Blood gas results in respiratory disease - a useful “rule of thumb”

Respiratory acidosis and alkalosis are characterized by primary change in pCO₂ (increase in the case of acidosis and decrease in the case of alkalosis). Secondary physiological compensation, aimed at returning blood pH to normal, is evident in blood gas results as a change in bicarbonate concentration (increase in the case of acidosis and decrease in the case of alkalosis). 

A useful set of “rules of thumb” has been proposed to estimate the extent of physiological compensation, i.e. change in bicarbonate concentration (∆ HCO₃^–), to be expected for a given pCO₂.

The proposed “rules of thumb” are:

• Acute respiratory acidosis - ∆ HCO₃^– = 1 mmol/l for each 1 kPa change in pCO₂
• Acute respiratory alkalosis - ∆ HCO₃^– = 2 mmol/l for each 1 kPa change in pCO₂
• Chronic respiratory acidosis - ∆ HCO₃^– = 4 mmol/l for each 1 kPa change in pCO₂
• Chronic respiratory alkalosis - ∆ HCO₃^– = 3 mmol/l for each 1 kPa change in pCO₂

For the purposes of the calculation normal HCO₃^– is taken as 24.4 mmol/L and normal pCO₂ is taken as 5.3 kPa. If measured HCO₃^– differs significantly (± 5 mmol/L) from the estimated value then the acid-base disturbance is not purely respiratory in origin.