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

calculate cardiac output by the indirect Fick principle, CO2 concentrations (CCO2) of mixed venous (C (v) over bar (CO2)) and arterial blood are commo nly estimated from PCO2, based on the assumption that the CO2 pressure-conc entration relationship (PCO2-CCO2) is influenced more by changes in Hb conc entration and blood oxyhemoglobin saturation than by changes in pH. The pur pose of the study was to measure and assess the relative importance of thes e variables, both in arterial and mixed venous blood, during rest and incre asing 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 C (v) over bar (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 LA T and to 7.13 at Max. Thus, as work rate increases above the LAT, changes i n pH, reflecting changes in buffer base, account for the major changes in t he PCO2-CCO2 relationship, causing C (v) over bar (CO2) to decrease, despit e increasing mixed venous PCO2. Furthermore, whereas the increase in the ar teriovenous CCO2 difference of 2.2 mM below LAT is mainly due to the increa se in C (v) over bar (CO2), the further increase in the arteriovenous CCO2 difference of 4.6 mM above LAT is due to a striking fall in arterial CCO2 f rom 21.4 to 15.2 mM. We conclude that changes in buffer base and pH dominat e the PCO2-CCO2 relationship during exercise, with changes in Hb and blood oxyhemoglobin saturation exerting much less influence.