Approach to Acid-Base Disorders

Abstract

This review on the approach to acid-base disorders uses the physiologic approach to assessing acid-base status, namely that based on the H2CO3/[HCO3–] buffer pair. A simple acid-base disorder is characterized by a primary abnormality in either carbon dioxide tension (Pco2) or serum [HCO3–] accompanied by the appropriate secondary response in the other component. The four cardinal, simple acid-base disorders are categorized into respiratory disorders and metabolic disorders. Respiratory disorders are expressed as primary changes in Pco2 and include respiratory acidosis or primary hypercapnia (primary increase in Pco2) and respiratory alkalosis or primary hypocapnia (primary decrease in Pco2). Metabolic disorders are expressed as primary changes in serum [HCO3–]) and include metabolic acidosis (primary decrease in serum [HCO3–]) and metabolic alkalosis (primary increase in serum [HCO3–]). A mixed acid-base disorder denotes the simultaneous occurrence of two or more simple acid-base disorders. Arriving at an accurate acid-base diagnosis rests with assessment of the accuracy of the acid-base variables, calculation of the serum anion gap, and identification of the dominant acid-base disorder and whether a simple or mixed disorder is present. Identifying the cause of the acid-base disorder depends on a detailed history and physical examination as well as obtaining additional testing, as appropriate. Key words: acid-base disorders; simple disorders; mixed disorders; anion gap; physiologic approach; physicochemical approach; base-excess approach