Nitrox, Skin Bends and Cozumel

[krbailey]

From my post in another thread...

"This past summer, I worked 50-60 hrs/week the 2 months before my annual Cozumel dive trip to get my workload in order. The day before my flight, I picked up a college friend at the airport and we had dinner with a few other buddies about an hour away. Got home around 9:00PM, finished packing and got to bed about 11:00PM. Woke at 2:00AM to drive to the airport and catch my flight (Friday). Arrived in Coz about 12:30PM and had booked a refresher dive for that afternoon.

Saturday: 2 AM dives and a night dive
Sunday: 2 AM dives
Monday: 2 AM dives

Had some knee issues before the trip and it was bothering me that day. Had some trouble getting off the boat and I sat on the dock to rest for a minute. 10 minutes later, I realized something was very wrong. Had another guest call the manager who called an ambulance and I spent the next week doing 7 chamber rides for Type II DCS.

In retrospect, there were several factors that caused the illness even though I was diving Nitrox on an air setting on my computer and was well within NDL on every dive. Slow ascent rate with a minimum 15ft/5 min safety stop (usually longer).

Stress from a large workload before my trip
Little/no rest immediately before my flight
Dehydration (No alcohol but I didn't drink enough water)
Overweight/out of shape (My knee issue kept me out of the gym for about 6 months and I had gained almost 30lbs - probably stress eating was a factor also)
Trying to pack as many dives as possible in a short amount of time

Lesson learned - vacation is for relaxing. Don't try to do too much. Drink much more water than you are used to. Stay in shape with a regular exercise program. Don't stress myself out immediately before the trip."

IMO - the last several factors I listed had a greater effect/cause with regards to my DCS than were offset by diving Nitrox with my computer set to "Air".

 

[ljwillia]

Do you have the ability to adjust the algorithm conservatism on your computer? If so, you would have the option to simply set and dive your actual mix and then adjust it to them more/most conservative setting to give yourself additional buffer on N2 loading.

You can also stay "X" minutes short of your computer's profile. i.e. Start ascending before your computer's calculated NDL/RBT falls to "X" minutes." An approximate "X" for diving EANx on AIR setting should be be pretty easily determined by comparing AIR/EANx tables for a given planned depth.

Unfortunately, no. I have a Pro Plus 2.1. I thought the algorithm was conservative until I developed skin bends. Found out it is one of the least conservative out there. I bought it with the rest of my gear during my first year of diving. I chose it because it had the largest screen available...old eyes

However, I can certainly dive more conservatively. Thanks for the suggestion!

 

[oly5050user]

Edit

 

[doctormike]

The absence of evidence is not evidence of absence.

The main concern about diving nitrox with air tables (NAT) is that you are underestimating your O2 exposure. That is a real issue in some situations, but rarely encountered in recreational diving.

As far as I know, there is no study that compares NAT to diving air for the same profiles in people who have a history of decompression sickness. This would be a very difficult study to do on ethical and logistical grounds, you would need a large population of people at risk willing to continue diving.

It is true that there is no detected advantage for NAT in reducing DCS in the general recreational diving population, this is because the incidence of DCS is extremely low in that population. So to pick up any advantage NAT for that group, you would need a massive study population.

However, that wasn't the question here. If you have a history of DCS, you shouldn't necessarily assume that reduced N2 loading (which absolutely happens with NAT) is of no potential benefit as a buffer to reduce your risk of DCS in the future. Yes, things like hydration and general fitness and work and water temperature and lots of other things are contributing risk factors, but a history of DCS is a big risk factor.

I believe that a reason why no agency will make a general recommendation for NAT is the risk of underestimating O2 exposure. But we are talking about an individual diver with a history of DCS who is not doing enough diving to make that a significant risk. Especially if they track their actual O2 exposure and watch their MODs, NAT would be a reasonable way of limiting N2 loading in recreational diving, reducing decompression stress and potentially reducing the chance of recurrent DCS. And it might be reasonable for a physician to make that recommendation.

 

[ReefHound]

The absence of evidence is not evidence of absence.

The main concern about diving nitrox with air tables (NAT) is that you are underestimating your O2 exposure. That is a real issue in some situations, but rarely encountered in recreational diving.

As far as I know, there is no study that compares NAT to diving air for the same profiles in people who have a history of decompression sickness. This would be a very difficult study to do on ethical and logistical grounds, you would need a large population of people at risk willing to continue diving.

It is true that there is no detected advantage for NAT in reducing DCS in the general recreational diving population, this is because the incidence of DCS is extremely low in that population. So to pick up any advantage NAT for that group, you would need a massive study population.

Exactly. I liken it to the lottery. Mathematically, we know buying 20 tickets greatly increases your odds of winning the powerball over buying 1 ticket but I know of no study of lottery winners that reflect this. In reality your odds are so low that you won't see the difference. I've never actually met anyone diving NAT. The dataset must be very small.

Just as I would agree with Don that exertion after diving elevates your risk and wearing your gear up the ladder is more exerting than taking it off and letting the crew lift it up. But I doubt there is an "study" that compares DCS rates in divers who exited with their gear with divers who removed their gear. The absence of a study proving a theory does not mean the theory is not true.

 

[MaxBottomtime]

And hydration. I've seen studies that suggest dehydration may be the single biggest factor in unexpected hits. Since alcohol is dehydrating, that means not showing up hungover.

Not true.

State of Hydration | Decompression Sickness - DAN Health & Diving

 

[ReefHound]

Not true.

State of Hydration | Decompression Sickness - DAN Health & Diving

You can follow a blanket statement from DAN that is unsupported with actual data. I will follow an actual study published in a medical journal that provides methodology and results.

Dehydration effects on the risk of severe decompression sickness in a swine model. - PubMed - NCBI
HYPOTHESIS:
Dehydration increases DCS risk.

CONCLUSION:
Hydration status at the time of decompression significantly influences the incidence and time to onset of DCS in this model.

Btw, I said "may be" based on some studies. In any event, DAN agrees that good hydration is important. So rather than nitpick semantics, how about we agree that it would be a sound practice for divers to avoid dehydration?

 

[Bob DBF]

You're far more experienced than I, but one of the safety reasons for removing your kit in the water is to reduce exertion which can kick off a hit. I've read about some boats that even have mechanical lifts for divers.

My understanding of the reason for the lifts have to do with sea conditions and the danger involved with exiting the water, especially in doubles, rather than reducing exertion to avoid DCS.

Exactly. I liken it to the lottery. Mathematically, we know buying 20 tickets greatly increases your odds of winning the powerball over buying 1 ticket but I know of no study of lottery winners that reflect this. In reality your odds are so low that you won't see the difference.

Mathematically there is little difference in winning between buying and not buying a ticket. I like to point that out when my wife buys a ticket. She still dosen't laugh.


It seems to me, if decompression science works the way I understand, that anything which reduces N2 loading below NDL will reduce the chance of DCS, not to say one can't get bent. Since the incidence of DCS is so low in recreational divers to begin with, a study would mostly likely show no difference, but that would not prove there is no difference.

I had skin bends once - not diagnosed - on a liveaboard, prior and subsequent trips I did not have the issue and I could discern no difference in my diving patterns during the trips. A few trips after the incident I started using Nitrox, I have had no recourances and I feel better after my dives, ymmv.



Bob

 

[Sevenrider860]

DAN’s statement is:

“Sound hydration is important for good health, both for general and for diving health, but your dive profile, thermal stress and exertion level are far more important risk factors for DCS”

The published study you reference did not alter the dive profile, did not alter thermal stress, and did not alter exertion level. What they did was confirm the hypothesis that “Dehydration increases DCS risk.” The DAN article does not dispute that.

If you remove 3 of the 4 risk factors, the only thing you would be left with is hydration. So in the test performed, hydration significantly influenced the onset of DCS...because no other known contributing factor was changed between the two groups.

The same test could be run with similarly hydrated pigs, but one control group has a conservative dive profile and the other control group put on an exceptionally aggressive dive profile. The conclusion would be that aggressive dive profiles significantly increase the risk of DCS.

DAN is not saying diving dehydrated is OK, what they are saying is that dehydration is not the “single biggest factor” for DCS in recreational diving compared to the other known risk factors.

 

[DandyDon]

And hydration. I've seen studies that suggest dehydration may be the single biggest factor in unexpected hits. Since alcohol is dehydrating, that means not showing up hungover.

Not true.

State of Hydration | Decompression Sickness - DAN Health & Diving

In any event, DAN agrees that good hydration is important. So rather than nitpick semantics, how about we agree that it would be a sound practice for divers to avoid dehydration?

DAN’s statement is:

“Sound hydration is important for good health, both for general and for diving health, but your dive profile, thermal stress and exertion level are far more important risk factors for DCS”

DAN is not saying diving dehydrated is OK, what they are saying is that dehydration is not the “single biggest factor” for DCS in recreational diving compared to the other known risk factors.

Yeah, I avoided that issue, leaving it under my suggestions of being in good health and well rested. I think the old claim of "dehydration may be the single biggest factor in unexpected hits" was exaggerated, but agree that "not showing up hungover" is a very good approach. Many Cozumel doctors have extensive experience in treating the bends, much more than in preventing, but treated patients will understandably ask those physicians for answers on why still. I think they go with dehydration warnings instead of hangovers as a gentler touch.

One of my earliest trips to Cozumel was with a group of 30 from a singles group. The organizer wanted to make sure that everyone had fun so pushed the partying more than I liked. 10% of the group left in ambulances that week.