Acid-base status immediately following rapid changes of alveolar gas composition in awake dogs

To study the interrelationship between blood O2, CO2, and acid-base status during rapid changes of alveolar gas composition unanesthetized dogs were made to inhale high CO2 gas mixtures following air breathing or to rebreathe high CO2 and O2 mixtures following hypoxia. Before and immediately after each change in alveolar gases, sequential blood samples were taken from the carotid artery for measurement of pH, PCO2 and PO2. In the experiments at normoxia the calculated base excess (BE) decreased by about 0.7 mmol/L after 10 sec and then returned to baseline level. A smaller decrease (averaging 0.4 mmol/L) was found with hyperoxia following hypoxia. The changes in BE can be attributed to bicarbonate (or H+) exchange between blood and tissue. Lung tissue is probably responsible for the rapid initial change in BE.