The arterial pCO2 value is used to quantify the magnitude of the alteration in alveolar ventilation (assuming CO2 production is constant and inspired pCO2 is negligible). The arterial pCO2 alone is not satisfactory for assessing the magnitude of a respiratory acidosis in some cases. In particular, coexisting metabolic acid-base disorders cause compensatory changes in pCO2 and these must be accounted for.
The reason we have to allow for a metabolic acid-base disorder is that the pCO2 value changes from 40mmHg due solely to the body's compensatory ventilatory response to a metabolic acidosis or alkalosis so just using a value of 40mmHg as normal would be wrong and lead us to incorrect conclusions.
With an acute metabolic acidosis, the body responds by increasing alveolar ventilation. This response is compensatory because hyperventilation results in a decrease in arterial pCO2 which tends to return the arterial pH towards 7.4 partially correcting the acute deviation of plasma pH from normal. The value of pCO2 at maximal compensation can be predicted using a simple bedside 'rule of thumb' and this calculated value is the 'expected' pCO2 which we use to compare with the 'actual'(measured) pCO2 value.
If a metabolic disorder is present, we can calculate (using a simple formula) a new reference value of pCO2 ( the ‘expected pCO2’) that we would expect that would be present with typical levels of respiratory compensation. We use this calculated 'expected value' to compare with the actual measured value.
You will now note as a consequence of this approach something that you might think to be rather odd: that is, it is possible for a patient to have a significant respiratory acidosis and yet be hypocapnic! This seems counter-intuitive if you wrongly considered that the terms 'respiratory acidosis' and 'hypercapnia' to be synonomous.
A final point: There is a widespread use of the term 'respiratory alkalosis' to refer to the compensatory hyperventilation that occurs with a metabolic acidosis but this term is quite wrong in this situation. The terms 'acidosis' & 'alkalosis' refer to primary abnormal processes (by definition) and should never be used to refer to compensatory processes. (Refer to Section 3.1 for definitions & discussion).