Inspiratory airway CO2 loading in the pony

To determine if CO2-sensitive airway receptors are important in the control of breathing, CO2 was preferentially loaded into the respiratory airways in conscious ponies. The technique involved adding small amounts of 100% CO2 to either the latter one-third or latter two-thirds of the inspiratory air in an attempt to raise CO2 concentrations in the airway dead space independent of the arterial blood. Arterial blood gas tensions (PCO2 and PO2) and pH, as well as respiratory output (minute volume, tidal volume, and respiratory rate), were measured in a series of 20 experiments on 5 awake ponies. Elevation of airway CO2 to approximately 12% by addition of CO2 to the latter portion of the inspiratory tidal volume did not alter either ventilation or arterial blood gases. When CO2 was added earlier in the inspiratory phase to fill more of the airway dead space, a small but significant increase in minute volume (2.1 l X min-1 X m-2) and tidal volume (0.1 l X m-2) was accompanied by an increase in arterial PCO2, arterial PO2, and a fall in pH (0.96 Torr, 10.5 Torr, 0.007 units, respectively). A second series of 12 experiments on 6 awake ponies using radiolabeled 14CO2 determined that the increases in breathing were minimal when compared with the large increase that occurred when these animals inhaled 6% 14CO2 (12.7 l X min-1 X m-2). Also, stimulation of systemic arterial or central nervous system chemoreceptors cannot be eliminated from the response since significant amounts of 14CO2 were present in the arterial blood when this marker gas was added to the latter two-thirds of the inspiratory tidal volume. The results, therefore, provide no evidence for CO2-sensitive airway receptors that can increase breathing when stimulated during the latter part of the inspiratory cycle.