Decompression accident

Text Author: Laura Vernotico
Photo credit: Marco Daturi

To understand how it is possible to get into an accident
decompression we must first understand how the gas enters our
body (passage of the gas into solution) and how the inert gas is eliminated from the
our organism.
Let's start by analyzing how absorption occurs in our body.
organism.

Divers during safety deco

The basis of this concept is Henry's law which
statement is: at constant temperature, the amount of gas in solution is
proportional to the pressure the gas exerts on the liquid. The volume
total amount of gas dissolved in a liquid (when we are in a state of
saturation) depends on the solubility of the gas, the volume of the liquid and the pressure
with which the gas presses on it and the temperature.
In short: a gas that exerts pressure on the surface of a
liquid, it enters into solution until it has reached the same level in that liquid
pressure exerted on it. Once equilibrium is reached, the liquid
defines saturated with that gas at that pressure. This state of equilibrium
remains as long as the external gas pressure remains unchanged,
otherwise, if it increases, more gas will enter into solution; if it decreases, the
liquid will be in a supersaturated situation and the gas will be released
returning to the outside until the pressures are balanced again.

Air bubbles expelled by divers during deco

Let's now see how the inert gas is eliminated from our
organism. For the purposes of calculating decompression it is important to take into account
from the:

• duration of the dive calculated from the moment of entry
  in the water until the first stage

• speed of descent to the bottom (a rapid descent)
  crushes the microbubbles, a slow descent causes the absorbing fabrics
  nitrogen more slowly begin to charge the inert gas already during the
  descent)

There are two approaches to calculating decompression:

1. il compartmental system (elaborated by prof.
   Buhlman starting from the studies of Prof. Haldane creator of the US tables
   Navy),

2. il training and development control system
   some bubbles (Varying Permeability Model – VPM, Reduced Bubble Gradient
   Model – RGBM, Tissue Bubble Dynamics Model – TBDM)

What is the difference between these two models of
decompression?

The compartmental model predicts that during the
decompression all the nitrogen accumulated in the tissues is released into the blood as
dissolved gas and is then eliminated through exhalation from the lungs. The law
The basis of this model is Henry's. Bubbles form only if there is no
respect the decompression stops and the maximum ascent rate. According to
this model, in a dive in safety curve, the fabrics can
can withstand twice the nitrogen normally present on the surface (up to 1,6
bar). If the amount of nitrogen released from the tissues during ascent is greater than
at double the normal speed, then you have to stop for a safety stop, called
decompression stop (3 meters, 6 meters, 9 meters, etc.).
Field research, however, says that after every dive there are always some
bubbles (more or less numerous) and that bubbles do not always create problems. In
In our blood there are always microbubbles with a diameter of less than 10 microns
(therefore smaller than the diameter of a red blood cell which measures 8
micron).

Le micro bubbles present in our organism are of
two types:

1. short-lived microbubbles (a few hours at most);
   They arise from the swirling of the blood as it passes through the heart valves,
   from muscle and joint movements (be careful when doing physical activity
   right before the dive!)

2. long-lived microbubbles (up to a few days); they derive
   from previous dives.

The danger of the bubbles depends on their number and
their size. Translated into practical terms, we can say that after diving
without decompression stress (within 30-40 meters, in safety curve, only one
diving per day, etc.) few bubbles form and these do not cause problems.

As soon as the diver begins to ascend (the external pressure on the body is
reduces and the accumulated nitrogen leaves the tissues to pass into the blood) 90%
of the nitrogen contained in the tissues
diffuses into the blood to be eliminated with exhalation, while 10%
penetrates the bubbles that are always present in circulation.

What does the ability to enter depend on?

From the resistance that the bubble offers: the smaller the
(compressed) the bubble, the greater the resistance it offers to entry
of nitrogen. This is the principle of Laplace's law. Obviously when
nitrogen enters the microbubble, it starts to grow and if it grows
too much ends up breaking and creates bubbles of smaller size
inferior. The bubble, like the dissolved gas, reaches the lung. The capillaries
surrounding the alveoli hold bubbles with a diameter greater than 10
microns, at this point the nitrogen comes out of the bubble and is eliminated with the
exhalation. However, if you dive with decompression stress (dives
over 30 meters, multiple dives per day, multiple dives in multiple days
consecutive, reverse profile dives) or decompression errors, here
that many bubbles and large bubbles form. At this point the very large bubbles
they can directly damage the vessel wall, or our organism
recognizes the bubbles as foreign and attacks them (developing a real
inflammation). The larger the blisters, the greater the probability
to have problems.
Diving often (more than 40 dives a year) is good because it helps you
They crush the microbubbles which become more resistant to nitrogen ingress.

We have seen that bubbles begin to form at the moment of detachment from the
bottom. When can we consider them gone? Four hours after the end
of the dive. This is why it is important to respect a surface interval
at least two hours between dives.

Hyperbaric chamber in Sharm

After seeing how our body absorbs
nitrogen and how to eliminate nitrogen during ascent and for
minimize the chances of experiencing a decompression accident
(DCI), let's see what the causes of DCI are.
The cause of the decompression accident is the bubble, but not only her or
at least not always. If the bubble is large, then it will occlude
mechanically the blood vessel, but when it is small, smaller than the diameter
How does the blood vessel cause damage? In this case, it is our body.
which recognizes it as foreign and triggers a response
inflammatory. This also explains why sometimes, even when diving
identical, out of two subs only one experiences DCI (same bubbles, different response
inflammatory). The concept of inflammation also explains why some
decompression accidents occur within 24 hours of completion
of the dive.
Secondo Haldane (assumption on which the US Navy tables are based)
bubbles formed only if the maximum ascent speed was not respected
and when the decompression stops were skipped. This concept is based on
many computers (called compartmental) based on the professor's programs
Buhlman and called Haldane modified. Today we know that this is not exactly the case.
It is very important to recognize that our body makes bubbles.
It is essential to dive in good health to minimize the risks.
compromising factors; hence the importance of the suitability visit
to underwater activity, which must be conceived as a visit that
allows you to go into the water more safely.
Scuba diving, unlike other sports, is burdened by the so-called “risk
intrinsic” due to the environment in which it takes place. The thing that must be
It should be noted that scuba diving is a sport that is practiced in a "safe environment"
extraordinary”, that is in hyperbarism (underwater and even if the dives are
without decompression, it is still necessary to respect the times and methods for the
resurfacing).

Interior of the hyperbaric chamber in Sharm

In general, the dive itself may not be strenuous:
the fatigue problem arises when you find yourself diving in
current or with particular problems (deviations from the planned dive),
regardless of depth and decompression stops. If you are in
current…..you have to deal with it, while on the treadmill, during a
aerobics class or a swim in the pool, you can slow down or even
stop.
For these reasons I believe it is essential that a diver carries out the so-called visit
competitive with a sports doctor who has specific skills in sports medicine
underwater and scuba diving activities, as the diver often expresses doubts about
problems related to diving (headache, difficulty equalizing,
finning problems, etc.) which require specific expertise in the field.

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