Threshold alveolar-arterial Pco2 (PtCOCO2) 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 Po2, the lower the PtCOCO2. The relationship between the PaOO2 and the PtCOCO2 was curvilinear, PtCOCO2 showing an abrupt fall in the range of PaOO2 of 30–60 mm Hg. 2) The respiratory drive induced by hypoxia was completely extinguished by lowering alveolar Pco2. Thus it was proved that, during intense hypoxia, respiratory drive continues to be attributable to Pco2 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 PtCOCO2 differences and from the ventilatory coefficients. It was then inferred that a positive interaction between Po2 and Pco2 or cH stimulus for ventilation exists when PaOO2 falls below about 70 mm Hg. Pco2 levels in acute hypoxia; dogs
By observing respiratory discharges of the phrenic nerve, the threshold values of Pco2 (Ptco2) as chemical stimulus for the respiratory system were examined at varying arterial pH's on anesthetized dogs with and without vagotomy. 1) The reliability of the Ptco2 obtained by this method was high, the standard deviation being 0.48 mm Hg of Pco2. 2) The Ptco2 was lowered as the arterial cH (Ht) was increased by ammonium chloride infusion. The relationship between the Ptco2 and the Ht was linear and the regression line was calculated to be Ptco2 + 0.429 Ht = 44.0 for vagotomized and Ptco2 + 0.361 Ht = 46.1 for nonvagotomized dogs. 3) By combining this equation with that previously obtained expressing the partial effect of increased arterial cH and Pco2 on alveolar ventilation ratio (VR), the general chemical stimulation equation for ventilation in anesthetized normal dogs was derived to be VR = 0.194 Pco2 + 0.071 H – 8.94. 4) The Ptco2 of vagotomized dogs was several millimeters Hg lower than that of nonvagotomized dogs. Submitted on April 23, 1962
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