Effect of an increase in cardiac output on the in vivo C02 titration curve of mixed venous blood.

The slope of the in vivo CO2 titration curve of blood (pH-[HCO3-]p relationship, in vivo slope, unit: slyke) expresses the buffering capacity of blood for CO2 in vivo or the buffering capacity of extracellular fluid for CO2. It depends on the extent of contributions by chemical and physiological buffering processes for CO2 occurring during CO2 titration. Theoretically, the in vivo slope obtained for mixed venous blood is independent of cardiac output during CO2 titration. The present study was attempted to confirm this theoretical concept experimentally. Anesthetized dogs were allowed to breathe air and then 10% CO2-25% 02 in N2 for 60 min for the titration in vivo. During the CO2 breathing period the legs were exercised through direct electrical stimulation of the muscles; this also induced an increase in cardiac output 130% above the air breathing period. The in vivo slope (delta[HCO3-]p/delta/pH-]p/"pH) of the mixed venous blood at an apparent steady state in the CO2 breathing period was 16 slyke, which was 5 slyke higher than in other dogs breathing CO2 at rest with a 60% increase in cardiac output observed in our previous study. This finding was discussed in relation to a greater contribution of tissue fluid for buffering of CO2. The present study suggests that cardiac output is a significant determinant of the buffering capacity of extracellular fluid.