Measurements and normal values

The variables which we are interested in are:

• [H⁺] or pH

• pCO₂

• [HCO₃^-]

[HCO₃^-] is not easily measured and the surrogate used is [Total CO₂].  [Total CO₂] includes not only the [HCO₃^-] but also the CO₂ and other carbonates dissolved in the blood. As a result [Total CO₂] is 1-2 mEq/l higher than serum [HCO₃^-], although this will be greater with higher pCO₂ levels.

Serum [HCO₃^-] is not measured directly but derived from pH and pCO₂ using the Henderson-Hasselbach equation.  This is usually done on arterial blood and produces a lower value than the [Total CO₂] of venous blood.  This is because the pCO₂ of venous blood is higher than arterial which will also push up the [HCO₃^-].

Normal values:

pH and pCO₂ measured in venous blood will vary with local blood flow rate and therefore arterial values should be used.

Back