Metabolic Acidosis

Metabolic acidosis is the most common acid-base disorder. There is either an acid gain, absolute loss of bicarbonate from the ECF (that is, loss of base) or both. Bicarbonate and pH are both decreased. Metabolic acidosis can be identified by assessing the bicarbonate value together with anion gap.

Acid gain metabolic acidosis

Metabolic acidosis that is caused by a gain of acids is also referred to as a titration type of acidosis because bicarbonate is utilized to titrate the excess H⁺ of the added acid. On the biochemical panel decreased bicarbonate and increased anion gap are normally present.

Causes of acid gain metabolic acidosis: accumulation of organic acids e.g. ketoacids (as with diabetic ketoacidosis), lactic acid (e.g. grain overload, extreme exercise, poor tissue perfusion as with shock and ECF volume contraction), renal acids (as with renal failure); exogenous toxins (e.g. ethylene glycol, salicylates); and decreased renal excretion of NH₄⁺ (as with distal renal tubular acidosis).

Bicarbonate loss metabolic acidosis

Metabolic acidosis that is caused by a loss of base (i.e. bicarbonate) is also sometimes referred to as a secretory type of acidosis especially when excess bicarbonate is secreted (e.g. in saliva). On the biochemical panel the bicarbonate concentration is decreased, the anion gap is normal, and chloride may be normal or increased (to maintain electroneutrality).

Causes of bicarbonate loss metabolic acidosis: diarrhea (resulting in loss or sequestration of bicarbonate-rich fluid), or renal bicarbonate loss (as with proximal renal tubular acidosis). In cattle, salivary bicarbonate loss (e.g. esophageal obstruction [choke]) may result in metabolic acidosis due to bicarbonate-rich saliva in this species.

Respiratory compensation

Respiratory compensation for metabolic acidosis occurs by hyperventilation. The observed respiratory compensatory response in dogs and horses is approximately 0.7 mmHg decrease in pCO₂ for every 1 mmol/L decrease in HCO₃^–. The respiratory response of cats to metabolic acidosis is not defined, but there has been some evidence to suggest that the cat does not develop respiratory compensation to the same extent as dogs.

If the actual measured pCO₂ is higher than the expected pCO₂, then a mixed acid-base disturbance is likely, namely, metabolic acidosis and respiratory acidosis (see below for discussion of mixed acid-base disorders). If the actual measured pCO₂ is lower than the expected pCO₂, then metabolic acidosis and respiratory alkalosis are likely. These interpretations can be made providing there has been sufficient time for respiratory compensation to occur which is generally quite rapid.