"Reverse Profile" Paper - - -

[Dr Deco]

Hello readers:

It took a while for my mail to be forwarded from NASA to my home. I have read the paper concerning reverse profiles and now have the following comments.

Computer Profiles

I am not certain what the authors’ meant when they stated that some computers do not “correct” for reverse profiles. All current dive computers, to my knowledge, will calculate the gas loads for compartments for the dives as they are actually made. A deep dive at the end of a sequence will give different calculated gas loads than a shallow one at the end.

To give safe gas loads, the computer will indicate that the last dive must be short – sometimes very short. For this reason, most divers will elect to dive in a deep-to-shallow sequence, since it will allow more bottom time.

Guinea Pigs and Surface Intervals

It is known that multiple dives with short surface intervals are often possible, because the next dive in the sequence will act as a prophylactic recompression [reference below]. While this is a thirty-year old reference, it should not be obscure to professional barophysiologists. This “recompression effect” appears to have been a very real possibility in the reported series of tests.

Selected Profiles: Gas Loads

I calculated the gas loads for some of the profiles.

For the Forward Profiles of 98, 65, and 33 feet with 15-minute surface intervals, we get the following nitrogen tensions for the 5, 10, 20, and 40-minute compartments at the end of the series:

59, 58, 59, 60 feet nitrogen partial pressure.

For the Reverse Profiles of 33, 65, and 98 feet with 15-minute surface intervals, we get the following nitrogen tensions for the 5, 10, 20, and 40-minute compartments at the end of the series:

123, 114, 99, and 81 feet nitrogen partial pressure.

I would expect that these loads would make a considerable difference in the DCS outcome. It did.

Modified Profiles: Gas Loads

In order to obtain the same gas loads following the final dive of the series, the reverse profile (of the dives above) would have a bottom time of only two minutes for the last, 98-foot dive.

Small Animals

As I indicated in the former posting, small animals do not get limb-bends DCS. They have connective tissues that are too small and dump the dissolved nitrogen too quickly. What they do display is cardiopulmonary collapse from too many gas bubbles filling the heart. Essentially the heart is trying to pump foam, and there is no output of blood.

Field Experience

I have not heard of poor dive outcomes using the new guidelines. There may indeed have been some, and I am not arguing for or against the Smithsonian guidelines. I only am questioning the results of this small-animal study.

Dr Deco :doctor:


References :book3:

Gait D, Miller KW, Paton WD, Smith EB, Welch B. The redistribution of vascular bubbles in multiple dives. Undersea Biomed Res. 1975 Mar;2(1):42-50.

 

[Charlie99]

I am not certain what the authors’ meant when they stated that some computers do not “correct” for reverse profiles. All current dive computers, to my knowledge, will calculate the gas loads for compartments for the dives as they are actually made.

......

I calculated the gas loads for some of the profiles.

For the Forward Profiles of 98, 65, and 33 feet with 15-minute surface intervals, we get the following nitrogen tensions for the 5, 10, 20, and 40-minute compartments at the end of the series:

59, 58, 59, 60 feet nitrogen partial pressure.

For the Reverse Profiles of 33, 65, and 98 feet with 15-minute surface intervals, we get the following nitrogen tensions for the 5, 10, 20, and 40-minute compartments at the end of the series:

123, 114, 99, and 81 feet nitrogen partial pressure.

I would expect that these loads would make a considerable difference in the DCS outcome. It did.

Thanks for the analysis. A bit more time spent reviewing the design of experiments would save a lot of wasted effort.

I think the authors were referring to "Sunto RGBM" and "Mare-Wienke RGBM" computers as "correcting for reverse profiles". They track dissolved gas loads like any other neo-haldanian computer, but they do reduce the allowable compartment M-values in response to things like reverse profiles (also things like fast ascents and short SIs, and perhaps even just going deeper than 100'/30m).

Charlie Allen

 

[ArcticDiver]

Isn't this whole concern over what is called "Reverse Profile" really a function of available technology?

Back in the days when we had to use prepared tables and couldn't measure actual dive profiles it made sense from safety and calculation ease points of view to do the shallowest dive first. Plus, that meant the surface interval between dives 1 and 2 could be shorter than if the reverse were true. This also brought a desireable commercial outcome. The SI from the deep dive is occuring on the customer's time, not the boat's time.

But now we can measure dive profiles and apply actual dive data to the raw algorithm. We are not constrained by paper tables that require the shallowest dive to be first. So, the only effect of a "reverse profile" is to require a longer surface interval between dives 1 and 2 than would be the case if the shallower dive were first. As a commercial matter it may still be preferrable to do the shallow dive first. But that shouldn't be confused with safety.

So, what is all the fuss about discarding the old rule that apparently is no longer useful?

 

[Saturation]

Thanks Dr. D. You should consider sending your reply to the UHMS Letter to the Editor. I 'guestimated' the gas loads too but in reference to human dive tables, rather than for the guinea pig. The forward profile dives have the advantage of looking like an in-water recompression profile.

 

[ArcticDiver]

When I reread what I posted it looks like I must not have thawed yet after our cold snap.

What I was trying to say has to do with the concern over "reverse profiles". Sure there will be different gas loading. But they can be accomodated nicely by using appropriate surface intervals. This is possible because we are no longer restricted by the technology of paper tables. We can now plug dive data directly into the dive algorithm. Plus, the data we use now due to computers is more accurate.

Therefore, with modern technology we are not constrained by any particular profile. Thus, this information is certainly interesting and useful at the understanding level but is not needed at the field application level.

So, I don't understand all the fuss that some folks raise over any particular dive depth sequence. I do understand, I think, what this paper says but put it into a behind the procedure perspective.

OK?

 

[Dr Deco]

Hello ArcticDiver:

The authors of the Reverse Profiles paper did not take into account the differences in gas loads. In short, one finds that, in a sequence of multi-level dives or a series of single-level dives, the individual will surface [roughly] with the tissue gas loads of the last dive.

Thus, an initial dive to 100 fsw and a final dive to 30 feet will find the diver with nitrogen partial pressure indicative of the 30-fsw dive. If the 100 fsw dive is last (and with the same bottom time), the gas loads will reflect the 100 fsw nitrogen dose.

Dives cannot be shifted without regard to bottom time (or surface interval) with the expectation of the same nitrogen loads in the tissues.

Dr Deco :doctor:

 

[ArcticDiver]

Hello ArcticDiver:

The authors of the Reverse Profiles paper did not take into account the differences in gas loads. In short, one finds that, in a sequence of multi-level dives or a series of single-level dives, the individual will surface [roughly] with the tissue gas loads of the last dive.

Thus, an initial dive to 100 fsw and a final dive to 30 feet will find the diver with nitrogen partial pressure indicative of the 30-fsw dive. If the 100 fsw dive is last (and with the same bottom time), the gas loads will reflect the 100 fsw nitrogen dose.

Dives cannot be shifted without regard to bottom time (or surface interval) with the expectation of the same nitrogen loads in the tissues.

Dr Deco :doctor:

Absolutely. I do not disagree. But that is not the end of the story.

What is important is what happens next. Post dive activity just needs to take that gas load into account. In fact there may be a commercial benefit to doing a Deep Dive Last profile. In that profile the customer(diver) is off gassing on his own time and not on the company's time. If the dive is a boat dive that means less operation time, which means less fuel, less employee time, etc. and availability for some other revenue producing activity.

The diver just needs to follow the proper off gassing time before engaging in any activity where the gas load would be a factor.

So, taken in isolation there are certain apparent benefits to the deep dive first profile. But taken in context of current technology and economics deep dive last protocols become much more attractive.

 

[Dr Deco]

The economics of diving might well be different depending on the order of the dives. That is a different matter; I was commenting only on the physiology.

 

[ArcticDiver]

The economics of diving might well be different depending on the order of the dives. That is a different matter; I was commenting only on the physiology.

I understood that. My comments were just trying to broaden the focus and to make the paper more useful. At the same time point out that the idea of which sequence in which to perform dives of differing depths was product of technology, not physiology or economic necessity.

I enjoy your insights and the discussions you stimulate. So, I'm not shooting at you at all.

 

[BillP]

Thanks for your analysis of the paper. I would like to know what dive computers (if any) don't make adjustments for reverse profiles as the authors claim. Saturation's suggestion for a Letter to the Editor sounds like a good idea.

Bill