Understanding decompression when scuba diving. Peter Rachow

[Charles B.]

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 fo Peter Rachow llowing
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)
Peter Rachow 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 Peter Rachow 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 Peter Rachow (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 Peter Rachow
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 Peter Rachow 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.