Isobaric counterdiffusion

If you could send us an explanation about Isobaric Counterdiffusion in Tek diving in 3 or 4 simple sentences it would be just great!

Inert Gas Counterdiffusion or "Isobaric Counterdiffusion" was first described by Idicula and Lambertsen in 1973 in subjects who breathed one inert gas mixture while being surrounded by another. It implies the diffusion of gases in different directions, without variations of the environment pressure, and can produce the formation of bubbles without decompression.
Various forms of this phenomenon have been described.
Superficial Isobaric Counterdiffusion is generated when the inert gas breathed diffuses more slowly into the body than the inert gas surrounding the body (example: breathing air in an Heliox environment. Helium diffuses fast into the skin, while nitrogen is slower in diffusing from the capillaries to the skin and outside. The summation effect generates supersaturation in certain sites of the superficial tissues and the formation of bubbles). The phenomenon has also been observed in the helium environment while changing from helium to hydrogen as a breathing inert gas.
This phenomenon can also affect the vestibular system, with the risk of DCS at depth, as a consequence of the possible counterdiffusion between the middle ear and the fluids of the inner ear, or as a vestibular effect of the continuous gas formation involved in superficial isobaric counterdiffusion.
Deep Tissue Isobaric Counterdiffusion can occur, even when the breathed and the environment gas are the same and superficial counterdiffusion would not be justified, if different rates of tissue uptake and elimination of the different gas mixtures breathed in sequence lead to supersaturation or subsaturation in the isobaric state. For example, after prolonged exposure to a nitrox mixture, changing to heliox would result in a rapid entry of helium and a relatively slower elimination of nitrogen from tissues, with the possibility of a supersaturation effect without ambient pressure changes.
Switching Gases and Decompressin Sickness. DCS, which has been described in similar circumstances, can be triggered or worsened when a diver breathing heliox switches to air. Even if this can be difficult to explain with isobaric counterdiffusion principles, it is possible that the variations in gas density and pulmonary dynamics may slow down the elimination of helium.


Abstracted from: C. Edmonds, C Lowry, J Pennefather. Diving and Subaquatic Medicine ( 3rd Edition). Chapter 11, Historical and physiological concepts of decompression. Butterworth Heinemann, Oxford, 1992.