DIR Answer Required

[Cacia]

thanks. yea, I knew it was accepted knowledge, but did not understand how it worked. thanks, the core thing makes sense because in hypothermia, your body no longer compensates, right? Like the no shivering once hypothermic.


yes, I use the 120 rule as a cross-check.

 

[Vayu]

So, since we know cold drops your metabolic rate...why would cold water increase your 02 demand?

Because a cold body will try to heat itself up and spend lots of energy to do it. I was cold once and it made my consumption rate very high. I have also found that it is difficult to catch my breath when I am very cold. I think once hypothermia sets in or the dive reflex begins to lower the bodies metabolism there will be greater problems than the demand for 02.

Shivering is the primary culprit. It is designed to produce alot of heat. Intense cold will eventually lower your metabolic demands. We are lucky that the body resists this at first by doing everything it can to reach thermal homeostasis.

 

[TSandM]

When your core temperature begins to fall significantly (and I use that word because, apparently, core temperature can and DOES fall after diving, but it's a small decrease), it means you have exhausted your compensatory mechanisms -- the rate of heat loss exceeds the rate of heat generation by all means available. To get significant decreases in O2 requirement, what you really need is a cold brain (since it's the tissue that gets damaged the fastest when oxygen is deprived) and the brain cools slowly, because it's insulated inside the skull. By the time the brain is cooled by several degrees C, you aren't mentating very well, and you're probably at significant risk of ventricular arrhythmia.

 

[Cacia]

ventricular irritabilty in general? good explanation..thanks so much. Is it the acidosis that causes the irritability or the hypoxia?

 

[Thalassamania]

Point of information:

1) 95% confidence that you will not get bent is not the same as 5% incidence of bends.

2) According to Workman, the test group was actualy (as I mentioned earlier) overweight, hard-drinking, cigar chomping chiefs, not young, fit divers.

 

[TSandM]

I don't believe it's either -- ventricular irritability occurs even in people who are being ventilated if they are cold enough. Although, in the in-water scenario, the person would be likely to be hypoventilating by that time as well, and might be both acidotic AND hypoxic, which would increase the problem.

 

[Cacia]

clear.

Thal, once more, Workman based this on what? what about the control group, same?

 

[Thalassamania]

What he told me was the the young fit Navy diver story was not true that the test subject were all, "overweight, hard-drinking, cirgar chomping chiefs."

 

[TSandM]

The U.S. Navy Tables were created to prevent decompression sickness (not bubbles) in Navy divers. At their acceptance trials in 1956, the schedules produced an overall DCI rate of 4.6% (26 cases in 564 dives)for single dives. The schedules on which the DCI occurred were then recalculated and tested again (involving very few tests) until no cases of DCI occurred. All schedules were then recalculated in accordance to these modifications. The repetitive schedules were tested on 61 repetitive profiles involving 122 dives. Three cases of DCI occurred, an overall incidence of 4.9% (3 cases in 61). There is no mention in the report whether or not the repetitive schedules were recalculated and retested to produce no DCI -- it appears that they were not.

When some controlled dives were made in laboratory chambers, or in open water, to the No Decompression Limits, the incidence of DCI was four or more times greater than that reported by the Navy. Merrill Spencer and Bruce Bassett independently subjected divers to pressure in recompression chambers. They took the divers right to some of the NDLs of the U. S. Navy Tables, eg. 60 ft (18m) for 60 minutes, 70 ft (21m) for 50 minutes, and 80 ft (24m) for 40 minutes. The results suggested that if no-decompression dives are performed to the full limits of the Navy Tables, the incidence of DCI is about 5%.

Both of the above from Lippmann & Mitchell, Deeper into Diving. Note that the specific profiles mentioned in the second quotation are the center portion of the "120 rule". I also found it interesting that all the other decompression algorithms/models discussed in the book are more conservative than the Navy Tables EXCEPT for the algorithm used by Cochran.

I don't think you can't use the 120 rule; I just think if you are pushing the limits on it, you are probably a lot safer viewing your dive as a mandatory decompression dive, and managing your ascent accordingly.

 

[Cacia]

I concede I use it as an "outside parameter". I need to keep with what I can manage,
mentally and that may not be as much as some other diver. Smarter, no camera...whatever. maybe not DIR, I will hand you that.