To estimate the influence of ventilatory conditions on the CO
2 equilibration between the alveolar gas and arterial blood during steady state hypercapnia, we measured arterial and end-tidal
PCO2 (
Pa
CO2,
PET
CO2) of the anesthetized rat under the following three conditions: spontaneously breathing with CO
2 inhalation, artificial respiration with gas mixture containing CO
2, and artificial respiration with reduced ventilatory volume (hypoventilation). In each ventilatory condition,
Pa
CO2 correlated linearly with
PET
CO2. However, in spontaneously breathing animals, the
Pa
CO2-
PET
CO2 difference which was positive in a control condition (without CO
2 inhalation) became negative during CO
2 inhalation. The mean (±S.D.) difference was -3.6±1.5mmHg (n=9, p<0.001) at the
PET
CO2 range from 72 to 77mmHg. During artificial respiration with constant ventilatory volume, initial positive
Pa
CO2-
PET
CO2 difference approached zero when CO
2 was administered into inspiratory gas. In both ventilatory conditions the slope of the
PET
CO2-PaCO
2 relation line was less than 1.0, whereas the PaCO
2-
PET
CO2 difference remained positive when
PCO2 level was increased with reducing the ventilatory volume (accumulation of endogenous CO
2). These observations suggest that for a given increase in
PCO2 by administration of exogenous CO
2, the extent to which PaCO
2 increases is smaller than that of
PET
CO2. This peculiar relationship together with changes in breathing pattern during CO
2 inhalation likely results in "negative" PaCO
2-
PET
CO2 difference in the spontaneously breathing animal. We conclude that the PaCO
2-
PET
CO2 difference, either as positive or negative values, depends upon both the level of
PCO2 and the ventilatory condition to increase
PCO2.
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