Nitrox and Fatigue: the Evidence

[dmaziuk]

You are assuming that the amount of O2 absorbed into the blood as it passes through the lungs remains constant and equal to the metabolic need and therefore there is some mystery about how you could metabolize the "extra" O2 being offgassed since we know the metabolic rate is not going to increase.

f your blood/hemoglobin is saturated with O2 being offgassed, you will not be taking on as much O2 from whatever it is you are breathing. The hemoglobin is already bound up with O2 coming out of your tissues as you ascend.

I am confused: either the amount of O2 absorbed into the blood increases with increasing PPO2, and then it goes on and dissolves into tissues as a simple function of gas laws or you are not taking on as much O2 from whatever you're breathing because your tissues are already saturated. Having it both ways is magical.

 

[Storker]

either the amount of O2 absorbed into the blood increases with increasing PPO2, and then it goes on and dissolves into tissues as a simple function of gas laws or you are not taking on as much O2 from whatever you're breathing because your tissues are already saturated.

or the O2 dissolved in the tissues is rather quickly consumed by the tissues and is much less of an issue than the inert N2 is.

No magic required.

 

[jgttrey]

Your confusion is not due to magic or the false assumption that I am having it both ways, but rather because you are not distinguishing between dissolved O2 in solution (same as happens with nitrogen, helium, or any other gas) and O2 that is chemically captured by hemoglobin, something that does not occur with those inert gases.

1. The amount of O2 absorbed in solution/tissues increases as the inspired PP increases above the tissue PP. We seem to agree here, which is good because the gas laws require it. Conversely, it will come out of solution on ascent as tissue PP > inspired PP. This is a physical, gas law, process.

2. O2 bound to hemoglobin is bound chemically - a completely different phenomenon. And, for purposes of our discussion, you have X amount of hemoglobin with X amount of O2 capacity. That does not change with depth. They are two independent processes.

3. If the hemoglobin scavenges O2 being offgassed from tissues, then it is not available to scavenge it out of your lungs.

Result: whatever O2 saturation you have as a result of high PPO2 at depth offgasses on the way up, binds with the first available hemoglobin it finds, and thus takes the place of some of the O2 that would otherwise be captured by hemoglobin across the lungs in the aveoli. It is then metabolized.

That is a layman's description of course, but it's just physics and chemistry. It is also exactly what the authors of your goat study seemed to think was happening.

 

[dmaziuk]

I was confused about hemoglobin until a medical doctor set me straight.

"Hemoglobin" is 97% saturated at 1 atm. It's not going to "scavenge" any more than the remaining 3% regardless of PPO2. If you have enough dissolved oxygen in your tissues, the cells will metabolize that and leave heme alone (that was the part I didn't get until @rsingler spelled it out to me). What that means is the amount of O2 bound to hemoglobin remains constant, once you've saturated it to 100% at the start of the dive.

No matter how you cut it, the amount of O2 metabolized, hemoglobin transport or not, is constant, whereas the amount of O2 in breathing mix is variable. In normoxic/hyperoxic mixes and increasing ambient pressure the variable part is always greater than the baseline value: there is always "extra" O2. What you keep saying is extra O2 disappears "because metabolism hemoglobin".

@Storker has a much better explanation: that it never reaches the "critical mass" where it becomes a DCS concern. That's possible: you can't go too deep/too long because of oxtox so there is a practical limit on how much O2 Dr. Henry can push into my tissues. Maybe he can never push more than I can metabolize. But where I work we have programmers who though there will never be more than 2 digits in the year number, so I tend to be skeptical about "never" claims.

 

[huwporter]

Your goat study shows this isn't true - that with sufficient extreme conditions, O2 does bubble out. There's no question that it dissolves in tissues. That's a simple function of the gas laws. O2 is a gas like any other.

The likely explanation, which is the same one the authors of the study suggested, is that that offgassed O2 was metabolized quickly so that any DCS symptoms cleared quickly.

The actual quantity of inert gas dissolved in tissues, even in pretty serious technical dives, is tiny - of the order of a litre or two. I did the maths in another thread. If that also holds for Oxygen - which is roughly three times as soluble as Nitrogen in water - then back-of-napkin you'd be talking about something in the order of a litre of oxygen dissolved in tissues total, as compared to whole-body metabolic requirements of half a litre of O2 per minute, so it seems plausible that it could all be consumed by metabolism.

 

[jgttrey]

What you keep saying is extra O2 disappears "because metabolism hemoglobin".

What I keep saying, fairly clearly IMHO, is represented in my posts. I'm sorry you're not getting it, but I'll try again, even more simply:

I assume you understand that the hemoglobin transports O2 through the body, delivers it, and has to be "recharged" with O2 in the lungs. It is constantly doing this, along with delivering waste CO2 to the lungs.

When you are offgassing O2, much of that is likely metabolized in situ. When that happens, your metabolic need is being partially met and, of course, your hemoglobin no longer has to be stripped of that bit of O2. When it gets back to the lungs, instead of being "empty" it is still bound up with O2 and thus does not absorb any more.

This is all in response your evolving statements about the magical properties of nitrox and your questions about how you could metabolize offgassed O2 along with inspired O2 if the metabolic rate was constant. The answer, for the 8th time, is that if you are metabolizing offgassed O2, you are metabolizing a correspondingly lesser amount inspired O2.

What that means is the amount of O2 bound to hemoglobin remains constant, once you've saturated it to 100% at the start of the dive.

To be clear it's saturated before you start the dive because it's always saturated unless you have a medical problem like COPD.

@Storker has a much better explanation: that it never reaches the "critical mass" where it becomes a DCS concern. That's possible: you can't go too deep/too long because of oxtox so there is a practical limit on how much O2 Dr. Henry can push into my tissues.

And I totally agree with him, as well as @huwporter above. For normal diving, we never come close.

But, what started this little digression was your goat study and what the mechanism was that led to goats getting "oxygen bends" that resolved much more rapidly than typical DCS. The explanation in the study was it was metabolized and for several pages I've been trying to explain how that is true without any resort to voodoo nitrox magic or some metabolic miracle. It's simply the case that to the extent your metabolic needs are met in part by O2 you are offgassing, you will be consuming that much less O2 from what you are breathing.

 

[Fastmarc]

The last several posts were too technical for me to read through... eyes quickly glazed over, but I'm assuming this indicates that we are now closer to finally answering this whole fact or fiction debate.
YES!!!

 

[ofg-1]

I think that you all should go and get several goats, teach them to dive, and then obtain your own experimental data, don't believe the so called "experts". Just because 95% of scientists believe you can give goats transient O2 bends under extreme exposures, does not make it true.

 

[dmaziuk]

I think that you all should go and get several goats, teach them to dive, and then obtain your own experimental data

First you should teach them to talk: those researchers observed that goats were "evidently in distress" but never asked the goats what they were distressed about. For all we know they may have had terrible stomach cramps from the bad lab feed they were given that morning.