Alveolar Pco2 during rest and exercise in divers and non-divers breathing O2 at 1 ATA.

The extent of hypercapnia in oxygen-breathing subjects was investigated during rest and exercise (650 kpm/min). Divers, ex-divers, and non-divers were studied. Carbon dioxide retention was judged by end-tidal Pco2 values achieved after a few minutes at each state. End-breath-hold (EBH) PAco2 was also measured in the two states to assess the contribution of breathing pattern to CO2 retention. Findings indicated a pronounced hypoventilation and hypercapnia in the divers during exercise, and a mean exercise EBH PAco2 value 15 mmHg greater than that in non-divers. Ex-divers had group mean values not significantly different from those of active divers. Superimposed inspired CO2 load caused similar elevations in PAco2 values in divers and non-divers. We concluded that CO2 retention in exercising oxygen-breathing divers cannot be accounted for solely as a conditioned breathing behavior, may be due partly to a reduced central responsiveness of CO2, and is not dependent on current diving activity.