PADI Dive Tables

[Lost Yooper]

Hey Dr. D,

I've been pondering over some stuff here and got a bit curious. You said that in the testing done on PADI tables, most people had no bubbles detected.

So, a person who comes out and no bubbles are detected, then exercises, the doppler will then pick up some bubbles?

I seemed to have lost my notes, but I seem to remember that members of the WKPP underwent similar testing and no such bubbles were detected after excercise -- once the bubbles were silent (30-60mins after the dive). I could be mistaken because I don't have the notes. These guys don't worry about time to fly, repetitive deco dives, or any of that stuff.

My question pertaining to this is if these guys aren't hearing bubbles and they are able to do much more extreme repeat dives, then why do recreational divers have to worry about it if no bubbles are being detected? Could it be they (WKPP) are using different doppler detection equipment that can pick up micro bubbles and therefore they can target them in their profiles? I know that the Navy was involved in doing much of the testing on them including blood work.

Do you think that instead of reducing the NDL's for the tables, PADI could reduce their 60FPM ascent rate to 30FPM, throw in some short deep stops, advocate a super slow final ascent, and thereby produce safer tables for post dive activity? This is where I get confused, I guess. If there isn't any bubbles being detected already, how can they make them any safer other than reducing the NDL's? On the other hand, RGBM claims to be much safer.

Is there any talk about the recreational agencies giving in and going with the RGBM tables? Weinke is/was with NAUI, have they changed over to RGBM now?

Boy, I'm on a role now! Wahoooo!!

Mike

 

[Dr Deco]

Dear Mike:

Thanks for the question. First, the part about Doppler bubbles. One must recall that the Doppler system will detect gas bubbles in the circulatory system while Decompression joint pain is the result of a gas phase in the tissues, that is, outside of the circulatory system. The dissimilarity of the gas phases creates problems for Doppler systems. They are monitoring the wrong gas phase. When one looks at a group of individuals, it is possible to determine if the group as a whole is doing better or worse on a decompression schedule, but it is not possible to predict definitively who will get DCS. It is for this reason that the device has its greater use in research labs than fieldwork.

Therefore, when someone says that no bubbles were detected in the blood vessels, we really wish to know if they are in the tissues (such as the tendons and ligaments). If WKPP group does not worry about repetitive dives and time-to-fly restrictions, then it is clear that, for the type of dives that they are performing, the “restrictions” do not apply in their situations. It is also possible that they are very lucky, to date. To determine just what is happening, it would be necessary to know all details of the dive situations. One cannot extrapolate to ALL scuba divers.

As I have said on other occasions, there are not any “bends/no bends” limits in diving. There is instead a steady progression such that as the gas loads increase the number of individuals who will get DCS increases and the severity of the “hits” will increase. The NDLs are determined so that a very small percentage of individuals will get DCS under certain surfacing conditions. (Those conditions are not any strenuous activity following the arrival at surface.)

The early caisson works on the St Louis Bridge project (Eads Bridge, 1865 to 1869) worked in conditions of compressed air for several hours and decompressed with often more than two hours missing from their decompression according to US Navy tables. Did most of them come out OK? Yes. However, some of them were severely afflicted and 13 died. We could dive like that today, but we would not be happy with the results of those “decompression tables.” There may be conditions that work in one instance but will not work in others. Likewise, they might work for one group of individuals but not others.

The safety stops and slow ascents work because they prevent the growth of tissue micronuclei into larger gas bubbles that hold the free nitrogen and prevent its loss from the cells into the capillaries. Naturally any diver can perform a slow ascent of a safety stop at 15 fsw. We have spoken about this before in this FORUM.

The Reduced Gradient Bubble Model (RGBM) purports to produce safer schedules by essentially reducing the rate of ascent. This would really only function in dives requiring deep stops. It will also play some role in repetitive dives. In the case of deep dives, the deco time is reduced. In the case of repetitive dives, the surface intervals are longer. Both will produce safer dives (assuming that the surface conditions were identical, i.e., same activity levels).

What training agencies will do with all of this, I do not know.

Dr Deco

 

[Lost Yooper]

Great doc.

Thanks.

Mike

 

[A34735]

After a 20 year lay-off, I'm back into diving..and some of the changes, in diving practices and education are dramatic. SB for example, and other fora like it, are wonderful contributions to diver ed.
This thread in particular has changed my dim view of the overall quality and commitment of PADI to a much more positive one. But enough rambling and now to my question about micronuclei.

'...because they prevent the growth of tissue micronuclei into larger gas bubbles...'

Has any work been done to characterize tissue micronuclei at the cellular level? (I can see some obvious experimental difficulties - Would it even be possible to take a cohort of saturated mice to the brink of DCS, slaughter them and prepare tissue sections? Perhaps it'd be easier to look for very localized tissue damage, presumptively due to micronuclei expansion, in dead divers?).

I think I'm really asking about the quality of evidence for micronuclei being real, as opposed to (perhaps very good) hypothetical...

 

[Dr Deco]

Hello: A34735

Micronuclei cannot be seen at room temperature. Surprisingly, when water, e.g., is heated, the nuclei expand to form vapor bubbles that are very easily visible.

If they cannot be seen, where did the idea arise? Measurements of the tensile strength of water indicate that it requires hundred of atmospheres of negative pressure to form a bubble in denucleated water.

What is “Tensile Strength”?

When a material is pulled apart, it is said to be under tension. That solid can rip apart is clear, but the same occurrence in a liquid is not at all obvious. However, if a liquid is placed in a cylinder [with no air space present] with a piston, it is found that certain tension must be applied before the piston can be pulled out and the liquid essentially splits. It was found experimentally that a liquid has a different tensile strength depending on how much pressure was applied prior to applying tension [pulling the piston]. Naturally, the first thought is that the pressure applied forces minute bubbles into solution. These were considered to be micronuclei.

Calculations based on theory indicated that the true tensile strength of water must be more than 1,000 atmospheres, much higher than had ever been measured in the laboratory. Micronuclei were postulated to be present although their origin was unknown. Thermal movement of water molecules were postulated to provide a range of “holes” in water. Bigger holes were rare but possible. With increasing temperature, the holes became bigger and more numerous.

Nuclei in Tissue

These were introduced into decompression theory by Edmund Newton Harvey in the early 1940s. How they form in tissue and how long they last are points of contention.

Nuclei in Decompression

These are thought to reside on surfaces [wall nuclei] rather than in the fluids, themselves [stream nuclei]. They might form from hydrodynamic mechanisms from exercise [my theory]. They are gaseous and not visible under a microscope since they are so minute.

Dr Deco :doctor:

 

[HJeffreyM]

Dr. Deco,

As you probably know, a number of things you mention as "excessive musculoskeletal activity" like moving SCUBA tanks, weight belts, integrated BCDs, climbing a ladder with tank and weights on your back, or carrying gear to and from a shore entry are in fact endemic to diving.

Saying that the tests were done without such activities is like saying skiing would be perfectly safe if noone ever got on the lift!

So, knowing what is actually expected of divers getting out of the water, what can we do to lessen the chances of taking a DCS hit?

 

[Dr Deco]

Hello: HJeffreyM

You are absolutely correct. There is no question that those activities are part and parcel of diving. These are:

Moving SCUBA tanks, lugging weight belts and integrated BCDs, climbing a boat ladder with tank and weights on your back, or carrying gear to and from a shore entry.
​

Regrettably, in the late 1980s when the last table tests [in a laboratory] ceased, the effect of this surface activity was not known. Test subjects simply sat around and read books or magazines. Exercise at the bottom [for gas loading] was considered, but not micronuclei formation.

So. What's A Diver To Do Topside?

The best idea is to get some buddy to do everything for you - in the name of “diver safety.” I call this “Concierge Diving.” But, realistically. Do not move everybody’s SCUBA tank for the first couple of hours, or in the interdive interval. Do not climb ladders with all of your dive gear. Try to hand it up to someone on the boat.

Do not perform what are called Valsalva-like maneuvers. These would be breath holding while climbing or lifting, for example. These could promote arterialization of venous bubbles should they happen to be in you.

Dr Deco : doctor: