Dave Chamberlin's post:
After chowing down on some donuts, pastries, cheeses and other goodies provided by the GUE conference organizers, I headed in to the conference room and sat down next to one of the numerous bowls of candies, also courtesy of the conference. And thus began the lecture on "fitness and diving". :-)
The over-riding theme I gleaned from the various lectures on fitness was that it definitely does not pay to be a rat - at least not one involved in DCS studies. :-)
The benefits of fitness on reducing decompression were well-covered by all the speakers. Cameron's book and the wealth of e-mails he and others have written in the past cover the topic rather thoroughly so I won't go in much detail here. I'll cover a few quick points though.
There was a study done whereby they compared both mice and pigs that trained on a tread-mill for 2 weeks prior to an aggressive decompression event. In the case of the mice, the treadmill mice experienced a 40% incidence of DCS whereas the control group experienced an 80% incidence. The results with the pigs were very similar, with 42% for the treadmill pigs and 74% for the control group.
Clearly this did not have much to do with body composition as that did not change much in the short period of training time, nor were the animals high in body fat to begin with. At this point GI chimed in that the primary reason for this is likely the exercise within 24 hours of diving, not that they had been training for 2 weeks.
He continued, saying that the Navy found that there was an enzyme release that reduces bubble nuclei generation. As it turns out, the slides presented later confirmed what GI had said. Note that the bubble nuclei generation tied in with Erik Baker's decompression lecture later.
There were 2 interesting points on the exercise-before-diving topic.
I didn't hear any theories to explain the latter effect. I wonder if the exercise ends up creating more bubble nuclei through cavitation and/or mechanical stresses and the extra nuclei cancel out the beneficial effects of the enzyme release? Anyway, it'd be interesting to hear about more studies on this.
With exercise after diving, they found that weight lifting more than doubled the cases of DCS. The results for low-impact, aerobic exercise was less conclusive. But clearly weight-bearing exercise after diving is a bad idea - keep that in mind when climbing onto the boat in full gear after doing a deep dive ....
There were also some studies done on exercise *during* diving. With moderate exercise (swimming) at depth, it took almost 3 times the amount of decompression to eliminate cases of DCS. The rationale given for this is that the exercise increases the perfusion to the tissues and thus increases the gas burden in the tissues.
There was another study that looked at exercise during decompression. What they found was with moderate-light exercise during decompression, on a 100FSW dive they were able to decrease the length of decompression without affecting the number of DCS cases. On the same test to 150FSW they were able to reduce the length of decompression even further. Again, the rationale relates to perfusion of the tissues. The increased activity during decompression increases the perfusion, facilitating offgassing.
Thus the ideal would be very little activity during the dive and then light activity during decompression. If they had put the scooter workshop right after this lecture, one might have argued that it was partly a sales pitch. :-)
Note that in another lecture, a similar effect was shown but with temperature. Being warm during the dive, then cold during decompression greatly reduces the efficacy of the decompression. Reversing that and being cold during the dive and warm during decompression has the opposite (desirable) effect. Reinhard and Michael commented that this is the reason they do not turn on their heated undergarments during the dive - they only turn them on during decompression.
And just to tie back to the comment about Erik's talk and bubble nuclei ... Erik pointed out that creating bubbles in a liquid is actually pretty difficult purely through pressure differentials. Thus there must be a reason that bubbles are created much more easily in the body. David Yount found that under normal conditions there are surfactants in the blood and tissue.
Surfactants are molecules which are called "amphiphiles" which means they are hydrophilic on one side and hydrophobic on the other. As a result, they tend to clump together, forming a surface between fluid an air - which means they will in essence create bubbles (and in fact it's the surfactants that keep the bubbles from collapsing).
These little bubbles become nuclei for larger bubble formation with pressure differentials. Thus the presence/absence of the bubble nuclei has a very large effect on the number of bubbles that are generated during decompression.
Some studies that NASA did researching decompression when going into space showed that if you increase pressure first before dropping it, bubbles were less likely to form. The theory is that a number of the bubble nuclei get crushed during the inital pressure increase and thus the number of bubble nuclei present during decompression is reduced, reducing the likelihood of bubble formation.