ABG values: Measured vs calculated

Advanced, Critical Care, Organ-Based and Clinical Sciences

Arterial blood gas (ABG) analysis provides important information about a patient’s overall acid-base status, as well as information about oxygenation and ventilation. Most ABGs typically provide some basic electrolyte concentrations.  

Co-oximetry measures concentrations of oxygenated hemoglobin (oxyHgb), deoxygenated hemoglobin (deoxyHgb or reduced Hgb), carboxyhemoglobin (COHgb), and methemoglobin (MetHgb) as a percentage of the total hemoglobin concentration in the blood sample.

Most of these values are measured directly by ABG machine, except for bicarbonate concentration, which is calculated from measured pCO2 and pH values using the Henderson-Hasselbach equation.

Total CO2 exists in blood in multiple forms.  The bicarbonate value is a key figure in assessment of acid-base status because it is the most important buffer, accounting for the majority of total buffering in blood (the rest being performed by proteins such as Hgb).   

CO2 + H2O → H2CO3 → H+ + HCO3−

pH = pKa + log[HCO3−]/[H2CO3]

Total CO2 = [HCO3−] + [DissolvedCO2] + [CarbaminoCO2] + [H2CO3]

≈ PCO2 × 0.03 mmolCO2/L/mmHg

from above 2 equations, pH = 6.1 + log[HCO3−]/(PCO2 × 0.03)

Measured total bicarbonate = 0.03 x pCO2 x 10^(pH-pKa); assuming that the dissociation constant (pKa) and solubility coefficient (alpha) are constant.

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Author
Achillina Rianto, MD