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Expected HCO₃ in Respiratory Acidosis / Alkalosis

Expected HCO₃ for a primary respiratory disorder per 10 mmHg change in pCO₂ from 40: acute acidosis +1, chronic acidosis +3.5, acute alkalosis −2, chronic alkalosis −4 mmol/L. A measured HCO₃ outside ±2 suggests an additional metabolic disorder.

Arterial pCO₂ (mmHg or kPa)
Duration: Acute (< 24–48 h); Chronic (> 3–5 days)
Measured HCO₃⁻ (optional) (mmol/L or mEq/L)

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Interpretation

Result	Meaning
Acute respiratory acidosis	HCO₃ ↑ 1 per 10 mmHg pCO₂ rise
Chronic respiratory acidosis	HCO₃ ↑ 3.5 per 10 mmHg
Acute respiratory alkalosis	HCO₃ ↓ 2 per 10 mmHg pCO₂ fall
Chronic respiratory alkalosis	HCO₃ ↓ 4 per 10 mmHg

Formula

Expected HCO₃⁻ = 24 + k × (pCO₂ − 40); k = 0.1 (acute acidosis), 0.35 (chronic acidosis), 0.2 (acute alkalosis), 0.4 (chronic alkalosis). Range ± 2 mmol/L.

Clinical pearls

Published slopes vary (chronic acidosis 0.3–0.4, chronic alkalosis 0.4–0.5); treat the ±2 boundary as approximate.
Renal compensation takes 3–5 days; intermediate durations fall between the acute and chronic values.

Examples

respiratory compensation pco2 60 chronic hco3 31 expected hco3 acute respiratory acidosis paco2 70 copd patient chronic co2 retention pCO2 8 kPa bicarbonate 34

FAQ

How do I tell acute from chronic respiratory acidosis?

Acute: HCO₃ rises about 1 mmol/L per 10 mmHg pCO₂ rise. Chronic: about 3.5–4 mmol/L per 10 mmHg after renal compensation.

How much does bicarbonate fall in respiratory alkalosis?

About 2 mmol/L per 10 mmHg fall in pCO₂ acutely and 4–5 mmol/L chronically.

Evidence

Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med 2014;371:1434–45.

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