Alveolar Gases in Rapid Decompression to High Altitudes

Abstract

During and immediately after rapid decompression to high altitudes breathing air the composition of alveolar gas undergoes changes involving an increase in the volume fractions of CO2 and O2 and a decrease in N2. At altitudes above 39,400 ft. more O2 was present in the lungs than in the ambient atmosphere. While breathing O2 in similar expts. the alveolar O2 fraction was partly displaced by CO2 which comprised more than half of the lung gas content at 52,500 ft. By relating the alveolar gas pressure to the altitude it is shown that the benefit gained by breathing O at moderate altitudes is less pronounced in rapid decompression to higher altitudes. This is not only the result of the reduction in total pressure but also due to the relatively high O2 content in the lungs after decompression breathing air, where O2 from the blood returns to the lungs and temporarily augments the alveolar O2 pressure at the expense of N2. In effect there is little difference in alveolar O2 pressure whether breathing air or O2 in rapid decompression to altitudes above 52,500ft. Nevertheless, the decisive importance of breathing 100% O2 previous to and continuously after decompression for the chances of survival on descent is emphasized. In rapid decompression "equivalent altitudes" cannot be estimated from tracheal O2 tensions, as these do not reflect the conditions in the lungs. On applying the data obtained in this study to previous comparable investigations on rapid decompression, it appears that survival time and time of useful consciousness reach a minimum and remain constant at altitudes where the alveolar O2 pressure drops below 18-20 mm. Hg. Evidence is presented that O2 passes back into the alveoli from the blood in the pulmonary capillaries whenever the alveolar O2 pressure is lower than that of mixed venous blood. The low CO2 pressures encountered immediately after rapid decompression indicate that hypocapnia of the blood is not a secondary event but is simultaneous with hypoxia under these conditions.