Decompression chamber question

[Gene_Hobbs]

Even Table 19-4 in the USN Manual (rev6) will give up to 10 min at 50fsw while breathing 100%. Donald's work did show that short exposures to deeper depths can work (Summaries of Donald's work). His work is also what showed us the effects of immersion, workload, and temperature as discussed earlier in the thread.

I loved hearing Chris Lambertsen discuss a dive to 80fsw in 1942 on his LARU system to test the CO2 absorbent… “My pressure tests went very well. CO2 absorption was fine but O2 poisoning came on at 80 feet. I was almost a goner”(1) I am really going to miss his stories.

Interesting. How much of a factor do you believe or suspect that physical conditioning plays? Here is why I ask: Military non-combat divers are usually in pretty decent shape (excluding alcohol), but nothing compared to SEALs. Commercial divers are usually strong but often have lower cardio fitness.

I doubt physical fitness plays much part in it at all. Individual day to day variation is just too high to think it does. It is much more influenced by workload (cardiac output) and CO2 (vasodilation in the brain). The testing of oxygen tolerance for US Navy divers has gone away.(2)

Also, have you noticed or suspect a correlation to the first symptom being convulsions at higher PPO2 levels? Max was 2.8 ATA/60' in my career, except for the French divers I observed and described earlier.

Donald's work was really the first time these symptoms were described in great detail and the symptoms were pretty consistent as long as they were high but again they were highly variable from day to day in the same individuals.

My experience has been more like DDM's in that most patients go to the convulsion pretty quickly. (Our time in the Duke lab clinically does not overlap)

I was also lucky enough to be a standardized patient for Ed Thalmann when he first arrived at Duke and his briefing was to make our lips quiver, count to five, and seize. That was based on the time course observed through his career.

1. Vann, RD. (2004) Lambertsen and O2: beginnings of operational physiology. Lambertsen Symposium. Undersea & Hyperbaric Medicine Spring;31(1):21-31 RRR ID: 3987
2. Walters, KC; Gould, MT; Bachrach, EA; Butler Jr, FK. (2000) Screening for oxygen sensitivity in U.S. Navy combat swimmers. Undersea & Hyperbaric Medicine Spring;27(1):21-6. RRR ID: 2358

 

[Duke Dive Medicine]

I'll echo Gene in that I don't think physical conditioning has much to do with O2 tolerance. There's no evidence for it, and it's not really borne out by what we know of the physiology.

A lot of my personal experience has been with commercial diving students, and we had a few of them seize at 60 FSW in the chamber while doing sham TT6As. Only one described feeling anything before the seizure. I do think that the likelihood of seizure without warning increases with higher PO2.

The vast majority of our patients are treated at 33 FSW. The only one I've ever seen seize at that depth was a poorly-controlled diabetic who liked to take his insulin and then not eat before treatment. Hypoglycemia increases the risk of O2 toxicity, and you could almost set your clock by this fellow on Monday mornings (he was a bit of a drinker also)... about 30 minutes in, he'd seize, we'd put an IV in, give him some ativan, then he'd want to drive home afterwards. Again, that's resting and dry vs. working and wet, with a patient at significant risk, so take it for what it's worth.

Re full face mask: again in my experience, when I was diving the Mk-16 with Navy EOD a number of years ago, we went over to the FFM exclusively. Personally, I was glad for the extra safety margin.

Gene, it's great to see you back here!

 

[Akimbo]

…I doubt physical fitness plays much part in it at all. Individual day to day variation is just too high to think it does. It is much more influenced by workload (cardiac output) and CO2 (vasodilation in the brain)…

I am surprised that you and Duke Dive Medicine so readily doubt the roll of physical condition. A lay person like me would think that an athlete’s blood CO2 levels would be lower at the same workload compared to an average person. Granted, there is a very small sample that indicates high day-to-day individual variation, but I don’t understand how that brings the advantages of exceptional cardiopulmonary conditioning into doubt. One thing for sure, individual and daily variations are so great that investigating O2 tolerance would be the study from hell!

…The testing of oxygen tolerance for US Navy divers has gone away.(2) …

I am confused. The Author’s Note on Page 26 indicates that the SEALs discontinued O2 tolerance testing in April 1999 but this link still shows it. Military.com isn’t an official source but it is the closest list of requirements I could find.

BUD/S Warning Order - Military Fitness - Military.com
Under:

Submit the Following With Your Request:
a. A certified copy of your ASVAB test scores
b. Your physical screening test results
c. Pressure and oxygen tolerance test results (if completed)
d. Your completed diving physical (Form SF88-SF93)
e. Certified copy of your latest performance evaluation report

Does anyone know if they also eliminated it for Navy (salvage) divers? Given that hardly anyone ever fails and 60' for 30 minutes doesn’t appear to be a reliable predictor, it probably should be.

 

[Thalassamania]

I did a fair number of O2 tolerance tests back in the day, all because of my involvement with various SCCRs and CCRs.

I thought the Navy dropped them completely now, for all personnel ... but I've never seen that in writing.

 

[Gilldiver]

Interesting. How much of a factor do you believe or suspect that physical conditioning plays? Here is why I ask: Military non-combat divers are usually in pretty decent shape (excluding alcohol), but nothing compared to SEALs. Commercial divers are usually strong but often have lower cardio fitness.

Some of the guys I worked with were ex-Seals and British Commandos. They often talked about dropping below their 25' limit on O2 rebreathers — the deepest at 25 Meters/82' — pushing 3½ ATA. Considering that they were deep because they concluded the alternative was much more dangerous, the stress level had to be pretty high. I always wondered how much their extraordinary physical condition may have protected them from oxygen toxicity.

Also, have you noticed or suspect a correlation to the first symptom being convulsions at higher PPO2 levels? Max was 2.8 ATA/60' in my career, except for the French divers I observed and described earlier.

The Navy O2 rebreather is rated for excursions to 90' - how do you think they get out of, and back into an Ohio class sub converted for Special forces?

The missile tubes also have room for stowage canisters that can extend the forward deployment time for special forces. The other two Trident tubes are converted to swimmer lockout chambers. For special operations, the Advanced SEAL Delivery System and the Dry Deck Shelter can be mounted on the lockout chamber and the boat will be able to host up to 66 special operations sailors or Marines, such as Navy SEALs. Improved communications equipment installed during the upgrade allows the SSGNs to serve as a forward-deployed, clandestine Small Combatant Joint Command Center.[7]

 

[Akimbo]

The Navy O2 rebreather is rated for excursions to 90' - how do you think they get out of, and back into an Ohio class sub converted for Special forces?...

Are you sure they are using pure O2 rebreathers and not mixed gas eCCRs? Going below 25' on O2 rebreathers was a really big deal to all the SEALs and British combat swimmers I ever spoke with. The US Navy Diving Manual Revision 6, page 19-13, Table 19-3, Excursion Limits doesn’t even approach that depth.

Depth	Maximum Time
21-40 fsw	15 minutes
41-50 fsw	5 minutes

(I just learned how to insert tables)

 

[Duke Dive Medicine]

I am surprised that you and Duke Dive Medicine so readily doubt the roll of physical condition. A lay person like me would think that an athlete’s blood CO2 levels would be lower at the same workload compared to an average person. Granted, there is a very small sample that indicates high day-to-day individual variation, but I don’t understand how that brings the advantages of exceptional cardiopulmonary conditioning into doubt. One thing for sure, individual and daily variations are so great that investigating O2 tolerance would be the study from hell!



I am confused. The Author’s Note on Page 26 indicates that the SEALs discontinued O2 tolerance testing in April 1999 but this link still shows it. Military.com isn’t an official source but it is the closest list of requirements I could find.

BUD/S Warning Order - Military Fitness - Military.com
Under:


Does anyone know if they also eliminated it for Navy (salvage) divers? Given that hardly anyone ever fails and 60' for 30 minutes doesn’t appear to be a reliable predictor, it probably should be.

Gene and I were talking about the PnO2 test yesterday. His memory is better than mine, but from the best that we can recall, the oxygen tolerance test for all Navy dive candidates went away around 2000.

Re CO2 level and O2 tolerance: the body works to maintain arterial pCO2 between 35 and about 40 mmHg on the surface. Immersion changes things a bit, but let's start with the exercise. In your example of two divers doing equal work, both divers' lungs will be working to maintain the pCO2 within normal limits. The only difference is that the diver who's not in good shape will reach maximum exercise tolerance before the diver who's an athlete.

With immersed exercise, gas density and airway resistance come into play. Cherry et al (2009) and Peacher et al (2010) demonstrated that arterial pCO2 rises with exercise at depth. They used 37 meters (~120 FSW), and showed that pCO2 can rise from an average of about 35 mmHg to about 45 mmHg. They didn't differentiate between athletically trained and untrained divers, but gas density and corresponding airway resistance would be similar between the two.

Another factor that comes into play is hypercapnic ventilatory response, which has nothing to do with physical condition. For example, we recently evaluated a diver who complained of severe, migraine-like headaches after diving. He is in a profession that requires him to maintain a relatively high level of physical fitness, but when we looked at his hypercapnic ventilatory response curve, it was almost flat. In other words, his respiratory rate did not increase significantly in response to increases in inspired pCO2. This resulted in elevated arterial pCO2 and corresponding CO2 toxicity with diving. This diver would presumably be at a higher risk of O2 toxicity, despite his good physical condition.

So, if you put all that together, you really can't generalize that divers in better physical condition are less susceptible to O2 toxicity. Hope this helps answer your question.

Best,
DDM