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Estimates the expected serum bicarbonate during acute or chronic respiratory acidosis and alkalosis using the Winters/Merck compensation rules, helping to identify whether a metabolic disturbance is appropriately compensated or a mixed disorder is present.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
The compensation rules quantify the normal bicarbonate response to chronic alterations in PaCO₂. When the measured bicarbonate deviates beyond the expected range, a mixed acid–base disorder should be suspected.
Specialty
Internal MedicineCategory
Internal MedicineDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Depends on the disorder type (see classification)
| Classification | Range |
|---|---|
Acute respiratory acidosis Merck / classic rule | +1 to +2 mEq/L HCO₃ per 10 mmHg PaCO₂ |
Chronic respiratory acidosis Merck / classic rule | +3 to +4 mEq/L HCO₃ per 10 mmHg PaCO₂ |
Acute respiratory alkalosis Merck / classic rule | −1 to −2 mEq/L HCO₃ per 10 mmHg PaCO₂ |
Chronic respiratory alkalosis Merck / classic rule | −4 to −5 mEq/L HCO₃ per 10 mmHg PaCO₂ |
Estimates the expected serum bicarbonate during acute or chronic respiratory acidosis and alkalosis using the Winters/Merck compensation rules, to identify appropriate compensation or a mixed acid–base disorder.
Expected bicarbonate = 24 + k x ((PaCO2 - 40) / 10), where k = 1 for acute respiratory acidosis, 4 for chronic respiratory acidosis, -2 for acute respiratory alkalosis, and -5 for chronic respiratory alkalosis. The expected value is compared with the measured bicarbonate to identify mixed disorders.
Selects the acute or chronic respiratory disorder, which determines the compensation coefficient k.
Arterial partial pressure of carbon dioxide in mmHg, entered into the (PaCO2 - 40) / 10 term.
Measured serum bicarbonate in mEq/L, compared with the expected value to assess compensation.
Expected HCO₃ = 24 + k × (PaCO₂ − 40)/10, with k = 1 (acute respiratory acidosis), 4 (chronic), −2 (acute respiratory alkalosis), and −5 (chronic). The measured bicarbonate should be within about ±2 mEq/L of the expected value; larger deviations suggest a mixed acid–base disorder.
Compensation rules quantify the normal metabolic response to chronic PaCO₂ alterations. Deviations beyond the expected range unmask mixed acid–base disorders that would otherwise be overlooked.
A 65-year-old man with acute hypercapnic respiratory failure has a PaCO₂ of 50 mmHg and a measured bicarbonate of 25 mEq/L.
Inputs:
Expected Result: Expected HCO₃ = 24 + 1 × (50 − 40)/10 = 25 mEq/L. The measured value (25) is within ±2 mEq/L — appropriate acute respiratory compensation.
It means the measured bicarbonate should fall within roughly 2 mEq/L of the calculated expected value for the patient's PaCO₂ and disorder type. Larger deviations suggest a mixed acid–base disorder.
Acute changes rely mainly on cellular buffering, which raises HCO₃ modestly. With time, renal compensation becomes much more effective, so chronic respiratory acidosis and alkalosis produce larger changes in bicarbonate.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Acid–base interpretation requires the full clinical picture, including electrolytes, albumin, and the anion gap.