Saturation and desaturation: Basics
During every dive, the body absorbs inert gases from the breathing gas. As already mentioned, inert gases are gases that are not metabolized in the body, but are simply inert. Their behavior is in contrast to that of oxygen, for example, which can be chemically bound by the body and ultimately metabolized to carbon dioxide (CO2). The ability of body tissues to absorb inert gases and thus build up a potentially high inert gas pressure in the tissue has important consequences for us divers.
This chapter deals with the basics of how inert gases get into the body and how they get out again. How to turn the findings into a model that can be used to calculate decompression. And we discuss where the limits specified by the decompression models lie (which we often call “M-values” or “M-lines”), and how the model and the body are compatible.
Gases and liquids
How do gases get into liquids?
To understand what is going on in the body during diving, the uptake and release of nitrogen and other gases is fundamental. Gases and liquids that are adjacent to each other are visibly clearly separated from each other – but there is always an exchange between them. Gases dissolve in the liquid, or escape again into the gaseous phase. This happens even continuously, but as long as the pressures of the individual gases in the liquid and in the gas phase are in equilibrium, the same number of atoms or molecules pass over in both directions per time. In net terms, therefore, there is no change in the concentration. However, the situation is different if there is a pressure difference. Then there will always be an exchange between gas and liquid in the direction of the lower pressure, until the pressure of each gas is the same on both sides. This pressure equalization can be described by two physical principles:
“Principle of Le Chatelier”: It is in nature to avoid constraints. If the pressure of a gas increases on one side, the gas will move to the side with the lower pressure.
At the end of this process, a state is reached that we know in diving as “Henry’s law” : The concentration of a gas dissolved in a liquid is proportional to the partial pressure of the gas above the liquid – so this is the state after pressure equalization has taken place.
If several gases are involved, Dalton joins the game: Each individual gas in the mixture exerts a part of the total pressure, the partial pressure. A high proportion of a gas naturally means that more of it will dissolve.
Tissue saturation
Saturation curves
The greater the pressure difference, the faster gases pass from the air into the bloodstream and also from a tissue with higher to one with lower pressure.
The pressure equalization thus takes place quite quickly at the beginning, then more and more slowly. This results in a slowly flattening curve.
What is a half-life?
The saturation curve shows on the vertical axis (y) the saturation in %, on the longitudinal axis (x) the elapsed time. Each tissue reaches 50% saturation after a certain time: This time is a half-life. The term may be more familiar to some in the context of radioactivity than diving. There, a half-life means the period of time in which a radioactive metal has lost half of its activity. Such radioactive half-lives are of quite different lengths: from fractions of a second to hours, days (in the case of iodine-131, notorious from tragic reactor accidents) to billions of years, as in the case of naturally occurring uranium-238 and the waste from nuclear energy.
In diving, on the other hand, we fortunately don’t have to deal with such extreme times. For us, periods of minutes to hours are decisive. So back to our diagram: if you follow the curve further, you can see that after another half-life, 75% of complete saturation has been reached. Sure – the tissue halved the remaining pressure difference just as quickly as the first time.
After 6 half-lives we arrive at over 98%, for our purposes we can then assume complete saturation.
Desaturation
Desaturation of a tissue
Here we will introduce a type of graph that is very often used in various publications, including Erik C. Baker: “Understanding M-values”[1] – a paper that is well worth reading. It is a little different from what you might be used to from such diagrams: there is no axis with time and one with any values whose course over time you want to look at, but the axes represent the ambient pressure and the inert gas pressure in the tissue – and the time in which model tissue moves through this graph is not shown directly at all.
Since it is more than complicated to have all model tissues (compartments) in view at once, we will first take a look at the saturation and desaturation of a single model tissue. The diagram therefore shows the ambient pressure on the x-axis and the inert gas pressure in the model fabric on the y-axis.
[1] (Baker, Understanding M-values, 2019)
We spend a dive at a certain depth, saturate a model tissue to a certain point – and then, of course, want to end the dive. In doing so, we will inevitably have tissue saturation above ambient pressure at some point during the ascent. Of course, only then can you desaturate.
The relevant question, in fact the only really important question, is now: how far can you reduce the ambient pressure without the nitrogen (or any other inert gas in mixed gas diving) causing problems in the body? This is exactly what the important topic of M-values is all about.
M values
The limits of oversaturation
Unfortunately, gases do not always diffuse out of the body unimpeded, but can form bubbles if the pressure change is too abrupt, and if some unfavorable factors combine. A certain amount of these bubbles is usually tolerated by humans. However, many and large bubbles in the body can also cause a variety of problems, precisely that decompression sickness (DCS).
It is impossible to predict exactly when this will happen. But one can determine values at which the statistical risk of falling ill is low – very roughly summarized, this is exactly the way many decompression models have come about. Statistically determined risks are used to set limits up to which the risk is considered to be “acceptable.” Acceptable is a level of risk accepted by the diver:s following the model, and where exactly that lies is a matter of agreement – risk is never zero, and the outcome for a specific person on a specific dive is unpredictable.
THIS risk is what is meant by the M-value: from here on, your risk of getting DCS with clear symptoms is higher than accepted by the modelers – and the risk grows more and more from here on.
Where exactly do these limits lie? How are they even known? And why is the search for limits like poking in the fog?
Thinking of the process of surfacing in our diagram, we have in the left triangle the area that we have to dive through as we ascend: As the ambient pressure drops, at some point the pressure in the tissue is above the ambient pressure
We are faced with the question of exactly how far we want to go with our current tissue saturation every time we ascent. The fact that overpressure inevitably occurs in various tissues cannot be avoided. The higher this overpressure, the greater the risk of DCS – at the same time, it is necessary if you want to surface.
