Acid-Base Physiology

6.5 Respiratory Alkalosis - Compensation

The compensatory response is a fall in bicarbonate level.

As can be seen by inspection of the Henderson-Hasselbalch equation (below), a decreased [HCO3-] will counteract the effect of a decreased pCO2 on the pH. Mathematically, it returns the value of the [HCO3]/0.03 pCO2 ratio towards normal.

pH = pKa + log {([HCO3]/ 0.03 pCO2 }

Key points regarding compensation in respiratory alkalosis:
Compensation in an ACUTE Respiratory Alkalosis
  • Mechanism:Changes in the physicochemical equilibrium occur due to the lowered pCO2 and this results in a slight decrease in HCO3-. There is insufficient time for the kidneys to respond so this is the only change in an acute respiratory alkalosis. The buffering is predominantly by protein and occurs intracellularly; this alters the equilibrium position of the bicarbonate system.
  • Magnitude: There is a drop in HCO3- by 2 mmol/l for every 10mmHg decrease in pCO2 from the reference value of 40mmHg.
  • Limit: The lower limit of 'compensation' for this process is 18mmol/l - so bicarbonate levels below that in an acute respiratory alkalosis indicate a co-existing metabolic acidosis. (Alternatively, their may be some renal compensation if the alkalosis has been present longer than realised.)

Compensation in a CHRONIC Respiratory Alkalosis
  • Mechanism: Renal loss of bicarbonate causes a further fall in plasma bicarbonate (in addition to the acute drop due to the physicochemical effect and protein buffering).
  • Magnitude: Studies have shown an average 5 mmol/l decrease in [HCO3-] per 10mmHg decrease in pCO2 from the reference value of 40mmHg. This maximal response takes 2 to 3 days to reach.
  • Limit: The limit of compensation is a [HCO3-] of 12 to 15 mmol/l.