Chapter 6 - Metabolic Acidosis

Abstract

The diagnostic approach to metabolic acidosis utilizes information from the history and physical examination, and laboratory studies in a stepwise fashion. Step 1: Review history and physical examination—Identify potential processes in the history, recognize the situations that often are associated with metabolic acidosis, and review the physical examination for clue to diagnosis. Step 2: Evaluate serum electrolytes and calculate serum anion gap—Diagnosis of the underlying causes of metabolic acidosis can be simplified by determining whether the disorder is associated with an increased or a normal anion gap. Metabolic acidosis occurring with a normal anion gap is usually caused by either gastrointestinal bicarbonate loss or renal tubular acidosis and the use of carbonic anhydrase inhibitors. Normal anion gap acidosis has also been reported in ureteral diversions, NH4Cl infusions, and during hyperalimentation. The anion gap acidosis can be divided into five major categories: (1) renal failure; (2) ketoacidosis (diabetes, hypoglycemic ketoacidosis); (3) lactic acidosis (hepatic failure, circulatory failure, sepsis); (4) drugs and poisons (salicylate, paraldehyde, ethanol, ethylene glycol); and (5) inborn errors of metabolism. Step 3: Assess the degree of respiratory compensation—Is the respiratory system compensating adequately? Step 4: Identify concomitant metabolic disorder—Is the anion gap increased above normal value (> 16 mEq/L)? If yes, there should be concordance between the fall in the HCO3− concentration and the increase in anion gap. This relationship works because, for each mEq increase in the anion gap, there is a mEq decrease in the serum HCO3 concentration.