Metabolic acidosis

Question 1

Why is the anion gap important in metabolic acidosis?

Answer 1

It can be helpful to calculate the anion gap in order to identify the cause of the metabolic acidosis.

Question 2

How is the anion gap calculated?

Answer 2

This is calculated as the difference between plasma cations (Na+ and K+) and anions (Cl- and HCO3-).

Question 3

What is the normal range of the anion gap?

Answer 3

The normal range is 10-18mmol/L.

Question 4

What does a raised anion gap signify?

Answer 4

If the anion gap is raised, this suggests that there is increased production, or reduced excretion, of fixed/ organic acids e.g.

• Lactic acid (sepsis, tissue ischaemia)
• Urate (renal failure)
• Ketones (diabetic ketoacidosis, alcohol)
• Drugs/ toxins (salicylates, methanol, ethylene glycol)

Question 5

What is happening if there is a metabolic acidosis with a normal anion gap (hyperchloraemic metabolic acidosis)?

Answer 5

Either due to loss of bicarbonate, or accumulation of H+ ions. Causes include:

• Renal tubular acidosis
• Addison’s disease
• Gastrointestinal bicarbonate loss: diarrhoea, ureterosigmoidostomy, pancreatic fistula
• Drugs: e.g. acetazolamide
• Ammonium chloride injection

Question 6

How can you further divide metabolic acidosis?

Answer 6

Metabolic acidosis secondary to high lactate levels may be subdivided into two types:

1) lactic acidosis type A: shock, hypoxia, burns
2) lactic acidosis type B: metformin