Primary Changes and Compensations in Simple Acid-Base Disorders

Metabolic acidosis

< 7.35

Primary decrease

Compensatory decrease

1.2 mm Hg decrease in Pco₂ for every 1 mEq/L (1 mmol/L) decrease in HCO₃⁻

  or*

Pco₂= (1.5 × HCO₃⁻) + 8 (± 2)

  or

Pco₂= HCO₃⁻+ 15

  or

Pco₂= last 2 digits of pH ×100

Metabolic alkalosis

> 7.45

Primary increase

Compensatory increase

0.6–0.75 mm Hg increase in Pco₂ for every 1 mEq/L (1 mmol/L) increase in HCO₃⁻ (Pco₂ should not rise above 55 mm Hg in compensation)

Respiratory acidosis

< 7.35

Compensatory increase

Primary increase

Acute: 1–2 mEq/L (1–2 mmol/L) increase in HCO₃⁻ for every 10-mm Hg increase in Pco₂

Chronic: 3–4 mEq/L (3–4 mmol/L) increase in HCO₃⁻ for every 10-mm Hg increase in Pco₂

Respiratory alkalosis

> 7.45

Compensatory decrease

Primary decrease

Acute: 1–2 mEq/L (1–2 mmol/L) decrease in HCO₃⁻ for every 10-mm Hg decrease in Pco₂

Chronic: 4–5 mEq/L (4–5 mmol/L) decrease in HCO₃⁻ for every 10-mm Hg decrease in Pco₂