A comparison of carbon dioxide titration curves of arterial mixed venous and coronary sinus blood.

Two types of titration curve (log PCO2 vs. pH and whole blood [HCO3-] vs. pH) were obtained in anaesthetized dogs by altering FICO2. Both plots reveal a significant progressive decrease in slope from arterial(a) to mixed venous (v) to coronary sinus (cs) blood. Lines drawn between corresponding points on the arterial and mixed venous or coronary sinus curves have slopes determined by the Respiratory Exchange Ratio (R) and are called R lines. Differences in slope between arterial and venous (v or cs) curves may be due to changes in blood flow or of delivery of CO2 to the blood. The former is indicated by constant R values at different CO2 tensions whilst the latter is a reflection of changing R values due to fluctuations in CO2 stores. The myocardium is not an important store for CO2 but CO2 has a profound effect on coronary blood flow and it is this that causes the difference in a-cs slopes. The reverse is true for skeletal muscle where the charging and discharging of CO2 stores brings about the difference in a-v slopes.