Metabolic Alkalosis and Respiratory Acidosis

Occurs in: 

• Diuretic therapy (with low potassium) and COPD (poor gas exchange with retention of CO₂)
• Vomiting (loss of H⁺) and COPD

To distinguish between compensated primary metabolic alkalosis or respiratory acidosis, and a primary mixed disorder, the clinical picture must be considered. Also, if the HCO₃^– is greater than 45 mmol/l, it is much more likely to be a mixed disorder. [11,12]

For example, in a patient on long term thiazide medication for hypertension who presents with an acute exacerbation of COPD:

• pH   7.36
• pCO₂   7.8 kPa
• pO₂   7.8 kPa
• bicarbonate  48 mmol/L
• potassium  3.2 mmol/L

It would be easy to be reassured by this patient’s normal pH, but the low potassium and acute presentation of the breathing difficulty should point to this being a more complicated picture. The bicarbonate is unusually high, even for metabolic compensation of a respiratory disorder. The clinical condition of the patient should be treated rather than simply relying on a normal pH. [11,12]