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Calculates the expected compensatory respiratory response to a primary metabolic acidosis: expected PaCO2 = 1.5 × HCO3 + 8 ± 2 mmHg. It is used to determine whether the respiratory response is appropriate, inadequate (respiratory acidosis), or excessive (respiratory alkalosis).
Expected PaCO2 (mmHg) = 1.5 × HCO3 (mEq/L) + 8 ± 2
Expected PaCO2 ± 2 mmHg
Winter's formula is a compensation equation: it predicts the physiologic ventilatory response (approximately 1.5 × HCO3 + 8 ± 2 mmHg), not a target to be achieved.
Specialty
Internal MedicineCategory
Internal MedicineDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Expected PaCO2 ± 2 mmHg
| Classification | Range |
|---|---|
Appropriate compensation primary metabolic acidosis | Measured PaCO2 within expected ± 2 |
Inadequate compensation concurrent respiratory acidosis | Measured PaCO2 above expected + 2 |
Excessive compensation concurrent respiratory alkalosis | Measured PaCO2 below expected − 2 |
Calculates the expected compensatory PaCO2 in primary metabolic acidosis using Winter's formula (expected PaCO2 = 1.5 × HCO3 + 8 ± 2 mmHg), helping identify appropriate, inadequate, or excessive respiratory compensation.
If measured PaCO2 is within expected ± 2 mmHg, compensation is appropriate. Above the range: concurrent respiratory acidosis (inadequate compensation). Below the range: concurrent respiratory alkalosis (excessive compensation).
Winter's formula quantifies the expected ventilatory compensation in metabolic acidosis, allowing clinicians to detect superimposed respiratory acid-base disorders that would otherwise be missed.
A 32-year-old man with DKA has serum bicarbonate 10 mEq/L and arterial PaCO2 28 mmHg.
Inputs:
Expected PaCO2 = 1.5 × 10 + 8 = 23 mmHg (range 21–25). Measured 28 mmHg is above the range — compensation is inadequate, suggesting a concurrent respiratory acidosis.
Compensation is inadequate — evaluate for a concurrent respiratory acidosis, such as hypoventilation from sedation, neuromuscular disease, or lung disease.
No. In metabolic alkalosis the expected compensation follows a different relationship (PaCO2 ≈ 0.7 × HCO3 + 20), capped near 55 mmHg.
First-step acid-base classification
High anion gap acidosis evaluation
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Acid-base interpretation requires correlation with pH, the clinical picture, and the timing of blood sampling.