In physiology, isobaric counterdiffusion (ICD) is the diffusion of different gases into and out of tissues while under a constant ambient pressure, and the physiological effects of this phenomenon. The term inert gas counterdiffusion is sometimes used as a synonym, but can also be applied to situations where the ambient pressure changes.
Isobaric counterdiffusion was first described by Graves, Idicula, Lambertsen, and Quinn in 1973 in subjects who breathed one inert gas mixture (nitrogen or neon) while being surrounded by another (helium).
In medicine, ICD is the diffusion of gases in different directions that can increase the pressure inside open air spaces of the body and surrounding equipment.
An example of this would be a patient breathing nitrous oxide in an operating room (surrounded by air). Cuffs on the endotracheal tubes must be monitored as nitrous oxide will diffuse into the air filled space causing the volume to increase. In laparoscopic surgery, nitrous oxide is avoided since the gas will diffuse into the abdominal or pelvic cavities causing an increase in internal pressure. In the case of a tympanoplasty, the skin flap will not lay down as the nitrous oxide will be diffusing into the middle ear.
In underwater diving, ICD is the diffusion of one inert gas into body tissues while another inert gas is diffusing out. If the gas that is diffusing into a tissue does so at a rate which exceeds the rate of the other leaving the tissue, it can raise the combined gas concentration in the tissue to a supersaturation sufficient to cause the formation or growth of bubbles, without changes in the environmental pressure, and in particular, without concurrent decompression. Two forms of this phenomenon have been described by Lambertsen:
An example of this would be breathing air in an heliox environment. The helium in the heliox diffuses into the skin quickly, while the nitrogen diffuses more slowly from the capillaries to the skin and out of the body. The resulting effect generates supersaturation in certain sites of the superficial tissues and the formation of inert gas bubbles. These isobaric skin lesions (urticaria) do not occur when the ambient gas is nitrogen and the breathing gas is helium.
Deep tissue ICD
Deep tissue ICD occurs when different inert gases are breathed by the diver in sequence. The rapidly diffusing gas is transported into the tissue faster than the slower diffusing gas is transported out of the tissue.
An example of this was shown in the literature by Harvey in 1977 as divers switched from a nitrogen mixture to a helium mixture they quickly developed itching followed by joint pain. Saturation divers breathing hydreliox switched to a heliox mixture and developed symptoms of decompression sickness during Hydra V. More recently, Doolette and Mitchell have described ICD as the basis for inner ear decompression sickness and suggest "breathing-gas switches should be scheduled deep or shallow to avoid the period of maximum supersaturation resulting from decompression".
Lambertsen made suggestions to help avoid ICD while diving. If the diver is surrounded by or saturated with nitrogen, they should not breathe helium rich gases. Lambertson also proposed that gas switches that involve going from helium rich mixtures to nitrogen rich mixtures would be acceptable, but changes from nitrogen to helium should include recompression. However Doolette and Mitchell's more recent study of Inner Ear Decompression Sickness (IEDCS) now shows that the inner ear may not be well-modelled by common (e.g. Bühlmann) algorithms. Doolette and Mitchell propose that a switch from a helium-rich mix to a nitrogen-rich mix, as is common in technical diving when switching from trimix to nitrox on ascent, may cause a transient supersaturation of inert gas within the inner ear and result in IEDCS. A similar hypothesis to explain the incidence of IEDCS when switching from trimix to nitrox was proposed by Steve Burton, who considered the effect of the much greater solubility of nitrogen than helium in producing transient increases in total inert gas pressure, which could lead to DCS under isobaric conditions. Recompression with oxygen is effective for relief of symptoms resulting from ICD. However, Burton's model for IEDCS does not agree with Doolette and Mitchell's model of the inner ear. Doolette and Mitchell model the inner ear using solubility coefficients close to that of water. Burton departs from this inner ear model and uses the solubility coefficients of lipids (fats) to model the inner ear. 
A decompression planning software tool called Ultimate Planner attempts to predict ICD through modeling the inner ear as either aqueous (Mitchell and Doolette's approach) or lipid tissue (Burton's approach).
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