EFFECT OF AN INCREASE IN CARDIAC OUTPUT ON THE IN VIVO CO₂ TITRATION CURVE OF MIXED VENOUS BLOOD

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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% O₂ in N₂ for 60min 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 (Δ[HCO₃^-] P/ΔpH) 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.