SAC, air vs. nitrox

[scubadada]

Hi Dr Deco,

Is there a difference in SAC related to partial pressure of oxygen inspired? What drives your SAC, O2 requirement or CO2 elimination? Thanks in advance for answering my naive question.

Best, Craig

 

[Dr Deco]

Hello Craig :

Varying Oxygen Consumption

The consumption of oxygen is a function of several factors. Naturally, physical activity is the first that comes to mind; all measurements are therefore made in resting individuals.

Nervousness is a factor and indicates a diver who is not relaxed underwater; training is usually the answer for this problem. Cold and shivering will also increase consumption of oxygen.

Being in good physical conditions is the best answer for low consumption.

Oxygen Content of Mix?

The oxygen content will not change the breathing rate. This rate is controlled by the carbon dioxide in the blood and is indicative of muscle activity. Surprisingly, oxygen is not controlling. People have walked into low-oxygen environments and died while breathing normally. This can occur in rooms where the air is replaced by something with low oxygen [for fruit storage].

Poring liquid nitrogen in a shallow area will drive out the oxygen and replace it bynitrogen. Breathing nitrous oxide for recreational purposes has resulted in death when the oxygen [in a separate cylinder] became exhausted.

A few years ago [October 1999], a Learjet with low oxygen caused all of the passengers,including pro golfer Payne Stewart, and the pilot to pass out and die. The plane crashed near Aberdeen SD.




Dr Deco :doctor:

 

[scubadada]

Thank you Dr Deco, I appreciate the response.

 

[turnerjd]

As a simpler answer, yes, I notice that my SAC goes down when I dive on nitrox. It might not be too visible when diving on less than 32% but as the % gets closer to 40 it is certainly noticeable. I did the calculations back when I first started diving nitrox and the difference as I remember it was round about, or slightly less, than 10% lower sac on rich nitrox.

As a general rule a smaller 13.5L tank of nitrox is for me the equivalent of a 15L tank of air. This should be 10cuft difference in the amount of available air for a 3000psi tank if my maths is correct and a 15L tank is roughly 100cuft at this pressure. (although the 13.5L tanks are a bit rare!)

Jon

Sorry I forgot to say that this is for 2 dives to the same depth air vs nitrox. So the simple fO2 can be used rather than considering different depths and ppO2s to give the same effect.

 

[TSandM]

As I recall, there is a little bit of literature supporting the idea that high ppO2s can induce CO2 retention in some divers. This would result in a lower SAC rate, but at the cost of running a higher pC02, which has significant impacts on cognition, anxiety levels, and potential for O2 toxicity. There is no other reason why breathing a higher FiO2 should affect volume per minute breathed.

 

[turnerjd]

Hi TSandM

I know there is absolutely no scientific reason for the lower sac on nitrox, as it is the CO2 that controlls respiration, however this was my observation. Do you have the reference for the CO2 retention? Or is it something that I will find on pubmed?

I had noticed that when I have a high nitrox mix, that I am significantly calmer on a dive, and whilst I am not usually anxious, these dives just feel better, perhaps this is the CO2 impacting on cognition so that I don't notice things that would normally make me look a bit closer at somebody (or something) during the dive!

Jon

 

[drrich2]

...I had noticed that when I have a high nitrox mix, that I am significantly calmer on a dive, and whilst I am not usually anxious, these dives just feel better...

Speculation alert - is it just possible that mild nitrogen narcosis or a sort makes you less comfortable, and that lower body nitrogen levels with nitrox lead you to a calmer, more relaxed state with a mildly lower SAC for that reason?

Richard.

 

[turnerjd]

Speculation alert - is it just possible that mild nitrogen narcosis or a sort makes you less comfortable, and that lower body nitrogen levels with nitrox lead you to a calmer, more relaxed state with a mildly lower SAC for that reason?

Richard.

I don't think that narcosis is a reason. By high fO₂ I am talking about nitrox 38 or 40 (the max that I can get, MOD nitrox 40 = 25m). I used this regularly when assisting on OW dives. As these are all <18m dives, often with the majority of the dives around 10-12m. I don't think that narcosis comes in. Certainly task loading does.

As an addition, I knew about the effect of CO2 on latency to CNS OxTox, but haven't yet managed to get a copy of Arieli R and Ertracht O. Latency to CNS oxygen toxicity in rats as a function of PCO₂ and PO₂. Eur J Appl Physiol Occup Physiol 1999; 80:598-603. (my institutional springer license is 2000 and onwards.) so if anybody has a copy it would be great!

I also knew about the retention of CO₂, but I though that this was a general phenomenon independent of fO₂

It is well established that endtidal PCO2 increases in a diver due to increased metabolic CO2 production caused by physical exertion of swimming, working, and breathing underwater, and due to reduced ventilatory efficiency (alveolar hypoventilation) caused by the added dead space of a breathing apparatus and the increased airway resistance caused by increased density of gases at increased ambient pressure (79, 147, 194).

However, for those interested, there is a nice paper from a group in Russia that have shown how an elevated ppO₂ causes CO₂ retention and the role that carbonic anhydrase and nitric oxide play in this. the reference is Gutsaeva DR, Moskvin AN, Zhilyaev SY, Kostkin VB, Demchenko IT. The roles of nitric oxide and carbon dioxide gas in the neurotoxic actions of oxygen under pressure. Neurosci Behav Physiol. 2005 Sep;35(7):751-6 (PMID: 16433071)

Jon

as the French say: je me coucherais moins bête ce soir (literally, I will go to bed less stupid tonight)

 

[Rick Murchison]

...I had noticed that when I have a high nitrox mix, that I am significantly calmer on a dive, and whilst I am not usually anxious, these dives just feel better...

This is a constant point of disagreement between my wife and me. She reports what you do... calmer, feels better etc (with a concomitant better SCR). I insist it's strictly a placebo effect as I can't tell a speck of difference between a dive on air and a dive on Nitrox. She says I'm just insensitive
Nevertheless, I'd love to see a double-blind study on the matter...
Rick

 

[turnerjd]

This is a constant point of disagreement between my wife and me. She reports what you do... calmer, feels better etc. I insist it's strictly a placebo effect as I can't tell a speck of difference between a dive on air and a dive on Nitrox. She says I'm just insensitive
Nevertheless, I'd love to see a double-blind study on the matter...
Rick

I can see the smiles on the ethical review boards faces as I present the project, the depths, profiles and nitrox mixes of the dives....... Ok, so you're taking a bunch of your students off diving for a crapload of fun and you call this research....................

I don't know if it is placebo effect, but when I calculated SAC rated there was a significant p<0.05 (students T-Test) difference when comparing dives (spot the )... I think I did the analysis only on OW1, 2, and 5 dives.

Jon

---------- Post added April 18th, 2012 at 05:02 PM ----------

Science hasn't advanced at all today in my lab, however, I have found some real nuggets of information.......

How about having 2 or 3 double expressos before going diving just to make sure you don't have any CNS O₂ Tox problems?

Bitterman N and Schaal S. Caffeine attenuates CNS oxygen toxicity in rats. Brain Res 1995; 696:250-253. PMID: 8574677

Apparently it works in the opposite manner to CO₂ retention by preventing and delaying the breakpoint of the initial cerebral vasoconstriction and as such preventing cerebral PO₂ from rising.

Jon