ABG values: Measured vs calculated

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.