Metabolic acidosis

Metabolic acidosis is an acid-base disorder characterised by a blood pH < 7.35 which is caused by a metabolic problem.

Respiratory acidosis is similar but is rather due to a respiratory problem. Metabolic alkalosis is sort of the opposite of metabolic acidosis.

Types

There are two types of metabolic acidosis: Normal anion gap metabolic acidosis (NAGMA) and high anion gap metabolic acidosis (HAGMA).

HAGMA = High anion gap metabolic acidosis. NAGMA = Normal anion gap metabolic acidosis. The figure to the left shows the normal situation

Anion gap

The anion gap is a fancy term we use to include all anions in the serum that are not bicarbonate and chloride. The reason they’re called that is because the levels of these anions aren’t measured directly, but rather calculated by the following calculation:

Anion gap = [Na] – ([Cl–] + [HCO3–])

The reason this works is because we know that the plasma is electroneutral, meaning that there must be an equal amount of positive and negative charges. Almost all the positive charge in plasma comes from sodium, while the negative charges come from chloride, bicarbonate and other anions that aren’t measured in the lab, like lactate, phosphate, sulphate, proteins).

The anion gap can be increased due to elevated levels of acids like lactic acid, ketone bodies, ethylene glycol, etc. The extra acids increase the “gap”.

The anion gap is normal in case of increased loss of bicarbonate or decreased loss of H+. No extra acids means that there is no increased “gap”.

High anion gap metabolic acidosis

High anion gap metabolic acidosis (HAGMA) or elevated anion gap metabolic acidosis indicates a metabolic acidosis where the [HCO3–] is reduced while the concentration of the anions that comprise the anion gap is increased. It occurs when there is overproduction or ingestion of organic acids or decreased acid excretion in the kidney.

Normal anion gap metabolic acidosis

Normal anion gap metabolic acidosis (NAGMA) occurs when HCO3– is lost in significant amounts, but no excess acid accumulates in the blood. When bicarbonate is lost will chloride be retained to uphold the electroneutrality. Because of the increased chloride is it sometimes called hyperchloraemic metabolic acidosis. One can distinguish between a hypokalaemic and a hyperkalaemic type of NAGMA, but this distinction is rarely used.