Chart — ABG & Acid-Base
Expected Compensation Chart
Once you name the primary disorder, this chart tells you what the other system should be doing to defend the pH. Compare your measured value to the expected one — a number outside the window is the fingerprint of a second, hidden disorder.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Expected Compensation by Primary Disorder
| Primary Disorder | Expected Compensation | Rule | Worked Example |
|---|---|---|---|
| Metabolic acidosis | PaCO₂ falls | Winter's: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (±2) | HCO₃⁻ 12 → expected PaCO₂ 26 (24–28) |
| Metabolic alkalosis | PaCO₂ rises | Expected PaCO₂ ≈ 40 + 0.7 × (HCO₃⁻ − 24) (±5) | HCO₃⁻ 35 → ≈ 47.7 (43–53) |
| Respiratory acidosis (acute) | HCO₃⁻ rises | +1 mEq/L per 10 mmHg PaCO₂ ↑ | PaCO₂ 60 → HCO₃⁻ ≈ 26 |
| Respiratory acidosis (chronic) | HCO₃⁻ rises | +3.5 mEq/L per 10 mmHg PaCO₂ ↑ | PaCO₂ 60 → HCO₃⁻ ≈ 31 |
| Respiratory alkalosis (acute) | HCO₃⁻ falls | −2 mEq/L per 10 mmHg PaCO₂ ↓ | PaCO₂ 30 → HCO₃⁻ ≈ 22 |
| Respiratory alkalosis (chronic) | HCO₃⁻ falls | −4 to −5 mEq/L per 10 mmHg PaCO₂ ↓ | PaCO₂ 30 → HCO₃⁻ ≈ 19–20 |
How to Use This Chart
Identify the primary disorder, calculate the expected value from the rule, then compare it to what the blood gas actually shows. If the measured value lands inside the window, compensation is appropriate and a single disorder explains the picture. If it lands outside, there is a second primary disorder driving the value the rest of the way.
- Measured vs. expected.A measured PaCO₂ above the Winter’s window adds a respiratory acidosis; below it adds a respiratory alkalosis.
- No overshoot. Compensation never normalizes or pushes the pH past 7.40 — if it appears to, you are looking at two primary disorders, not one being over-corrected.
- Acute vs. chronic.The kidneys take 48–72 hours to shift bicarbonate, which is why the respiratory rows split into acute and chronic rules. Use the patient’s timeline to choose the right one.
Every rule in this chart is built into the ABG Interpreter, which runs the expected-compensation math automatically and flags a second disorder when the measured value falls outside the window.
Related Resources
Sources
- Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312-322.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.