CO2 and bicarb are interchangable. CO2 does not travel in the blood as a gas, but gets changed into bicarb in the tissues and then back to CO2 in the lungs by a reaction with H2O and the enzyme carbonic anhydrase. So when they say CO2 they are actually measuring bicarb, but the bicarb level represents the level of CO2 in the blood. So basically its 2 ways if referring to the same thing.