure Nitrox in SC (semi-closed) mode.
In both modes, it is impossible to get hypoxia with such a rebreather...
Instead hypoxia is a quite frequent incident with CC rebreathers using an air cylinder as a diluent.
Which indeed I never used, so I am not an expert on them.

 

[Mike1967]

@Angelo Farina the TDI AN course isn't JUST nitrox, it allows you to use up to 100% O2. It's actually a prerequisite to doing any TDI CCR course.

 

[Zef]

Now that the spoiler is posted I have a question. If the stimulus to breathe is solely dependent on CO2 levels, why is it one has a greater urge to breathe with a low lung volume than with a full lung volume?

From what I recall from my days studying exercise physiology nearly 30 years ago:
The reason why CO2 drives the stimulus to breathe is because in its dissolved form, in the blood, it affects the blood's Ph...the higher the CO2 concentration the more acidic the blood becomes, the stimulus to breathe is not to increase oxygen concentration but instead to rid the body of CO2...the parasympathetic nervous system will not, under normal conditions, allow the body to accumulate CO2 to the point that the acidity level of the blood reach a concentration level that is damaging to tissues, hence the stimulus to breathe.

One reason why with a larger lung volume of a forced large breath there is less urge to breathe could be the fractional percentage of CO2 in the body compared to the percent of O2. If the hemoglobin component of the blood is able to bond with available O2 then the blood maintains an acceptable Ph level and the stimulus to exhale is not present. One can somewhat prolong their time on a breath held dive by slowly exhaling to reduce the amount of CO2 built up in the body/lungs. As one moves about in the water though, the metabolic rate increase causes an increase in the amount of CO2 to build up which starts to rapidly change the Ph level of the blood to be more acidic, thus stimulating the response to breathe. If the available O2 concentration drops below a certain level for a sustained period we lose consciousness as part of the parasympathetic mechanism to cause us to cease resisting the urge to breathe...if this happens underwater, one aspirates water into their lungs and drowns.

-Z