Many processes that are crucial for understanding decompression can be explained quite simply by the behavior of inert gases, as they do not interfere with the metabolism. However, as soon as gases such as oxygen and carbon dioxide come into play, things become more complex, as their metabolism must also be taken into account. This requires additional considerations and limits the validity of Dalton's law for these gases. A perfect example for this is the oxygen window. There is probably no diving physiology concept that is more often misunderstood. This blog post attempts to shed light on the oxygen window and differentiate it from other decompression physiology concepts with which it is often confused. And you will learn another technical term that is good for bragging.
Henry and tissue respiration
Every oxygen molecule (O2) that is metabolized in tissue respiration is converted into carbon dioxide (CO2) and is thus replaced. As CO2 is much more soluble than O2, the same concentration of CO2 in a liquid leads to a much lower partial pressure than with O2 (according to Henry's law). In addition, excess CO2 is further processed in the body - it reacts with water to form carbonic acid and converts into bicarbonate (HCO3-), which then goes into solution. As a result, a large proportion of the CO2 in the blood or tissue is no longer present as a gas and therefore does not generate any partial pressure.
As a result, the sum of the tissue pressures of O2 and CO2 must be lower than their sum in the alveoli or arterial blood. This is illustrated by the following numerical example:
The table illustrates partial pressures in various tissues under air respiration at an ambient pressure of 1 bar (values idealized). Despite a drop in the partial pressure of O2 from 0.14 to 0.04 bar, the partial pressure of CO2 in the tissue only increases from 0.05 to 0.06 bar.
The total of the partial pressures of O2 and CO2 is lower in the tissue than in the alveoli.
The partial pressure of CO2 therefore does not compensate for the drop in the partial pressure of O2 in the tissues. The CO2 partial pressure remains almost unchanged. This results in a lower total pressure in the tissues (0.90 bar vs. 1.0 bar in the alveoli in this example). This difference is referred to as the oxygen window.
It follows that the total of all partial pressures in a tissue must always be slightly lower than the ambient pressure (at equilibrium conditions, when the ambient pressure does not change rapidly). This phenomenon is referred to as “inherent unsaturation”. The reason for this is, as shown above, the metabolization of O2 to CO2.
The “inherent unsaturation”
The significance of the “inherent unsaturation” lies in the fact that it describes a minimum decompression that does not cause bubbles to form. As long as the decompression does not exceed the range of the “inherent unsaturation”, no oversaturation occurs. Theoretically, you could therefore ascend in such a way that this threshold value is not exceeded during decompression, which would guarantee bubble-free decompression. In practice, however, this is difficult to implement and impossible when diving. However, when decompressing in hyperbaric chambers, the ambient pressure could be lowered so slowly that the tissue always remains in the “inherent unsaturation” range.
Tip: Mention the "inherent unsaturation" casually in tekkie discussions!
The higher the inhaled partial pressure of O2 (PiO2), the larger the oxygen window. Because O2 is first bound by hemoglobin and the O2 partial pressure therefore remains low, it is small under normal pressure conditions when breathing air. When diving, however, the hemoglobin is 100% saturated with O2. As a result, above a haemoglobin saturation of 100%, any increase in PiO2 causes the partial pressure of O2 in the blood to rise, thereby enlarging the oxygen window.
The figure shows the oxygen window as a function of the oxygen content in the blood. The orange line represents the CO2 partial pressure. This remains (almost) constant even at high CO2 content.
The first, steeply rising part of the curve for O2 (blue) is defined by hemoglobin binding. A small increase in the O2 partial pressure leads to a strong increase in the oxygen content in the blood in the form of hemoglobin-bound O2, while the O2 partial pressure does not increase significantly. When the haemoglobin is completely saturated (approx. from 0.2 bar oxygen partial pressure/air respiration at normal pressure), any further increase in the O2 partial pressure leads to a proportional increase in the oxygen dissolved in the blood. This is represented by the linear part of the oxygen content curve. The oxygen window is therefore much larger at 100% O2 breathing during the decompression stop at 5 m (1.5 bar O2 partial pressure). It reaches its maximum at approx. 2.2 - 2.3 bar O2 partial pressure and remains afterwards constant.
Does the oxygen window shorten the decompression time?
Does enlarging the oxygen window really help to shorten the decompression time, as is often claimed in diving forums or taught in technical diving courses? - No! The true driving force for desaturation is the difference in inert gas partial pressures between tissues and lung, which is increased by ascent or the choice of suitable decompression gases. Although the oxygen window widens due to the higher oxygen partial pressure with hyperoxic breathing gases, it plays no role in desaturation and shortening decompression. It is merely a concomitant phenomenon.
In the next blog post, you will discover why the oxygen window is nevertheless very important when diving and why it often causes so much confusion.
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