The Oxygen Window

 Sooner or later, all divers who are trained in the use of oxygen first-aid, or learn about recompression on oxygen, will encounter the term "oxygen window".  In fact, 'oxygen window' is actually the name of our Newsletter for DAN Oxygen First Aid Instructors.

So, what is the "Oxygen Window"...?

Essentially, the oxygen window may be thought of as the 'missing' gas tension caused by the conversion of oxygen to carbon dioxide (as a result of their different solubilities in blood), which allows more nitrogen (or any other inert gas) to be dissolved in venous blood to take 'in its place' and increases the rate of nitrogen (or inert gas) gas elimination.

If this did not make sense, please read on. We'll try not to be too technical, but some foundational principles do need to be explained:

As we breathe, the inspired gas partial pressures  (O2, N2, CO2 and H2O vapor) within the alveolar sacs of the lung are in a dynamic balance with the dissolved gas tensions in the blood flowing through the lung.  Basically, as per Dalton's Law: In a gas mixture, the sum of the partial pressures of the constituent gases equals the ambient pressure. Perhaps, more simply stated: the total is equal to the sum of the parts. The same is ultimately true for dissolved gas tensions (i.e., gases in solution) at equilibrium or the point of saturation, but gas tensions always lag behind partial pressures changes by varying degrees. This is because Henry's Law involves certain time-related mechanisms, i.e., diffusion and perfusion. In fact our decompression tables are the mathematical models for this phenomenon. Essentially they attempt to predict these 'lags' in uptake and release of gas tensions, in response to changes in ambient pressure and inspired gas partial pressures, to protect us from getting decompression illness.

Now, here is the mental bridge that needs to be made: Although partial pressure and gas tensions are expressed in the same units of measure - milimeters mercury or kilo pascals typically - they are not identical: Partial pressure is the force a gas exerts per surface area within a gaseous environment, whereas gas tension is the pressure exerted by a gas within a solution. In a gas mixture, a given number of molecules of any gas will exert the same partial pressure (i.e., as a function of Avogadro's Law): so, for example, 20 oxygen gas molecules wil exert the same partial pressure as 20 carbon dioxide gas molecules. Once dissolved in solution, however, Avogadro's Law no longer applies: The gas tension is now determined by the gas' solubility within that given solution. So, for instance, if one gas is more soluble in e.g., water or blood than another gas is, a larger number of moleculeswill enter the solution (i.e., a higher dissolved gas concentration) for the same partial pressure driving it into solution (i.e., Henry's Law). Put differently: If both gases are driven into solution by the same partial pressure, more actual gas molecules of the soluble gas will dissolve before it reaches saturation. Therefore, unlike the the situation in a gas mixture, where oxygen and carbon dioxide exert the same partial pressure for the same number of molecules, in water or blood the gas tension of 1 oxygen molecule would be the equivalent of 20 carbon dioxide molecules (i.e., carbon dioxide is twenty times more soluble in water than oxygen). 

Now for the oxygen window: In the body, oxygen is metabolised into carbon dioxide. But, because carbon dioxide is 20 times more soluble, as oxygen is converted to carbon dioxide, the large drop in oxygen gas tension is mirrored by only a very minor (theoretically 1/20th) increase in carbon dioxide gas tension. This is due to the relative solubility difference between the gases. In truth, it is slightly more complicated than this under normal physiological conditions due to the carrier and buffering effects of hemoglobin: In other words, when we breathe air, much of the oxygen our bodies use is  supplied from hemoglobin rather than from the dissolved oxygen portion. However, when we increase the body's gas tensions of oxygen, by breathing 100% oxygen, and even more so when we breath it under hyperbaric conditions (e.g., during recompression at 2.8 atmospheres), more of the body's demands for oxygen can be met by a reduction in the amount of dissolved oxygen. Under these conditions, there is a stark contrast between the plummeting oxygen gas tension (as the blood moves through tissues from arteries to veins), with only a minor carbon dioxide gas tension rise being evident.

So, what is the oxygen window?  Essentially, it is the 'missing' gas tension created by the conversion of oxygen to carbon dioxide (as a result of their different solubility in blood); this allows more nitrogen (or inert gas) to be dissolved in venous blood to take 'in its place' and increases the rate of nitrogen (or inert gas) gas elimination.

Read more on wikipedia.