RMV (or SAC) and Bends: Deco software Vs. Diver's Physiology

[Guacharo]

Few weeks ago I was talking with a colleague about a DCS accident among a group of tech diving students that has lead us to make a further research.

The case is that 5 divers went into a tech diving training dive (the 5th since the beggining of the course) which was somehow physically demanding. All of them were young and relatively equally shaped but one of them consistently had a much higher gas consumption than the rest of the group in all previous dives.

A couple of hours after the dive, the guy developed a DCS that needed two recompression treatment to resolve. Despite that the higher consumption was not taken as a factor for a more conservative dive planning (perhaps because it was a skill oriented dive), it made me simulate the profile in several decompression programs.

I have to admit that although one teaches to have in mind higher RMV as a risk factor, in recreational diving is very very rare to have such an DCS due a higher air consumption (something very common in new divers). Yet in Tech diving now it seems to me a real concern, and further more because no deco software take it in count for planning a deco profile.

Assuming that higher consumption leads to higher DCS risk in deco profiles, my questions are:

A- Does anyone know which RMV or SAC are used as "control" value within the most used deco program like Decoplanner, GAP or Vplanner.?

B- Is this value used just to predicts how many liters a diver would demand?

C- Exist, for any deco Algorithm, a "control" value like 18 lts per minute or something?

D- Being above or over this value should be a factor when planning a dive. If so how you can manage it in a computer profile ?


PD: I spent few hours looking for a thread the could cover my doubts with no success so if my question is duplicated, my apologizes.

 

[amascuba]

I've never heard of anybody saying that having a higher consumption rate as a risk factor in DCS. Having a higher consumption rate may contribute to CO2 retention, but DCS? It may also mean that the diver is uncomfortable with the dive (if that's the case then he shouldn't be doing the dive in the first place), out of shape cardio wise, or maybe even smokes.

The dive planning applications don't take into account the consumption rate when figuring the decompression obligations. It's simply a tool to figure out how much gas you will need to bring with you during the dive.

 

[MookieMoose]

Increasing RMV does not increase DCS risk. How quickly you empty your tank has no impact whatsoever on how you ongas. Only partial pressure (depth) and exposure time affect ongassing. RMV will affect how long you can stay down, because, at a very simple level, you have to come up when you run out of gas. As mentioned above by amascuba, increasing RMV may also lead to increased CO2 retention if one is taking shallow, rapid breaths as opposed to filling their lungs, but this does not really affect DCS risk.

 

[grazie42]

Deco programs do not adjust deco depending on RMV (at least v-planner doesen´t)...
RMV in itself shouldn´t effect risk of dcs but it may be (depending on the individual) that a higher RMV is a symptom of something else (bad shape, bad technique, fatigue, high workload, anxiety etc) that may in turn predispose a person to dcs...

 

[Guacharo]

First, thanks to all for taking a time to post.

Looking for more information, I found this at scuba-doc:

"There is a hypothesis that attempts to relate the surface air consumption (amount of nitrogen breathed) with the amount absorbed and thus to the risk of a decompression accident. Although surface air consumption is quite important in developing a dive plan and decompression tables, it has little to do with nitrogen absorption at depth - a function of depth/time and partial pressures. (CO2 and exercise might have some part to play in this situation)."

 

[Guacharo]

From the same article...

In this hypothesis, nitrogen, as the inert air component, is not a driving factor for surface air consumption (SAC), rather 02 and increased CO2 are; conditions that increase demand for 02 or production of C02 increase air, or gas, consumption and thus, nitrogen uptake is faster at depth until the tissues are supersatured.

Thus, if a diver had an inherently lower SAC rate, say less than the average of 0.5 cfm, the result would be that the chances of getting DCS are lower than someone with the average SAC, which is assumed to be the SAC rate operant when dive tables were tested and designed. Conversely, for those with SAC above the average, the likelihood of getting DCS is greater even if dive times are substantially lower than the dive table values, as these divers supersaturate sooner. .

As allways in deco issues, there is too much to research everyday

 

[Dr Deco]

Hello Guacharo:

A- Does anyone know which RMV or SAC are used as "control" value within the most used deco program like Decoplanner, GAP or Vplanner.?

These algorithms are based on previously published tables [or possibly dive data from the field]. As such, the divers are a general population and exhibit all degrees of Surface Air Consumption. There is not a standard SAC for table development.

B- Is this value used to predict how many liters a diver would demand?

Air consumption is not a part of any dive table.

C- Does there exist for any deco algorithm, a "control" value, e.g., 18 liters per minute or something?

Again. air consumption is not a part of any dive table.

D- Being above or over this value should be a factor when planning a dive. If so, how you can manage it in a computer profile ?

Very little nitrogen is removed from the lungs and transported into the body; therefore, air consumption is a trivial factor. However, if air consumption changes because of workloads, then this is an indicator that heart rate is elevated. This being true, it could indicate an increase in tissue perfusion [blood flow]. Compartment halftimes could be shifted, but this is not done in any model for diving [only for the NASA model for “spacewalks.”]

Dr Deco :doctor:

 

[lmorin]

....
Very little nitrogen is removed from the lungs and transported into the body; therefore, air consumption is a trivial factor. .....

I'd appreciate further explanation about this. It would seem to me that the nitrogen accumulating in the blood during a dive has to come from outside the body (i.e., through air intake) and is not the metabolic product of internal processes. Maybe that is really only a "very little" amount, as you state. However much it is, clearly it becomes very non-trivial for some people. Isn't the principle of nitrogen displacement with other gases behind the use of nitrox and trimix?

I have also wondered about the relationship between incidence of DCS and air consumption. I was on a boat recently with a very slim, petite woman who consumed only 1500 lbs after a 50 min dive to 100 ft during which her instructor used 2000 lbs. Instructor was blown away by how little air the woman used. Wouldn't that effectively be the functional equivalent of using enriched air, i.e., lower nitrogen exposure? Or, has this not been studied?

 

[Dr Deco]

Hello Readers:

Air consumption is a bad guide to the quantity of nitrogen dissolved in tissues. Some people are nervous and huff and puff a large amount.

There is a possible relationship if an individual was working hard, e.g., swimming against a current, and this activity was reflected in the amount of air consumed. Here, however, we are really talking about an increase in tissue perfusion as capillaries dilate.

When the individual later slows down during the off gassing phase, the capillaries close and the blood flow is reduced. This is not what most people think of when discussing air consumption. Remember that little nitrogen is removed. At complete saturation, the body dissolves about one quart of nitrogen per 33 fsw. That is not much in a cylinder containing many cubic feet of compressed air.


Dr Deco :doctor: