Effects of low intrapulmonary PCO2 on ventilatory sensitivity to PaCO2 in chickens

We found conflicting reports regarding the relationship between intrapulmonary PCO2 and sensitivity to PaCO2 in the chicken. To resolve this, we anesthetized eight cockerels with sodium pentobarbital (25-35 mg/kg), cannulated the cutaneous ulnar vein and carotid artery, opened the thorax and ventilated right and left lungs independently. We established PaCO2 by over-ventilating the denervated, perfused right lung with the following: 94.7, 39.9, 31.6 and 19.0 Torr PCO2 balanced with oxygen. At each right lung CO2 tension (PRCO2), we measured blood pressure and ventilatory responses to five CO2 tensions (PLCO2) ventilating the non-perfused left lung. PLCO2 ranged from 70.3 to 15.7 Torr. We found a linear relationship between the amplitude of sternal deflections and PaCO2-1, and that the slope depended upon PLCO2. Respiratory period was also linearly related to PaCO2-1, however the intercept, not the slope, was a altered by PLCO2. Stepwise regression analyses revealed an important interaction term (that is, (PaCO2.PLCO2)-1) in the determination of respiratory amplitude, but not in the determination of period. We conclude that low intrapulmonary PCO2 enhances the sensitivity of respiratory amplitude to changes in PaCO2. Further, we propose that intrapulmonary chemoreceptor discharge mediates this change in sensitivity to PaCO2.