Ventilatory response to inhaled and infused CO2: relationship to the oscillating signal

The purpose of this study was to determine the relationship between the respiratory oscillations of CO2 and the ventilatory response to CO2. Cats anaesthetized with chloralose and urethane were given CO2 either by exchange transfusion into the inferior vena cava or by inhalation. Changes of amplitude of the respiratory oscillations of CO2 (amp PCO2) were assessed from measurements of the amplitude of the arterial pH oscillations (amp pH). For short periods of CO2 loading (2-8 min), the ventilatory response to CO2 was greater for venous than for airway loading; this difference was statistically significant at low and intermediate loads but not at high loads. The dependence of the respiratory response to the route of administration of CO2 could not be related to amp PCO2. For long periods of CO2 loading (20 min or more) there was no marked change of minute ventilation when the route of administration was changed from airway to venous route, despite an increase of amp pH at both low and intermediate loads. We suggest that the dependence of ventilatory response to CO2 on the route of administration observed in this study was mainly due to a longer period required to attain steady state conditions for CO2 inhalation, and that the respiratory oscillations of CO2 do not affect the ventilatory response to CO2.