AskCalc
Home › Nephrology & Electrolytes › pCO₂ in met alk

Expected pCO₂ in Metabolic Alkalosis

In metabolic alkalosis pCO₂ rises about 0.7 mmHg per 1 mmol/L rise in HCO₃: expected pCO₂ ≈ 40 + 0.7 × (HCO₃ − 24) ± 5 mmHg. Compensation rarely pushes pCO₂ above about 55 mmHg.

  • Serum bicarbonate (HCO₃⁻) (mmol/L or mEq/L)
  • Measured pCO₂ (optional) (mmHg or kPa)
—

Interpretation

ResultMeaning
Measured > expected + 5Additional respiratory acidosis
Within ± 5Appropriate compensation
Measured < expected − 5Additional respiratory alkalosis

Formula

Expected pCO₂ (mmHg) = 40 + 0.7 × (HCO₃⁻ − 24) ± 5 (≈ 0.7 × HCO₃⁻ + 23). Upper limit of compensation ≈ 55 mmHg.

Clinical pearls

  • Compensation for metabolic alkalosis is less predictable than for acidosis; published slopes range 0.6–0.75 mmHg per mmol/L.
  • A pCO₂ above ~55 mmHg usually indicates a primary respiratory acidosis as well.

Examples

expected pco2 metabolic alkalosis hco3 34met alk compensation bicarb 40 pco2 50vomiting with bicarbonate 36 and PaCO2 6.5 kPa: is compensation appropriate

FAQ

How much does pCO₂ rise in metabolic alkalosis?

About 0.7 mmHg for each 1 mmol/L rise in bicarbonate, rarely above 55 mmHg.

Can Winters' formula be used in metabolic alkalosis?

No. Winters' formula applies to metabolic acidosis; use the 0.7 × ΔHCO₃ rule for alkalosis.

Evidence

  1. Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med 2014;371:1434–45.
  2. Merck Manual Professional. Primary changes and compensations in simple acid-base disorders (table).

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology