Does higher RMV cause higher DCS risk?

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However, if exercise level (and therefore increased perfusion) was a real issue, then it would be the leading cause of injury, and figure heavily in the case reports. Where is the identifiable trend line in case reports, with a common exercise component? It does not seem to exist.

As I have told you Ross, it absolutely exists. We see cases of DCS in divers who have worked hard on the bottom all the time. But why would I or any of my colleagues think to report it as anything special? Work at depth as a risk factor for DCS is an established part of the "wisdom" of our field, and for good reason. For a start, there are experimental studies (which you erroneously dismiss as old or aggressive) which confirm what is barn-door obvious from gas physiology 101. Enhanced perfusion during work at depth will cause tissue to load inert gas more quickly.

Simon M
 
As has been noted previously in this thread, in response to the initial question, increasing respiratory minute volume without a change in the work the diver is doing (i.e. hyperventilating) would not be expected to have much effect on inert gas exchange or on the risk of decompression sickness (DCS). The hyperventilation will blow off a little CO2, and that could alter blood flow in some tissues a bit, but at the levels in the question, probably not enough to make a difference in DCS.


On the other hand there is a reasonable amount of good evidence that increasing work rate (exercising) can change the risk of DCS or change the decompression requirements. So for instance exercise on the bottom increases the decompression requirement and light exercise during decompression can reduce the decompression requirement. Military decompression procedures usually account for this by always developing and testing decompression procedures with divers working on the bottom and resting during decompression – the worst case condition – so that the schedules will be conservative if used without exercise on the bottom. The evidence for exercise as a risk factor is summarized in a conference proceedings I wrote with Dick Van titled Risk factors for decompression sickness, which is in the DAN Technical Diving Workshop proceeding, available from DAN


https://www.diversalertnetwork.org/files/Tech_Proceedings_Feb2010.pdf


a nice illustration of the effect of exercise on the bottom is : Figure 3 on page 125.


The mechanism is assumed to be an increase in blood flow to the tissues, and as a result greater uptake of inert gas into the tissues. Exercise results in increased blood flow to the exercising muscles (exercise hyperaemia) to satisfy the increased metabolic demand, and also increases blood flow to some other tissues. The principal determinate of the rate of tissue uptake and washout of nitrogen and helium (often characterized by half-times) is the blood flow per volume of tissue (perfusion). These tissue inert gas kinetics seem to me modified by diffusion, but the main determinate is perfusion. I spent a lot of my professional life studying exactly this, and this paper here (excuse the typo in the title):

Altering blood flow does not reveal differences between nitrogen and helium kinetics in brain or in skeletal miracle in sheep

shows how we change the rate of helium and nitrogen uptake and washout in tissues by changing only the blood flow to the tissues. If you look at figures 3 for instance, this compares gas uptake and washout in high and low blood flow states in the brain, and the difference in the rates in the high (right panels) and low (left panels) blood flow states is obvious just from inspection.

David Doolette
 
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Thanks David,

It is good for everyone to hear this from the world's premier decompression modeller. Some extremely basic misunderstandings of some of the most fundamental concepts of decompression theory have been exhibited in this thread.

Simon
 
https://www.shearwater.com/products/perdix-ai/

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