First we have to know about the concept of anion gap which is the
difference between plasma concentration of Na+ and the sum of chloride
and bicarbonate [Na+ — (CL + HCo 3 - ) = 6 — 16 mmol]. This gap
represents substances which combine with Na+ other than CL- and HCo 3 -
which are not measured in routine chemistry such as amino acids.We may classify metabolic acidosis into those with high anion gap
[Na+ — (CL + HCo 3 - ) > 16 mmol] and those with normal anion gap:I- Metabolic acidosis with high anion gap:
The high anion gap is due to the addition of anionic toxic substances
into the circulation which combine with Na+ at the expense of chloride
and HCo 3 -. Since these substances are not measured, the anion gap will
be high.Causes of metabolic acidosis with high anion gap are:- Lactic acidosis; the anion toxic substance here is lactate
- Diabetic ketoacidosis with accumulation of acetoacetic acid; B-
hydroxybuteric acid - Intoxication with methyl alcohol; Ethylene glycol, paraldehyde and
salicylates. - Renal failure with accumulation of sulfates; phosphates and phenols.
II- Metabolic acidosis with normal anion gap (hyperchloraemic metabolic
acidosis).
This could be due to renal, gastrointestinal or other defects.
A. Renal causes of metabolic acidosis with normal Anion gap: