Rapid Ascents and DCS

[BillP]

Hi Dr. Deco:

Current conventional wisdom seems to be that rapid ascents increase the risk of decompression sickeness (DCS). Forgetting the risk of arterial gas embolism (AGE) with rapid ascents for the moment, can you discuss the mechanisms by which slow ascents reduce the risk of developing DCS on a dive?

I would suppose that the level of tissue nitrogen loading would be directly related to the risk of DCS from a rapid ascent, but do you have to be at or near the no decompression limit (NDL) on a dive before a slow ascent becomes important in reducing the risk of DCS? (IE, if you're not at or near the NDL on a dive and the risk of AGE could be ignored, would a slow ascent still be important in reducing the risk of DCS?) Is there any difference in the importance of making a slow ascent after multiple repetitive dives over several days?

TIA,

Bill

 

[Dr Deco]

Bill:

The question of slow ascents is often linked to the concept of the no-decompression limit (NDL), and some problems can arise when viewed only in the light of gas supersaturation. This “limit” is linked to, or based upon, the original concepts of JS Haldane who, at the turn of the century, developed his decompression system based on the idea that limiting supersaturations would prevent gas bubble formation. Thus, if you were below the NDL you were below the “metastable limit” (an idea from the German physical chemist Wilhelm Ostwald, and decompression gas bubbles would not form.

Today we recognize that idea as being flawed by virtue of the fact that a free gas phase (i.e., a bubble) does not actually form during decompression, rather the bubbles grow from micronuclei (i.e., micro bubble “seeds”) that are already present in the body fluids. In reality, all fluids that move will have micronuclei because fluid motion promotes the formation of microbubbles. (This is referred to as “hydrodynamic cavitation.” You know as a fact that you could not put carbonated beverages into a blender and expect much effervescence to remain after a few seconds of stirring; stirring promotes microbubble formation)
The presence of these microbubbles is what determines whether a gas phase will grow (it does not really form "de novo") during the ascent portion of the dive. Whether a micronucleus grows depends on the initial size of the micronucleus (because of the internal, constricting pressure caused by surface tension) and the level of supersaturation (as we have all been taught). The supersaturation must be sufficient such that this constricting pressure (this collapsing pressure is termed the “Laplace pressure”) is less than the gas supersaturation.

Now, and here is the rub, we wish as few nuclei to grow into bubbles as possible. The faster the ascent, the greater is the local supersaturation and the more micronuclei can grow at that moment. Some can grow to the point where they remain stable (for a while anyway) and remain in the tissue. The slower the ascent, the fewer micronuclei will grow. (An extremely slow ascent would result in not nuclei growth.)
Thus when we come to multiple dives, a slow ascent will prevent the growth of micronuclei into tissue microbubbles that could remain in the tissues for a while (several hours) and participate in the following dives. What rapid ascents do then is to CONVERT more micronuclei into decompression gas bubbles.

These ideas are based on micronuclei theory which is a different viewpoint that supersaturation, although they are coupled closely. Nuclei control is being used by my group at NASA to control decompression sickness during extravehicular activity in astronauts (when they are in the low-pressure space suit). It is, in reality, more than a theory. This idea is a practical one that is derived from the space program with application to the earth-based SCUBA diver.

 

[BillP]

Hi Dr. Deco:

Another helpful relpy. Thanks. To be sure I understand, you are saying that it is important for divers to ascend slowly even if they are not at or near the NDL's? A rapid ascent increases the risk of DCS.

To expand on your carbonated beverage analogy to illustrate the point, if you unscrew the cap on a shaken Coca-Cola very slowly (ie give it a "slow ascent" to 1 ATA), it doesn't fizz and overflow as much as it would if you removed the cap suddenly. Similar concept for divers?

Again, TIA,

Bill

 

[Dr Deco]

Bill:
This is what actually happens both with soda pop and divers. A rapid depress allows much smaller micronuclei to expand. When nuclei expand, the internal pressure from the surface tension is reduced, and the gas supersaturation now can cause them to grow. Thus, while slow ascents may not appear to give much of a result on gas loading analysis, in terms of nuclei expansion into decompression bubbles, the effect is considerable.

 

[old_mike]

this is a seriously good forum we have here! i have the following question: Would the level of hydration (from drinking plenty of water or the flip side: dehydration due to caffeine, seasickness or too many beers) have much effect on the surface tension in tissues and hence, the probability that any one micronucleus would grow and persist?

 

[Dr Deco]

I am pleased that you are finding this forum of value and also of interest. Please tell your dive buddies about this new service from Scuba Source......

It is generally agreed that dehydration, either from lack of intake water or from excessive output of water, is a factor that adds to the risk of decompression sickness. What is lacking is good, quantitative data from the laboratory on this subject. We might guess that there are more questions that arise than laboratory tests performed to explore them; it is very often a matter of research funding. [This last statement is meant to be a subtle message from a professional scientist regarding federal monies for scientific research.] The actual reason for the effect of dehydration is generally ascribed to a reduction in blood flow (local perfusion) to tissues and a hindrance in the offgassing process. Another possibility is a reduction in tissue fluids and a concomitant concentration of surfactants. Surfactants (biomacromolecules) are present in all of the body’s fluids and are the cause for persistence of bubbles in fluids such as urine, blood, saliva, etc. if shaken with some air. The highest surface tensions would be found in pure water and the lowest where there was a high concentration of surfactant(s). [Adding soap to pure water, e.g., lowers the surface tension and makes bubble formation easier.]

In the late 40’s, Dr. Denis Walder performed experiments that indicate dehydration decreased serum surface tension (=concentration of surfactants) and increased the risk of DCS in individuals exposed to altitude in a hypobaric chamber. These experiments have not been duplicated, although some data has recently appeared from the laboratory of Dr. Alf Brubakk linking serum surface tensions and the formation tendencies of decompression gas bubbles.

The actual fact is that several processes would play a role. First, surface tension (ST) would affect the transition of gas embryos (micro-micronuclei) to micronuclei by musculoskeletal activity (hydrodynamic cavitation from fluid flows). Second, ST would influence the lifetime of a micronucleus after it was created by stress (= muscle activity). Third, ST would affect the bubble’s internal pressure (“Laplace pressure”) and whether a micronucleus could grow under a given gas supersaturation. Therefore we have several possible junctures where ST could play a role; currently this is speculative until actual data arrives. My colleagues and I do plan to look into this question in the near future in the laboratory. We all need to keep an eye on this one.

 

[old_mike]

I would offer a null hypothesis that dehydration could be a double whammy for geting bent. Reduced surface tension would prolong the life of the bubbles (in addition to facilitating their creation) and reduced circulation to certain tissues would allow the N to persist at dangerously high concentrations and feed high concentrations of N into circulation as one ascended and even sat on the boat for that matter?

oh and cheer up on the funding, i had to fund my own thesis project back in '96 when i started collecting data ; )

 

[Dr Deco]

Mike:
That is a very good summary of the situation. Hydration is a portion of the story for good blood flow. Lee Somers, PhD, at the University of Michigan once wrote a piece entitled "If You Can't Spit, Don't Dive." By that he meant, you can not always tell when you are underhydrated, but when you are, the production of saliva is slowed.