Oxygen Window: Explanation and Purpose?

[Dr Deco]

Hello charlie99:

How much of this "partial pressure vacancy" is present in the tissues as compared that in the venous system?

The “partial pressure vacancy” (ppv) will actually vary within the tissue. It will be more “arterial-like” in the tissue at the arterial end of the capillary and more “venous like” at the venous end of the capillary. The ppv will also drop as one moves away [outwards] from the capillary. This reflects the consumption of oxygen as it diffuses outwards from the capillary (“”radial diffusion”). There is likewise an “axial diffusion” as one moves parallel to the capillary.

The magnitude of the effect is not straightforward as the capillaries bend and sometimes overlap. The tissue geometry is not simple and neat. I am not certain that it has actually been measured at the cell level. It has been measured with larger pockets of gas being formed within a rat.

Does the oxygen window (the partial pressure vacancy) have any affect on bubble formation within body tissue in addition to the postulated (ever proven?) effect of lower venous bubbling?

The total pressure of all dissolved gasses will affect bubble formation in tissues – or any liquid in , eg, a shaken test tube. The dissolved gases diffuse into the nascent bubble and stabilize it [for a few seconds]. The ppv will remove oxygen as one dissolved gas and therefore would reduce the tendency to form bubbles by hydrodynamic methods.

Preoxygen Days

Please remember that the “oxygen window” was a small effect when first described. It treated the loss of oxygen present in a diver's breathing air. Oxygen was not used in the fractions used today. In fact, until the later 1950s, oxygen was not even used in the treatment of DCS. High concentrations of oxygen are relatively new in diving.

Dr Deco :doctor:

 

[rjack321]

As I have conceptualized this term...

Its the difference in sum saturation between arterial and venous blood which creates a large gradient for inert gases to diffuse from tissues to blood and the get to the lungs capillary beds. The arterial blood having O2, CO2 plus some water vapor in it while the venous blood has only a small amount of CO2 and water vapor as well. Hence plenty of "room" for inerts to diffuse into it.

The magitude of the gradient between blood/lung gases is distinct from the magnitude of the tissue/blood gradient. Hence you can accelerate off-gassing of N2 even when using 21/35 with 50% for deco. The inspired N2 has not changed much but there is a vacancy created by the O2 which enhances N2 offgassing.

Of course the He gradient goes to maximal on this switch but that's indpendent of O2 concentration.

I could be totally wrong though

 

[gcbryan]

As I have conceptualized this term...

Its the difference in sum saturation between arterial and venous blood which creates a large gradient for inert gases to diffuse from tissues to blood and the get to the lungs capillary beds. The arterial blood having O2, CO2 plus some water vapor in it while the venous blood has only a small amount of CO2 and water vapor as well. Hence plenty of "room" for inerts to diffuse into it.

The magitude of the gradient between blood/lung gases is distinct from the magnitude of the tissue/blood gradient. Hence you can accelerate off-gassing of N2 even when using 21/35 with 50% for deco. The inspired N2 has not changed much but there is a vacancy created by the O2 which enhances N2 offgassing.

Of course the He gradient goes to maximal on this switch but that's indpendent of O2 concentration.

I could be totally wrong though

I think you're wrong. Isn't the off gassing of any gas only dependent on that gas and independent of any other gas in the mix?

 

[Charlie99]

Isn't the off gassing of any gas only dependent on that gas and independent of any other gas in the mix?

This is my understanding also.

IMO, "partial pressure vacancy" is a better term than oxygen window, since it specifically refers to the change in ppO2 between arterial and venous systems, as opposed to the straightforward effects on offgassing caused by replacing inert gas with O2 in a high FO2 mix.

To the extent that the inert gas in the tissues stays in dissolved form, then partial pressure vacancy should not have any effect on the level of offgassing, since that is controlled strictly by the relative partial pressures of each gas in the tissues and the blood.

So what DOES the partial pressure vacancy/oxygen window do??? Some articles on partial pressure vacancy/oxygen window hypothesize that the lower sum of partial pressures in the venous system allow the blood to accept more dissolved gas without forming bubbles. A problem I have with accepting that hypothesis is figuring out how one can increase the partial pressure of some inert gas in the blood by a huge amount without already having the sum partial pressure in the tissue so high(at least in the areas that have ppO2 closer to arterial levels) that there is a lot of hydrodynamic bubble formation going on in the tissues already.

In most cases, if someone clearly understands a phenomena, then they are able to clearly describe it. So far I haven't found any clear, lucid descriptions of the oxygen window, other than some early papers by Momsen (1939/1942) and Benhke (1967), but those papers rely on some assumptions about on/offgassing and bubble formation that have generally been rejected by later investigators.

Charlie Allen

 

[Charlie99]

Here's the thoughts of Ross Hemingway, the author of the popular VPM decompression program V-Planner.

He reviews the paper most often referred to by people promoting the concept of oxygen window, and comes to this conclusion:

The term oxygen window can be used is many different ways. This topic discusses the paper "Gas Exchange, Partial Pressure Gradients and the Oxygen Window" by Johnny E. Brian, Jr., MD. This paper is the commonly used reference by tech divers and is part of GUE / DIR training. download here or read here (http://www.tek-dive.com/portal/upload/brian_oxygen_window.pdf)

The paper offers arguments to support the concept that the O2 window can enhance off gassing of inert gas from the tissues. The paper starts out explaining the mechanisms for gas to propagate through tissues. But then I believe the paper contradicts its own initial explanation and the arguments offered.

With those points considered and applied to the paper, then I believe no relevant argument remains to support the concept of increased off gas of N2 and He in the O2 window for decompression.

The oxygen window concept does exist as described in the paper.... except it has no practical effect on decompression rates.

==========

I was happy to run into this link (thanks to TS&M who linked to it in a recent post on deep stops), because I had also read that article and also felt that, although JE Brian had lots of pretty graphs and calculations on partial pressure vacancy, that he then concluded that it didn't have any effect. Yet this is the very article that a couple different people have pointed to as the definitive article that shows how the oxygen window works!

Charlie Allen

 

[ianr33]

Interesting then that V-Planner has a selectable box for "O2 Window Affect" in the Configure window ??


And I thought it was common knowledge that the only correct oxygen window is 1.6

. The 80/20 mix is in fact totally useless and contraindicated as a deco gas. At thirty feet it is only a 1.52 ppo2 (the real 1.6 ppo2 gas would be 84/16) and as such does not either provide the right oxygen window, nor does it work as well as pure oxygen without an inert gas at any depth.

 

[Charlie99]

Interesting then that V-Planner has a selectable box for "O2 Window Affect" in the Configure window ??

Checking the "O2 Window Affect" configure box doesn't affect deco algorithm or calculations.

It controls whether or not the program will check to see if the next shallower stop can be skipped when the user has activated extended stops.

Extended Stops

Add to stop time - this controls if the time specified is a minimum time, or will add extra to the required stop time.

All deco setting will cause the extended time to occur, even if a stop was not required at that level. On simple dives, when little deco is actually required, this option should be disabled, as it will put a stop in where it is in conflicting with the underlying plan.

O2 window affect causes to program to check if sufficient deco was carried out during the extended time and to possibly bypass the next stop all together. i.e. if an extended period was used on a 50% switch, then the adjacent 60ft/18m stop might be eliminated. Without this option, all stops from the extended depth onwards will be printed will at least the minimum stop.​

 

[ianr33]

So are we saying that in V planner it has nothing to do with the O2 window as it is normally understood (i.e a "magical" property of O2 at 1.6 ATM,partial pressure vacancy etc) but rather just skipping a stop if the preceeding stop has been long enough to allow sufficeient offgassing? In which case it is a rather confusing name.

But then everything to do with the Oxygen Window is confusing !

 

[limeyx]

But then everything to do with the Oxygen Window is confusing !

That's why I ignore it and just go diving!
Honestly, I never understood the O2 window junk in the first place, but I know that the schedules and deco plans generated from whatever basis they may be, work for me from recreational dives through "tech1" dives down to 20-30 mins at 190-200 feet (and for others far deeper/longer)

That's all I need to know and it's enough for me in a "selfish, me go dive kind of way"

Of course, that doesn't mean there is no room for further research and in-depth study, but what I have works and that's that.

 

[Dr Deco]

Hello Readers:

[Sorry that I am still running on “slow” because of the surgery and my meds.]

Oxygen Window and Washout

As far as I am aware [for the past several decades], the oxygen window [OW] and decompression applies ONLY to the gaseous phase and bubbles. It does not deal with some “magical” ability to effect a rapid release of dissolved nitrogen or helium.

The OW will help with the shrinkage of bubbles in tissues and the venous system [because of surface tension effects].


Dr Deco :doctor: