Metabolic acidosis classification
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Metabolic acidosis can be classified as:
- Low anion gap metabolic acidosis
- Normal anion gap metabolic acidosis (Hyperchloremic Acidosis)
- High anion gap metabolic acidosis
Normal Anion Gap (Hyperchloremic Acidosis)
Usually the HCO3- lost is replaced by a chloride anion, and thus there is a normal anion gap. In normal anion gap acidosis, the increased anion is chloride, which is measured, so the anion gap does not increase. Thus, normal anion gap acidosis is also known as hyperchloremic acidosis. Urine anion gap is useful in evaluating a patient with a normal anion gap.
Coexistent Elevated Anion Gap and Normal Anion Gap Metabolic Acidosis
- An elevated anion gap can coexist with a normal anion gap metabolic acidosis.
- In a single acid-base disorder of elevated anion gap metabolic acidosis, serum bicarbonate (HCO3) will decrease by the same amount that the anion gap increases.
- However, a situation in which the anion gap increases less and serum bicarbonate decreases significantly indicates that there is another metabolic acidosis present, which is decreasing the the serum bicarbonate, but not affecting the anion gap i.e. normal anion gap metabolic acidosis is also present.
- Thus, it is advised to compare the changes in the anion gap with the changes in the serum bicarbonate.
- This is often referred as the delta-delta equation, or the corrected bicarbonate equation.
- Delta-Delta equation: Change in anion gap = Change in bicarbonate