What is missing though (?) is an inner ear decompression model.
Well, David Doolette had a crack at this in a paper we published together in 2003.
Doolette DJ, Mitchell SJ. A biophysical basis for inner ear decompression sickness. Journal of Applied Physiology 94, 2145-2150, 2003
I would be happy to send it to you if you PM me an email address.
Are there other sites like this in the body? Reservoirs of badly/nonperfused liquid? At the joints?
None that are clearly functionally significant in the same way as the inner ear that we know of. I say this because decades of experience have demonstrated that inner ear symptoms can appear during decompression in temporal relation to helium to nitrogen gas switches (and the putative mechanism of such events are explained in the above paper), but there are no other manifestations of DCS that clearly do this. That is not to say that other symptoms of DCS don't occur during deco, but none have emerged that consistently do so in clear relation to gas switches.
The subtext here is that all the fuss over "ICD" is probably only relevant to inner ear DCS, and even then, only when the inner ear is already considerably supersaturated with inert gas. This situation typically occurs during decompression from very deep dives (nominally > 100m), and hence the widespread perception (probably correct) that ICD is mainly a problem on such dives.
In fact, for the reasons I explained in the post I linked to, assuming that prevalent assumptions about helium and nitrogen exchange are correct (but see below), in most tissues gas switches from helium to nitrogen should accelerate decompression rather than cause problems. That is one of the reasons that use of such switches has been very common in military and commercial diving. In my Navy days we would routinely run surface supply dives to a maximum training depth of 80m using oxygen-helium 16:84 and change
straight to air during decompression on a table prescribed by the DCIEM. In providing medical support to well over 100 such dives I never saw inner ear DCS in relation to the switches (though we always knew it was possible) or at any time during the decompression for that matter.
The risk of ICD in general has been conflated by the technical diving community during discussions which seem to embrace the assumption that many forms of DCS are caused by ICD. This is almost certainly not true, and not consistent with real world observations which confirm a link with inner ear DCS, but nothing else. That is not to say that one should ignore the risk of IEDCS when gas switching because, as I point out, these events do occur - just not as commonly as you might think given the catastrophizing about switching to nitrox on this thread (and many others on technical forums). Nevertheless, I don't get bothered by the obsessing over the issue, even though it is almost certainly excessive, because careful attention to gas switching "rules" and ICD alarms in dive planning software etc is probably harmless at worst, and is likely protective to some degree.
The main reason for not using gas switches (unless you are on OC and worried about cost) in the modern context is that the assumptions about helium and nitrogen exchange in tissues that underpin the practice may not be true. David and I discuss this in this article:
Doolette DJ, Mitchell SJ. Recreational technical diving part 2: decompression from deep technical dives. Diving and Hyperbaric Medicine 2013; 43:96-104.
...which stems from original work he performed sometime ago, but only recently (ie last month) published here:
Doolette DJ, Upton RN, Grant C. Altering blood flow does not reveal differences between nitrogen and helium kinetics in brain or in skeletal muscle in sheep. Journal of Applied Physiology Dec 18:jap.00944.2014. doi: 10.1152/japplphysiol.00944.2014. [Epub ahead of print].
The title of the above paper tells the story. In other words, gas switches may not appreciably accelerate decompression.
Simon M
