Isobaric inert gas counter diffusion is an unusual, hazardous and newly discovered circumstance of modern diving in which differential rates of inert gas movement in opposite directions in a tissue can lead to gas super saturation and gas bubble disease without change in pressure (isobaric state) (1)(2). The expressions of the phenomenon include severe cutaneous itching, development of gas lesions in the skin, incapacitating vertigo from vestibular derangement, and lethal gas bubble embolism. Since the problem has been recognized for only a few years, it is not yet commonly understood. Because it has implications to safety in diving operations, in decompression and in therapy of decompression sickness, this description will define conditions in which potential hazard due to isobaric counter diffusion exists and conditions in which it does not.
Over the more than 100 years that decompression sickness has been known, the gas used in most diving has been air. When helium diving was introduced in about 1935, it was used, with necessary oxygen, from start to finish in a dive, without gas switching during decompression. Such use of helium alone continued in naval diving, and then expanded with increasing interest in deep diving for commercial purposes. Its use in conjunction with other inert gases was suggested to facilitate deep diving or decompression from it.
Among the methods proposed to complement use of helium-oxygen alone were those providing for speeding the elimination of helium by breathing another inert gas-oxygen mixture during decompression (3)(4). In this manner, switching to air breathing during decompression after a deep helium dive enables the more rapidly exchanged helium to be eliminated faster than the more slowly exchangingli1trogen is taken up by the same tissue compartments. The advantage - a more rapid lowering of tissue total inert gas pressure is aimed at providing more rapid decompression or safer decompression. While more rapid elimination of helium must result, it has been noted that the switch to nitrogen may be associated with development of vertigo, indicating vestibular dysfunction (5). This occurrence does not yet have a clear or demonstrated explanation. The method should be a rational and effective procedure for reducing tissue total gas pressure in all tissue compartments as long as abrupt switch of airs is avoided, precautions are taken against isobaric counter diffusion, and the pressure of the switch to air is low enough to avoid prominent nitrogen narcosis or oxygen toxicity. It differs entirely from the use of nitrogen in a mixture with helium for voice modification or other purpose.
The second form of gas switching involves changing to helium breathing after prolonged periods of work while breathing air or nitrogen oxygen mixtures at increased pressure (Fig. 1). This procedure has been employed experimentally (6)(7)(8)(9), in excursion from nitrogen saturation states and in special situations in the therapy of decompression sickness.