Effectiveness of a breath during exercise in a hyperbaric environment

During vigorous foot-pedal exercise at 6.75 ATA, three subjects had lower total ventilation, larger functional residual capacity (FRC), and higher P(CO₂) in end-expired and mixed-expired gas than during the same exercise at 1.5 ATA. Compartmental analysis of multiple breath washin suggested that ventilation was more evenly distributed during the high pressure exercise. Mass-balance analysis of inert indicator gases in single breaths showed that at a given pressure, low-diffusivity gases did not mix in the lung as well as high-diffusivity gases. It did not follow, however, that a particular gas was better mixed at low pressure than when its diffusivity was decreased by high pressure; the data showed just the opposite during exercise. The apparent paradox seems to be explained by the change of other conditions for mixing at high pressure, especially the enlargement of the FRC.