Learn the difference between metabolic and respiratory compensation in ABG interpretation, including how the lungs and kidneys respond, when compensation occurs, and how to recognize the expected pattern on arterial blood gases.
Compensation is the body's attempt to reduce the pH disturbance caused by a primary acid-base disorder. In ABG interpretation, compensation does not fix the underlying problem. It simply helps move the pH closer to normal.
The key idea is simple: the lungs compensate for metabolic disorders, and the kidneys compensate for respiratory disorders.
Metabolic compensation happens when the kidneys adjust bicarbonate levels to respond to a primary respiratory disorder.
This is seen in respiratory acidosis and respiratory alkalosis. Because the kidneys work more slowly, metabolic compensation takes time.
Respiratory compensation happens when the lungs change ventilation to adjust PaCO₂ in response to a primary metabolic disorder.
This is seen in metabolic acidosis and metabolic alkalosis. Because breathing changes quickly, respiratory compensation happens much faster.
Decide whether the patient is acidotic or alkalotic first.
Determine whether PaCO₂ or HCO₃⁻ best explains the pH change.
If the primary problem is metabolic, the lungs compensate. If the primary problem is respiratory, the kidneys compensate.
Use compensation formulas to see whether the response is appropriate or if a mixed disorder is likely.
pH 7.28 / PaCO₂ 28 / HCO₃⁻ 12
This is a primary metabolic acidosis because the pH is low and HCO₃⁻ is low. The PaCO₂ is also low because the lungs are compensating by blowing off carbon dioxide.
See Winter’s Formula →pH 7.33 / PaCO₂ 60 / HCO₃⁻ 31
This is a primary respiratory acidosis because the PaCO₂ is high and the pH is acidotic. The bicarbonate is elevated because the kidneys are compensating.
See Compensation Formulas →Use the PulmTools ABG calculator to identify the primary disorder, evaluate compensation, and recognize possible mixed acid-base patterns.
Open ABG Calculator