Ventilatory response to CO2 inhalation and intravenous infusion of hypercapnic blood

The ventilatory responses to intravenous infusion of hypercapnic blood (equilibrated with 80% CO2: 20% O2) and inhaled CO2 (1.5-2.0% CO2) have been compared in anaesthetized rabbits. A control infusion of blood equilibrated with 4% CO2: 21% O2 produced no change in V, PaCO2 or PaO2. The ventilatory response to a given rise in PaCO2, deltaV - (delta PaCO2)-1, was greater during infusion of hypercapnic blood (69 +/- 33 ml-min-1-mm Hg-1, mean +/- SD) than during CO2 inhalation (28 +/- 16 ml-min-1-min Hg-1, P less than 0.01). There was no difference between the rates at which the final levels of ventilation were reached during inhalation of CO2 or infusion of hypercapnic blood. It is suggested that the difference between the responses may be accounted for by differences in the pattern of PaCO2 oscillations in the two situations.