Carbon dioxide pressure-concentration relationship in arterial and mixed venous blood during exercise

Sun, Xing-Guo, James E. Hansen, William W. Stringer, Hua Ting, and Karlman Wasserman. Carbon dioxide pressure-concentration relationship in arterial and mixed venous blood during exercise. J Appl Physiol 90: 1798–1810, 2001.—To calculate cardiac output by the indirect Fick principle, CO2 concentrations (CCO2) of mixed venous (Cv#CO2) and arterial blood are commonly estimated from PCO2, based on the assumption that the CO2 pressure-concentration relationship (PCO2-CCO2) is influenced more by changes in Hb concentration and blood oxyhemoglobin saturation than by changes in pH. The purpose of the study was to measure and assess the relative importance of these variables, both in arterial and mixed venous blood, during rest and increasing levels of exercise to maximum (Max) in five healthy men. Although the mean mixed venous PCO2 rose from 47 Torr at rest to 59 Torr at the lactic acidosis threshold (LAT) and further to 78 Torr at Max, the Cv#CO2 rose from 22.8 mM at rest to 25.5 mM at LAT but then fell to 23.9 mM at Max. Meanwhile, the mixed venous pH fell from 7.36 at rest to 7.30 at LAT and to 7.13 at Max. Thus, as work rate increases above the LAT, changes in pH, reflecting changes in buffer base, account for the major changes in the PCO2CCO2 relationship, causing Cv#CO2 to decrease, despite increasing mixed venous PCO2. Furthermore, whereas the increase in the arteriovenous CCO2 difference of 2.2 mM below LAT is mainly due to the increase in Cv#CO2, the further increase in the arteriovenous CCO2 difference of 4.6 mM above LAT is due to a striking fall in arterial CCO2 from 21.4 to 15.2 mM. We conclude that changes in buffer base and pH dominate the PCO2-CCO2 relationship during exercise, with changes in Hb and blood oxyhemoglobin saturation exerting much less influence.