Ventilatory sensitivity to changes in inspired and arterial carbon dioxide partial pressures in the chicken

To determine if intrapulmonary chemoreceptors could be the sole peripheral chemoreceptors responsible for ventilatory responses to inhaled CO2, we studied the relationships of minute ventilation, tidal volume, and respiratory frequency to inspired and arterial partial pressure of CO2 (P(I)CO2 and PaCO2) in decerebrate upright chickens when the birds inspired gases containing four low partial pressures of CO2 (0.02, 5.0, 8.2, and 11.4 torr). Because of variability in the measured variables from time to time in the same birds, as well as between birds, and because of the limited precision in measuring PaCO2, a 4 x 4 Latin square design and four statistical methods of data analyses (modified reduced major axis estimator, maximum likelihood estimator, average slope method, and summary slope method) were used. Tidal volume, minute ventilation, and PaCO2 increased, but respiratory frequency remained unchanged, as gases containing increased partial pressures of CO2 were inhaled. The results indicate that intrapulmonary chemoreceptors are not the sole receptors stimulated by inhaling gas containing even low partial pressures of CO2 and that stimulation of arterial and central chemoreceptors also occurs. Stimulation of these latter chemoreceptors may account for the increase in ventilation. Further, the results demonstrate the importance of maintaining a low level of environmental CO2 in poultry houses to minimize its influence on ventilation.