Delayed Approach of Arterial to Alveolar Nitrous Oxide Partial Pressures in Dog and in Man

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Abstract
Simultaneous measurement of arterial (Pa) and end-tidal (Pet) nitrous oxide partial pressures in dog and in man revealed differences of 10-20 per cent [1 - (Pa/Pet) =0.1 to 0.2] following a 1/2 min. of ventilation with nitrous oxide. These differences decreased to 5 to 10 per cent by 4 minutes and to less than 2 per cent after 23 minutes. The differences were increased by onset of anesthesia and were reduced by raising the alveolar nitrous oxide concentration. The results suggest that the end-tidal partial pressure of a gas is less representative of the arterial partial pressure when uptake of that gas is great.