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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