Deep Stops Increases DCS

[rongoodman]

Hi rivers. Hey expaain something to me. You blew my mind with the statement about min deco being for 32% ir 30/30. Are you saying that the 2 gasses are the same? If not then i s my understanding about deco getting the N2 out wrong cause 32% has 68% N2 and 30/30 has 40% N2. How are teh 2 gasses similar enough to work with the same table? Lookin to be edumacated.

A quick comparison of the two on a 100' 30 min dive with MultdDeco VPM +3 shows them within a minute of each other. They're both about 70% inert gas, so it makes sense that they are very close. A larger proportion of He might cause the two run times to diverge more.

 

[Dr Simon Mitchell]

The disadvantage of tactically compensating for the deep stop slow tissue loading with elective extra O2 deco time of around 10 to 20 minutes maximum was as both you & I expected: an uncomfortably high CNS OxTox figure result of 300 to over 500 max as tracked by the Petrel dive computer on 30/85 GF.

Hello Kev,

Not to say there is no risk (because obviously there is), but we published this last month which might make you feel slightly more comfortable:


Aviat Space Environ Med. 2015 Jan;86(1):41-5. doi: 10.3357/AMHP.4113.2015.

End Tidal CO2 in Recreational Rebreather Divers on Surfacing After Decompression Dives.

Mitchell SJ¹, Mesley P, Hannam JA.
Author information

• ¹Department of Anaesthesiology, Faculty of Medical and Health Sciences, University of Auckland, New Zealand.

Abstract

INTRODUCTION:

Deep dives using rebreather devices result in oxygen exposures that carry a risk of cerebral oxygen toxicity. Elevation of arterial CO2 levels increases this risk. CO2 retention may occur during the deep working phases of dives, but it has not been investigated in 'real world' dives at the end of resting decompression when oxygen exposures are peaking, often to levels higher than recommended maxima.

METHODS:

We conducted an observational field study to measure end tidal CO2 (Petco2) in divers surfacing after decompression. Sixteen rebreather divers conducted two dives and two completed one dive (a total of 34 dives) to depths ranging from 44-55 msw. Bottom times ranged from 35 to 56 min and time spent on decompression ranged from 40 to 92 min. The first breaths on reaching the surface after removing the rebreather mouthpiece were taken through a portable capnograph. The Petco2 was recorded for the first breath that produced a clean capnography trace. Petco2 measurement was repeated for each subject 2-3 h after diving to give paired observations.

RESULTS:

There were no differences between mean surfacing Petco2 [36.8 mmHg (SD 3.0)] and the mean Petco2 made later after diving [36.9 mmHg (SD 4.0)]. One subject on one dive returned a surfacing Petco2 higher than a nominal upper limit of 45 mmHg.

DISCUSSION:

We found no general tendency to CO2 retention during decompression. It is plausible that breaching oxygen exposure limits during resting decompression is less hazardous than equivalent breaches when exercising at deep depths. Mitchell SJ, Mesley P, Hannam JA. End tidal CO2 in recreational rebreather divers on surfacing after decompression dives. Aerosp Med Hum Perform. 2015; 86(1):41-45

If you want the full paper just pm me.

Simon

 

[Kevrumbo]

That preliminary impression from the above study is somewhat reassuring. Still, was very worried breathing O2 at 9m for over an hour on an IWR session last Oct-Nov, even with Air Breaks. (That's an Oxygen ppO2 of approx 2.0).

Also the previous three type I DCS incidents experienced on my Truk trip last Oct-Nov 2014 -all were upper Right arm/Shoulder classical acute "pulsing" symptoms with increasing pain within 90min time post-dive, and occurring within three to four days of starting Deep Air deco dives (two dives per day with a 3 hour SIT). Possible contributing factors were dehydration, insufficient "acclimatization" to the tropical environment, and no prior "work-up" practice deco dives to sensitize the body's immune/inflammatory response system to inert gas loads & micro-bubbles in tissues & venous blood vessels (first AM deep dive with deco of that trip was SF Maru at two-and-a-half hours runtime, after trans-pacific flight from LA arriving late in the night before). Used Ratio Deco w/ Deep Stops or GF 30/85 per Petrel Computer, without initially utilizing extended O2 deco profiles as was tried on this currently completed trip described above.

All DCS Pain Symptoms at that time last Oct-Nov were resolved with IWR -the modified Australian Method as taught by UTD- with either 30, 60 or 90min of elective O2 breathing at 9m depth (10min O2:with 5min Air Break); and then slow 0.1m/min ascent to surface (same breathing 10min O2:with 5min Air Break). Went with 60 minutes O2 time at 9m (Air Breaks do not count or accrue credit into the O2 time at 9m; on the slow 0.1m/min ascent you have to hold at depth during the 5min Air Break, so after 10 minutes and 1 meter delta of ascent on O2 you switch to Air and stay at that depth for 5 minutes). The entire IWR treatment session took three-and-a-half hours (and the CNS readout on the Petrel Dive Computer was "pegged" at 999).

 

[PfcAJ]

Kev, any reason for the 2.0 vs a 1.6? Did symptoms resolve at 9m and not 6m?

---------- Post added February 18th, 2015 at 11:47 PM ----------

Kev, any reason for the 2.0 vs a 1.6? Did symptoms resolve at 9m and not 6m?

 

[Kevrumbo]

Kev, any reason for the 2.0 vs a 1.6? Did symptoms resolve at 9m and not 6m?

---------- Post added February 18th, 2015 at 11:47 PM ----------

Kev, any reason for the 2.0 vs a 1.6? Did symptoms resolve at 9m and not 6m?

Pain immediately -on a scale of 1 to 10- reduced from a "10+" to a "4" upon reaching 6m. After the three-and-a-half hour IWR session, it was an dull "achy" 2 and the next morning just general musculo-skeletal sore & stiffness. But was always falling "behind the 8-ball" on that trip; seems like I couldn't effectively eliminate the inert load and was becoming susceptible to later DCS hits on that limb. By this latest trip however, I stayed on top of it by starting & staying with extended O2 profiles with surfacing GF's of 60 or less. (The first two deco dives, I kept everyone waiting on the dive skiff by surfacing with a GF 40 . . .I effin' didn't care about everyone else's impatience).

2.0 ATA in my opinion is a good compromise IWR pressure that's not as OxTox risky as going in-water to 18m & start replicating a standard USN Chamber Table 6 profile, and yet still deep enough for relief of most simple type I DCS musculo-skeletal limb/joint pain-only symptoms. By Boyle's Law, this is relatively equivalent to reducing pathological bubble size by roughly 20%, and of course squeezing inert gas volume by 50%.

Below for reference only and FYI with caveats:

. . .We removed the Exceptional Exposure Oxygen tables from the NOAA diving manual 4th editon because there was fear that if the general public saw them printed that they might take it as an endorsement to use them.

The NOAA exceptional exposure limits are set for extreme emergencies only and are not for routine use. i.e. --should be used for life saving only.

These are for a working dive meaning with light exertion. Remember that there are a variety of factors that come into oxygen toxicity, and crossing the 1.6 atm 45min line does not guarantee convulsion, it also does not guarantee it won't.

NOAA OXYGEN
EXCEPTIONAL EXPOSURE LIMITS
PO2 Minutes

2.8 5
2.4 10
2.0 30
1.9 45
1.8 60
1.7 75
1.6 120
1.5 150
1.4 160
1.3 240

As you can see the exceptional times allow you a fairly large margin to use this method for an "escape." The table is NOT linear. Note that exceptional exposures are DANGEROUS and can only be done once in a day. . .

Joel Silverstein

 

[rossh]

Extended shallow stops Increases DCS

Hello,

I am late to respond to this thread. Sadly there appears to be a few mistakes in the opinions offered above.

Here are some facts to consider:


1/ The Nedu test did NOT test deep stops. What that really tested was "Extended time shallow stops" which is then miss represented to be deep stops.

The test profiles comprise of shallow stop segments only. The VPM / RGBM / RD / DIR stops were never applied or used in the test.


Beware: The "longer and shallower" style ascent being recommended, is the same type that produced the increased DCS in this test. Repeating myself .... higher risk came from extended shallow stops - not deep stops.


2/ The profiles and plans in the test come from experimental Nedu designs (both shallow stop models), and do not represent anything we use today.





3/ These tests profiles do NOT have high supersaturation. In fact they are the opposite, and from a gas pressure perspective, these test profiles offered a very low risk. The current "plausible explanations" given that have attributed the causes to supersaturation, are plain wrong. The underlying cause of injury appears to be with other factors.





Our latest V-Planner and MultiDeco programs, give diagrams that show supersaturation throughout the ascents (shown in the diagram above). It's a new and unique view on deco diving and risk. Press the bubble icon on the right side. Make some plans and decide for yourself.


More diagrams taking a close look at this Nedu test are here: nedu test review

 

[PfcAJ]

Ross, can you tell us a little about your formal scientific training and experience in testing decompression models?

 

[rossh]

Ross, can you tell us a little about your formal scientific training and experience in testing decompression models?

Stick to the topic please. Do you find anything factually incorrect with the above? It is the same math and comparison methods used in formal dive testing and modelling. The Nedu test needs to be fitted into the larger picture of deco testing and results. Sadly this seems to have been over looked in this case, and we are given a fallacy instead.

 

[PfcAJ]

When the leading experts in the field come to a conclusion based on data, I tend to go with that.

You're not a leading expert. You've for no formal training nor experience in testing models. You have a financial interest in a "deep stops" model. You not only have no real qualifications, you have a conflict of interest.

 

[kr2y5]

When the leading experts in the field come to a conclusion based on data, I tend to go with that.

...which is why credible conferences tend to be double-blind, to reduce bias