Serial observations of arterial and mixed-venous blood gases after step change in ventilation

In 22 dogs, subjected to a step change in ventilation, serial changes in blood gas composition and lactate and pyruvate concentrations of arterial as well as mixed venous blood were studied. The change of PaCO2 was approximately 20 mm Hg both in hypo- and hyperventilation. During hypoventilation the difference in various forms of CO2 between arterial and mixed venous blood showed first a downward shift and then gradually increased, whereas during hyperventilation they progressively increased and reached a constant level within 10-20 min. This difference was assumed to be mainly due to more efficient CO2 elimination through lung ventilation in hyperventilation as compared with CO2 accumulation from tissue metabolism in hypoventilation. In vivo buffer slopes for CO2 during hypoventilation were about half those in vitro, whereas during hyperventilation both slopes were approximately the same. In vivo arterial buffer slope was higher during hypoventilation and lower during hyperventilation as compared to that of mixed venous blood in the respective state of ventilation.