Nitrogen elimination during steady-state hyperbaric exposures

Nitrogen elimination was measured in six divers during steady-state exposures in an oxygen-nitrogen atmosphere at 1, 2, and 3 ATA using both oxy-helium and pure O2 as washout gases. This was accomplished by using mass spectrometry to measure the expired N2 concentration breath-by-breath over periods of 120 min in all experimental conditions except for O2 breathing at 3 ATA, which was limited to 30-min periods. In all cases the area under the elimination curve increased with pressure. Total area under the curve was also greater when breathing O2 than when breathing oxy-helium, but this difference decreased with depth and washout time. Nitrogen elimination on a semilogarithmic plot falls rapidly during the first four minutes and then shows a slow linear fall for the remainder of the measurement period. Effective elimination of nitrogen decreased with depth and oxygen was more effective than oxy-helium in washing out nitrogen at all depths studied. Possible causes of the different variations noted in the washout curves during the experiment are discussed.