viernes, 18 de octubre de 2013

Velocidad de Ascenso Lenta VS Ideal tras una Inmersión

Muy buenas,
Ayer me topé con este artículo que trata sobre la conveniencia o no de hacer un ascenso lo más lento posible: 

Referencias (en el orden presentado arriba):

Al parecer, tiene sentido el hacer un ascenso lo más lento posible pero también lo tiene el hacerlo lo más rápido posible dentro de la velocidad máxima de ascenso marcada por el ordenador ya que conseguimos empezar a desaturar antes.
Habrá que estudiarlo pero parece una teoría interesante.

Actualización a 20/10/2013: Ok, ya me lo he empollao!
Lo que dicen los manuales de nuestros ordenadores de buceo (Suunto Vyper y Gekko) sobre la velocidad máxima de ascenso:
There must be an optimal ascent rate that would reduce unnecessary exposure to depth and provide a sufficient decrease in pressure to allow offgassing but be slow enough to protect divers from DCS. This ascent rate is thought to vary depending on the depth, tissue saturation and breathing gas. In saturation diving, the decompression rate is on the order of several feet per hour, while in short, deep diving it is on the order of feet per minute — faster at depth and slower close to the surface. In recreational diving, where the depth and exposure are limited, a maximum ascent rate may be specified without regard to depth.
while in short, deep diving it is on the order of feet per minute — faster at depth and slower close to the surface. In recreational diving, where the depth and exposure are limited, a maximum ascent rate may be specified without regard to depth.
The U.S. Navy and the National Oceanic and Atmospheric Administration (NOAA) use a rate of 30 feet per minute, and recreational dive-training-agency recommendations range from 30 to 60 feet per minute.
Es decir:
-Reducir exposición inmecesaria a la profundidad y disminuir la presión para permitir la desaturación a la vez que proteger contra la DCS (decompression sickness).
-30 to 60 feet per minute = de 9 a 18 ms por minuto. &name=DLFE-133.pdf
Reduced gradient bubble model:

The reduced gradient bubble model (RGBM) is an algorithm developed by Dr Bruce Wienke for calculating decompression stops needed for a particular dive profile. It is based on the Varying Permeability Model.

It is used in several dive computers, particularly those made by Suunto, Mares, HydroSpace Engineering, and Underwater Technologies Center.

Manufacturers such as Suunto have also devised approximations of Wienke's model. Suunto uses a modified haldanean nine-compartment model with the assumption of reduced off-gassing caused by bubbles. This implementation offers both a depth ceiling and a depth floor for the decompression stops. The former maximises tissue off-gassing and the latter minimises bubble growth.
Suunto Reduced Gradient Bubble Model:

Deep Safety and Decompression Stops:

It has long been known that deep stops benefit divers performing decompression dives.  The same rationale behind deep decompression stops holds true for safety stops.  What technical divers know, but many recreational divers don't, is that all dives are decompression dives whether the dive requires a mandatory stop or not.  The reason all dives are decompression dives is simple - the diver is ascending from under pressure (i.e. decompressing). 

What must be kept in mind is that even though a dive is within NoStop time limits, the diver is probably experiencing asymptomatic bubbles (minor bubbling that does not cause DCI).   This is not unusual and causes no harm since the body can be forgiving.  This brings us to our first topic...

Why Safety Stops?

Every diver has been told to perform safety stops, yet the vast majority of divers I see never do one.  Why?  Probably because they are not understood.  Many divers have been informed that performing a safety stop allows them to slowly eliminate nitrogen.  This is simply not true.  A safety stop assists the body in rapidly eliminating nitrogen!  Why?  The reason is very simple.  Bubbling does not occur in a diver under pressure, it only occurs when the pressure is reduced "too much".  Once bubbling occurs, gas elimination is reduced.

To put it even simpler, a diver that performs a 3 minute safety stop after a dive will have less nitrogen in their body immediately upon surfacing as compared to a diver that did not perform a safety stop, but has been on the surface "off-gassing" for 3 minutes. 

Therefore, one of the best things a diver can do for themselves is to perform a safety stop, no matter how short, after every dive.

How Deep?

Since the goal is to promote nitrogen elimination prior to surfacing so the risk of DCI is lessened, the next question is how to calculate the depth of a safety stop.  Years ago,  a safety stop was recommended at 10 feet after every dive.  This recommendation wasn't so much a depth recommendation other than a recommendation to just perform a safety stop.  The depth of 10 feet was chosen because the final stop on decompression tables was at 10 feet. 

Later, the recommendation turned to 15 feet and is now currently 15 to 20 feet for 3 to 5 minutes.  A deeper depth of 15 feet was chosen for several reasons, but a driving force behind this deeper recommendation was overwhelming information that deeper stops promote greater nitrogen elimination.  My favorite example was a fairly well controlled study where two control groups were taken to the same depth for the same time and then performed safety stops for the exact same time, but at different depths.  One group performed a safety stop for 5 minutes at 10 feet while the other group performed a 1 minute stop at 20 feet and then a 4 minute stop at 10 feet.  Even though both groups did 5 minutes of safety stops, the group that started at 20 feet had significantly less bubbling not only upon surfacing, but also over the hours that followed the dive.

If you haven't really performed safety stops before or haven't paid much attention to them, the procedure I just described may sound boring.  But try it.   I don't think you will find it boring.

 When the safety stops are actually broken up into different depths, the time passes very quickly.  If you also understand why safety stops are one of the best things you can do to reduce the risk of DCI, it shouldn't seem boring and a waste of time, especially when compared to the time that could be spent in a recompression chamber and knowing that you will be surfacing with less nitrogen in your body as compared to not performing a stop.

Ascent Rates
Sport Diving

So what should the ascent rate be?  If just one ascent rate is chosen, then it should be 30 feet per minute.  But for those that want to be more technical and advanced, two ascent rates can beused.  When diving deeper than 60 feet, use an ascent rate of 30' per minute.  And when diving to 60 feet or less, use an ascent rate of 20 feet per minute.

Getting More Advanced

80' dive and deeper, the first stop is one-half depth plus 10'
Less than 80' the first stop is one-half max depth.

Nitrox 32:
Can do stops as if on air, can use an Equivalent Air Depth for your diving depth to calculate the first stop, or may do the following:
80' dive and deeper, first stop is one-half depth; and less than 80' the first stop is one-half max depth minus 5'.

Air Dives to 70' or deeper:
A 20' stop is always required.

Air Dives to 80' or deeper:
Two minutes at 20' is always required.

Air Dives to 100' or deeper:
Instead of doing 2 minutes at the first stop, a diver may do a 1
min. stop at first depth and then ascend 5' to do the 2nd minute of the initial stop.
This should not be done when diving to less than 100'.
Decompression Strategies

The starting point is to understand more about what causes DCS. Everyone understands that excessive bubbles will cause DCS.  But what a lot of divers don't realize is how these bubbles start ... which will help a diver to understand how to decrease their risk. 

When a diver goes underwater, the diver's tissues start taking in inert gas due to the pressure.  So when the diver surfaces, the tissues have an excess of inert gas in them.  But, contrary to what divers believe, this is not the problem.  A diver could take up a very large amount of inert gas and never get DCS if it wasn't for one thing ... the presence of micronuclei ... or bubble seeds.  In other words, a diver could dive to very deep depths for very long times without ever needing to decompress if it wasn't for the micronuclei.  These micronuclei act as a source for bubbles to start occurring.  They may be viewed as very tiny bubbles themselves.  What happens while ascending is that the gas built up in the tissues from the dive are now in a supersaturated and high pressure state.  This gas wants to start leaving the tissue and escaping.  This gas will go wherever possible.  One route is for it to enter the blood and exit that way.  Another possible route is that the gas leaving randomly bumps into and enters a nuclei.  This nuclei will continue to grow as more gas enters it and/or the diver ascends towards the surface causing it to expand according to Boyle's law.  If this nuclei gets too big, a bubble results.  So why is this being mentioned?  In addition to the obvious point of slowly ascending / decompressing through the water column towards the surface, the control of nuclei is a topic that should be discussed.

 It has been shown that activity increases resulting bubbling from a decompression.  Since micronuclei can't be seen, they are postulated due to the known increase in bubbling from activity. 

Most activity generates micronuclei, but the activity performed by divers are especially bad for generating micronuclei such as hauling gear to and from the water or climbing up the ladder onto a boat.  The greater the activity, the more nucleation that will occur.  It doesn't matter when the nucleation occurs.  It can occur from pre-dive activity or post-dive activity.  As long as there is an excess of gas in the tissues from the dive, nucleation will generate more places for gas to enter while trying to leave the tissues ... and the more places gas has to enter, the greater the number of resulting bubbles ... and the greater the risk of decompression sickness.  So the moral of the story is not to believe that a model is responsible for your safety, but instead understand that your own activity can be too and be aware that pre and post dive activity (as well as that during the dive itself) can substantially increase the risk of getting decompression sickness.

The conditioning

As much as many of us would hate to admit it, the condition of our bodies do make a difference.  Age has been shown to affect the degree of bubbling from a dive.  Older divers have more bubbles from a dive than younger divers.  The reason is not exactly known but may be accounted for by having more adipose tissue.  Adipose tissue can act as a gas storage area and could result in an overload and release of gas later.

The physical shape of a diver is also important.  It has also been shown that the greater the body uses oxygen, the less risk one has for DCS.  The usage of oxygen simply shows  the oxygen used by the tissues which is an indication of blood flow rates.  Obviously, the better the blood flow the better the off-gassing during decompression or after a dive.

Decompression Tactics

In addition to the rate of decompression through the stops and making sure enough pressure is kept on the diver during decompression, divers can also speed up their off-gassing (as well as in-gassing if not careful at depth) by doing very super mild movements.  We hesitate to call it exercise because that makes it sound like work must be performed ... which is exactly what a diver does not want to do.  Performing work or exercise will generate micronuclei and make decompression worse.  Instead, very slow and ultra gentle movements are desired, such as bending the arms and legs slowly.  It is the change in muscle form that will open up capillaries and make blood flow occur and result in better off-gassing due to the increase in blood flow.  But again, anything more that amounts to activity can generate micronuclei and make things worse ... so always error on the side of caution by doing ultra gentle movements   The exercise that some divers think is helpful during decompression is a big no-no. 

Putting it together

Divers should be aware that they have control over nucleating events and they should minimize pre and post dive activity (including underwater activity).  Minimizing activity may help reduce decompression risk and performing activity will certainly increase it.  Of course dives should be planned in a conservative fashion since factors such as age and body fat can also increase decompression risk.  But putting this aside, some individuals are simply more susceptible to DCS than others.  Since most of these divers do not know who they are until it is possibly too late, dives should always be planned in a conservative fashion.  Very mild muscle movement can also be done to eliminate gas faster during decompression.  But if this is started too deep, there is a possibility that it will also result in more gas being taken in. 

Divers can also remain on their final decompression gas while at the surface to increase the gas elimination rate before the gas can enter bubbles leading to bubble expansion. 0/SPUMS_V33N1_7.pdf
Introduction: The practice of safe decompression in recreational divers has not been thoroughly studied. Among the important variables are ascent rate, decompression or safety stops, and oxygen breathing underwater to eliminate nitrogen.

Actualización a 20/10/2013: Bueno, estamos teniendo un debate interesante en el grupo PADI Pros de Linkedin y me ha servido para entender algunas cosas.
Las páginas 4 y 7 del documento que puse antes (Suunto Reduced Gradient Bubble Model) son bastante reveladoras:

Algunos debates en castellano sobre el tema:
Fíjate, curiosamente, una escuela de buceo técnico americana ya desde hace bastantes años viene divulgando el modelo que tú comentas de realizar SIEMPRE al menos 4 paradas a -12,-9,-6 y 3m. sea la inmersión que sea y como mínimo de un minuto en -12 y -9 y 3 minutos en -6 y -3m.
Conviene diferenciar entre los modelos de descompresion:
- Modelos haldanianos: gestionan el gas disuelto, y se busca la desaturacion mas rapida posible, maximizando el gradiente. Generalmente esto se hace haciendo paradas cercanas a la superficie y extendidas en el tiempo.
- Modelos de gestion de burbuja: buscan reducir el gradiente de manera que se minimice el numero y tamaño de las burbujas en el cuerpo. Incorporan paradas mas profundas y de menor duracion que en el modelo anterior.

El metodo propuesto de 1,1,3,3 es del 2o tipo. Y aun aceptando que se minimice la aparicion de burbujas, no tengo claro que la carga de N2 residual sea menor. Me gustaria ver estudios cientificos al respecto.
Los buceadores técnicos de NAUI han venido utilizando el modelo RGBM en los últimos años y no han registrado ningún caso de ED. Con estos datos y los aportados por DAN, a partir del 2003 NAUI ha sugerido que en el buceo recreativo se haga una parada de 1 minuto a mitad del recorrido de ascenso y luego una parada de seguridad de 2 minutos a 6 m.
Con estos nuevos datos, DAN ha empezado a probar este sistema y se está a la espera de los resultados.
Y habrá que tratar de conseguir el libro Deco for Divers - A Diver's Guide to Decompression Theory and Physiology de Mark Powell:

Actualización a 29/10/2013: Ok, consulta hecha a Suunto. A ver qué me responden!

Actualización a 14/08/2014: Ok, libro comprado:

Actualización 11/09/2014: Pruebitas con el planificador de inmersiones del Suunto DM4:
Por ahora, he empezado probando lo que implica el usar mezclas con aire enriquecido durante la deco.
Se me ocurre que, por ejemplo, también podría simular la influencia de la velocidad de ascenso (el tema del que habla esta entrada del blog).

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