Mixed Acid-Base Disorders

The most common mixed acid-base disorder recognized on biochemical panels is a concurrent metabolic acidosis and metabolic alkalosis. This type of disorder may be seen when metabolic alkalosis, for any of the reasons described above, is associated with significant fluid volume depletion from lack of water intake and/or increased water losses (e.g. concurrent diarrhea). Dehydration may lead to lactic acid and possibly renal acid accumulation resulting in acid gain metabolic acidosis with a high anion gap. The typical biochemical abnormalities are hypochloremia, increased anion gap, and a bicarbonate that may be normal, increased, or decreased depending on the relative magnitude of the two acid-base disturbances. A normal bicarbonate value can be misleading in these cases, since this seems to suggest that there is no alkalosis or acidosis. However, the presence of an increased anion gap or hypochloremia are clues that indicate a mixed acid base disorder. Inappropriate level of compensation may be another clue to indicate the presence of a mixed acid-base disorder.

An example of a mixed acid-base disorder identified on blood gas analysis would be a patient with both respiratory acidosis and metabolic alkalosis. In this situation, respiratory acidosis would result in increased pCO₂ and decreased pH, while metabolic alkalosis would result in increased HCO₃^– and increased pH. Despite the presence of two acid-base disorders, the pH could be normal.