Recreational Trimix NDLs

[guyharrisonphoto]

Hi Everyone,

My wife has a medical issue that resulted in her doctor recommending that she use significantly lower 02% nitrox mixes to avoid a risk of 02 toxicity. The mixes are so low that she will lose a substantial portion of the increased NDLs from nitrox.

My question is whether she can regain some of this lost benefit by using recreational trimix. Yes, even for dives 60' or deeper (above that, we can just use air). Will the reduced fraction of nitrogen restore some extra bottom time, or will the "helium penalty" offset that? I have read that there is new thinking on the helium penalty being more conservative than necessary. But, these discussions have been in tech diving forums and do not seem to relate to much more simple rec diving.

I know trimix is much more expensive, but hopefully not prohibitive for an HP 100 tank, and we want to explore whether it would be worth it.

I have looked for no-decompression tables for a standard trimix like 21/35, but have not been able to find anything. I do not, right now, have a trimix computer and so cannot use the dive planning function to compare to nitrox NDLs

What software would you recommend so that I could find NDLs for trimix down to 130' max? Also, what agencies teach recreational trimix without requiring full tech training complete with doubles, which we will never be doing?

Any insight would be appreciated!

 

[northernone]

Unfortunately you won't find the increased NDL you hope in trimix. With the helium penalty ignored (unwise currently) it would be nearly equal to nitrox with the same o2%.

For shallow single tank trimix training:
RAID
IANTD
UTD
PSAI

More great info on this thread:
https://www.scubaboard.com/community/threads/training-for-recreational-trimix.443159/page-1

 

[RainPilot]

The problem is that since the atmosphere has almost 0 helium in it, the surfacing pressure differential is higher for helium than for a similar tissue pressure of nitrogen. In other words, if you dived 21/79 helitrox (no nitrogen at all) you would have lower NDL than for air since the surface inert partial pressure would be zero rather than 0.79 for nitrogen.

The only benefit to helium is narcosis management (as well as gas density but thats for depths outside this OP).

Subsurface is free and can do trimix planning.

For 2 dives to NDL at 20m, one on 21/50 and one on air, the NDL for the trimix is 24 min and for air is 36.

 

[KenGordon]

Hi Everyone,

My wife has a medical issue that resulted in her doctor recommending that she use significantly lower 02% nitrox mixes to avoid a risk of 02 toxicity. The mixes are so low that she will lose a substantial portion of the increased NDLs from nitrox.

My question is whether she can regain some of this lost benefit by using recreational trimix. Yes, even for dives 60' or deeper (above that, we can just use air). Will the reduced fraction of nitrogen restore some extra bottom time, or will the "helium penalty" offset that? I have read that there is new thinking on the helium penalty being more conservative than necessary. But, these discussions have been in tech diving forums and do not seem to relate to much more simple rec diving.

I know trimix is much more expensive, but hopefully not prohibitive for an HP 100 tank, and we want to explore whether it would be worth it.

I have looked for no-decompression tables for a standard trimix like 21/35, but have not been able to find anything. I do not, right now, have a trimix computer and so cannot use the dive planning function to compare to nitrox NDLs

What software would you recommend so that I could find NDLs for trimix down to 130' max? Also, what agencies teach recreational trimix without requiring full tech training complete with doubles, which we will never be doing?

Any insight would be appreciated!

I am going to suggest learning to do the stops and dive air. Maybe dive air and deco on 50% but only on the shallow stops. Another plan is to use a weak Nitrox.

Helium generally makes deco worse. Pershap that is a mistake, but that is how the numbers come out, mostly, at the moment. Helium is expensive, hard to get and cools the diver more than air. It is a load of hassle for no benefit.

You might want to get a second opinion too.

 

[tomfcrist]

As stated before, helium isn’t going to give you any more or less bottom time....

You say that her medical condition could cause her to Tox easier, which confuses me. The only conditions physically that come to my mind that would increase the chance of toxicity are neurological (read epilepsy?) or some blood flow/lung issue that limits CO2 elimination....I’d be hesitant to dive without a lot of forethought in example A, and hesitant to run ppo2 above say 1.2 in the latter.

If the issue is CO2 retention, helium does have the benefit of lower work of breathing, thus lessCO2 production ..but whether that’s enough to outweigh physiological limitation??? I don’t really know.

 

[Billy Northrup]

I think it would be a great idea to have her Doctor tap cones with the good people at DAN

 

[DogDiver]

Anicdotal studies show women using sudafed or birth control pills have a lower oxygen toxicity threshold. Research is ongoing.

 

[guyharrisonphoto]

Thanks everyone for the insight and information. My wife would be on a medication that would dilate her blood vessels which would increase her blood flow and so increase O2 absorption more than "normal". The diving doctors at Duke University told her that she would have to adjust her PO2 computer setting to no more than 1.0 as opposed to the standard 1.4. They also generated a set of custom nitrox MOD/NDL tables. Basically, when everyone else is diving 35%, she will be diving around 28%.

Looks like trimix might not be the answer, but it was worth asking and I appreciate the responses for sure.

 

[stuartv]

@guyharrisonphoto: Something else to consider: There seems to be growing evidence that a Ketogenic Diet (KD) has significant benefits in lowering the risk of CNS Ox Tox (as well as many other health benefits).

Ketogenic diet for high partial pressure oxygen diving. - PubMed - NCBI

https://www.nature.com/articles/s41598-018-20933-w

News: ONR-Supported Research Fights Oxygen Toxicity in Navy Divers - Office of Naval Research

You can google it yourself for more info. Those are just some quick links I found.

To summarize my VERY layman interpretation: Ketogenic Diet means high fat and low carb (roughly 75% fat, 20% protein, and 5% carbs). The benefit that may reduce risk of OxTox is that it produces a higher level of antioxidants in the body - and especially the brain. Apparently, it is pretty well accepted that a KD is very useful for treating epilepsy patients and reducing their incidence of seizures. At the very least, it seems that there is general agreement that a KD does not introduce any additional risks into diving - i.e. it can't hurt.

Not sure how your wife eats now, but a KD seems like something to consider.

 

[guyharrisonphoto]

Actually, one of the interns at Duke mentioned the ketogenic diet. His take was to do a ketogenic diet beginning three days before a planned dive. He indicated your body gets rid of a lot of water at the beginning of such a diet and this could help (my wife's condition is the risk of immersion pulmonary edema). We have done some research on that diet and it seems good for the short term (weight loss, water removal) but I cannot think it is a healthy long term diet with the extreme lack of fruits, vegetables and grains. The doctors also were clear that there was no verifiable evidence at this time that it would decrease the risk of O2 toxicity, which needs to be handled by the special tables.

Those articles are interesting, though. I think doing a short-term ketogeic diet, maybe twice a month for 5 days, might be something worth trying.