INFO SHEETS > PERFECT YOURSELF
Prevention of desaturation accidents (ADD)
The text below is taken from the book Diving for Pleasure 4.
Studies and evaluations on ADD show a relatively low risk, of the order of 1 in 10,000 with complete remission, after hyperbaric treatment, in most cases. If the speed of the alert and the intervention (oxygen, evacuation to a hyperbaric chamber) is crucial, prevention is essential.
See also: Déshydratation et plongée, diurèse d’immersion.
The formation of bubbles during desaturation, which begins with each ascent, involves:
- The actual amount of inert gas (nitrogen) dissolved during the dive, which depends on cardiac output (air consumption). It is specific to each individual and depends on the diving conditions. Exertion, cold, shortness of breath, stress are factors that increase this quantity of dissolved nitrogen, at the risk of being outside the assumptions of your computer model.
- The pre-existence of pockets of gaseous germs or gaseous micro-nuclei which promote the appearance and then the enlargement and the release of bubbles.
During desaturation, the amount of gas actually dissolved and the number of gaseous seed pockets dictate the potential for bubble creation. The conditions of desaturation (speed of ascent, levels, profile, behavior, gas breathed on ascent and / or level) and individual factors (some people are more "bubbly" than others) will induce the level of bubbles. real.
These do not cause an accident when they remain small and few in number: they are eliminated by the pulmonary filter on exhalation. We speak of "silent bubbles".
But these microbubbles can also:
- grow in size during ascent or associate with each other to form emboli or gas sleeves which then become pathogenic;
- pass through the arterial circuit, even if their number is relatively low, for example during a Valsalva maneuver at the landing if there is a PFO.
The signs depend on the location of the pathogenic bubbles.
While the risks are relatively low, studies (FFESSM and DAN) show that 75% to 90% of ADDs occur despite compliance with procedures.
This means that the constant improvement of the models now makes it possible to benefit from a safe desaturation, but that the cases which persist are mainly located outside the domain of validity of the protocols (computer, tables).
To dive safely, it is therefore necessary not only:
1) follow a desaturation protocol;
but also take into account:
2) the existence of individual risk factors;
3) dangerous dive profiles;
4) risky behavior.
The greater the amount of nitrogen accumulated, the greater the likelihood of ADD risk. The main factors to take into account are:
- Dive time.
- The depth.
- Air consumption. Stress, cold, shortness of breath, stress increase this consumption and therefore the level of nitrogen saturation. In addition, some individuals consume more air than others or eliminate nitrogen less well during desaturation (effectiveness of the pulmonary filter).
Julien Hugon, in his thesis defended in 2010 in Marseille on the Biophysical modeling of decompression, sums up the situation perfectly: “The fact of exerting efforts during an exposure increases the risk of generating a decompression sickness (Vann and Thalmann 1993). This is due to the fact that an activation of the circulation increases the saturation speed of certain tissues (skeletal muscles, skin, tendons, etc.). Also, for the same type of exposure, the amount of gas that can be dissolved in the body can be significantly higher if exercises are performed, compared to a situation at rest. To protect against decompression sickness effectively, the decompression times must then be increased (Vann and Thalmann 1993) ”. - Sont également à considérer :
- The surface interval between two dives. The shorter it is, the less time the body has to eliminate excess nitrogen from the previous dive.
- The number of dives carried out during the day, during the week ...
Every diver should keep in mind that their nitrogen saturation level and therefore the risk of ADD depend not only on the depth and time of the dive, but also on the air consumption and the reactions of the organism. While desaturation models are reliable, they still cannot offer an absolute 100% guarantee of divers in 100% of dives. You should not therefore blindly trust a computer or desaturation tables.
Individual risk factors are factors favoring ADD risks. By definition, they are not taken into account by the desaturation models because they cannot be generalized. By way of illustration, here is a list freely inspired by that of Comex and which does not commit it:
- Poor physical shape, long trips to go diving, lack of sleep, excessive fatigue, including psychic stress: stress at work, long-lasting family or professional problems, job loss, divorce, etc. ;
- Poor lifestyle: tobacco (this would increase blood viscosity by 3 or 4, alcohol, too rich food, etc.);
- Age> 40 years old;
- Weight ;
- History of serious illness, regular medication;
- Lack of recent practice: previous dives dating back several weeks. For French recreational divers in Metropolitan France, the typical case is that of the dives in May, at the start of the season. This is the period when the CROSS observes the most serious diving accidents because many divers are sedentary during the winter, rather than planning a phase of gradual readjustment to the
depth, take the opportunity to dive to the limit of their prerogatives. Common sense would however want us to be careful and that after an interruption of a few weeks or months, we resume the activity gradually, like what is practiced in mountaineering or in aeronautics for example. Profile at risk: 50 years old, 50 m, seasoned diver.
Depending on whether you have 2, 3, 4 or more of these contributing factors, you will need:
- limit the depth (eg 20 m);
- limit diving time;
- use nitrox;
- respect an interval between two dives of at least 3 to 4 hours;
- increase the levels (for example by using the "personalization" mode of your computer);
- only do one dive per day;
- or even temporarily giving up diving when too many factors are present.
Fast ascents, yo-yos and sawtooths cause the creation of bubbles and increase the risk of desaturation accidents. These profiles are inherently dangerous and are not taken into account by desaturation models (and are therefore not taken into account by dive computers).
It has now been proven that certain dive profiles involve risks, by increasing the
number of bubbles they cause.
These are :
- The fast lifts.
- The "yo-yo" or "sawtooth" dives (even if the ascent rate is slow), an ascent of 10 m (1 bar of pressure variation, regardless of the depth) being sufficient to create bubbles whose the physiological effects are not neutralized by a descent.
- Consecutive dives, and close successive dives. DAN Europe's study on the subject showed that the quantity of bubbles detectable by the Doppler effect was almost twice as large in the context of repetitive dives than for unit dives. In order to limit this phenomenon, it is recommended to respect a delay of at least 3 or 4 hours between two dives.
consult Pleasure Diving Info Sheet on the subject.
The case of inverted profiles is more controversial. During a simple dive, this consists in reaching the greatest depth at the end of the dive, then to rise to the surface (for example, diving along a drop off). In repetitive diving, this amounts to descending deeper than during the first dive. Hamilton and Thalmann indicate on this subject: no convincing evidence has shown that inverted profiles in no-decompression diving lead to an increased risk of decompression sickness […] for no-decompression dives below 40 meters and whose depth differential is less than 12 meters.
The dangerousness of this type of profile therefore depends on the depth, the dive time and the difference in depth between the two dives.
In the absence of certainty on this point, we cannot recommend enough to dive directors and dive guides to remain cautious.
Beyond respecting the correct diving procedures and profiles, individual behavior is also a key element in accident prevention.
Here are some rules of good practice:
- Avoid chest hyperpressures, they can cause the opening of cardiac (patent foramen ovale) or pulmonary shunts, which allows the passage of venous bubbles in the arterial circuit. Valsalva maneuvers on ascent or landing must therefore be prohibited, as well as violent efforts (for example, lifting an anchor by hand, force-raising a bottle on board a tire, etc.).
- Avoid playing sports, and in general, any violent effort (for example, pulling up a large mooring), within 2 hours of a dive. In an organism supersaturated with nitrogen, this can only promote the creation of bubbles.
- Also avoid freediving less than 6 hours after scuba diving. This hinders the natural elimination of nitrogen, with unmeasured consequences on repetitive dives. In addition, the multiple phases of descents and ascents, often rapid, increase the risk of the appearance of bubbles.
- Climbing to altitude or flying after a dive can promote the onset of a desaturation accident (see Plane, Altitude and Diving Info Sheet).
- Generally, it is advisable not to make more than 2 dives per 24 hours, with a break every 6 or 7 days. However, the now widespread use of computers and the development of tourism over short periods (1 to 2 weeks) mean that practitioners want to dive 3 or 4 times a day, in order to fully enjoy their vacation.
Faced with this reality, remember that a computer calculates a desaturation profile from a mathematical model valid for 2 dives per 24 h, successive or not. Beyond that, a computer still performs the calculations but there is currently no indication that the data displayed is reliable. Under these conditions, the use of a computer does not exempt you from respecting the “2 dives per day rule”… or from taking responsibility.
Vidéo
Voir également la vidéo sur la reprise progressive de la plongée.
© Extract fromPleasure Diving booksby Alain Foret at Editions GAP.
Any reproduction prohibited on any medium whatsoever without the written consent of the publisher and the author.
