Relationship between whole blood base excess and CO2 content in vivo

Empirical relationships are demonstrated for whole blood base excess (BE) and CO2 content (CCO2), both calculated from in vivo measurements of PCO2, pH, hemoglobin concentration and O2 saturation. Comparisons are provided by measurements from three separate studies: (1) supine exercise (arterial and mixed venous samples); (2) chronic obstructive disease patients (arterial samples) breathing air and 100% O2; and (3) maximal seated exercise on a bicycle ergometer with and without added inspired CO2 (arterial samples before, during and after). Two standardized values of CCO2 (vol.%) are derived which closely relate to BE (mmol/l). The CCO2 at a PCO2 of 40 mmHG [CCO2(40)] for all samples (n = 220) demonstrated a curvilinear relationship: CCO2 (40) = 45.37 + 1.48(BE) + 0.0156(BE)2, r = + 0.996, SEE = 0.88 vol.%. The CCO2 at a pH of 7.4 [CCO2(7.4)] gave a linear relationship: CCO2(7.4) = 45.09 + 2.58(BE), r = + 0.998, SEE = 1.19 vol.%. Empirical computations for the Haldane factor from studies 1 and 2 gave values of 0.285 in terms of CCO2 (vol.%/vol.%) and 0.266 for BE (mmol/l/mmol reduced Hb). The BE values can serve as useful estimates of lactate concentrations during exercise and the excellent relationships between standardized CCO2 and BE demonstrate their equivalency and either can be utilized, depending on whether quantification of the CO2 dissociation curve or acid-base status is desired.