Metabolic Alkalosis

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3


In a metabolic alkalosis, a high pH is caused primarily by either a raised [HCO3] or reduced [H+]. By adding a base to the equation, it binds to H+ removing the H+ from the system. The equation then moves to the right and the HCO3 rises. A similar shift will also occur following direct loss of H+.

Causes of metabolic alkalosis
  • Direct loss of H+ in gastric secretion:

    • Vomiting
    • Nasogastric suction
  • H+ loss from the kidneys where high aldosterone causes Na+ reabsorption and H+ secretion – Conn’s syndrome
  • H+ shift into cells – hypokalaemia
  • Excess alkali

    • IV bicarbonate administration in large amounts
    • Ingestion of antacids

For example, in a patient with profound vomiting:

  • pH   7.49
  • pCO2   5.4 kPa
  • pO2   10.6 kPa
  • bicarbonate  37 mmol/L

The pH is high indicating an alkalosis due to the direct loss of H+ from the gut. Because the equation moves to the right, the bicarbonate is high, indicating a metabolic disorder.

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