Acid-Base Physiology

5.2 Metabolic Acidosis - Causes

5.2.1 Classification by Patho-physiological Mechanism

A decrease in plasma bicarbonate can be caused by two mechanisms:

All causes of a metabolic acidosis must work by these mechanisms. The gain of strong acid may be endogenous (eg ketoacids from lipid metabolism) or exogenous (NH4Cl infusion). Bicarbonate loss may occur via the bowel (diarrhoea, small bowel fistulas) or via the kidneys (carbonic anhydrase inhibitors, renal tubular acidosis).

5.2.2 Classification by Anion Gap

An alternative to the above, is to classify the causes of metabolic acidosis into two groups depending on whether the anion gap is elevated or normal. These 2 groups are referred to as:

The term 'hyperchloraemic metabolic acidosis' is also often used for the 'normal anion gap' group but the terms are not really synonomous (as discussed in section 8.4).

This is the most clinically useful way to classify metabolic acidosis and it is used extensively when assessing metabolic acidosis. The further sub-divisions within this classification are outlined in the table below.

Causes of Metabolic Acidosis (classified by Anion Gap)

A: High Anion-Gap Acidosis

1. Ketoacidosis

  • Diabetic ketoacidosis
  • Alcoholic ketoacidosis
  • Starvation ketoacidosis

2. Lactic Acidosis

  • Type A Lactic acidosis (Impaired perfusion)
  • Type B Lactic acidosis (Impaired carbohydrate metabolism)

3. Renal Failure

  • Uraemic acidosis
  • Acidosis with acute renal failure

4. Toxins

  • Ethylene glycol
  • Methanol
  • Salicylates
B : Normal Anion-Gap Acidosis (or Hyperchloraemic acidosis)

1. Renal Causes

  • Renal tubular acidosis
  • Carbonic anhydrase inhibitors

2. GIT Causes

  • Severe diarrhoea
  • Uretero-enterostomy or Obstructed ileal conduit
  • Drainage of pancreatic or biliary secretions
  • Small bowel fistula

3. Other Causes

  • Recovery from ketoacidosis
  • Addition of HCl, NH4Cl