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Arterial to end-tidal PCO2 difference varies with different ventilatory conditions during steady state hypercapnia in the rat.

To estimate the influence of ventilatory conditions on the CO2 equilibration between the alveolar gas and arterial blood during steady state hypercapnia, we measured arterial and end-tidal PCO2 (PaCO2, PETCO2) of the anesthetized rat under the following three conditions: spontaneously breathing with CO2 inhalation, artificial respiration with gas mixture containing CO2, and artificial respiration with reduced ventilatory volume (hypoventilation). In each ventilatory condition, PaCO2 correlated linearly with PETCO2. However, in spontaneously breathing animals, the PaCO2-PETCO2 difference which was positive in a control condition (without CO2 inhalation) became negative during CO2 inhalation. The mean (+/- S.D.) difference was -3.6 +/- 1.5 mmHg (n = 9, p less than 0.001) at the PETCO2 range from 72 to 77 mmHg. During artificial respiration with constant ventilatory volume, initial positive PaCO2-PETCO2 difference approached zero when CO2 was administered into inspiratory gas. In both ventilatory conditions the slope of the PETCO2-PaCO2 relation line was less than 1.0, whereas the PaCO2-PETCO2 difference remained positive when PCO2 level was increased with reducing the ventilatory volume (accumulation of endogenous CO2). These observations suggest that for a given increase in PCO2 by administration of exogenous CO2, the extent to which PaCO2 increases is smaller than that of PETCO2. This peculiar relationship together with changes in breathing pattern during CO2 inhalation likely results in "negative" PaCO2-PETCO2 difference in the spontaneously breathing animal. We conclude that the PaCO2-PETCO2 difference, either as positive or negative values, depends upon both the level of PCO2 and the ventilatory condition to increase PCO2.