Effects of small changes in PaO2 on the ventilatory response to CO2 infusion

The steady-state ventilatory responses to CO2 inhalation and two levels of intravenous CO2 infusion were examined in chronic awake dogs. Responses were obtained for four treatments: 1) air breathing, in which arterial O2 tension (PaO2) increased during infusion; 2) normoxia, PaO2 maintained at control levels; 3) hypoxia, PaO2 = 70 Torr; and 4) hyperoxia, PaO2 = 160 Torr. In three dogs, the ventilatory responses were obtained for treatments 1, 2, and 4 and in an additional three dogs for treatments 2, 3, and 4. Analysis of the data by directional statistics indicates that the response to infusion was hypercapnic for all treatments, and the slope of the response (change in minute ventilation divided by change in arterial CO2 tension) was identical to that of CO2 inhalation. Also, the slopes of the responses for the low infusion rate, CO2 output (VCO2) = 50%, and the high infusion rate, VCO2 = 270%, were identical, which suggests that the CO2 response is not significantly curvilinear near the eucapnic region. Finally, changing PaO2 between 70 and 160 Torr had no significant effect on the response slopes. Thus the ventilatory response to CO2 infusion in the awake dog is a hypercapnic hyperpnea that is not due to ventilatory inhibition arising from an increase in PaO2.