Many of the decompression algorithms we are familiar with as technical divers use something similar to the Buhlmann ZH-L16 compartment structure in which the helium half-times are about 2.65 times faster than nitrogen half-times in every compartment, and these differences are attributed to the difference in diffusivities of the two gases. In a very few places in the body, gas uptake is diffusion-limited, and in these places, the helium and nitrogen half-times will be determined by the differences in diffusivites. However, in most places in the body, any differences in helium and nitrogen half-times have little to do with differences in diffusivities. In most tissues in the body, the distances between capillaries blood vessels (and therefore the distance over which gas must diffuse to equilibrate blood and tissue) are sufficiently small that diffusion is not the rate-limiting process. The rate-limiting process, and therefore the process which determines the half-time, is the delivery (or removal) of gas in the blood. The half-times are related to the blood flow times the solubility of the gas in the blood divided by the tissue volume times the solubility of the gas in the tissue. The blood flow and tissue volume are (more or less) independent of the gas, so any differences in half-times arise from differences in the ratio of blood solubility to tissue solubility (partition coefficient) of the gases. Helium is approximately equally soluble in blood and most tissues (partition coefficient=1). Nitrogen is a little more soluble than helium in blood and tissues, but, in for most tissues the nitrogen blood:tissue partition coefficient is approximately 1. In these tissues, helium and nitrogen half-times should be the same. Unlike helium, nitrogen is very much more soluble in fat than is blood, so that the partition coefficient=1/5. In tissues with a high fat content (i.e. fat), the nitrogen half-time will be slower than the helium half-time. The high diffusivity of helium does play a minor role, but it actually acts to slow down blood:tissue equilibration of helium because some helium diffuses directly between arteries to veins ‘outside’ of the tissue – so during uptake, some helium bypasses the tissue and during washout some helium circulates back to the tissue.
There is no reason to believe that the gas kinetics at sites at which DCS occur are any different to the gas kinetics observed in experiments such as the one cited at the beginning of this thread (which is the full write-up of the conference UHM 2005 conference presentation cited above). However, you can never be sure because we do not really know where these DCS-sites are. Therefore, it is important to compliment gas kinetic studies with actual human decompression experiments. There is mounting evidence indicating that there is no difference in decompression requirements for helium and nitrogen for bounce dives (there is a difference for saturation dives where decompression s controlled by gas washout from the very slowest exchanging tissues, from which nitrogen probably washes out slower).
To actually determine whether there is any true difference in the decompression requirements for heliox, trimix, or nitrox dives, experiments comparing dives using the same depths and the same schedules need to be conducted. The NEDU trimix study (Doolette et al. NEDU Technical Report 15-04) found that dives conducted breathing either heliox resulted in 0/50 (DCS/dives) and identical dives breathing trimix resulted in 2/48 (DCS/dives) – i.e. no evidence of a “helium penalty” resulting from faster uptake of helium than of nitrogen, instead evidence that indicates that the uptake and washout of helium and nitrogen are similar in faster exchanging compartments, and consequently, little difference in decompression requirements for helium-based and nitrogen-based breathing gases. In addition to the trimix study, there are two studies which inform the issue. First, Thalman, Flynn and colleagues (reported in Hamilton RW et al. NMRC Technical Report 2002-002.) compared air and heliox (21% oxygen / 79% helium) at 60 fsw with various no-stop bottom times. They found similar incidences of DCS for both gases: for BT of 80, 90, 100 air resulted (DCS/dives) in 1/14, 2/21, 2/13 and heliox resulted in 0/20, 3/10, 5/20. It is possible that faster uptake of helium at depth was compensated for by faster washout of helium (and therefore less prolonged supersaturation and bubble formation) on the surface, but nonetheless, these data indicate no significant difference in the decompression requirements for nitrox or heliox. Second, another experiment of heliox-to-nitrox gas switch (Survanshi et al. NMRI Technical Report 98-09) provides some information. In this experiment dives were conducted to 300 fsw for 30 minutes breathing 1.3 atm PO2-in helium, with identical decompression either on 1.3 atm PO2-in helium or 1.3 atm PO2-in-nitrogen. Heliox decompression resulted in (DCS/dives) 1/32 and nitrox deco resulted in 3/16. Again, no significant difference between the two gas mixtures.
