[Variations of the sum of the arterial partial pressure of inert gases during changes in the composition of inhaled gas without a variation of the ambient pressure, in a hyperbaric atmosphere].

In hyperbaric environments, when inhaled inert gas composition is abruptly modified, the sum of the arterial inert gases partial pressures is different to the sum of these same gases in the inhaled mixture. While switching from a helium-oxygen to a nitrogen-oxygen mixture of same P1O2 and total pressure, the sum of the arterial inert gas partial pressure was transiently less than the one in the inspired gases: there was an arterial under-saturation: PaHe + PaN2 = 0.68 (P1He + P1N2). During the opposite switch (from nitrogen to helium), a reversed time course, namely a transient over saturation, was observed: PaHe + PaN2 = 1.31 (P1He + P1N2). Amongst the different possible explanatory hypotheses, the most probable is that inert gas partial pressure equilibrium through the alveolo-capillary membrane is not achieved when the blood leaves the pulmonary capillary.