A comprehensive review of metabolic acidosis

Arterial blood gas analysis is used to assess and monitor patient acid-base status. Disturbance of acid-base balance is classified to one of four main types depending on the pH, pCO₂(a) and bicarbonate results generated during blood gas analysis; the four types are respiratory acidosis, respiratory alkalosis, metabolic acidosis and metabolic alkalosis. 

A recent review article focuses on one of these disturbances, metabolic acidosis, which is characterized by primary decrease in bicarbonate and compensatory decrease in pCO₂(a). pH may be either reduced (if compensation is incomplete) or normal (if compensation is complete). This wide-ranging, comprehensive review includes discussion of epidemiology, pathophysiology, clinical consequences and management of metabolic acidosis.

The authors distinguish acute metabolic acidosis (lasting hours/days) from the much less common, chronic metabolic acidosis, which can last for years. Acute metabolic acidosis is a common feature of serious illness; a study quoted in the review suggests it affects around two thirds of all patients admitted to intensive care. In broad terms metabolic acidosis arises as a result of net loss of bicarbonate or excessive addition of acid.

Discussion of pathophysiology includes detailed consideration of the role of the kidney in regulating acid-base balance, focusing particularly on mechanisms involved in the maintenance of sufficient bicarbonate in blood to buffer metabolic acids. Distinguishing pure metabolic acidosis from a mixed respiratory/metabolic acidosis on the basis of the hypoventilatory response (i.e. magnitude of the reduction in pCO₂(a)) is a focus of the diagnosis section.

The value of measuring anion gap in identifying the cause of metabolic acidosis is also discussed, and the review includes a useful list of other laboratory tests that are helpful in identifying the cause of metabolic acidosis. The many adverse effects of acute metabolic acidosis discussed in this review include, decreased cardiac output, hypotension and change in mental status.

Chronic metabolic acidosis can lead to accelerated progression of kidney disease, disordered bone metabolism and muscle wasting. Discussion of treatment focuses on the controversies surrounding the use of sodium bicarbonate to correct acute metabolic acidosis; the authors provide their detailed recommendations. The use of bicarbonate to correct chronic metabolic acidosis is apparently not controversial. Supported by 123 references, this paper provides a useful overview of many aspects of metabolic acidosis.