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Calculates the expected arterial PaCO₂ in metabolic alkalosis (PaCO₂ ≈ 40 + 0.6 × (HCO₃ − 24)) to assess whether respiratory compensation is appropriate or whether a concurrent respiratory disorder is present.
Expected PaCO₂ = 40 + 0.6 × (HCO₃ − 24), capped near 55 mmHg; ±5 mmHg is considered appropriate compensation
PaCO₂ within ±5 mmHg of the expected value
In metabolic alkalosis, hypoventilation is the physiologic compensation and produces a rise in PaCO₂ of roughly 0.6 mmHg per 1 mEq/L rise in bicarbonate. The respiratory response has a ceiling near 55 mmHg.
Specialty
Internal MedicineCategory
Internal MedicineDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
PaCO₂ within ±5 mmHg of the expected value
| Classification | Range |
|---|---|
Appropriate compensation classic metabolic alkalosis rule | expected PaCO₂ ±5 mmHg |
Limits of compensation compensatory ceiling | PaCO₂ rarely exceeds ~55 mmHg |
Calculates the expected arterial PaCO₂ in metabolic alkalosis (PaCO₂ ≈ 40 + 0.6 × (HCO₃ − 24)) to assess whether respiratory compensation is appropriate or a concurrent respiratory disorder is present.
Expected PaCO₂ = 40 + 0.6 × (HCO₃ − 24), capped near 55 mmHg. The measured PaCO₂ should be within about 5 mmHg of the expected value. A value well below expected suggests a concurrent respiratory alkalosis; well above suggests a concurrent respiratory acidosis or a blunted ventilatory response.
Metabolic alkalosis is compensated by hypoventilation, raising PaCO₂ by roughly 0.6 mmHg per 1 mEq/L rise in bicarbonate up to a ceiling near 55 mmHg. The expected-PaCO₂ comparison detects inappropriate ventilation and mixed disorders.
A 50-year-old woman with vomiting-related metabolic alkalosis has a bicarbonate of 40 mEq/L and a measured PaCO₂ of 50 mmHg.
Inputs:
Expected PaCO₂ = 40 + 0.6 × (40 − 24) = 49.6 mmHg. The measured value (50) is within ±5 mmHg — appropriate respiratory compensation.
The ventilatory response has an upper limit — PaCO₂ rarely exceeds ~55 mmHg in pure metabolic alkalosis even at very high bicarbonates — so the linear rule flattens at the extreme.
It suggests a coexisting respiratory alkalosis (e.g., pain, anxiety, hypoxemia, or over-breathing). A PaCO₂ well above expected suggests a coexisting respiratory acidosis.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Acid–base interpretation requires the full clinical picture and should guide, not replace, clinical judgment.