Threshold Pco2 for respiratory system in acute hypoxia of dogs

Threshold alveolar-arterial Pco 2 (Pt CO CO2 ) for the respiratory system in acute hypoxia of anesthetized dogs was examined by observing phrenic nerve discharges. It was found that 1) the lower the alveolar Po 2 , the lower the Pt CO CO2 . The relationship between the Pa O O2 and the Pt CO CO2 was curvilinear, Pt CO CO2 showing an abrupt fall in the range of Pa O O2 of 30–60 mm Hg. 2) The respiratory drive induced by hypoxia was completely extinguished by lowering alveolar Pco 2 . Thus it was proved that, during intense hypoxia, respiratory drive continues to be attributable to Pco 2 or cH, even when they have been reduced far below normal. 3) Ventilatory response related to the hypoxic drive with other parameters constant was calculated from the nonhypoxic and hypoxic Pt CO CO2 differences and from the ventilatory coefficients. It was then inferred that a positive interaction between Po 2 and Pco 2 or cH stimulus for ventilation exists when Pa O O2 falls below about 70 mm Hg. Pco 2 levels in acute hypoxia; dogs