Arterial-alveolar difference in PCO2 during air and oxygen breathing.

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Abstract
The mean arterial-alveolar difference in Pco2 was found to be higher during O2 breathing than during air breathing in 62 subjects breathing successively both gases in a random order. This increase resulted from a change in PAco2 [arterial CO2 tension] and Paco2 [alveolar CO2 tension]. PAco2 decreased in all subjects independently of the PAco2 value during air breathing Paco2 increased proportionally to its value during room-air breathing. The increase of Paco2-PAco2 is discussed in terms of Haldane effect and in terms of redistribution of ventilation-to-perfusion ratios throughout the lung during O2 breathing. It is concluded that the Haldane effect, although always present, plays only a minor role, leading to a moderate increase of Paco2, unless the arterial O2 saturation was quit e low during air breatning. Changes in ventilation-to-perfusion ratios distribution secondary to a shift of blood flow during O2 breathing are an important factor to increase Paco2-PAco2. They effect significantly both Paco2 and PAco2, but more the latter than the former. Any increase in ventilation, as occurred in these experiments, during O2 breathing tends to increase the arterial alveolar difference in Pco2.

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