Mr Carcharodon
Contributor
@Mr Carcharodon: I would be satisfied if you could tell me what tissue in the human body correlates with the "slow" compartment and how perfusion rate can be accurately measured through that specific compartment.
The idea that gas transport to some tissues (e.g. muscle, skin, nerves) is limited by perfusion goes back at least to Haldane’s time. Also the idea that diffusion limits gas transport to other tissues (e.g. bone and the ear) seems to be nearly as old. Perhaps one of the more accessible discussions of perfusion and diffusion limited transport is provided in Wienke’s text on Basic Decompression. Haldane discussed perfusion rates of various tissues in his 1908 paper in the context of estimating gas loading, and time constants, of those tissues. So given the date of his paper it seems safe to conclude that MRI is not the sole technique available to measure perfusion rates. The diffusion limited tissues are the “slow” tissues. Clearly if you accept that those tissues are diffusion limited you will conclude that their gas loading does not change as a function of perfusion. And equally clearly the rate of gas loading will change for perfusion limited tissues with exertion provided they are not saturated to begin with.
The idea that circulation varies with activity seems almost too obvious to defend, but if you really need backup I would suggest looking at the v-dot studies of the exercise physiologists. The idea that circulation can be locally modulated is a bit less obvious but can be demonstrated by considering what is happening with flushing skin or shock. As far as specific ratios of change of circulation with activity I would have to point you back to Powell and Hennessey again. The 7x ratio is a single value I used to simply express a chart from Hennessey. Perhaps a range would have been better but it still seems like a reasonable value to me.
But this is tangential since the objective is answering the original posters question. Ianr33 is right to think that the lungs will be near ambient pressure. But the rest of the body needs some time to catch up. For recreational dives some of the tissues in the body never catch up entirely. Exertion will increase the rate with which they do. I do not know enough to say how significant this is to DCS, but will point out that the DSAT/PADI approach is to add padding for strenuous dives. Also Buhlmann’s ZH-16A tables are not thought to be conservative enough. And one complaint about those tables is that they were derived from chamber trials with subjects at rest. Subsequent work tightened up the m-values to envelope the gas loading of more typical levels of exertion. What changed were the mid compartments which are the ones that would be expected to represent unsaturated perfusion limited tissues.
So I conclude that exertion does increase gas loading and does increase DCS risk but freely admit that I do not know how much. Strenuous dives are not what the tables are based upon and it seems prudent to do safety stops or min deco to mitigate the risk associated with the extra gas load that exertion causes. In the context of Mr. DeVlieger’s accident we do know with certainty that a safety stop would have helped and would have only taken a few cubic feet of gas.