The Oxygen Window/Gradient: Got a simple explanation?

[King Kong Matt]

I've seen a couple of explanations re: the terms "gradient" and "oxygen window" and I *think* I understand it, but I would be very appreciative if someone could post an explanation in its simplest form.

I know this is a big topic and a bit of a tall order...

thx,
kkm

 

[Doppler]

King Kong Matt once bubbled...
I've seen a couple of explanations re: the terms "gradient" and "oxygen window" and I *think* I understand it, but I would be very appreciative if someone could post an explanation in its simplest form.

I know this is a big topic and a bit of a tall order...

thx,
kkm

Matt:

attached is a txt file... it's long and contains some technical concepts but it is worth reading as an intro. Once you have that done, we can do the 10 cent version here... but read it to begin with.

DD

 

[canuckdiver]

thanks doppler for a GREAT piece of info!

gonna take me a while to get through that

 

[King Kong Matt]

Doppler once bubbled...
Once you have that done, we can do the 10 cent version here... but read it to begin with.

I started this thread after reading this very same piece in a back issue of Quest (2.1 http://www.dirquest.com/02_01_toc.shtml - need to be a Quest member to access).

LOL, after reading that piece, I thought I would ask for the *simple* version.

Oh well, I will go back and try to read that again...

Thanks

 

[Soggy]

I'll check this thread next week for the simple version. It's taking a bit of time digesting that text file.

 

[Dr Deco]

Dear kkw:

Gases in the Tissue

A simple version of the “oxygen window” might be as follows. Normally there are present in tissues the following gases: water vapor, oxygen, carbon dioxide, and nitrogen. All of these gas species will be present in a gas bubble sitting in a solution of these gases. Thy will diffuse in and out until equilibrium is reached, typically in a few seconds.

With Metabolism

If one were to breathe oxygen, that gas would be entirely lost because of metabolism. If the oxygen were to be converted to some non-gaseous substance, e.g. water, we would eventually have present only water vapor and nitrogen, the later slowly being reduced as it is carried off in the blood stream. However, as we know, oxygen is converted into water and carbon dioxide. What is interesting is that an equal number of carbon dioxide molecules are not produced for each oxygen molecule consumed. It would be equal if all oxygen was used for the production of energy but this is not the case. Other biochemical processes occur in the body and not all produce carbon dioxide as the final product. One metabolic process is the conversion of protein and carbohydrate to fat. That is certainly one process that could be reduced, in my case at least. Because of this inequality, there exists a deficit of gas (carbon dioxide) produce for gas (oxygen) utilized. That inequality is termed the “oxygen window.”

There is actually a bit less dissolved gas in you body (when it is alive) than if you were to remove the tissue fluids and equilibrate them by bubbling air through them. This slight reduction, coupled with the internal pressure produced by surface tension (the Laplace pressure) is why gas bubbles are not stable in your body.

This is a less elegant explanation than in the enclosure above, but is probably closer to what for which you were looking.

Gradient

The gradient is the difference in pressures of the gases, be they dissolved or in free form. It is similar to a hill with its gradient. A ball placed at the top of the hill will roll down and “move with the gradient.” It will not go up hill “against the gradient” unless outside energy is supplied, i.e., someone kicks it up hill.

The difference in partial pressures of dissolved gases is the “partial pressure gradient” in your body. During on gassing, nitrogen moves from the blood in the capillaries into the tissues. This is down the gradient. During off gassing, the nitrogen moves from the tissues to the blood, again this is “down the gradient.”

If it were to go “up the gradient” that would be termed “active transport” and this does not occur in our bodies with gases. It does occur in nerve cells with regard to the sodium ion.

Dr Deco :doctor:

 

[The Iceni]

Hi Matt,

I can see why you are confused.

I must admit I had not previously looked in detail at the concept of "the oxygen window". It would appear that I had wrongly assumed that this term means something other than what it does. I suspect others may have made the same mistake.

If I can return to first principles, the main aim of any decompression mix is to reduce the inert gas counterpressure, since this is the most important factor in offgassing rate. The obvious choice for any deco mix is (high fractions of) oxygen because oxygen is readily available, non-narcotic, easy to control and relatively safe.

As readers know it has two main disadvantages. Firstly it is toxic to the CNS at the high partial pressures seen at the start of any accelerated deco schedule (>=1.6 bar) and, secondly, it produces whole body toxicity after prolonged exposure (in which potentially irreversible pathological changes also reduce offgassing efficiency).

My naive impression of the "oxygen window" was therefore the window in a decompression schedule during which high oxygen partial pressures and CNS toxicity are avoided at the start of its use, and the time-dependent toxicity which inevitably develops with prolonged exposure at these high partial pressures.

If my assessment is correct, the oxygen window - as defined above - is a more intimate additional facilitation in offgassing produced by the biochemical and physiological effects of high partial pressures of oxygen itself (on the Hb dissociation curve and others) over and above that of any other inert diluent ( that has not been used in the bottom mix) but is used in the decompression mix to maximise the inert gas offgassing pressure.

Very basic biochemistry;

Glucose + oxygen => water and carbon dioxide
C6 H12 O6 + 6 (O2) = 6 (H2O) + 6 (CO2)

In normal cellular respiration of glucose, for each molecule of O2 used one molecule of CO2 is produced

However other substrates can be used, including glycogen and lipids. Using alcohol as an example;-

C2 H5 OH + 3(O2) = 3 H2O + 2 (CO2)

Ethanol plus three oxygen molecules gives water and only two carbon dioxide molecules;-

Is that a fair assessment Dr Deco, or am confusing the matter even further? :boozer:

 

[Saturation]

In quick terms and layspeak, the difference between oxygen content in the arteries versus its content in veins represents the oxygen window.

Because the partial pressure of gases must reach a balance between blood and tissues, supersaturated tissues will give up its excess inert gas into veins to balance out the absence of 02 in the veins, thus aiding off gassing of inert gases and keeping inert gas in solution.

The reason for a difference between artery and vein is that 02 is metabolized by tissues. Thus, the 02 window is also depedent on the rate individual divers metabolize 02. 02 remaining in veins represents 02 gas consumption not necessarily actual consumption due to other gases and water vapor in balance, i.e., equilibruim, in following the law of partial pressures.