1. The acid-base and respiratory status of fifteen healthy male subjects living in an atmosphere of 1% CO2 in air was studied over the course of 44 days, during an operational patrol in a nuclear submarine.

2. Observations were made during a control period before exposure to CO2, at 4 day intervals during the patrol, and finally during the period after its completion. Samples of arterial blood from each subject were analysed for Pa,co2, pH and Po2 immediately after collection. Concurrently mixed expired Pco1 and Po2, together with minute volume and respiratory rate, were measured.

3. A mild uncompensated respiratory acidosis in which arterial pH was depressed by 0·02 pH unit was demonstrated throughout the period of exposure to CO2. This was associated with an acute rise in Pa,co2 of 0·14 kPa, accompanied by an increase in minute volume from 11·5 to 15 l/min. By mid-patrol minute ventilation had returned to control value and a further elevation in Pa,co2 of 0·31 kPa was detected together with a rise in plasma bicarbonate of almost 1 mmol/l.

4. On return to air after completion of the patrol, the acid-base changes appeared to be quickly reversed. Minute volume decreased slightly initially, but it too subsequently returned to the control value. A fall in Pa,o2 of about 2·5 kPa was recorded at this time, together with a reduction in forced vital capacity of 8%.

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