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Problems related with breathing air under high ambient pressure

Understanding decompression when scuba diving.

A lot of research has been done past the last centuries on the topic of coping with pressure-related problems encountered by the human body. In the following thesis we will try to summarize the current standard of scientific discussion for the sports diver with average medical knowledge.

A look on physiology

The human body consists of a lot of of different tissues, which are mainly formed by proteins. These tissues are capable of absorbing gases like nitrogene, oxygene, carbon dioxide and others. Some of these gases are used for chemical reactions inside the cells of the body (e. g. oxygen), others are present in our body but having no physical or chemical effect under normal pressure conditions (e. g. nitrogen).

We will taker a closer look on those gases that don’t take part in bio-chemical reactions. We will refer to these gases as „inert gases“.

Considering quantities

The total amount of the various gases that can be soluted, and therefore stored in the respective tissues mainly depends (on the basis of a mathematical function) on the partial pressure of the single gases, i.e. of the fraction of this gas in relation to the overall compound of all gases.

Other co-factors are the time this pressure is exerted to the tissue and the characteristics of the respective tissue itself (i.e. lenght of the particular so-called half time period).

Partial pressure

Air mainly consists, roughly spoken, of oxygen (21%) and nitrogene (78%). On the surface of the earth the overall air pressure is in the range of about 1 bar (9,81N/cm²). Since breathing air consists of 21% O2, we can say that the partial pressure of oxygen (ppO2) under atmospheric conditions is around 0,21 bar (21% of 1 bar). The same can be deduced for nitrogene, where the partial pressure can be calculated to be around 0,78 bar.

Gas solution in liquids and body tisues

If we breathe, the various gases that are incorperated by our lungs are soluted by our blood and transported to all cells of the human body. What happens with inert gases (i.e. nitrogen)?

Our cells permanently absorb certain quantities of the inert gases. On the other hand, statistically viewed, a certain quantity of soluted gas permanently leaves the tissues per each time unit and travels away with the stream of blood and is excorporated by the lungs. So, if no changes in ambient pressure take place, the saturation of gases in the tissues is nearly constant. But the quantity that is soluted is permanently decreasing according to a exponential curve. The quantity that is soluted depends on the amount of gas that already has been soluted so far and is approximating to a final value.

In opposite to inert gases, some gases undergo changes on their way through the body, for example oxygen that is converted to carbon-dioxide and supplying the oxidant for the energy generating cells, the mitochondriae.

Do all tissues have the same behaviour when soluting gas?

Tissues are sometimes also refered to as ‚compartments‘. We have to become aware of the fact that the speed, in which a certain amount of gas can be soluted in a compartment differs with the specific tissue (i.e. the tissue’s characteristic cell structure).

What does this mean for scuba diving?

While scuba diving (where circumstances are different from apnoe diving), the diver breathes air that is supplied by his diving regulator. This regulator consists, besides a lot of other parts (valves, hoses etc.) of a membrane working as a pressure transferring barrier between the inside of the regulater (supplying the diver’s breathing air) and the outside, where the water exerts a certain pressure depending on depth. So, the diver’s air supply delivers air under conditions of the current ambient pressure of the given depth in water. This is to say that the diver breathes his air under the same pressure like his environment currently holds. If this would not be the case, breathing would be completely impossible, since the diver’s lungs would have to work heavily agains ambient pressure, which they could not do, if the pressure difference between inside and outside of the lungs would be higher than about 0,01 bar (10 mbar). Physiological damages would occur as well.

Effects of changes in partial pressure

If the toal pressure of a mixture of gases increases to a certain degree, the partial pressures of these single gases increases by the same rate.

Example: Under atmospheric conditions the partial pressure of nitrogene is about 0,78 bars. In a depth of 10 m seawater the ambient pressure is 2 bars (1 bar air pressure from the atmosphere plus 1 bar from water pressure). Hence to this increase in ambient pressure the partial pressure of nitrogen also doubles. It is now 1,56 bar.

Because of the fact that in this case the partial pressure of nitrogen in the body tissues has been 0,79 bar so far up to the time the dive started, it will now rise as well, depending on diving depth. The diver, who breathes air under higher pressure, will now quickly find a higher partial pressure of nitrogen in his body. If he had an appropriate tool of measuring the partial pressure of nitrogen in his blood, he would find (in 10 m depth) a scale reading of a ppN2 nearly doubled from that on surface.

Nitrogen as an inert gas

Nitrogen, as we mentioned before, in opposite to oxygen, does not take part in any bio-chemical reaction in the human body. Therefore it is referred to as an inert gas. Due to this characteristic behaviour there are two consequences that arise if air is breathed under higher pressure:
Transported with blood nitrogen comes to all cells of the body. If the ambient pressure increases the partial pressure of N2 also increases. This means that more nitrogen will commence to penetrate into the cells of the body than the amount that is currently leaving. How fast this absorbing process can take place depends on the half.time of the particular tissue.

Nervous cells for example are very prone for fastly absorbing N2. Their half-time in addition is fairly short (in the range of some minutes)

=> The phenomenon of N2-Narcosis
=> The problem of decompression disease (DCS)

The phenomenon of N2-Narcosis

If a certain partial pressure of N2 is reached, the nitrogen can cause effects with the nervous system. These effects are generated by interferences that takes place in the synapsae, which can be understood as connection zones between the single nerves. These synnapsae are responsible for transmitting information through the body by connecting the nerves and transmitting electrical potentials. So, in a simplified manner, they can be seen electrical relais or switsches.

In the human brain and in central nervous system, the densitiy of synnapsae is very high, so that the negativ effects of N2-narcosis mainly show in the behaviour of an effected diver.

N2-narcosis goes along with a certain dizziness and (frequently) a serious misestimation of the environment. Some divers describe this special state of mind as a very easy feeling (similar to being slightly disoriented by consuming too much alcohol), in grave cases some were seen trying to do regulator sharing with a fish.

It is stated that a diver can get accustomed to N2-narcosis to a certain degree loweinr the seriousity of the symptoms in a given case. In addition we found out by own experiments, that reducing the symptoms of N2-narcosis can be achieved by mental training and by constantly anticipating the situation under water. In other words: if you know about the risk of getting struck by N2-narcosis, this will decrease your risk to a certain degree. But these possibilities should not be overestimated.

As an instant cure ascending has proved to be highly effective. But it should be avoided to exceed maximum ascending speed (normally 10 m/min) and skipping decompression stops.

Decompression sickness

The second problem that arises when deeper dives are exerted using air is decompression sickness. As we have pointed out before the various tissues of the body are constantly absorbing nitrogen when the ambient pressure becomes higher.

The mathematical function of this absorbing process has exponential character approximating to a certain peak value (state of saturation). This peak value can be understood as the current ambient pressure. Given infinite time, the partial pressure of nitrogen inside the tissues would be sooner or later the same as the partial pressure of nitrogen in the respired breathing air. But with scuba divng this never takes place and this so-called state of saturation only approximately can be reached for very fast compartments (e.g. the nervous system).

After having spent some time under higher ambient pressure, when a diver finally ascends again towards the surface the ambient pressure decreases and the partial pressure of nitrogen inside the tissues is now higher than that outside. So, the reverse process of absorption starts: Nitrogen is released from the single compartments into the blood.

The quantity of nitrogen that is set free from the tissues depends functionally on the following parameters:

- Partial pressure of nitrogen in a given compartnent (ppN2)
- The ambient pressure

As the process of saturation is functionally dependent on the specific half-time of a compartment (i.e. the time, that it will take to increase/decrease the partial pressure by factor 2 if changes in ambient pressure are applied) it is clear that tissues with comparitively long half-time periods are not capable in storing larger quantities of nitrogen, because the dive time with an average scuba-dive ist too short. So, for these „slow“ compartments we can deduce that the quantities of nitrogen they release during the ascention phase of the dive can be more or less neglected.

For scuba divng the „fast“ compartments with half-times between 2 and 20 min. (e.g. nervous tissues) are more affecting. A fast tissue can, under certain conditions, be satured more or less with nitrogen to ist peak value. When ascending rapidly this nitrogen is released from the cells into the blood. Blood, as every liquid, is capable of soluting only a certain quantity of gas (depending on a large number of parameters).

When ascendng to fast (and by this lowering ambient pressure rapidly) now there is more nitrogen released from the cells than can be soluted by this time in the blood. Now nitrogen bubbles are likely to occur. These bubbles that now occur in the streaming blood can be transported into the nervous symptom and block arteriae for example, thus partly or completely cutting off the blood stream in a given area of the human body and leading to damage of tissues. The effects are often going along with damages of the nervous system. We often face paralyses of extremeties or the whole body down from the upper regions as a result of decompression sickness.

What can be done to avoid nitrogen bubbles to occur in the body

The main problem going along with nitrogen bubbles is, as we pointed out, ascention speed. If a certain (still to be calculated) speed is not exceeded the release of nitrogen from the tissues is slow enough, so that all nitrogen can be transported to the lungs by the blood and can leave the body using the normal way.

Peter Rachow