Can expected pCO2 be calculated by pCO2=HCO3+15 formula in central venous blood gas samples?

In mixed acid-base disorders, it is essential to identify the dominant disorder, either metabolic or respiratory. The calculation of expected partial carbondioxide (pCO2) value obtained from arterial blood gas sample can give a clue to the physician about the main disorder. There are several formulas to calculate the expected pCO2 which are not practical to use and require an arterial blood gas sample. The aim of this study is to investigate whether expected pCO2 could be calculated with a simple formula by adding 15 to the bicarbonate (HCO3) value obtained from a central venous blood gas sample.

50 (42.7%) female and 67 (57.3%) male patients aged 18 years and older, hospitalized in the Intensive Care Unit (ICU) between January 2022 and June 2022, whose arterial and central venous blood gas samples were drawn at the same time, were included in this study. Expected pCO2 values were calculated with both Winter's (pCO2 = 1.5 × HCO3 + 8) and simple (pCO2 = HCO3 + 15) formulas from the data obtained from arterial and jugular central venous blood gas samples.

A statistically significant strong positive correlation was identified between arterial and venous expected pCO2 values, which were calculated by using both Winter's and simple formulas [Pearson's correlation coefficient (r) = 1, p<0.001].

In ICU patients, (pCO2 = HCO3 + 15) formula can be used to calculate expected pCO2 in central venous blood gas samples to identify the primary disorder as metabolic or respiratory in mixed acid-base disorders.