The “Oxygen Window”

by Matt London

The Benefits of 02 for Decompression

The practice of breathing OEA (oxygen enriched air) or 100% oxygen in order to speed up or enhance decompression is commonplace in both sport and technical diving today. This procedure benefits the decompressing diver by increasing the partial pressure gradient or difference between inert gas accumulated in body tissues over the course of the dive and that being inspired. As you may remember from your open water training curriculum, oxygen is metabolized and does not accumulate like inert gases such as nitrogen, helium, argon etc.

This oxygen-derived gradient increase coined the “oxygen window” (Behnke 1967) can be somewhat simplified if we consider the Equivalent Air Depth (EAD) of a common decompression gas. The concept of EAD was first introduced by Dr. Morgan Wells in 1970 and represents the depth at which breathing normal air would result in the same partial pressure of nitrogen.

Let’s take for example an – OEA51 – (51% oxygen/ 49% nitrogen) a gas usually breathed during decompression starting at its MOD (maximum operating depth) of 21MSW (meters of seawater). This gas has an EAD of 9.3 MSW (meters salt water) at it’s MOD of 21 MSW and can be calculated as follows:

EAD = [(Depth MSW + 10 MSW) . (1.0 – FO2) / 0.79] -10 MSW
EAD = [(21+10) x 0.49] / 0.79] – 10
EAD =9.23 MSW

So theoretically, as in the example above, off-gassing will occur at a much faster rate, while maintaining a hydrostatic pressure of 21 MSW on the body and any bubbles or bubble nuclei present (Boyle, 1652). Benefits of accelerating the rate of decompression include: reducing the divers exposure time underwater and a substantial decrease of breathing gas requirements.

How much?

The higher the Pa02 (partial pressure of arterial 02) the larger the oxygen window is considered to be, and the greater the possible benefit. There are of course two ways in which to increase Pa02:

  • by increasing depth
  • by increasing the percentage of oxygen in the mix

Oxygen toxicity will without doubt limit the size of the oxygen window to 1.6 Pa02 during submersion (K. Donald 1945), but it is possible to extend this advantage to 3.0 in the safety of a “dry” recompression chamber where the convulsions associated with CNS oxygen toxicity will not cause inhalation of water and subsequent drowning. So the best strategy for in water decompression is to stay at high ppO2 (within CNS toxicity limits) to accelerate elimination of dissolved gas, while at the same time maintaining maximum hydrostatic pressure (on Six and 3-meter stops

Generally, in decompressing diving the bulk of the time decompressing is at the six and three meter stops. In this depth range 100% Oxygen gives the highest decompression benefit when compared to other breathing mixtures simply because of a total absence of inert gas in the lungs (partial pressure vacancy). In other words it’s not necessarily the presence of oxygen, but the total absence of any inert gases, that speeds up off gassing!

The arterial partial pressure of inert gas when breathing pure oxygen is considered zero regardless of the depth (EAD is generally considered around minus -10 MSW). Pure oxygen produces the largest possible outward gradient, allowing no inert gas to diffuse back into your body while it’s diffusing out of your body. Maintaining maximum hydrostatic pressure is the primary reason for pulling your 3 meter stop at 6 meters on oxygen!

If for example, if a 50% or 80% OEA mixture were breathed during the 6 MSW decompression ceiling, then N2 (or any other inert gas) would occupy some of the partial pressure of the possible oxygen window. Simply, inert gas exchange when breathing pure oxygen is a one way event (outward) as opposed to breathing mixtures that contain N2 or other inert gases.

Breathing pure oxygen can also give you additional advantages by reducing venous blood super-saturation. For practical purposes, venous blood is considered to be equilibrated with the lungs alveoli instantaneously (Barnett and Elliot 1993). So for example: When an inert gas bubble is introduced into the circulatory system, while breathing pure oxygen, the bubble will rush to equilibrate with the inert gas tension of the surrounding tissue (blood), so the bubble will more likely shrink and disappear before it becomes a problem (Behnke 1951).

Pure Oxygen is really the optimal extra gas to have on hand at the dive site as the administration of pure oxygen is standard practice in the aftermath of almost all forms of diving related accidents. A 50% or 80% OEA gas blend for example would be more or less useless in the unlikely event of a diving related accident.

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