MedCalcHub stores your favorites, history, and saved calculations locally in your browser using localStorage. This data never leaves your device.
Optional advertising, when enabled, may use third-party cookies or similar technologies. You can accept or reject advertising below.
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.
Expected HCO₃ = 24 + k × ((PaCO₂ − 40) ÷ 10), where k = 1 (acute respiratory acidosis), 4 (chronic), −2 (acute respiratory alkalosis), −5 (chronic)
Depends on the disorder type (see classification)
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 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 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.