The preceding discussion applies to an acid-base problem involving a single primary metabolic or primary respiratory abnormality, with or without body attempts at compensation. Unfortunately, in some cases the laboratory picture is more complicated; for example, when there are superimposed attempts at therapy or when two different acid-base processes coexist (referred to as “mixed acid-base disorders”). An example is diabetic acidosis (metabolic acidosis) in a patient with chronic lung disease (compensated respiratory acidosis). In this circumstance it is very important to decide what serious clinical condition the patient has (e.g., renal failure, diabetic acidosis, chronic lung disease) that might affect the acid-base status and then what other conditions may be superimposed (such as vomiting, diuretic therapy, or shock) that could alter the acid-base picture in a certain direction.

PCO2 Values in Metabolic and Respiratory Acid-Base Disorders
PCO2 NORMAL
An abnormality in pH means an uncompensated metabolic process.
PCO2 ABNORMAL
PCO2 decreased
Could be respiratory (hyperventilation) in origin (respiratory alkalosis). If so, pH should be increased (acute onset), or normal, but more than 7.40 (chronic-compensated).
Could be metabolic acidosis. If so, pH should be decreased (partial compensation), or normal but more than 7.40 (fully compensated).
PCO2 increased:
Could be respiratory (hypoventilation) in origin (respiratory acidosis). If so, pH should be decreased (acute onset), or normal but less than 7.40 (chronic-compensated).
Could be metabolic alkalosis. If so, pH should be increased (partial compensation), or normal, but more than 7.40 (fully compensated).

In some cases of acid-base disturbance, such as classic diabetic acidosis, the diagnosis may be obvious. In other cases the diagnosis is made from the first set of arterial blood gas measurements. In these two situations, continued acid-base studies are needed only to gauge the severity of the disorder and response to therapy. If the PO2 does not indicate respiratory impairment, it may be sufficient to obtain PCO2 or HCO3 values, with or without pH determination, on venous specimens rather than make repeated arterial punctures.