Effects of H+ versus CO2 on ventilation in the Pekin duck

Measurements were made of ventilation (VE) and arterial blood gases (PCO2 and PO2) and pH in awake Pekin ducks, Anas platyrhynchos, exposed to sequential cycles of 1% step changes in inspired CO2 from 0 to 5 and back to 0% CO2, as well as to a sustained period (5 h) of 5% CO2 breathing. Changing the background level of inspired PO2 was used to alter the range of PaCO2 over which CO2 response curves were obtained. The initial increase in minute ventilation following a sustained step increase in inspired CO2 (5%) began to decline immediately at a rate of approximately 1% every 5 min. This was closely paralleled by increases in both pH and [HCO-3] while arterial PaCO2 remained constant or increased slightly. CO2 response curves (VE vs PaCO2) generated in a sequential fashion shifted down and to the right (a paradoxical decrease in VE associated with an increase in PaCO2) while the relationship between VE and pH remained constant. These data suggest that VE is a single function of pH in birds under these conditions and that the relationship between VE and PaCO2 is altered due to metabolic acid-base compensation even during acute CO2 exposure.